Heartworm Disease in Dogs


Photo Gallery - Ascites from Heartworm Disease

Heartworm transmission

Question: Are heartworms in any way contagious?

Thank you Linda

Answer: Linda-

Heartworms are contagious, but not directly from an infected dog to another dog. In all cases, baby heartworms must be transferred to a mosquito, then injected into the next dog (or back into the same dog) by the mosquito, before they can grow into adults. So dogs acquire heartworms from being bitten by a mosquito who is carrying heartworms from an infected dog.

Mike Richards, DVM 9/14/2001

Photodynamic therapy for squamous cell carcinoma and other tumors

Question: Why doesn't Dr. Mike advise owners of cats diagnosed with squamous cell carcinoma about the new photo dynamic therapy being used?? My cat has a small lesion on the tip of his nose and was taken to Oklahoma State University, where they are carrying out a clinical study on PDT with great success. (Other facilities around the country are also using PDT with success, as are human medical facilities.) It bothers me to read so many questions from owners with cats that have similar lesions and know that they are not being advised of this ground-breaking therapy that could save their cats lives!

Answer: C-

Photodynamic therapy is a relative new procedure in veterinary medicine. It is considered to be promising for the treatment of superficial cancers, such as squamous cell carcinoma. Early studies of the response of cats with squamous cell carcinoma (SCC) to photodynamic therapy were only moderately successful (Peaston et. al., 1993) but more recent studies using newer photosensitizers seem to be more successful. This is an option for treatment of squamous cell carcinoma and other tumors, such as fibrosarcomas, that tend to occur on or near the skin surface.

Photodynamic therapy requires the use of specialized equipment and is not easily available in all areas of the country. It appears to be a viable option in areas in which it is available, though.

Mike Richards, DVM 9/14/2001

Heartworm disease in Mexico - continued

Question: Dr, once again i am very very grateful for all your comments, and once again i´ll be very please having you opinion.

Finally, i succeed getting a heartworm antigen kit test from idexx laboratory, so yesterday my Vet and i, made the test not only for “bungy” but my other two shnauzers too, being the result “positive” in the high antigen level for all of them, ¡WHAT A DAY!, you don´t even imagine how i am feeling.

That why, i have to decide as soon as possible for the best treatment to take, for that matter i need you to help me.

In my personal opinion, there are two choices:

1.- kill all adults worms with an adulticide (melarsomine) and all microfiliare with a microfilaricide (Ivermectin), with all of the complications and cares that this medicaments involves (adverse reactions), being honest this choice scares me for all of i have know. Also because they are not accustomed living lock up.

2.- kill just the all microfiliare with a microfilaricide (Ivermectin), and start all over again (a never ending treatment) to prevent babys worms become adults, doesn´t caring about the adults already exist and leaving them die from all age.

It is important to mention that two of them, have not present clinical signs, and the other one have presented moderate clinical signs.

Thank you. Luis

Ps. One question, does an adult heartworm is the result for each mosquito bite?????

Answer: Luis-

Not all dogs are infected with heartworm larvae, even in an endemic area. I do not know the percentages of infected mosquitos. On the other hand, if a mosquito is carrying heartworm larvae it can expose a dog that it bites to up to 12 larvae in one bite.

Mike Richards, DVM 3/19/2001

Prevalence of Heartworm disease and general information

Question: Dear Dr Richards, Could you please give me some information on the prevalence and incidence of heartworm infestation (ie full-blown, clinically presenting cases)? Are statistics on the incidence of this awful illness kept, or is it just something which practitioners in different areas come to know by experience? Are there certain areas which are more susceptible than others? (I guess the prevalence of mosquitoes is the governing factor?) Is there anything to be said for applying repellants to one's dogs before they go outdoors (in addition, of course, to maintaining them on heartworm preventative medication)? And could you please enlarge on the statement (reproduced below) which you made in your latest newsletter concerning the frequency with which heartworm testing should be done, and the possibility of resistance: "We heartworm check dogs at the time of their first year booster vaccinations and then every two years. From the stantpoint of an individual patient, it would be eaasy to justify a longer interval, but from the standpoint of dog health in general, it is pretty important to recognize resistance problems to heartworm medication early, if they ever occur...." I think I understand what you are saying - that we should all do our bit to help compile a detailed database and statistics on the effectiveness of heartworm meds, etc. If that's what you meant, I absolutely agree and am only too happy to pitch in and do my part, but I had always thought it was mandatory to test annually. Is it just that the drugs are getting better and more effective? Sorry to ask so many questions, but this is a subject on which it is difficult to get nuanced answers. Thanks for your help. Louise

Answer: Louise-

The only prevalence information that I can find comes from the American Heartworm Society and from a couple of studies of heartworm incidence in specific areas. The AHS web site is http://www.heartwormsociety.org/ . Their information states that the incidence of heartworm disease in areas along the Atlantic seaboard and the Mississippi River basin is about 45% in untreated dogs. In studies in Louisiana, the infection rate in unprotected dogs was nearly 100% but these dogs were penned, which seems to increase the incidence of the disease, probably because the dog can't move to an area where mosquitoes bother it less.

There are huge variations in infection rates across the country, based largely on where mosquitoes thrive. There are variations within small geographic areas, as well. In our county, which is the second smallest county in Virginia in land area, we see variations from one area of the county to another in incidence of heartworm disease in unprotected pets.

I don't think that mosquito repellants work all that well but any protection they do offer would be good for dogs not on heartworm prevention medications in an areas in which heartworms is endemic. The monthly heartworm prevention medications are very good and I don't think that additional protection is necessary, but the dogs might appreciate it, anyway, since getting bit by the mosquitos is no fun even when they aren't carrying a disease.

