Heartworm Disease - Treatment


Melarsomine (Immiticide Rx) and fertility

Question: How does Immeticide affect a dogs fertility??


Answer: Cherie-

The safety of melarsomine (Immiticide Rx) was not determined for dogs who might be used for breeding purposes in the studies done for FDA approval. It is an organic arsenical compound, which is similar to the original heartworm treatment medication, thiacetarsemide sodium ( Caparsolate Rx). While this medication was also apparently never really tested for use in dogs intended for breeding I know from our practice experience that it did not appear to impair future reproductive capability to treat dogs with thiacetarsemide, as we had a number of both female and male dogs who were successfully bred following heartworm treatment. To the best of my ability to remember these things, we have only had one female dog who has been bred following treatment with melarsomine, so far, and there were no problems with the bitch or the puppies.

There is also no information that I can find on treatment of pregnant bitches with melarsomine but I would strongly advise not attempting treatment of a pregnant bitch. We inadvertantly treated one pregnant bitch with thiacetarsemide and all of the fetuses were deformed and died. Perhaps this was coincidence, but I doubt it. I strongly suspect that the potential for similar complications with melarsomine would be high.

I am sorry that I was not able to find more specific information.

Mike Richards, DVM 10/17/2001

Heartworm medication and Collies

Question: Dr. Richards,

I wrote to you before regarding heartworm prevention. I told you I have a Borzoi and a Collie that I give Interceptor. I made mention that I knew that Ivermectin was downright dangerous to give a Collie, and your reply was that "you give it to Collies without problems all the time." I'd like you to investigate the use of Ivermectin in Collies with the Collie Club of America. It is not acceptable and has caused many untimely and unnecessary deaths. I trust you will, I know you care.


Answer: Vox

Collies are more sensitive to ivermectin toxicity than other breeds. However, the level of ivermectin necessary to induce toxicity in collies is still very high compared to the level used in Heartgard or Heartgard Plus (tm). The dosage of ivermectin necessary to prevent heartworm disease is 6ug/kg, although it is possible to administer 12ug/kg using the recommended dosing on the tablets, if a collie weighs close to 50lbs. The dosage of ivermectin at which toxicity problems start to become likely in collies is around 50ug/kg (based on the study reported on the Collie Club of America web site). Most of the original problems reported occurred when ivermectin was used for killing heartworm microfilaria, which are baby heartworms circulating in the blood stream after heartworms have already invaded a dog's body. The dosage originally used for this purpose was 50 ug/kg of ivermectin -- and there were some severe reactions in collies and collie related breeds at this dosage. Ivermectin is available in formulations for large animal use which some people try to compound into medications usable for small animals, leading to some dosing errors that have severely intoxicated collies. I have personally treated five or six collies or collie related breeds that people poisoned using large animal ivermectin preparations in an attempt to prevent heartworms. I have also treated a number of beagles and hound breeds for this problem, as it is a common practice among people owning many dogs, such as hunt clubs. Lastly, ivermectin is used at daily doses of 250 ug/kg for demodex fairly commonly and ranging up to 600 ug/kg to treat resistant cases of demodecosis. While most dogs can tolerate this dosage, it will injure or kill some patients. This is not something that we would try in a collie, either. I am not aware of documented problems with Heartgard or Heartgard Plus at the recommended dosages for the purposes of heartworm prevention. These dosages are low enough that they do not cause problems.

Milbemycin toxicity has been studied in collies, too. The dosage of milbemyin that causes toxic signs in some collies is 5 times the usual dosage, according to an article in the 1991 issue of the American Journal of Veterinary Research, by Tranquilli, Paul and Todd (I mentioned all three names because Dr. Paul is the author who reported the studies on ivermectin on the Collie Club of America web site). This is approximately the same level of overdosage that causes problems with ivermectin.

The Collie Club of America recommendation to use daily heartworm medication containing diethylcarbamazine (DEC) as an alternative is poor advice. I practice in an area in which heartworm disease is endemic. During the time that DEC was the only heartworm preventative available, we had between 5 and 20 patients a year, ON heartworm preventative medications, who developed heartworms. Since the two monthly preventative medications have become available, we have averaged less than 1 patient a year who is on heartworm prevention medication who develops heartworms.

I have not had a single client who we have dispensed Heartgard Plus (Rx) or Interceptor (Rx) for their collie report any reaction to the medication, at all. We have had complaints in several other breeds, usually suspicions that seizures might have been induced by the monthly dose of heartworm medication.

Mike Richards, DVM 11/29/2000

Heartworm Treatment for Dogs

Heartworm treatment should be much safer for dogs now that a new medication, Immiticide (rx), is available to veterinarians. This medication has fewer side effects and kills a higher percentage of heartworms than Caparsolate (rx), which has been the only medication available until now.

Heartworms live in the heart and the large blood vessels which go to the lungs from the heart. When they are killed by medications, a danger of embolism results if the dead worms block the flow of blood to the lungs. This is a common cause of death during a heartworm treatment. This risk still exists with Immiticide, but to a much lesser extent, due to differences in the timing of heartworm deaths. In addition, Immiticide does not appear to damage the liver or kidneys, which was sometimes a problem with Caparsolate.

If your dog has heartworm disease and you have been putting off treatment due to the higher risk of complications associated with Caparsolate, it would be a good idea to call your vet and discuss this new treatment. Dog feel better after heartworm treatment and their expected lifespan is the same as a dog that was not affected with this parasite if treatment is successful. Even though there is a small risk of death during treatment, it is still usually a much better choice than not treating for heartworms.

Mike Richards, DVM

Heartworm treatment in Westie

Question: We just received a diagnosis of an adult heartworm in our Westie. The vet is going to give her 2 shots and advised we must keep her quiet for 3 weeks.

How quiet is quiet? Can medication be given to her for 3 weeks to assist in calming her down? Does someone need to be with her at all times? When is it more critical to keep her quiet after the treatment? The first few days or several days after?


