Heartworm and flea control medication and Your Dog


Collies and Monthly heartworm Preventative

Question: I have recently been informed that collies should not be given once a month heartworm preventative because it can cause organ damage, seizures and even death in some collies. Would this also apply to shelties or any other breeds? My source was the Collie club of America. Thank you Diana

P.S. I do enjoy your Vetinfo Digest. Best newsletter I get.

Answer: Diana-

This is simply an error on the Collie Club of America web site. It is easy to verify this by going to the PubMed web site (http://www.ncbi.nlm.nih.gov/PubMed/) and searching for the author of the quoted studies using "Paul AJ" as the search term. These are the articles that this information was based on and ivermectin (Heartgard Rx), milbemycin (Interceptor, Sentinel Rx) and moxidectin (ProHeart Rx) are all considered to be safe for use at the recommended dosages and did not start to cause problems in collies until about 10x the normal dosage for ivermectin and milbemycin and toxicity was not seen with moxidectin at up to 30X the recommended dosage. This makes collies more susceptible to the effects than many breeds, since many dog breeds can tolerate dosages that are closer to 100X the heartworm prevention dosage but it still provides a very adequate safety range.

The once monthly heartworm prevention medications are much more reliable than the daily medications in real life use. Both will work if given according to directions, but I practiced when the daily medications were all that was available. We had between 5 and 20 dogs a year who would turn up positive for heartworms, whose owners really felt they were being given the medication, each year. With the monthly heartworm prevention medications we have seen approximately 5 positive patients since these medications became available. If you live in an area in which heartworm disease is likely it is a much better choice to use the monthly prevention medications.

Please check this information for yourself before deciding not to use one of the monthly heartworm prevention medications -- it is easy to do that.

Mike Richards, DVM 1/22/2001

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 and flea control medication

Question: Doctor would you let me know if there is a substitute for Sentinel (milbemyonoxime+lufnuron) to prevent heartworm desease and fleas in my dog. I am retired and find that Sentinel is real expensive.

Thank you, Ray

Answer: Ray-

Sentinel (Rx) is a combination of lufenuron, the active ingredient in the product Program (Rx) and milbemycin, the active ingredient of the product Interceptor (Rx). Program controls fleas by preventing flea reproduction and Interceptor prevents heartworms, hookworms, roundworms and whipworms.

It is possible to substitute other products that control these parasites and often it is less expensive to use other medications in combination, although it is a little less convenient.

There are four approved products for heartworm control in dogs at the present time, ivermectin (Heartgard Rx), moxidectin (ProHeart Rx), selamectin (Revolution Rx) and milbemycin (Interceptor Rx, Sentinel Rx). All work well for heartworm prevention. They vary in their ability to control other parasites but usually heartworm prevention is the major concern. The cost of these products differs some and you may find you can save money by using a different heartworm preventative.

There are a great number of flea control products. The major advantage of using a separate flea control product, rather than a combined product, such as Sentinel (Rx) or Revolution (Rx), is that you can save money by treating for fleas only during the flea season, even if you must use heartworm preventative all year in your area. Where we live, most of our clients do fine controlling fleas from June to November, so it is only necessary to use both a flea control and heartworm control product for six months of the year.

The best of the flea control products that we have used are Frontline (Rx) and Advantage (Rx). I think that Program and Sentinel work well for dogs that are confined and for house cats but are not as effective for pets that have free roam of the neighborhood or who spend time in places where other dogs are present, such as parks. If fleas are the only concern, it is probably less expensive to use Frontline, since it sometimes works for 6 to 8 weeks to control fleas, especially for dogs that are not flea allergic and can tolerate a small number of fleas prior to the time the product is reapplied. If it is possible where you live to use Frontline every other month and to use a less expensive heartworm prevention medication, you should be able to save some money on heartworm and flea control. This is even true if you have to use Frontline or Advantage for six months of the year and heartworm medicine all year, although the savings would be smaller.

There are a number of over-the-counter flea control products but these do not work as well as Advantage or Frontline and good flea control usually saves money since fleas cause a lot of skin disease.