The new monthly heartworm prevention medications are safer to use than the older diethylcarbamazine (DEC) based medications. DEC could cause serious illness and even death in a patient given the medication while harboring heartworm microfilaria. Preventing this occurrence was the original reason for annual testing. The newer monthly heartworm medications do not cause serious reactions in the great majority of cases in which microfilaria are present and are actually used commonly as microfilaricidal treatments after the adult heartworms are eliminated. Due to this, many vets are comfortable testing at longer intervals now. I think that the American Heartworm Society recommendation is "every two to three years" at this time.

I view heartworm testing as a way to screen for the possibility of resistance developing to the medication and I think that it is reasonable to ask people to test every two years or so in order to be sure that any resistance problem that does occur is caught very early, so that is what I was trying to say. It is also a good way to pick up the occasional dog who has learned to hold the tablet in its mouth 'til no one is looking and then bury it in the yard or hide it somewhere else and who manages to get heartworms due to this behavior. I am not seeing any evidence of resistance to the medications at this time and I suspect that if we focused entirely on the needs of a particular individual dog and we were pretty sure that it was getting the heartworm medication we would be comfortable testing only if clinical signs appeared. This is a case in which we try to focus on the bigger picture and to screen for resistance more carefully, though.

Please don't hesitate to ask for clarification of anything that was confusing about this message or other messages.

Mike Richards, DVM 3/13/2001

Heartworm disease in Mexico - continued

Question: Dr, once again i am very very grateful for all your comments, and once again i´ll be very please having you opinion.

Finally, i succeed getting a heartworm antigen kit test from idexx laboratory, so yesterday my Vet and i, made the test not only for “bungy” but my other two shnauzers too, being the result “positive” in the high antigen level for all of them, ¡WHAT A DAY!, you don´t even imagine how i am feeling.

That why, i have to decide as soon as possible for the best treatment to take, for that matter i need you to help me.

In my personal opinion, there are two choices:

1.- kill all adults worms with an adulticide (melarsomine) and all microfiliare with a microfilaricide (Ivermectin), with all of the complications and cares that this medicaments involves (adverse reactions), being honest this choice scares me for all of i have know. Also because they are not accustomed living lock up.

2.- kill just the all microfiliare with a microfilaricide (Ivermectin), and start all over again (a never ending treatment) to prevent babys worms become adults, doesn´t caring about the adults already exist and leaving them die from all age.

It is important to mention that two of them, have not present clinical signs, and the other one have presented moderate clinical signs.

Thank you. Luis

Answer: Luis-

I think that it is usually a mistake not to treat for the heartworms. This is my reasoning:

Heartworms cause damage by living in the heart and the big blood vessels going to the lungs. They do not eat the blood vessels or cause direct damage by their own actions. They just live in a bad place. The blood following through this area is moving fast and some blood cells break when they hit the heartworms. This leads to a release of chemicals that attract blood clotting agents. This causes mini blood clots to form in these vessels, which then bounce off the blood vessel walls, slowly damaging them over time. After a while, the blood vessels are scarred enough that they don't have their normal elasticity and they also become narrower. This causes an increase in blood pressure. Think how you increase the pressure of water in a hose --- you constrict the size of the opening it must go through at the end of the hose. The narrower the constriction, the higher the blood pressure. Eventually, the blood pressure gets higher than the heart can comfortably pump against and heart failure results. This process goes on no matter how many worms are present. More worms cause more damage in a shorter period of time, but a small number of worms can cause a lot of damage if they live in the vessels long enough. Heartworm prevention MIGHT shortent the life of adult worms (this is not proven for sure yet) but even if it does the estimates are that the lifespan of the worms is reduced from 3 to 5 years to perhaps 18 months to 30 months. This is long enough for a lot of additional damage to occur. The end result of some attempts to avoid treatment is having to treat a patient who is in much worse shape than if the treatment had been done at the time that a diagnosis could be made through testing.

Some dogs do luck out and get through the heartworm lifespan without clinically apparent damage. I am nearly certain that in my area, this is a smaller percentage of dogs than those that get into trouble because we tried the approach of putting dogs on heartworm medications and not treating and had some very bad experiences from doing that.

I would treat all three dogs if they were mine, keep them as quiet as I could and do my best to keep the surrounding temperature below 70 degress Fahrenheit and hope for the best. The odds are good that all three dogs will do fine but you are correct that complications can occur during treatment and that they include death in about 1 to 3% of dogs treated with melarsomine (Immiticide Rx) and about 8 to 20% of dogs treated with thiacetarsamide (Caparsolate Rx).

Mike Richards, DVM 3/13/2001

Heartworm disease treatment for Min Schnauzer in Mexico

Question: Hello, I am very desesperate I need a favor and that is if you can help me to diagnose an illnes that "Bungy" my miniature schnauzer might have. He is aprox. 7 or 8 year of age and waist around 11 kg (24.3 pounds).