Answer: Sheryl-

Based on our experience with melarsomine (Immiticide Rx), the newest treatment for heartworms, the most important time to keep a pet as quiet as possible is from the third day post injection to the 14th day post infection -- but we actually advise clients to keep dogs quiet for 6 to 8 weeks after treatment.

The two most important things that you can do to help your Westie survive heartworm treatment are to keep her as quiet as possible and to keep her cool. I tell clients that they must keep their pet in an air-conditioned house and that they should take the shortest walk possible, no more than three to four times a day, to allow her to urinate and defecate. I honestly think it would be best for dogs to stay in a crate, if they are quiet in the crate, for the first three to four weeks, but I understand that this is not practical for many pet owners.

I am very reluctant to give medications to calm pets down during this treatment, because these medications have cardiovascular effects. I have dispensed sedatives to a few dogs, when it seemed absolutely necessary, though. I have used acepromazine in these circumstances but there may be other medications that work well, too.

It would be best if someone could be with your Westie, but usually the complications do occur slowly enough that it is OK to do normal things like go to work, as long as you know where to go to get help at night when you come home and find a problem.

The things to look for are loss of appetite, depression, yellow color to the gums or eyes, coughing excessively, coughing up blood (no one misses the importance of this one), or anything else that doesn't seem right to you. It is always better to have a recheck examination at your vet's than to take a chance during the first few weeks of a heartworm treatment.

I hope all goes well.

Mike Richards, DVM 9/15/2000

Heartworm treatment

Question: Dear Dr. Richards,

I thank you from the bottom of my heart for your reply. I can tell that you are a compassionate soul, not to mention a very knowledgeable and skilled veterinarian. I saw Pepe' only twice today. His gums were much improved, with the yellow tinge disappearing and the "healthy pink" hue returning ever so slowly. He ate 4 cans of a/d for me, 2 at breakfast and lunch. His breathing is still quite "heavy", and was started on theophylline this a.m.. Your diagnosis of heartworm disease is probably right on the money, although I will mention the other diseases to our vet and discuss them with him.

I don't want to "wear out my welcome" here, but if you would only answer one more question regarding Pepe', I will forever be indebted to you. It concerns overall fitness for heartworm treatment. Would you please give me the clinical markers or parameters you would employ in deciding whether or not to treat a dog for the disease. What about body weight? Hematologically? What tests are performed during the treatment? If you would, could you describe the treatment process for me or direct me to the area within your website where I can find this information? Is it done differently for dogs who are "fragile" as opposed to those who are otherwise healthy?

Dr. Richards, thank you for your time and concern. I won't bother you with other e-mails and questions on Pepe' unless I feel that it is absolutely necessary. Again, my wife and I send you our thanks. Please keep up the good work and I look forward to your reply. I am certainly going to tell our friends about your site. It is, without question, the most informative site for pet owners on the internet.



Answer: Greg-

To move on to heartworms, I am aggressive about heartworm treatment. I do not routinely do a lot of pre-treatment lab work. Many dogs will have elevations in liver enzyme levels but these usually resolve after treatment. Chest X-rays are a good idea in symptomatic patients (coughing) just to be sure that there isn't another problem. It isn't a good idea to treat for heartworms to stop a cough that is actually due to lung cancer. I do not worry much about the dog's weight, although I like thin a little better.

To me, there are really two critical issues when deciding when to treat a dog for heartworms. 1) Can the dog be kept cool for the next eight weeks? Temperatures less than 75 degrees are best. 2) Can the dog be kept very very quiet for six to eight weeks after treatment? It is best if a dog is confined and walked only long enough to urinate and defecate and then confined again. The more serious the condition of the dog prior to the treatment, the more it will benefit from being kept very quiet during the treatment.

The safest way to use melarsomine (Immiticide Rx), the better of the two approved heartworm treatment medications, is to use three injections of the product. The first one is given on the first day of treatment. The next one is given approximately 30 days later and the third on the day after that. The patient should be confined and kept cool for a minimum of eight weeks, starting after the first injection. Twelve weeks is even better.

I don't do lab work after the treatment, either, unless it seems necessary based on the dog's response to treatment. If there is a loss of appetite, jaundice or any other reason that lab work seems necessary we will do lab work as often as it seems necessary to track the course of the complication. The reaction determines the type of lab work we do and the frequency, so it varies a lot. I believe that we have complications requiring some lab work in about 20% of cases.

I have treated well over 1000 dogs for heartworm disease. I have treated a dog successfully one week after it arrived at our practice comatose from the effects of heartworm disease and have had serious complications in dogs as young as eighteen months of age, who had no real clinical signs of heartworm disease other than positive heartworm checks.

The prior condition of a patient does impact the prognosis --- dogs with clinical signs such as coughing, weight loss and fainting do not have as good a prognosis as dogs without clinical signs. However, I have not yet found a lab test that really gives a good indication of which specific dogs will have trouble with heartworm treatment. I would really like to have a test that would give me this insight and if I discover it I will put it on the web site.

I do try to get severely ill patients reasonably stable -- we do use diuretics to treat pulmonary edema, we would wait for a dog to eat well that was showing a lack of appetite and we want our patients to have a reasonably good attitude prior to treatment, if we can get to that point. But I will treat even if I can't achieve these goals, because I know that most of these dogs are going to die if I don't treat them and at least 80% , even of the serious ill ones, will live if I do treat them, based on tracking our patients over the years.

Heartworm treatment is scary, because it does hasten the death of some patients. The trade-off is that it gives the rest of the patients a normal quality of life and normal longevity. The trade-off seems clearly worth it, to me.

Hope this helps.

Mike Richards, DVM 8/4/2000

Heartworm treatment in pug - positive test results when tested

Question: Hi Dr. Mike...Pug Rescue in Alabama here again.

