Heartworm prevention: The questionable zero mosquito factor
Q. We don’t see a lot of mosquitoes in my practice area. Is it really necessary to push prevention?
A. Veterinarians often refer to their locales as being “endemic” or “nonendemic” for heartworm. For those in many of the Western and Mountain states, the assumption is that if heartworm historically hasn’t been a problem in the region, there’s no reason to recommend yearly testing or heartworm prevention now. I moved to Phoenix—which is in a supposedly nonendemic region—less than 18 months ago. During that time, I’ve performed several surgeries for heartworm caval syndrome on dogs that had never left the Phoenix area.
It is true that the incidence of heartworm is lower in this and other parts of the country than others, but it is clear that heartworm is very much a threat. In my experience, where incidence of a disease is thought to be lower, so is vigilance. When many unprotected pets live in such an area, just one heartworm-positive animal can have a large impact. To understand why, we first need to examine the factors that cause heartworm to spread.
> Reservoir of infection. To be a reservoir, the heartworm-positive animal needs to have circulating microfilariae. The culprit could be the dog next door or a feral animal, such as a coyote or fox. Here in Phoenix, as in many urban areas, we have a high coyote population, and coyotes are an important potential reservoir for heartworm. Infected animals brought in from other parts of the country are another important reservoir.
> Mosquito vectors. Mosquitoes that feed on a host animal with microfilariae are soon in a position to transmit infective larvae to an unprotected animal. Most mosquitoes don’t travel long distances; instead they spend most of their lives in a 400-meter circle. But a lot can happen within that circle. If an owner has a reservoir in the form of an infected neighborhood dog, or if the owner lives near a golf course where coyotes hang out in the evening or even sleep behind the house, all it takes is the presence of mosquitoes attracted by standing water on the property to put mosquitoes in proximity to infected host animals.
> Access. Infected mosquitoes need access to unprotected pets. Unless a dog is receiving a heartworm preventive, there’s risk. Whether the dog only goes outside for brief bathroom breaks or enjoys long daily hikes, it is exposed to mosquitoes. Taking precautions that limit mosquito exposure certainly helps, but these precautions can’t completely eliminate the risk of heartworm transmission.
> Protection. As noted, the above factors can be almost completely nullified if pets receive year-round prevention. The American Heartworm Society (AHS) guidelines also recommend 12-month prevention to control other pathogenic or zoonotic parasites and to enhance compliance.
One small step for canine kind
As veterinarians, our sphere of control is really our practice area, but within that area, we have several important responsibilities. First, we need to recommend year-round use of preventives. Second, when we do diagnose an infected dog with circulating microfilariae, we need to take prompt action to treat it. By eliminating the microfilariae, we decrease the risk that that dog will serve as a source of heartworm infection to other dogs.
We may not be able to do everything to reduce risk—we can’t give preventives to coyotes or prevent infected dogs from moving into a neighborhood. But each time we work with a client to reduce the risk of heartworm transmission in a 400-meter circle where an infected mosquito might live, we’re making a positive difference for the pets in our practice area.
This question and others are the focus of a new series of 15-minute recorded talks designed to offer practical information on heartworm prevention, diagnosis and treatment. The “Eye on Heartworm” videos, which are made available by the AHS, can be found at https://www.heartwormsociety.org/veterinary-resources/veterinary-education/videos.