About 2 months ago he started to have chronic diarrhea with blood which lasted for 2 days until the doctor gave him vitamines, coagulants, antiparasital medicines and an IV (whey). at the same time my dog began to cough like saliva over and over again until he was coughing up blood. This lasted for a two days aprox. and we did not know how to stop it. We thought that all the medications for the diarrhea might have causes an ulcer and we gave him milk and yogurt. After about one or two months, one day he began to have about ten coughs spell again but this time with out blood. and about ten days later he did it again until we gave him more milk. Yesteday he began to cough about other ten times and when he stopped there was a worm hanging out of his mouth, i thought it was his slaver. The worm was round, white, thin, and about a foot in length. Almost like a long thin linguini noodle. We put it in a jar and brought it to a Vet to diagnos, but the doctor told us that he had never seen this before. He said it was ver rare for dogs to have worms. Normally they only have parasites and bugs but not worms. And what was ever more rare was that the worm was expelled from his mouth. The doctor took a sample of excrement and analyzed it under the microscope. He told me that he did not see any reminantes of parasites. Later he took a sample of blood and sent to the lab be studied (he asked for a hematic biometry). The doctor said that the parasites name could be (in spanish) "Dirofiliaria immitis", but it would be best to to wait for the test results. The tests came back normal. They said seemed to be normal. I can no longer see this Vet because he has left the city. Yesterday I began to investigate over the Internet to what this worm could be, and was very surprised to see pictures of heartworms, because they looked like the same worm that came out of his mouth. As well I learned that coughing is a symptom of heartworms, where I lived there are many mosquitos so it really could be anything. From what I read, I think that "Bungy´s" illness is in the 2nd or 3rd stage of heartworms. As well I saw many types of testing that can be done to diagnose these types of illnesses, and here in Mexico they do not do these tests, specially in my small city. I need to know for sure if this parasite that I saw is in fact "Dirofiliaria immitis" in order to correctly treat him. I did read too that exist a worm called Dipetalonema Reconditum that is too similar than a heartworm, so i guess maybe could be one of it. Does a simple test exist that can help diagnose my dog ?, i mean simple cause where I do live is a small city in Mexico that dont have acces to those test, Are there other parasites that look like "Dirofiliaria immits" ?, Could this be another type of worm ? I have spent a lot of time reading and researching my available resources about "Dirofilaria Immitis", and nothing said anything about a worm being expelled from the mouth. Another problem I have is that here in Mexico the do not sell these types of medications for dogs (Melarsomine, Ivermectin). I have the means to obtain these medicines from the States, but the problem is how will they sell the medicines to me and who will sell them ? Will a presciption from a a Vet here in Mexico work to buy the medicines in the U.S. ? I am worried because besides "Bungy" I have other two miniature schnauzers, "Rocket" and "Nena", which have not yet shown any signs of the same, except for a recent suffocate.

I read that there is a simple diagnostic method of analyzing the blood under a microscope in order to dilect larve. Even though the blood was sent to the lab and they said that iw looks normal, the test that they performed was only to watch the blood trough the microscope. I apreciate your time and would be ever thankfull if you would be able to answer my questions.

Any further information concerning this problem would be of great help I will await your response. I thank you again for your help.

Sincerely, Luis MEXICO

Answer: Luis-

Dirofilaria immitis is the scientific name for heartworms. It seemed like you knew that from your note, but I wasn't sure. In any case, heartworms are occasionally coughed up from the lungs. Presumably this happens because they develop outside of the pulmonary arteries or because of rupture of a blood vessel allowing the heartworm into an airway. Many dogs with advanced heartworm disease have blood clotting problems and that might lead to the gastrointestinal bleeding that you have been seeing (or this blood may have been from the lungs, swallowed as it was coughed up and then vomited later). Usually by the time this is happening there are other obvious signs of heart failure, such as coughing, tiring easily when exercised, abdominal swelling, lethargy, decreased appetite, weight loss or fainting.

It is not unusual for an examination of a blood smear or drop of blood to fail to show the typical microfilaria (baby heartworms) that occur with heartworm disease. At least 20% and probably more like 40 to 50% of the cases of heartworm in dogs do not have microfilaria in the blood stream. A more accurate test is an ELISA test (Snap tm, Witness tm) or a latex agglutination test (VetRed tm). These test directly for adult worms. This is the easiest way to differentiate between Dirofilaria immitis and Dipetalonema reconditum for many vets, too, since the ELISA test will only be positive for heartworms. The female worms of both D. immitis and D. reconditum are about 10 to 11 inches long so either one is possible. Dipetalonema worms are usually considered to be non-pathogenic, or to contribute secondarily to things like flea allergy dermatitis and glomerulonephritis by upsetting the immune system. In a situation in which clinical signs of heart disease are present, heartworms would be the best explanation for the presence of the worm and the signs that you are seeing.

It may be possible to get thiacetarsemide (Caparsolate Rx) where you are (I have heard it is available in Mexico) and that would be an acceptable treatment for heartworms, although with more risk than melarsomine. It would be a good idea to treat for the heart disease and to try to get Bungy more stable before attempting heartworm treatment, if the current signs are due to heartworm disease, though.

It would be a really good idea to try to figure out how to get your other dogs on heartworm preventative medications. I have no idea whether US mail order pharmacies would honor an international prescription for heartworm medications. I would be pretty surprised if a US veterinary hospital could do that, because I know for sure it is illegal for a veterinarian in Virginia to sell heartworm medications without a direct doctor/patient relationship.

I will try to check into the possibility of other types of worms but you have already found the two that seem most likely.

Mike Richards, DVM 3/10/2001

Heartworm positive and pregnant

Question: Dr. Mike, I recently found a female miniature American Eskimo, approximately 3 years of age. I have a trusted vet that examined her and found her to be expecting and positive with heartworms. I am anxious to commence treatment, however her pregnancy is obviously a factor . The vet will see her again in three weeks and make a determination on the type of treatment. I would like to see her deliver her puppies, but frankly I am more concerned about her welfare. what are your thoughts on heartworm treatment, (particularly the new Immiticide) on preganant dogs with heartworms? I keep my house very cool and the dog is not excitable and tends to be quiet. She has a very good appetite, no cough, although her breathing is abit on the heavy side. I am worried about her, as is understandable, so please respond in kind at your earliest convenience. (She tested positive quickly). Thank You.

Answer: Sid-

It is MUCH safer for her to let her have the puppies and to raise them to weaning age and then pursue treatment instead of trying to treat her when she is pregnant or nursing puppies.