In March of this year we treated a little pug for heartworm disease. He was a owner surrender to a shelter and was in pretty rough shape. We did Imiticide two days in a row, and then back in three weeks for a treatment of Ivomec. Testing the following week proved negative. I just got a call this week from the adoptive family who told me that their vet tested him again for heartworms and found him to be a strong positive. I have read different opinions on this, but I am concerned that the testing was done too soon after the treatment (less then four months). The adoptor's vet is starting him on aspirin therapy and planning to reinitiate the same treatment the middle of July (which will be exactly four months). He has been on monthly preventative since getting an all clear in April. Please give me your opinion on this scenario as we rescue quite a few heartworm positive pugs in the south, and I need to better prepare the adoptor if this happens routinely.

As always..thanks for being there!


Answer: Pam-

I think that it would be a serious error to treat this dog with melarsomine (Immiticide Rx) at this time. All of the currently available heartworm test kits test for cells lining the reproductive tract of adult female heartworms. This is the "antigen" that the test is sensitive to. These cells do not disappear from the dog's body for up to four months after heartworm treatment even when it is successful. I try to wait at least six months after the last treatment before deciding that the heartworm treatment did not work.

If the dog has no clinical signs but has a positive antigen test after being treated for heartworms I think that I might even consider just leaving the dog on preventative medication for a year and retesting it at that time, since the heartworm treatment may weaken the surviving worms, limiting their survival time. However, I would have a hard time arguing with going ahead with a re-treatment in this situation and have chosen that course of action myself on several occasions.

In any case, it is important to wait at least four months after treatment before deciding that the treatment didn't work.

Mike Richards, DVM 6/29/2000

Heartworm treatment

Question: Thank you for your help - the treatment went very well and Binh the dog seems extremely well. Please can you tell me how long he will continue to test postive for Heartworm,ie when I will finally know he has the all clear. Many thanks and you may be interested to know that through your advice several other dogs in Saigon have had successful treament with immiticide that were otherwise going to be left untreated as their owners thought the treament proposed by the vets here was too risky

Answer: Mary-Clare-

I am not sure exactly how long it takes, on the average, for the heartworm antigen to disappear after heartworm treatment but there is general agreement that an occult heartworm exam should be negative four months after treatment, if it was successful. We usually test dogs about six months after treatment, mostly because I am reluctant to treat again prior to that time.

The microfilaria have to be killed separately and their presence does not interfere with testing for adult heartworms. We usually just put dogs on the monthly heartworm prevention and wait for the heartworms to die, now.

Mike Richards, DVM 6/10/2000

Heartworm treatment for "high risk" dogs

Question: Dear Dr. Richards, I realize that Immitacide is the treatment of choice for heartworm positive dogs, and that the Immitacide treatment split by a month is the fall-back postition for "high risk" dogs. However, I am hearing about protocols using just ivermectin, or ivermectin for X number of months, then Immitacide. As far as is known at this time, does ivermectin have any adultacide action, or is the ivermectin-only treatment entirely a treatment to kill filaria and allow attrition to take the adults? My motivation to ask is, of course, my own HW+ 12 yr old shelter rescue (Peke) with an enlarged heart, a mild cough, and no obvious signs of CHF (per two vets). Her EOS are elevated and her AST is about double, otherwise her chemistries and hematology are WNL. This is a rural area with no 24 hr hospitalization as recommended for a dog getting Immitacide. So far I am going with ivermectin (1:10 in prop. glycol; 1ml/30 lbs.). She has only completed her first month so far (no adverse effects). Any words of wisdom you can offer??...other than "stop fooling around and just give her the Immitacide". Thank you for your time, Sue

Answer: Susan-

I guess it would be best to start with the facts "as we know them now" and then move on to opinion.

As far as anyone has shown through scientific studies, ivermectin has little to no effect on adult heartworms. There is a small amount of information that seems suggestive of some effect on lifespan of the worms but this has not been developed to the point that it is clear. If ivermectin does have an effect it probably is to shorten the average length of a heartworm's life from about 3 to 5 years to something like 2 years. At the present time, no one really knows for sure if this happens, or what reduction in lifespan is possible to achieve.

Melarsomine (Immiticide Rx), does kill adult heartworms. If a single intramuscular dose is given the first time it is used and then two intramuscular injections one day apart are given a month later, it is very close to 100% effective in killing heartworms. It has approximately a 1 to 2% death rate during the six to eight weeks after administration. We have treated approximately 80 dogs with this medication and have had one death from complications associated with the die-off of adult heartworms. We do not hospitalize patients we are treating with Immiticide overnight, even during the two injections one day apart portion of the treatment. We do keep patients for about four hours after injections because some dogs are so sore that they will bite if touched around the injection site for a couple of hours and because there are reported to be some reactions to the medication, which we have not seen, yet. We do make sure that our clients understand that the quieter their dog stays during the two months after the treatment, the more likely it is to survive.

We have treated three or four dogs who were over ten years of age and we have not lost one of these patients, yet.

Heartworms cause dogs to die by causing small blood clots and embolisms (including dead worms from natural death or from heartworm treatment). The blood clots damage the lining of the large vessels leading to the lungs, eventually causing enough scarring to lead to stiffness in the blood vessels and to decrease the diameter of the interior of the vessel. When the same amount of fluids must pass through a smaller diameter blood vessel in about the same amount of time the blood pressure has to rise. Eventually the blood pressure in these vessels gets high enough that the heart can't pump that hard. The result is heart failure. This process goes on until the worms die or the dog dies. The detrimental effects are a combination of the number of heartworms and the length of time they are present. A large number of worms can do a lot of damage in a short time. A small number of worms can do a lot of damage over a longer time period.

When heartworms die they become embolisms. The embolism moves with the blood flow until it hits a blood vessel small enough to stop it. The blood flow is blocked to this vessel. If enough of the lung blood supply is obstructed in this manner, the dog dies. In addition, embolisms cause the body's natural defenses to be triggered, which can lead to massive blood clotting and shock, providing another way for death to occur. This is why heartworm treatment increases the SHORT TERM possibility of death in an affected dog.