Mike Richards, DVM 9/25/2000

Heartworm Prevention and Flea and Tick Control

Question: Hi Mike:

I was reading the article that you had written in the June issue of the Vetinfo Digest on Medications for Heartworm Prevention and Flea and Tick Control. We live in an area of NE OH where ticks, fleas and heartworms are all prevalent. I have been hearing a lot about a product called Defend (EXspot INSECTICIDE FOR DOGS). It supposedly: Kills and repels Fleas up to 4 weeks, Kills and Repels Deer Ticks (lxodes dammini) Vector of lyme Disease up to 4 weeks. Kills and repels Mosquitoes. aids in prevention of Bkiid Feeding by Mosquitoes. Vector of Heartworm up to 4 weeks. Kills and repels Brown dog ticks. (Rhipicephalus sanguineus) up to 4 weeks. Kills and repels American Dog Ticks (Dermacentor variabilis for 2 to 3 weeks. Contains two 1cc applicators. We also use Heartguard Plus and I notice in your article as a combination you like to use Frontline and Heartguard Plus. In your opinion how does the product Defend as I described above compare to Frontline. Are their any advantage or disadvantages that you can see in using Defend.

Thank you for your time, consideration, and cooperation. I hope to hear from you soon, David

Answer: David-

We used Defend (tm) prior to the time that Advantage (tm) and Frontline (tm) were available. Based on our experience, Defend is a very effective tick control product which works well for 10 to 14 days. Defend (tm) is a moderately effective flea control product which rarely eliminates 100% of the fleas present on the dog. It is not effective enough to use on a pet with flea bite hypersensitivity or flea bite allergy but it may be an effective enough product to keep fleas to a liveable level for pet without these problems. I do not know what the current label says, but when it was sold through veterinarians the label said it could be applied at two week intervals. This interval would be necessary to get good control of ticks and moderate control of fleas. This product definitely does not control mosquitos well enough to consider stopping heartworm control medications.

Defend is less expensive than Frontline, which is its sole advantage, in my opinion. Frontline is a much better product for flea control and is as effective for tick control. For pets with skin disease, Frontline is usually effective enough to decrease the number of times treatment for skin disease is necessary, making total care of the pet less expensive despite the cost of the product. For pets who are not sensitive to flea bites and where a low flea population may be tolerable, Defend may be less expensive to use.

Mike Richards, DVM 8/24/2000

Program and lower seizure threshold

Question: hi gang... i thought i remembered that someone or some folks out there give their dogs program?....someone in the clicker solutions list wrote the following:

" after doing quite a bit of research on my own I found out that Program can lower a dogs seizure thresh hold. "

obviously i don't know what research she did, but i thought i'd pass it along for those of you who do use it or might use it. i don't know if it's a big deal, but it's always good to check these things out, right? ----------------- So, Dr. Mike, are there any health concerns with using Program, month after month? Ought I to discontinue using it?

Thanks so much.

Best regards, H

Answer: H-

Dr. Clemmons at the University of Florida has apparently made the statement that Program (tm) and Advantage (tm) can lower the seizure threshold in dogs prone to seizuring, based on several emails I have received over the last few months. There is no published data to support this claim, that I am aware of, and I haven't heard of similar experiences from other veterinarians. That doesn't mean it isn't possible --- but it probably isn't a major concern without at least a few other people making the connection.

If Program does lower the seizure threshold this would be a concern for pets known to have seizure episodes but not a problem for the general population of dogs. Fleas are a much bigger concern for the average dog and preventing flea infestation is a major boost to good health. If Program is working well to control the fleas in your household it is almost certainly better for your dogs to continue using it. If one of your pups does develop seizures that would be the time to worry about whether or not it might be worthwhile to continue with the Program in that particular dog.

I am not aware of any other long term health problems associated with the use of Program at this time.

Mike Richards, DVM 122099

Heartworm medication reaction ?