We have treated one pregnant dog, inadvertently, for heartworms. It was a near total disaster, except that we did finally manage to help her live through the complications. All of the puppies died.

Anything that might lead to bleeding should be avoided during a heartworm treatment. For this reason, if spaying is contemplated, we do the surgery prior to the heartworm treatment, because even going into heat can really complicate a heartworm treatment.

Heartworms cause damage that slowly leads to congestive heart failure. Since she doesn't have symptoms of heart failure at this time, it is likely that you can afford to delay treatment for several months without increasing the risk to her significantly. Being pregnant does increase the risk of heart failure somewhat. If this is an early pregnancy and if losing the puppies is acceptable to you, the safest course of action for her may be to go ahead and spay her now and then treat two to three weeks after surgery.

Sorting through the options is hard in these cases. I would look at them this way:

1) Best for her: Spay her if the pregnancy is not advanced. Treat two to three weeks later. She must be kept cool constantly and she must be kept quiet. 2) Best for the puppies and reasonable for her: Wait to treat her, let them nurse to weaning age and then treat her a week or so after the puppies are weaned

I hope this helps in your decision making. If something isn't clear, I'll be glad to try to explain this in another way.

Mike Richards, DVM 8/4/2000

Heartworm medication dosage

Question: Dear Dr Richards, I have faithfully administered heartworm preventive medication to my dogs over the years, going from the days of one-a-day DEC to Filaribits to Heartgard to Heartgard-Plus and now to Interceptor, as prescribed by the various vets we've had in the various places we've lived. I never worried much about the dosage until a few years ago, when our vet at that time recommended we give Heartgard Plus, which of course in addition to heartworm prevention is effective against hook and whipworms. Because we have Scottish Deerhounds, which as a breed have a tendency to react idiosyncratically to various drugs (sulfa antibiotics, anesthetics, etc.) I began thinking hard about the wisdom of dosing so heavily each month on the presumption of infestation with round, hook and whipworms. (I forget exactly what milligram dosage of Heartgard Plus we were administering, but it was the dosage used for dogs weighing 100lbs or more.) From what I have read, microfilariae are a great deal more easy to knock down than hooks or whips, and the dosage of ivermectin/Heartgard (or milbemycin/Interceptor)which is commonly prescribed may be way, way more than is needed strictly to kill off any circulating microfilariae. I understand the wisdom of dosing for intestinal parasites in general, but am really not convinced that it is essential to dose on a monthly basis. I would much rather dose monthly at a lesser level, aiming strictly to deal with microfilariae, and perhaps less often give a stronger dose to deal with intestinal parasites as well. Please would you adivse on the wisdom of such a course of action, and also help me to calculate what milligram dosage I would need to deal strictly with circulating heartworm microfilariae in my three deerhounds, each of which weighs around 100lbs, and who are currently getting one 23mg tablet of Interceptor per month. Many thanks, Louise

Answer: Louise-

The dosage of the heartworm preventative medication ivermectin in Heartgard Plus (Rx) is based on the dose necessary to kill the infective larvae which the mosquito injects into the dog. This is the stage after the microfilaria stage of the heartworm life cycle. This dosage is very low in comparison with the dosages of ivermectin used for other purposes. There is a second medication in Heartgard Plus (Rx), pyrantel pomoate, which is included to kill the hookworms and roundworms. This medication is considered to be very safe to use and is not likely to cause complications. It is dosed at the appropriate dosage for killing hookworms and roundworms. I think that it is still possible to get Heartgard (Rx), which does not contain the pyrantel.

There is not a big difference between the effective heartworm preventative dosage for milbemyin (Interceptor Rx), which is 0.25mg/kg, and the recommended package dosage, which is 0.5 to 0.99mg/kg. I have to admit that I thought the difference between the effective dosage for hookworms and for heartworm larvae was much higher than this, just as you thought. I am not sure why I had that impression.

I don't think that it is likely that many dogs need the roundworm medication on a monthly basis after they are several months of age. Hookworms are a problem in the South, where I practice, but are less of a problem in other areas of the country. You could certainly discuss the likelihood of this problem in your area with your vet and make a determination of the need for monthly dosing with pyrantel or the higher dosage of milbemycin.

There are alternative medications for heartworm prevention. Moxidectin (ProHeart Rx) is only labelled for heartworms and contains a single ingredient. Selamectin (Revolution Rx) is the newest heartworm prevention medication. It also kills fleas, some ticks, ear mites, roundworms and hookworms. I do not know the minimum heartworm dosing requirement for this product.

I don't think that this is really a major worry. The heartworm preventatives are not time-released medications, nor do they have extremely long half-lives. They work by killing infective heartworm larvae that have accumulated in the dog's body during the month prior to administering the medication. They can kill a wide range of ages of the larvae, which is why they can be used in this manner. Diethylcarbamazine could only kill one stage of the larvae, which lasted about 48 hours of the larva's life cycle. This made it necessary to give the medication daily, so that when the larvae reached the stage it could kill, the medication was in the dog's body already.

I have not heard of any long term problems associated with the heartworm medications at this time, in any dog breeds. Hopefully that will continue to be the case.

