Making the Case for Heartworm Prevention in the Western U.S.


My veterinary practice is located in the San Francisco Bay Area of California, where neither heartworm nor mosquitoes top the list of veterinarians’ and pet owners’ concerns. Nevertheless, the most recent Heartworm Incidence Survey by the American Heartworm Society1 determined that the number of heartworm-positive patients per clinic is increasing in states like California and Arizona.

Seek heartworm—and find heartworm

My heartworm epiphany began with a lecture I attended ten years ago on zoonotic intestinal parasites. I learned that T canis and T cati were a much greater threat than I had previously realized, with prevalence rates in dogs of up to 11 percent in the West.2 Knowing that roundworm infections can cause blindness and brain damage in people, and that sero-prevalence rates in the U.S. population are almost 14 percent,3 I decided to make prevention a priority and documented my recommendations. The easiest way to protect both my patients and their owners was to place the pets on a monthly heartworm preventive that was also effective against intestinal parasites. Since patients need to be tested before starting heartworm preventives, I began running heartworm tests.

To my surprise, I began to find heartworm disease, and I found it where I least expected it: in cats. Over the next ten years, I diagnosed 39 cases of feline heartworm infection, including ten infections in strictly indoor cats. While I happened to find more heartworm-positive cats than dogs in my practice, I knew that feline heartworm is only found where canine heartworm infections are also present. My long-standing case of “heartworm denial” was quickly reversed.

Heartworm incidence on the rise in the West

At one time, heartworm was not considered endemic in the western U.S.; however, we know that disease endemnicity can change over time. Approximately 60 percent of dogs travel with their owners,4 thus increasing the potential for heartworm exposure. In addition, heartworm carriers—both unprotected dogs and coyotes that live along the California coast and in the Sierra foothills—are present. Finally, watering practices in western desert communities have created conditions conducive to mosquito proliferation.

Make heartworm compliance easy for clients

Today heartworm testing and prevention are part of my routine recommendations. My approach is to include heartworm testing in the “wellness” screening I offer, and to position heartworm prevention in combination with prevention of zoonotic intestinal parasites. By educating my clients and making prevention and testing part of their pet’s wellness routine, I have good compliance and both my clients and my patients are protected.



1 American Heartworm Society,
2 Blagburn BL, Lindsey DS, Vaughn JL, et al. “Prevalence of canine parasites based on fecal flotation”. Comp Contin Educ Vet Pract 1996; 18:483-509
4 Summer Pet Travel survey (2010), Trip Advisor,