Question: What are the side effects, if any, and/or risks from once-a-month heartworm pills? I have heard that occasionally dogs die from taking these. And just how common are heartworms anyway? My dog (a Rottweiler) only goes outside two-three times a day to do his business, and I rarely see any mosquitoes, even in the summer.
_ G.A., New Port Richey
Answer: Before any heartworm prevention program is initiated, the dog must be tested for heartworms. Heartworm-positive dogs may experience side effects resulting from the action of the chemical on existing heartworm larvae. In some cases, these side effects can be quite serious.
Even dogs on a heartworm prevention program must be tested routinely (once a year, generally) to make certain they still are heartworm-free. Two different once-a-month medications exist that are used to prevent heartworms.
The first is Heartgard-30 (ivermectin), made by Merck. The second is Interceptor (milbemycin), made by Ciba-Geigy. Both are in pill form, and one currently is also available in a chewable form (Heartgard-30). The chewable form of Interceptor should be available soon. Both of these medications are available from veterinarians by prescription only.
According to the Heartgard package insert, the following adverse reactions may occur: vomiting, diarrhea, lethargy, loss of appetite, dilated pupils, staggering, convulsions and excess salivation. These adverse reactions, if they occur at all, are very short-lived.
According to Ciba-Geigy, more than 100-million doses of Interceptor have been sold since its inception in 1990. At the 60-million dose mark, 240 cases of adverse reactions were reported to the manufacturer. This is about 0.0004 percent. These reactions included diarrhea, vomiting and lethargy. Again, they usually were short-lived.
Both manufacturers and the National Animal Poison Control Center report no deaths from either product when used at the proper dose according to package directions.
I feel that both of these products are extremely safe and effective if used appropriately.
Mosquitoes have been, and still are, a significant problem in our area. With a budget of $2-million and a staff of over 30, Pinellas County Mosquito Control still wages a daily war on the pest. Our practice treated seven cases of heartworm disease last year. The vast majority of our clients give their dogs the appropriate heartworm preventive medication. If that were not the case, I suspect that we would have treated many, many times that number
_ Scott M. Lamb, DVM, Highland Animal Hospital, Dunedin