Mike Richards, DVM 5/11/2000

Heartworm disease in older dog

Q: Dr. Richards, We were adopted by a 12 year old yellow lab who has suffered with numerous health problems. We continued with a Vet who knew our dog and her history. We discovered she had heartworm which our Vet recommended treatment in hopes it would abate her health problems. She had the usual lab and radiology work which was within normal limits. She tolerated the first injection far better than we had anticipated (we gave her Prednisone for about the first 2 weeks), however, at about the fourth week she lost her appetite and was listless. Our vet ordered lab work which revealed elevated liver enzymes. He treated her with antibiotics and Prednisone...she responded rapidly and seemed to be fine. About two weeks off the medications she fell ill again with the same symptoms described earlier. He checked her liver enzymes and found them to be elevated - Alk Phos 453, SGOP 1357, SGOT 184. GGT 20, and bilirubin was elevated (bilirubin was WNL previous lab). Our vet explained our options are: 1) treat her symptoms with Prednisone and special diet 2) Do X-Ray &/or ultrasound to rule out obvious pathology 3) Go all the way with Ultrasound guided biopsy and cultures. Our Vet has consulted specialists and all agree not to continue heartworm treatment but do recommend biopsy. We are retired, therefore, money is a consideration. We understand her life expectancy to be 13 - 15 years and of course that is now impacted by the heartworm condition. We would be appreciative of your views on our dilemma. Respectfully, K. & P. N.

A: P. and K.-

We treat heartworm disease fairly often in our practice. It is a difficult choice to make when dealing with geriatric patients but we have treated dogs as old as 13 years of age, when it seemed appropriate.

I think that older dogs do have more problems with the treatment, possibly due to the effects on the liver and overall immune system compromise in older patients.

There is a very good chance that the first injection cleared enough of the heartworms to be a big benefit to your Lab if melarsomine (Immiticide Rx) was used for the treatment. If at a later date your Lab is doing much better it is still possible to continue the treatment at that time.

The time period in which the heartworms die and symptoms of reactions to this, such as lethargy, coughing, weakness, hemorrhage, inappetance or other signs occurs lasts for as long as eight weeks. So at this point, you are still in the time period in which these problems occur. Keeping your dog as quiet as possible is the best thing to do for these problems. Cool conditions help a lot, too. We advise air conditioning anytime the temperature exceeds about 75 degrees.

Prednisone can cause elevations in the alkaline phosphatase levels (SAP, AlkP, alk phos, etc.). It can sometime cause more liver damage and elevations in the ALT (SGPT) levels are indicative of this. Melarsomine can cause liver damage, too. It is less likely to than caparsolate, the original heartworm treatment, though. Heartworms and the reaction to dying heartworms in the circulation can cause elevations in liver enzyme levels, as can circulatory changes induced by the parasites. Since all of these causes are possible, and all should improve with time, I tend to just monitor the values a while unless I think the patient is getting worse instead of better based on overall clinical signs. (So I guess I'm saying I wouldn't do anything at all but wait to see what happens MOST of the time, in my patients. I am big believer in doing nothing, sometimes.) However, your vets will have a feel for this, too -- so trust their advice because they can see how your Lab is doing, on person.

Hope this helps. I would go ahead and use a monthly preventative medication even though the treatment wasn't finished. It will prevent further infection with heartworms and MAY (this is a big MAY) weaken the remaining heartworms further and hasten their demise.

Mike Richards, DVM 9/10/99

Occult Heartworm test

Q: Hi Dr Mike! I thank you in advance for all your valuable help. I have a question for you. Is there any quantitative test for occult HW disease?

My three year old dog "Chiquito" was diagnosed with HW last may, and went through the HW treatment last june ( two Immiticide injections one day apart from each other, and one Ivermectin injection three weeks later). Four weeks after the Immiticide shots, the Vet performed a test for occult HW , the result was positive and the Vet was surprised of such a result ( he was expecting a negative one). It worries me the fact that apparently he didn't know that the antibodies to HW remain in the dog's organism for several months after the treatment. I wonder if he administered the treatment the right way or the infestation is just getting worse as the time goes by.

I would like to "monitor" (without having to wait four more months) the decreasing antibody titers in my dog's blood to make sure the treatment was successful . It might be important to mention that the X- ray examination revealed an early stage infestation, and that Chiquito has been taking Milbemycin oxime once a month for the last two years (except for the time of the HW treatment).

I feel so sad thinking of the possibility that all the pain, the risks to his health, and hard time that Chiquito went through were useless, and that he might have to go through all that again .

A: Angelica-

I am not sure if it is still being produced but Idexx made a test which detected high and low antigen levels for heartworm antigen. If I remember correctly, the low antigen indicator was positive when there were approximately five or fewer worms and the high antigen indicator positive when there were more than five worms.

The new heartworm tests for dogs are not antibody tests. This would be a test for antibodies produced by the body in response to the presence of heartworms. The tests now test for antigen, which is a recognizable portion of the heartworm itself. Most of the tests use female reproductive tissues as the recognizable antigen so they actually only detect female heartworms. While all male heartworm infections certainly exist they are probably rare enough that this is not a major drawback of the tests.

The reason that is important is that it impacts on how quickly the tests become negative after heartworms are successfully killed. An antibody test would remain positive for a very long time. An antigen test becomes negative as soon as all of the worms have been disposed of by the body. This takes about 3 months, so we usually retest after 4 months, just to be sure to allow time for all of the antigen to be cleared from the dog's system.

Using melasomine (Immiticide Rx) the kill rate for heartworms is very high. It is greater than 97% with the two treatment method and 100% or very close to it using the three treatment method. It is very unlikely that the few heartworms which survive treatment will cause significant continuing damage in a dog. If monthly heartworm preventatives are administered to prevent reinfection it is usually safe just to wait for the female heartworms that remain to die from old age.

This is a situation in which you are almost certainly worrying more than you really need to. Melarsomine is very effective. Very small numbers of heartworms cause very limited damage so even if one or two female worms did survive the treatment they are unlikely to cause major damage. Continuing to administer milbemycin (Interceptor Rx) should prevent new infections. So the odds are very very good that Chiquito is going to be fine. I would recheck the occult (antigen) heartworm test results three to four months after the date of treatment just to be sure and if the test does remain positive periodic physical examinations to be sure that there is not a progression of clinical signs attributable to heartworms would be a good idea but retreatment is probably not necessary and probably wouldn't even be a good idea.