Over the long term, heartworms will kill most dogs who are infected with them. This means that viewed from a long term perspective, treating for heartworms produces a huge increase in expected lifespan for almost all dogs who are treated. In our area, dogs who are never put on heartworm preventative medications have an expected lifespan of about four to six years. The earliest we have ever seen a dog die as a direct result of heartworms was about fifteen months of age.

Most dogs with heartworm disease will return to nearly normal cardiovascular function within a few months after heartworm treatment, if they do not die during the treatment.

That is the factual part of the note.

I think that I would repeat the heartworm test, using a different testing method to help confirm the diagnosis. I would carefully examine the chest X-rays to look for X-ray evidence of heartworm disease. The eosinophil rise, enlarged heart and cough are all suggestive but not completely diagnostic. Assuming that I was convinced that heartworms were indeed present, though, what would I do with a twelve year old patient who I was pretty sure had heartworms but who had no clinical signs associated with heartworms?

A dog over ten years of age is one of the few times when I can see administering ivermectin heartworm preventative once a month and waiting to see if clinical signs of the disease develop, or worsen, especially if it is not possible to know if the infection is likely to be a recent one or not. Hoping for a recent infection that could be kept to small numbers by using ivermectin seems reasonable in this circumstance.

To be honest, though, I still think it is best to treat for the heartworms with Immiticide, most of the time, even at this age. Especially in a small dog, because they have an expected lifespan of fifteen to sixteen years, or more, which gives the heartworms lots of time to cause more damage. This decision has to be based on an overall evaluation of the dog's total health, though. It is also good if you can determine an exact age for the patient, which might not be possible in your peke's case. I don't worry much about increased liver values as our experience has been that they are usually due to the heartworms when they are present and they will drop after treatment in most cases.

This isn't an easy decision. If death were the only concern, I might be more likely to fall on the side of giving ivermectin and waiting to see what happens. But death isn't the only concern. Dogs with heartworms have heart failure symptoms, which are inhibiting their comfort and enjoyment of life. It is possible to reverse this process in the case of a dog with heartworms. You can provide a better quality of life, as well as the potential for an increased quantity of life. You take about a 1 in 50 chance of making things worse in order to get these benefits.

Whichever way you choose to go, there is some chance you will regret the decision. For this reason, you have to do what you think is best, because you have to live with the consequences.

Hope this helps instead of making things all the more confusing.

Mike Richards, DVM 5/27/2000

Heartworm disease and treatment

Question: Dear Dr Mike I have a Phu Quoc dog here in Vietnam - he is 7 years old. He has just been diagnosed with the first stages of heartworm. The Vet here has prescribed 7 days of 140mg of Aspirin a day before starting worm treatment. Is this normal treatment - my inclination is to treat the worms ASAP before they get any worse? After administering the aspirin yesterday he vomited all the food he had eaten that day. After the aspirin treatment he will use a French product called Levamisole to treat the worms. He has not heard of Immiticide which seems to be the drug of choice now - should I insist on using Immiticide? ( I am sure I can find a way of getting hold of the drug via Singapore) Many thanks for your help.


Answer: Mary-Clare-

I have to explain a little bit about heartworms prior to trying to explain the rest of this answer. Heartworms have three major stages in their life, from a clinical perspective. These are the adult stage, the microfilaria and the infective larvae.

Adult heartworms are full grown heartworms living in the heart and pulmonary arteries of the infected animal. These cause most of the damage associated with heartworms by causing very small blood clots which damage the lining of the blood vessels in this region, eventually leading to high blood pressure in the pulmonary circulation. When the blood pressure gets high enough, the heart can't work well and heart failure starts. This is a slow process, usually taking a year or more to produce clinical signs (but not always). Adult heartworms are six to fourteen inches long but very thin and flexible.

The adult heartworms produce babies, which are referred to as microfilaria. There are often as many as 60 or more microfilaria in one cc of blood, which means there could be hundreds of thousands of microfilaria in the blood stream of one dog. Parasites can only survive as a species if they don't kill the host they live in before they can reproduce. For this reason, it is important that all these microfilaria stay microscopic. If they all grew up, the dog would die. For this reason, microfilaria must be picked up by a mosquito and activated in the mosquito in order to grow up. Once the mosquito has activated the microfilaria, it is referred to as an infective larvae. Until this happens they just float around in the circulation until they are picked up by a mosquito or die a natural death.

The infective larvae are carried by a mosquito from the dog it took blood from to the next dog it bites (or the same dog, if it bites it again). They are injected into the skin and begin a long migration to the heart region, which takes about 5 months. During this time, they can be killed with little to no effect on the dog. This is how heartworm preventative medications work -- the dog is infected with the larvae until the medication is administered and then it kills all that are present. The newer medications can kill infective larvae up to about 50 days of age, which is why they are given once a month.

Both the use of aspirin and the use of levamisole are controversial when treating heartworms, at the present time.

In theory, aspirin should reduce the damage that is caused by heartworm disease, since it interferes with blood clotting, but this effect is not consistently found in clinical studies of its use. In fact, at least one of the studies found that aspirin use prior to adulticidal therapy for heartworms cause more complications than not using it. In our practice, we will sometimes use aspirin prior to treatment, or to treat inflammation after treatment. We tend to use a low dose of about 5 mg/lb once a day at the current time.

Aspirin causes gastrointestinal irritation severe enough to cause ulcers in some dogs (less than 5% of dogs treated with it). When there is evidence of GI irritation it is best to discontinue the aspirin, or to use medications to protect the stomach, such as misoprotol (Cytotec Rx) or cimetidine (Tagamet Rx).

Levamisole, as it is formulated in the United States, is a very inconsistent adulticide medication for heartworms. Several studies have shown that the formulation available in the US will not kill the adult heartworms in most cases. However, there are studies from Australia that support the use of levamisole. Unfortunately, I can not tell you what the differences are in the formulations, nor can I tell you how to tell if the levamisole will work. If your vet in Vietnam has had good success with the treatment he is using, in the past, I am not sure what to advise you.