Q: My toy poodle was fine until recently when we started her on her heartworm medication, the one month chewable variety. The next day she became very lethargic and weak almost to the point of not being able to stand. I rushed her to the vet early this morning and they diagnosed her with a very low red blood cell count something like 14 instead of the normal 35. Her temperature was normal at 102, according to the vet. She was put on a large dose of steroids at 10:30 AM and this evening the vet called and said she had gained her red blood cell count very slightly. He seems to think she may be out of danger, but I'm still very concerned. The vet has no idea what may have caused this, but my suspicions lie with the heart worm medication having triggered this. The medication is Heartgard Plus by Merck which contains ivermectin/pyrantel. Could you please give me your opinion on this and any further recommended treatment? Also, does this sound curable? Thanks, Tom

A: Heartgard (Rx) and Interceptor (Rx) both come under a lot of scrutiny as potential causes of immune medicated hemolytic anemia (IMHA) and immune mediated thrombocytopenia (ITP) because they are often the only medications that a pet is on when these occur. There is no evidence at this time of an increase in the cases of IMHA or ITP in dogs on these preventives. Recently, the once monthly flea treatments have also been subjected to some suspicion since they are the newest medications many dogs are on when these problems occur. There is also no evidence that they cause either of these conditions, presently. That doesn't mean new information won't come to light at some future time but I honestly do not suspect these medications as culprits in these diseases. Mike Richards, DVM

Heartgard (Rx) and Interceptor (Rx)

Heartgard (Rx) and Interceptor (Rx) both come under a lot of scrutiny as potential causes of immune medicated hemolytic anemia (IMHA) and immune mediated thrombocytopenia (ITP) because they are often the only medications that a pet is on when these occur. There is no evidence at this time of an increase in the cases of IMHA or ITP in dogs on these preventatives. That doesn't mean new information won't come to light at some future time but I honestly do not suspect these medications as culprits in these diseases.

Mike Richards, DVM

Program - side effects?

Q: I have a 12 year old (18 lbs. Schnauzer) and a 3 year old (75 lbs. collie) - The collie is presently on heartworm medication and hypothyroid medication (2 pills daily - .05mm each) - the collie is prone to seizures approximately every 8-9 weeks, we believe from the hypothyroidism. Will putting the collie on The Program flea control pill monthly cause any reaction or side effects with the other medications the dog is on? Would it be safe? And the 12 year old is on no medications but on a special diet due to fatty stones just surgically removed. I want my animals safe and I don't like giving them medications unless absolutely necessary. Frontline flea control was expensive and did not work for my animals last flea season. Topical treatments are just not enough and I am suspicious that some of the flea bombs and dips that I have used in the past may have been toxic enough to have caused these seizures in my collie. The seizures have only been noticable since January of this year. Thank you for your time. Please respond. RJ in Pennsylvania

A: RJ - There is no evidence that I am aware of that Program (Rx) will induce or facilitate seizures. It should be safe to use in this situation and does not interact with the medications listed.

It is always hard to figure out what is causing seizures and many times it is impossible to do so. That leaves a lot of room for guessing at causes. It is probably unlikely that previous insecticide use is the cause of the seizures but it is probably possible. Hypothyroidism has been implicated as a cause of seizures but it is probably not a common cause, either. Supplementation of thyroid hormone should reduce the incidence of seizures if hypothyroidism is the underlying cause.

The topical version (vs. the spray version) of Frontline (Rx) has been more successful on long haired breeds such as collies, for us. If Program does not control the fleas you might consider trying this - or using both Program and Frontline or Advantage (Rx).

I hope that the seizures do diminish as time goes on.

Mike Richards, DVM

A note from Dr. Richards:

As of January, 2004, there is evidence that collies have a specific genetic defect that makes them more susceptible to reactions to ivermectin, as well as some other medications, including loperamide (Immodium AD tm) and some chemotherapeutic agents. However, this discovery does not change the information related to dosing. At the dosage recommended for prevention of heartworm disease using ivermectin, there do not appear to be substantiated reports of toxicity. Use of the FDA approved products (Heartgard Rx, Heartgard Plus Rx and Iverhart Rx) is reasonable. Trying to dilute large animal ivermectin for use in collies is not reasonable. At higher dosages, such as those used to treat sarcoptic mange or demodectic mange, the potential for reactions is much higher and the use of ivermectin in collies is not recommended for these purposes. There are some very good reasons to consider using ivermectin based heartworm prevention medications even with the knowledge that collies may be more susceptible to overdosage reactions. One very good reason is the ability of ivermectin to prevent heartworm infection better when a month of medication is accidentally skipped than some of the heartworm prevention medications, making it very useful when this situation seems likely based on the past history of medication administration. Whether or not to use ivermectin in an approved form for an individual collie is a decision that is best made based an analysis of that particular collie's overall needs.


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...