I'm willing to bet that you'll find that the test results are negative in two or three months.

Mike Richards, DVM

Heartworm medication - lapse in medication

The monthly heartworm preventatives work to kill heartworm larvae that are 45 days or less in age and probably closer to 50 days. Since an every 45 day dose would be hard to remember and provide no leeway for error, the pills are approved for monthly use. We tell clients to give another pill if they are unable to remember if they dosed their pet and we can not help them determine if that happened based on our records (usually we can help in a single dog family but it is harder when several dogs are on the same pills in a household). We just check the dates the medication is sold and count pills from there. Administering another pill is safe if one was given at the beginning of a month because the medication is not time release. It doesn't last in the body a month it just kills all the heartworm larvae that have accumulated in the last 30 days. It is pretty much completely cleared from the body in 72 hours. Since it takes 6 months for heartworms to develop to the age they can be tested for an immediate heartworm test is not necessary prior to restarting the medication. It is a very good idea to check for heartworms 6 months or so after a lapse in preventative medications.

Mike Richards, DVM

People and Heartworms

Q: I have a question to ask you. Can humans get heartworms?

David and Karen

A: David and Karen-

Yes, humans can get heartworms. Usually in people they do not ever find their way to the heart and pulmonary arteries as they do in dogs. After being deposited into a human by a mosquito, the infective larvae usually develops partially and encysts in lung tissue or inside the eye. The worms can be removed from eyes. When there is a lung infection it may resemble cancer on an X-ray and result in a lung biopsy surgery for the unfortunate human victim. Fortunately, these infections are rare. It is another good reason to use heartworm preventative medications, though.

Mike Richards, DVM

Heartworm Host

Q: I understand that this form of worm is transmitted by mosquitoes but how does the host animal get the worm in the first place? Andrew

A: Andrew- I am not sure how the first case of heartworms originated, since it is necessary for the baby heartworm to go through a mosquito in order to grow. This was probably an adaptation from another method of reproduction that evolved over time. It is to the heartworm's advantage that the microfilaria (first stage of life) can not grow up. We find as many as 10 or 15 microfilaria in one drop of blood at times. That probably works out to about 100 microfilaria per cubic centimeter (cc) of blood. Since there are about 2000cc of blood in a 50 pound dog that would be 200,000 heartworm microfilaria in a severely infected dog! If they all grew up it would obviously kill the dog and the heartworms would die when the dog died.

It's confusing but it ensures that the heartworms will survive as long as possible. I am glad we have good heartworm preventatives now.

Mike Richards, DVM

Heartworms - lifespan

Q: I have a 5 year old cocker spaniel. He tested occult heartworm positive approximately 2 years ago. He has severe seizures whenever stressed, even when on pheno-barbital. He is also very aggressive in strange situations. Because of this my vet felt he was not a good candidate for heartworm treatment and instead put him on Heartguard +. We have him occult heartworm tested every year and the test remains positive. At this time B.J. shows no outward signs of the disease. How long is the typical lifespan of a heartworm? I've spoken to several vets and I get answers ranging from 1 year to 5 years? I worry about not treating B.J., but just taking him to the vet for his annual checkup causes seizures for days afterwards. Lori

A: Lorie- The most consistent figure I see quoted for the longevity of heartworms is three to five years in dogs. In cats the lifespan of the heartworm appears to be lower and I have seen one to three years quoted for heartworms in this species. Hopefully, the heartworm burden is low and will not cause problems while you wait for them to die off. This approach has not worked really well in our practice but we have had several patients who have done OK and not had any significant problems. I hope this works out for your dog, too. It does sound like it may be the best approach with the difficulties you mention.

Mike Richards, DVM

Heartworm Disease - heart attack

Q: Please help, I have a 9 and a half year old yellow lab/mutt. She tested positive for heartworm and is wheezing( the vet said). I am scared for her age about the new treatment. The vet said he thought she would make it, but no guarantees. She is 72 pounds, and has gotten very lazy since her partner died last week. Our other dog was 9 and started having seizures during thunderstorms, she had heartworm, but the vet said that it wasn't that bad, but we were going to start treatment when she had a heart attack during the seizure and died. Could the seizures have been from the heartworm? She also was 70 lbs. Please help, I am so confused about what to do. I don't want to lose both of my dogs.

A: Dayna- The heart attack could have been caused by heartworm disease but heartworms seldom cause seizures.

I hope you did decide to go ahead with treatment. We have had very good luck with melarsomine (Immiticide Rx) so far and I think a 9 year old dog that is otherwise in good health should be treated for heartworms. No one can guarantee it will work but the odds are very good and heartworms cause both disability and death -- so treatment is a good choice.

Mike Richards, DVM

Heartworm disease testing

Q: Dear Dr. Mike, Sparky,my 8 year old, male, long-haired Dachshund was diagnosed with heartworms two days ago. I'd last had him tested in February 1996 and the test was negative. He's been on the monthly preventative pill ever since. I can only think that I must have missed a month for this to have happened. My other dog, a Corgi, tested negative. The test of Feb 1996 was done at a local Petsmart where they have vet facitities. I wish I hadn't gone there. My current Vet thinks that the test they use in Petsmart is only 60% accurate. I don't know the name of the test done in the office two days ago but I do know it took five minutes and two out of three dots were blue. Chest x-ray showed some right ventricle enlargement. However, on a brighter note, Sparky showed no outward symptoms.