I do know that melarsomine (Immiticide Rx) is a very good adulticide medication with the best margin of safety for adulticide medications found to date.

It is usually safe to wait a few days, or even a few months, prior to treatment for heartworms in dogs that are not showing clinical signs (coughing, lethargy, exercise intolerance). If there are clinical signs waiting may not be a good idea, but a few days won't matter much.

Some veterinarians in the US prefer not to treat the adult worms and just put dogs on the ivermectin based heartworm preventative (Heartgard Rx) and wait for the adult worms to die. Personally, I don't think this is a good plan in most cases, since the damage to the arteries continues as long as there are adult worms. Some dogs do make out OK with this approach, though. If you can't get melarsomine, make sure you do use the monthly heartworm preventative medications after treatment. You should do this, anyway, but it would be especially important in this case.

Good luck with all of this.

If you need more information or clarification of something in this note, please let me know.

Mike Richards, DVM 4/29/2000

Heartworm Disease - treating large older dog

Q: Dear Doctor, My dog is 14 years old and has been diagnosed with heartworms. She is part Lab and Rott. Should I treat her? My vet thinks she could die after the treatment as the worms are breaking up, should I attempt it anyway? She has labored breathing and severe arthritis in her hips which makes it difficult for her to walk. She still has control over her bodily functions and still eats. Plus, she recognizes us and wags her tail. Thanks, Jan

A: Jan-

I would have serious doubts about treating a 14 year old large breed dog. If she is heartworm positive and has no clinical signs it is likely that she would not live long enough for clinical signs to appear, although there are no guarantees of that. If she is 14 years old and has clinical signs it would be hard to figure out whether or not the signs were due to heartworm disease alone or heartworm disease in combination with chronic heart failure, cancer or other systemic illness. A very careful evaluation to rule out other diseases would be best prior to a decision to treat, whether signs of illness are present or not. It is important to remember that treating the heartworms won't make her a young dog -- she will still have arthritis and other problems associated with age.

With all of that said, I have treated a couple of large breed dogs in this age range when all of the circumstances were right -- the owners wanted to and didn't mind the cost, the dogs were healthy otherwise and seemed to have a reasonable life expectancy to justify the treatment and they did not have serious clinical signs such as coughing up blood, excessive weight loss, etc. Unless my memory fails me, I believe that these treatments were successful in that the heartworms were eliminated without causing the death of the patients during the treatment or post-treatment period.

It is a tough decision to make. Most of my clients would opt not to treat at this age due to the cost and relatively short expected lifespan of a large dog of this age. I would advise against treating many dogs due to expected complications after physical evaluation or pre-existing conditions that appear to be life threatening already. Your vet is in the best position to advise you on these problems and then you have to decide how to calculate the cost (in money and in discomfort during the treatment) vs. benefit decision for your family.

Mike Richards, DVM

Reaction to Heartworm treatment

Q: Hello Dr. Mike, My name is Amy.. My husband and I have a question regarding the heartworm treatment of our dog. About 3 weeks ago I rescued a beagle from the local shelter. Upon examination, I was told she was approx. 5-7 years-old and had heartworm disease. My vet opted for traetment the following week. The first 3 days following treatment, Betsy was doing wonderfully. Then she took a turn for the worst. She started hacking, while gaging up a combination of saliva and phlegm, and she ultimately began vomiting blood. I rushed her to the vet, and he suggested keeping her in order to do intensive steroid treatment. When I picked her up today she seemed much better(I,m sure from the steroids). However, the vet told me that if we did not see a visible difference in her within 3 days, we should consider other alternatives. My question to you is: Does Betsy's reaction sound abnormal? And if so, what do you think her chances of a recovery are at this point? We would appreciate your opinion. Thank you very much. Amy & Rainer

A: Amy- The reaction you saw was more severe than most dogs experience but still relatively "normal" for reaction to a heartworm treatment. In many instances just keeping a dog very quiet will allow them to make it through this sort of reaction but most vets use corticosteroids during these episodes and most of the time I do, too. Most dogs only have one bad episode, especially if they are kept very quiet after the first one. I am hoping Betsy only had the one and is doing well now.

Mike Richards, DVM

Heartworm treatment - comparing costs

Q: Dr. Mike, Could you discuss the cost of this new drug in comparison to Caparsolate? Our Vet advised us it was twice as expensive to use. I have two dogs that have been diagnosed with heartworms. They didn't have heartworms last year when they had their yearly checkup. One dog is an Austrialian Shepherd and the other is a Lab. Both dogs are three years old. They do not exhibit any signs of being sick and we wouldn't have known they had heartworms except for the blood test. Please adivse me to how to treat them. Should I get another test to be sure they do have heartworms, or is the blood test the best method to determine they have heartworms? Thank you, Rita

A: Rita - Immiticide (Rx) costs a lot more then Caparsolate (Rx) but that is only part of the real cost of heartworm treatment. The really expensive part of heartworm treatments occur when reaction to the injections or to the death of the heartworms occur. In severe reactions to heartworm treatment hospitalization may be required for several days and treatment may include use of an oxygen cage, blood component therapy and other expensive therapeutic measures. Immiticide is much less likely to cause a severe reaction than Caparsolate, based on the veterinary literature and our experiences with it. The death rate during treatment is markedly lower as well.

If a dog has only minor problems with the heartworm treatment Caparsolate is less expensive to use. If reactions occur it is likely to be much more expensive to use. It is true that using Immiticide is not a guarantee of no reaction or a guarantee that death will not occur -- but it does increase the odds of a safe treatment.

Cost is only part of the issue from my perspective. Why tolerate the increased risk of death or difficulty during treatment unnecessarily? I do not even stock Caparsolate anymore as I have no intention of going back to it unless some problem I am currently unaware of is reported in the veterinary literature.

