Does Your Dog Really Need Two Medications to Prevent Heartworms?

This advertisement is circulating widely in veterinary journals these days. The company is promoting their flea and tick preventative, which also acts against mosquitoes. Their premise is by using their product in addition to a monthly tablet, the risk of getting heartworm disease is greatly reduced. And to convince vets of that, they did a study. The results are presented in this graphic.

To summarize the study, they divided Beagles into 4 groups. The control group received no treatment, one group was treated with the insecticide Vectra, one group with heartworm preventative Interceptor (milbemycin), and the final group with both products. The dogs were exposed to heartworm carrying mosquitoes twice, 21 and 28 days into the study. Interceptor was given on day 51 of the study.

When you look at the graphic, it looks like treatment with milbemycin (Interceptor) made no difference at all in the number of dogs infected, compared with the control (no treatment) group. But it did lower the number of worms in each dog. Only giving both products resulted in no heartworms in the dogs. But there are a few things the advertisement does not tell…

First, the mosquitoes used in the study were of the species Aedes aegypti. The CDC tells us that:

• These mosquitoes live in tropical, subtropical, and in some temperate climates.
• They are the main type of mosquito that spread Zika, dengue, chikungunya, and other viruses.
• Because Ae. aegypti mosquitoes live near and prefer to feed on people, they are more likely to spread these viruses than other types of mosquitoes.

You can see a map of where this species of mosquito lives in the US here.

Second, the mosquitoes were placed in a mosquito-proof container for one hour, while the dog in the test was sedated. The container measured approximately 29 inches by 16 inches by 13 inches. That’s right, a motionless dog in a confined space, not a real world scenario. Talk about a mosquito buffet! Yet I don’t see any mention of that in the ad…

Third, the strain of heartworm used in the study was one known to be resistant to milbemycin (Interceptor). The efficacy of the drug against this strain (JYD-34) has been found in other studies to be 52%. So of course one would expect some of the dogs given the oral preventative to become infected anyway. However, there is no mention in the ad of this, or the distribution of the resistant strain and its relatives. And a paper published in 2017 tells us that reports of resistant heartworms have come from Louisiana, Arkansas, and southern Illinois. (see map here).

It angers me that this ad (and others like it) is clearly meant as a scare tactic against veterinarians and owners who may see this version of it. In no way does the study bear any resemblance to real life exposure for our dogs, nor was it claimed to by the study designers. Yet that is how the manufacturer has spun the data. They realize that most veterinarians will not read the studies they cite in the fine print on the bottom of the page. Rather, they will be spurred into recommending another pesticide be applied to dogs each month — even dogs that are at little or no risk for exposure to any heartworm, let alone strains resistant to medications.