He was treated with 1 injection of Immiticide. In one month he's to get another followed by the last injection a day later. Now for my questions. Does the follow-up injection in one month have to be given in exactly one month or is it okay if it's 3 days late? I was told to keep him quiet for 3 weeks. So, I've put baby gates up in the kitchen and that's where he stays. That room measures approx 8 X 13 feet. He doesn't run around at all and stays quiet. Is it too much exercise it he comes out in the living room with us at night? I don't want to put him at risk but I don't want him depressed either. Do you think it's safe if he walks to another room in the house if he shows no signs of exertion? Thank-you ,Pat

A: Pat- Without knowing the type of test used at Petsmart it is hard to be sure of the accuracy, etc. Just a quick review of heartworm testing may enable you to understand how this situation could have developed, though.

There are several ways to test for heartworms. The first method developed was to put a drop of blood on a microscope slide and look for baby heartworms, which are microscopic and are known as microfilaria. This works if there are adult heartworms producing babies and if the dog's immune system is not killing the babies. It is probably an accurate test for adult heartworms in about 50 to 60% of infected dogs (just an educated guess on the percentages). The accuracy of this type of testing was improved by using "concentration" techniques --- methods of increasing the likelihood of finding microfilaria if only a few were present. Originally this involved centrifuging blood but later special microfilters were developed to allow filtering of baby heartworms out of a bigger blood sample. This was a better test but still was inaccurate if the heartworms weren't producing babies or if the dog's immune system was killing them. Estimates of the inaccuracy of this testing varied widely but the general consensus was that it missed between 20 and 30% of heartworm cases and I personally think it was probably higher than that. Testing for microfilaria was critically important when daily heartworm medications were in use. Ironically, the presence of microfilaria was actually more important than the presence of the adult heartworms when using these preventatives because reactions could occur that could cause death when daily preventatives were given to dogs with microfilaria in their circulation. There was not much incentive to get more accurate in testing for heartworms in order to use preventive but a number of dogs still had clinical signs of heartworm disease and yet tested negative on available blood tests. Tests for antibodies to heartworms were developed. These were problematic because they indicated exposure to the heartworms, so dogs remained positive on these tests even if heartworms had died naturally or after successful treatment. Finally, tests were developed that test directly for adult heartworm antigen -- but only from adult, sexually mature female heartworms. So now there are two situations in which the testing remains inaccurate -- all male heartworm infections and heartworm infections in which there are female heartworms present that are less than 6 or 7 months of age. There is also a small possibility of a false positive test with the antigen test kits -- estimates are that it is about 1 in 1000.

So how does this apply to your dog? The first test could be accurate and the second test a false positive. The odds of this are low but it is possible. The presence of clinical signs makes the odds very very low, in fact. The test may have been a filtration or concentration test for microfilaria and your dog may not have microfilaria despite having adult heartworms. The test may have been an antigen test and your dog may have been infected at the time of the first test but for less than 6 months -- so there were no adult females to make the test positive. Now, months later, the females have grown up and are causing problems. In any case, treatment is now indicated.

It doesn't make any difference at all if you wait the extra three days. It is slightly safer to treat dogs by injecting one injection of Immiticide (Rx) and then waiting a month because the first injection kills off the weaker worms, spreading out the effect of the medication. The two followup injections kill the stronger heartworms (usually the young females). It is OK to treat just by giving two injections one day apart as the whole treatment but since that kills all the worms at once it makes the possibility of complications associated with worm death higher. You don't want your dog to have heartworms longer than necessary to ensure safe treatment but a few days either way isn't a big factor.

It is physical exertion that complicates heartworm treatment. If your dog is calm then staying with the family is no problem. Some dogs are more active when they are confined (digging at the gates, crates, etc.). Those dogs need to be with their family instead of being confined. Other dogs jump up and down, on and off the furniture and generally get so excited around their family that confinement is the only way to help them through a heartworm treatment. You have to do what works best to keep your particular dog quiet. While I have not seen it published anywhere I can think of I am absolutely convinced that coolness is important too -- keep the air conditioning on!

Mike Richards, DVM

Heartworm testing and treatment

Q: Dear Dr. Mike, Thanks for your thorough explanation of heartworm testing. When the recent test was done, that was positive, blood was put on a white plastic thing with three dots in it. The dots formed a triangle. After waiting five minutes the top dot and the right dot were rather dark blue. The vet retested with the same test and the results were the same. He then looked at the blood under a microscope and said he didn't see anything. Do you think it would be prudent to have a blood test taken at the three week mark and have it sent out to the lab to make sure there are still heartworms before giving him the next two injections?

I know you said the first injection only kills the weaker ones (too bad, I thought it killed all adults). I just hate to give him any more drugs than is absolutely necessary. What if he had only female heartworms or only males. There'd be no microfilaria, right? Maybe I'm being overly optimistic. The air conditioning is always on here. In Florida it's soooo hot I won't even let Sparky sit out on the screened in porch until this heartworm ordeal is over. Other than the first day when he was uncomfortable from at the injection site he's acting totally normal. I've eased up on his restrictions but still no walks or playing. Thanks again for taking the time to write and explain everything. You've been a great help! Ever thought of moving to Florida? Sincerely, Pat

A: Pat- Your vet is probably using the Idexx Cite Snap (tm) heartworm test, based on your description. This is a very accurate test. It has a false positive rate of about 1 in 1000 tests. It would not be positive in an all male heartworm situation since the test kits all test for cells from the reproductive tract of adult female heartworms. It takes several weeks for the antigen to clear out of the dog's body after successful treatment so it is unlikely to be negative after three weeks even if all of the worms have been killed. It is best to finish out the treatment.