A positive blood test for microfilaria (baby heartworms) in the bloodstream is pretty accurate. An antigen test for adult heartworms (occult heartworm test) is also very accurate, but has about a 1 in 1000 chance of being a false positive. In an area in which heartworms are common this is a minor worry. In an area in which heartworms are not likely, it might be best to take chest X-rays and evaluate carefully for clinical signs of disease prior to initiating treatment.

Mike Richards, DVM

Seizures or fainting and Heartworm treatment

Q: Dr. Mike. We recently adopted from the pound a 1 year old chocolate lab/doberman mix to be a playmate for our 7 month old black lab. Upon his initial physical he was tested positive for Heartworms and immediately placed on the Immiticide treatment. He appeared to be doing well through the first three weeks of rest (other than not being thrilled about having to remain rested). During the end of the forth week (just prior to being injected with a dose of heartworm preventative mediciine) he started to develop seizures (appearing to be like epilepsy). These seizures would ensue after he would stand up when we were going to bring him out to do his buisness. They have gotten progressively worse and our vet does not know what to make of it. She says that this had happened with one other dog also and she can't explain it. Have you heard of any such side effect from this medicine. If so are the seizures a temporary side effect that goes away with time. If not how are the seizures treated (ie phenol-barbitol) and what would be the effect on the dogs activity and quality of life. Any help would be appreceiated. Brett

A: Brett- We have not seen seizures during the time period after melarsomine (Immiticide Rx) injection. The pattern of the seizures you are seeing suggests that heart disease may be present (fainting from heart problems can look an awful lot like a seizure). I would hope that this problem has resolved. If not, a careful review of the cause and then appropriate treatment is in order. Phenobarbital is the most commonly recommended seizure control medication, if seizure control does prove to be necessary. Mike Richards, DVM

Heartworm treatment and Collies

Q: Dr. Mike, What can you tell me about heartworm treatments for collies? I'm getting a different story from each vet I talk to. One says he sees more dead dogs with immiticide and won't even consider it. Another says the "old" treatment method would kill the dog. One has even recommended a cancer drug instead of conventional treatments. Who do I believe? This dog is about 5 years old and is a stray I found two weeks ago so I don't have much history on her. She was not exhibiting symptoms of heartworm disease, but I had her tested as the first step to begin caring for her. Her heart and lungs sound good and her BUN is 19.8. Her SGPT is 298. The hematology report I have shows boderline Eosinophilia. In your opinion, which treatment is easier on the dog? How many dogs die from the immiticide treatment? Thanks. Mary

A: Mary- Collies seem to be more sensitive to heartworm treatment than some breeds. I am probably risking angry email from collie fanciers all over the world, but I think that collies die when they are sick or injured more easily than other breeds, in general. At least the ones in our area. Despite that, we have treated a number of collies successfully for heartworms and I would not hesitate to do it again right after telling the owner that just what I said above.

I should explain something before going into the statistics we have kept on heartworm treatment. It is my opinion that almost all affected dogs experience a decrease in the quality of life that justifies treatment. Especially since most of them will eventually die from the disease. I do not accept the concept that a dog is "too far gone" to treat. I have treated a dog that arrived in our clinic comatose 20 days previously -- just taking long enough to get some stability prior to treatment. That dog lived. This philosophy does not seem to be the standard among veterinarians but no one has been able to convince me that ignoring heartworms is a better approach.

We kept track of over 280 heartworm treatments using sodium thiacetarsemide (Caparsolate Rx) and 8.5% of the dogs we treated during that time died during the 8 weeks following treatment. We didn't keep track of how each breed did but our impression was that collies did not do as well as other breeds.

We have treated 42 dogs with melarsomine (Immiticide Rx). One of those dogs died the day after we initiated treatment but it was so close to death on arrival in our clinic that it is unlikely melarsomine was the cause of death, although that is unprovable at this point. That would be about a 2% death rate, but the number of treated animals is still pretty small - so that figure could easily change somewhat as time goes on. I can not speak for other people's experiences but I certainly believe that Immiticide is superior at this point.

I am going to make a guess that the "cancer medication" recommended for heartworm treatment was levamisole (Levasol Rx). This medication has been used on and off for heartworm treatment since about the time I started in practice. In clinical trials this medication has not been an efficient treatment. I know of no effective alternative to thiacetarsamide or melarsomine.

Eosinophilia is just an indication that the collie is properly responding to a parasitic infection. The BUN is normal. The SGPT (ALT) is elevated but heartworm can do this and it will usually decrease post treatment. I do not see this as a significant reason to delay treatment or to decide not to treat.

Good luck with this situation. I'm glad your collie found someone who cares enough to consider treatment. Mike Richards, DVM

Heartworm treatment options

Q: Dear Dr. Mike, I am a new dog owner, so I don't currently have a vet picked that I have complete confidence in. My problem in my new dog, which I just got from the local animal shelter, has tested a weak positive for heartworms. He is a 2 year old brittany and shows no symtoms of the disease. After talking to three local clinics, I'm confused as to the most prudent treatment. Please look at the three choices and please let me know the one you feel makes the most sense.

1. Two injections of Immiticide followed by a treatment of ivermectin in 2 weeks, and then placed on Heartguard Plus with a retest in 6months.

2. Two injections of Immiticide followed by a retest in 4 weeks and placed on Heartguard Plus(no follow up of ivermectin recommended) and retested again in 6 months.

3. Put on Heartguard Plus and not treat the adult worms since a weak positive indicates a very minor infestation. The explanation is to prevent any new worms and let the small number of adults die over time.

Being a new dog owner I'm not sure I know which of these is best and I'm asking you for a second opinion of sorts. If this is too trival for your Q and A page, could you please E-mail me your thoughts.

A: Of the options you presented, I like the second one best except that I don't understand the recommendation to retest for heartworms in 4 weeks. Perhaps your vets meant 4 months?

I never really know what the term "weak positive" means. If this is the designation from a CITE Snap Test which has a high and low antigen spot, then I think retesting using a different test would be the best idea. False positive tests do occur. This is particularly important if you live in an area in which heartworms are not a frequent problem. If this means that one microfilaria was seen on a direct or filter examination then I strongly recommend treatment. We have seen a couple of dogs develop significant symptoms of heartworm disease while we kept them on heartworm preventative and waited for heartworms to die. I am just not comfortable taking this approach anymore.

Every vet has different experiences in practice and controversies over things like how to best treat heartworms are frequent. It is a very difficult thing to have to find a new vet in the middle of a problem like this in which conflicting advice is highly likely and you haven't got the advantage of a trusting relationship with a vet.

I wish you the best with this. The good news is that Immiticide really does seem much better than the old heartworm treatment. It is highly likely that you dog will do OK. Mike Richards, DVM

Heartworm treatment

Q: hi Dr. Mike...just found out today that my 8 yr old female lab has heartworms. our vet is going to run some prelim. tests on her to rule out any other major problems. she mentioned a new treatment just out for heartworms, and i was wondering if you might have any info on it. i would appreciate anything you might have, even web addresses...i've searched around some already, but keep coming up with the arsenic treatment info again and again. thank you very much. Ann

A: Ann- The new treatment for heartworms is melarsomine (Immiticide Rx). It seems to be much safer than the older heartworm treatment, Caparsolate (Rx). It is used in one of two treatment schedules. For dogs with minimal symptoms of heartworms it may be given by injection in the muscles of the back in two injections at 24 hour intervals. This treatment schedule is about 95% effective at eliminating the heartworms and is slightly more likely to result in complications than the alternate dosing schedule. It can save a significant amount of money in some dogs, though. The other dosing schedule is to give one injection then wait one month and give two injections 24 hours apart. This results in approximately a 100% kill of heartworms and is less likely to cause complications after the injections. The complications associated with heartworm treatment usually occur when the heartworms begin to die and they seem to die a little slower, spreading out the effect, using the three injection technique. Some dogs develop swellings or even sterile necrosis (death of some muscle tissue) at the injection site but even with this effect the new medication is much safer to use.

Hope that helps. Mike Richards, DVM

Heartworm treatment medication in Mexico

Q: Hello doc. I´m in Cancun, and I have my "Vizla" Penny.She is sick of this heartworm (dirofilia I think). Here in México is not posible to find any of these medicines.If there is any help for us I´ll be really apreciate your advice and opinion. thank you very much , Rafael

A: Rafael-It is hard to help when the medications for treatment are not available. I know that must be really frustrating.

I do not know of any substitute medications to kill the heartworms.

It may be potentially helpful to give an aspirin per day to delay blood clotting. This seems to slow the progress of the disease since much of the damage done is actually due to internal blood clotting in the heart and pulmonary arteries. Treating the heart failure that results from heartworms with diuretics such as furosemide (Lasix Rx) may also be helpful.

I have noticed that medications have different names in other countries after starting this webpage. Perhaps the medications are available but with different trade names. I'm sure you have probably asked your veterinarian about this but if not, it can't hurt to ask.

I wish I could help more.

Mike Richards, DVM

Best TO treat for heartworms

Q: Dear Doctor, Please help me, Today my DVM told me that my dog Samantha tested a weak positive for heart worm antigen.I have given her just one heartworm pill the day after the test. The DVM has told me because of this she must have the test repeated but not for a month then if necessary treatment for heart worm will begin, I have been studying about the risk in the treatment and also how sick it will make my dog, I read where the treatment may be of greater danger than not treating the dog, my children and husband love Samantha as much as I do, We would be devastated if anything happened to her,Samantha is a beautiful white Samoyed. She is six years old and the love of our life,we would do any thing for her but we don;t want her to suffer. Please would you help me decide what to do, Samantha is in perfect health and I want to have a long life but putting her through this process sounds cruel to me. PLEASE HELP ME

A: I suspect that you may have read Dr. Pitcairn's book. I can't remember the title but there is a chapter in which he states that the heartworm treatment may be worse than the disease. I couldn't disagree with this statement more. If death was the only consequence of heartworms and it occurred suddenly, there might be some slight logic to this statement. That isn't the case. Heartworm disease causes damage to the pulmonary arteries which eventually leads to heart failure. If you have any acquaintances with chronic heart failure, ask them how much joy there is in their lives. Heart failure robs the dog of its ability to play comfortably. Eventually it becomes difficult to just participate in normal activities. Serious damage begins to occur in other organ systems affected by the heart failure. Treatment of the symptoms alone fails to resolve the problem over the longterm and the dog eventually dies -- after having been miserable for some time. Some dogs do manage to live almost normal lifespans despite infection with heartworms but they are very lucky. A rough estimate from our practice experience: 80% of dogs affected with heartworms probably die from the heartworm disease before something else causes them to die and >90% eventually show some or all of the symptoms of the disease. The heartworm treatment does cause serious illness in some dogs for a short period of time, usually two to three weeks. We have not had a death associated with the use of the new heartworm treatment (Immiticide Rx) but we have only treated 32 dogs since it came out. If I remember the package insert correctly, about 1 to 2% of treated dogs died in their clinical trials. Three of those dogs had complications requiring hospitalization during the treatment period. The odds are very good that your dog can be successfully treated. When you compare those odds with the odds of death or disability from the heartworm infection, I think you can see why I strongly disagree with the statement that the treatment may be worse than the disease. There are two recommended treatment methods using Immiticide. A number of veterinarians, including me, feel that it is better to treat with a series of three injections. The first injection is given one month prior to giving an injection on two consecutive days. There seems to be less reaction to the treatment and it is reported to be slightly more effective when this treatment protocol is followed. Do confirm the test results. If another test is unequivocable it may be possible to discern evidence of heartworm disease on X-rays. There are some changes in the pulmonary artery size that are diagnostic for heartworm disease when present. Please treat your dog if the disease is confirmed. Mike Richards, DVM

Immiticide vs. Caparsolate

Q: Dr.Mike; Seka is my beloved 8 yr. old Female German Shepherd, yesterday my Vet advised me she tested positive for heartworms. I live in Texas where the mosquitoes grow quite large and are numerous. I checked your site last eve. and noticed Immitricide RX was safer than Caparsolate RX. I mentioned this to my vet and he told me it was not available. Should I get another opinion? And is Immitricide RX available in Texas? I opted for the treatment after reading your pages, and learned about the life expectancy. I need this info before Monday May 5th, as she is scheduled for treatment then.... Thank you. D.