Good luck with this! Mike Richards, DVM

Heartworm Disease

Q: I recently adopted a doberman/lab from our local SPCA, he had tested negative on a difil heartworm test. I work at a Veterinary Hospital so I promptly took blood and sent it to Antech for an occult test. The result was a weak positive, the blood was hemolyzed. I have received this same reuslt three times. I ran a CBC and all was normal except for Eos, they were slightly high. I also ran another difil, but no microfilarae. I am reluctant to treat for the heartworms without having a definite positive. Do you have any suggestions on how I may be able to determine whether or not this is heartworm disease or something else. Also, if I treat with Immiticide and he does not have heartworms, what are the risks to his health? THANKS!

A: In my area (Tidewater Virginia) I would probably treat a dog on the basis of a weak positive occult heartworm test in a situation in which heartworms seemed likely. Here, where heartworms are very prevalent, the odds are good that a positive test is accurate. This is not true everywhere. The odds of a false positive test vary from test procedure to test procedure but probably are 2% or less from all of the commonly used tests. If you live in an area where the natural infection rate is less than 2% it is actually more likely that a positive test result is wrong -- because the incidence of false positive results is higher than the incidence of the disease. So the decision about whether or not to treat for heartworms on the basis of a positive test in a dog that doesn't have symptoms of heartworms, such as coughing, tiring easily, weight loss or fainting, is harder in areas in which heartworms are not prevalent.

It is not at all unusual to have a dog who tests negative on a test for microfilaria (baby heartworms) such as the Difil test but who tests positive on an occult heartworm test. It is possible that as many as 40% of infected dogs will test negative for microfilaria. This is called occult (hidden) heartworm disease.

We use the VetRed test from Rhone Merieux most of the time. When we get a positive result that we question, Rhone Merieux will pay for more definitive testing. It is possible that your lab can arrange for this as well. We are sometimes too impatient for that and test with another test kit such as the Idexx Snap test in-house. If they are both positive, we feel more secure in the diagnosis. It is also possible to see the signs of heartworm disease, such as enlargement of the pulmonary veins, on X-rays. If the X-ray signs are present, that would be sufficient reason to treat, as well. An increase in eosinophils is a good indication that heartworms or other parasite infection is present, since eosinophils rise with these problems.

I do not consider Immiticide (Rx) to be totally benign as a medication. It seems to be much safer than Caparsolate (Rx) was, but it still can cause significant side effects. We have not had a fatality associated with effects of Immiticide itself, but we have had some pretty sore dogs after the injections. If you live in an area in which heartworm disease is unusual, it seems like a good idea to pursue further testing. If you live in an area in which heartworms are very likely, the high eosinophils and history may justify going ahead and treating, especially if X-rays are also suggestive of heartworm disease.

Hope you didn't want a short answer. Mike Richards, DVM

Heartworm disease in Mexico

Q: hi,my name is ma.luisa and i live in cancun,mex. Believe it or not, in this part of the country we have a lot of cases of the heartworm disease, i have three female dogs and the vet has told us that one of them has the worm and that probably the rest of them may as well have it. My dog is a vizla,she is one year old,the vet told us that he performed two tests, in the microfiliaria test the result was negative but in the other one it seems that she is infested with adult worms. We desperately need your help because we are not able to find the medicine caparsolate here and we need to know if we have another option, do you know of another medicine, or how can we get this one, can you help us? we will be very grateful for any kind of help that you can provide. By the way i was looking in your disease glossary and i was not able to read any of the info on the heartworm. THANK YOU

A: Luisa - Caparsolate may be hard to find because there is a new medication melarsomine (Immiticide Rx) which is a better heartworm treatment. It is more expensive but the complications of treatment are less and overall that probably makes it cheaper for most owners whose dogs are treated for heartworms.

These are the only effective medications I know of for removal of adult heartworms from the circulatory system so it will be necessary to find one or the other. Both are manufactured by Rhone-Merieux at the present time, I think. Mike Richards, DVM

Traveling with heartworm prevention:

Q: Dear Dr. Mike, This June we are travelling across Canada, (from B.C. to Newfoundland), with our newfies as part of the Great Newfoundland Dog Trek. The area of B.C. that we live in is free of fleas and heartworm, so we have never had to worry about treating or preventing these problems. However, we will be travelling through areas that have these nasties, and we will be with over 100 other newfs from all over N. America. Should we be considering some sort of preventative medications before we start or for while we're travelling? Thank you in advance for your advice.

A: I definitely think that you should put your dogs on heartworm preventive if you are traveling through an area in which it occurs. The monthly heartworm medications do not need to be started in advance of the trip but flea medications will need to be started before you leave. The monthly heartworm medications are not timed-release or long acting medications. They work by killing all stages of the heartworm's life less than about 45 days of age. So the dog collects whatever heartworm larvae it is going to collect for a month, a pill is given and they are all killed. In our practice, the monthly medications seem to be more reliable than the daily pills. To use these, you should start after you have been traveling for 30 days, then use them for at least two months after you get back, to ensure that all larvae are killed. If it is a short trip, just give them when you get back and for two more months, to be really sure that all larvae are killed. If you elect to use daily heartworm medication you do need to start it right before you leave because it works a little differently. It can only kill one stage of the heartworm larvae's life, so it must be present in the body when the larvae gets to that age. So the daily pills would need to be given throughout the time of the trip and about a month to two months after you get back.

Flea prevention is a little more straightforward. The best flea medications for short term use during a trip would be fipronil (Frontline or Topspot, Rx) or imidacloprid (Advantage Rx). Fipronil will last up to three months with one application and imidacloprid lasts a month with one application. I am not sure of the availability of these medications in Canada but you probably have them, too. You just apply them right before you leave and then again if you are gone longer than the period they work for. Your vet will probably want to check your dogs to be sure that none have heartworms prior to dispensing heartworm medication unless he or she feels really certain that is not at all possible where you live. Dogs less than 6 months of age can not be checked for heartworms using the currently available tests, so they would be excluded from this decision. Hope that helps. Mike Richards, DVM


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...