A: I am not sure why your vet thought that melarsomine (Immiticide Rx) is not available but I checked with Rhone Merieux and it is. Sometimes there are problems with distribution, ordering or billing between a veterinarian and the pharmaceutical company that make it temporarily difficult for a vet to get a product. Substitution of one product for another is not unusual in veterinary medicine because it is very difficult for any veterinary hospital to stock all of the available pharmaceuticals. However, in this case the difference in risk factor is significant enough in my opinion, that I would hold out for treatment with Immiticide in a pet of my own. Ask your vet to check on the availability of this treatment again. Mike Richards, DVM

Severe Heartworm disease - complicated treatment

Q: Dear Dr. Richards, Our dog Bronzie tested HW pos. Due to the enlarged heart as shown on X-ray, the vet opted for Ivermectin treatment first. She did not do well. She appeared to have had an immune-mediated hemolytic anemia (gross blood in urine for 1 day) with normal serum protein and drop in hematocrit to 15.5, rapid breathing, 3 fainting spells, vomiting blood with tubular shaped 1/2" clots on three occasions. At one point I did not think she would make it through the night. She was referred to an internist who performed an echocardiogram with report as follows: "Severe right heart enlargement. Severe enlargement of the main pulmonary artery with numerous heartworms visible in it. No heartworms evident in the right ventricle or caudal vena cava. Anemia may also be attributed to GI blood loss, given her history of hematemesis. Diagnosis: severe heartworm disease; tricuspid insufficiency; right sided CHF. "Treatment has been directed at right congestive heart failure (digoxin, enalopril, furosemide), HW-enduced pneumonitis (prednisone, then later aspirin), and hematemesis (Zantac, Carafate). Once her condition stabilizes and therapies have been judged to be adequate, continued HW treatment with Immiticide has been recommended. She appears to be getting better-no more fainting or vomiting, urine is clear, stool appears normal, appetite has returned (have been feeding her high-quality beef and beef liver). She is still weak and unsteady and has increased respiration, somewhat labored, especially when lying down, but she is alert, responsive, and quite able to get around. Exercise has been limited to very short walks in the backyard. It has been 10 days since Ivermectin treatment. She will have a repeat CBC,serum protein, etc and Digoxin level done in 6 days. I would appreciate your honest opinions on her prognosis and treatment.I look forward to hearing from you.

A: We have treated 36 dogs with Immiticide (Rx) since it came out. We have lost one dog during a treatment and it had a similar history to yours except that we couldn't even get it stable and finally elected to treat as a last ditch effort. It died the next day and we do not believe that it was due to the effect of the medication but more just the improbability of success in that situation. Even though we were not successful in that dog, I am not sure that would deter me from trying again in similar circumstances. We treated one dog with Caparsolate (Rx), the older heartworm medication, two days after it came in comatose from the effects of the heartworm -- and it lived. We treated well over 400 dogs with Caparsolate, in all stages of heartworm disease, and we had an 8.8% death rate due to treatment.

Immiticide is supposed to be much better and obviously has been better in our hands, so far. Of all the dogs we treated, I think only two owners felt that there was not a great deal of improvement in quality of life and longevity for their pets. I guess I'm not unbiased but I am a very strong advocate of treating dogs definitively for this disease. I do feel that stabilization prior to treatment is much better when it is possible. It sounds to me like you are taking the right course and getting the right advice. Mike Richards, DVM

Bronzie's HW treatment part2

Q: Dear Dr. Richards: Thank you for your response and opinions regarding our dog, Bronzie. Since my last E-mail, she had improved dramatically-Hematocrit 34, WBC 18.0, electolytes normal,etc. The vet performed the filter test again-still positive! (but not "as positive") She and the internist opted for a dose of Interceptor appropriate for her size. She tolerated it for four days and I thought there was light at the end of the tunnel. But she relapsed with the same symptoms-one fainting spell, red wine urine, lethargy. I don't completely understand what is going on. From what I do understand, the "immune-mediated hemolysis" is caused by the dead microfillaria attaching to the red blood cells and the RBC's then are lysed.

Does this happen often? I'm trying to understand the immunology behind this. We eventually want to treat her with Immiticide for the adults, but we can't seem to get to a point where we can. Hopefully her next filter test will be negative. Can Immiticide cause any immune reaction such as this? I don't know how much more she can take.I am afraid of trying anything else. One more question. I have heard that Collies can't tolerate heartworm treatments. What happens when the drugs are administered to them? >Bronzie is a mixed breed and I suppose there is a possibility she could have some Collie in there somewhere. Her face bears a resemblence to a Collie-long muzzle. Your opinions and suggestions are greatly appreciated.

P.S. She is still on Digoxin, Disal, Vasotec, and Prednisone

A: From your note it sounds like your dog may either have late-stage heartworm disease or possibly an aberrant form of heartworm disease known as "vena caval syndrome". These are the two conditions mos

also see Canine Heartworm


Michael Richards, D.V.M. co-owns a small animal general veterinary practice in rural tidewater Virginia. Dr. Richards graduated from Iowa State University's College of Veterinary Medicine in 1979, and has been in private practice ever since. Dr. Richards has been the director of the PetCare Forum...