Update on canine heartworm infection in the western United States (Sponsored by Merial)


No longer confined to the southeast portion of the United States, canine heartworm infection has spread across the nation-notably increasing its distribution and prevalence throughout the western states during the last two decades.

Evidence that heartworm infection has been locally acquired in all 50 states1 is encouraging more western practitioners to test annually for dangerous and potentially deadly heartworm infection.

While western prevalence remains low in comparison with the southeast, the rate at which ambient risk for canine heartworm transmission—especially autochthonous (locally acquired) transmission—has increased and is causing some western practitioners to re-evaluate their testing and preventive protocols (see Figure 1).2,3

Figure 1. Canine heartworm cases in the United States

Determining prevalence

Determining the risk for and the prevalence of heartworm infection in an area proves difficult because factors such as the time spent outside or traveling, heartworm preventive use, and annual testing vary among dogs. For this reason, local wildlife serves as a more accurate prevalence marker for an area, says Ben Sacks, PhD, director for the Canid Diversity and Conservation Group at the University of California-Davis, Davis, Calif.

Dr. Sacks says wild canids make great sentinels for canine heartworm disease because they are found in rural and urban environments, are outdoors year-round, are not on preventives, and have not come from or traveled to Louisiana for example.

To evaluate the recent change in prevalence and distribution of heartworm infection in California, Dr. Sacks compared the prevalence in coyotes within three foothill regions, throughout two time periods. His study showed that from 1975 to 2002, the heartworm prevalence in coyotes increased anywhere from 7% to 33% in these foothills.4 The prevalence in wildlife also can be an indicator that heartworms are endemic in parts of the western United States.

In a 2006 survey led by Dwight D. Bowman, DVM, MS, PhD, on heartworm prevalence, testing, and prevention in 11 western states, 6,585 surveys were distributed and 1,101 of those (16.7%) were completed. Results showed that 68% of the 2,224 heartworm-positive cases occurred in local dogs (see Table 1). Sixty-two percent of the total heartworm cases in local dogs occurred in dogs with no history of travel outside their local area. Each state reported heartworm positive cases.5

Table 1. Survey of heartworm disease prevalence, testing, and prevention protocol in 11 western states.5

Increasing testing

Both the American Heartworm Society and the Companion Animal Parasite Council recommend annual testing for canine heartworm infection for a variety of reasons; however, some clinics and hospitals currently do not test dogs annually. In Dr. Bowman's 2006 survey, only two of the 11 states had more than 50% or more of their responding clinics test dogs annually for heartworm infection (California 50% and Colorado 51%). Dr. Bowman says the rapid pace at which heartworm disease has spread throughout the West may account for the low percentages.

"It has happened so quickly that some areas may not be aware it has reached them," Dr. Bowman says. For that reason, Dr. Bowman encourages veterinarians throughout the West to increase testing.

"If you're not testing, or even if you are testing and just seeing low numbers, that doesn't mean your community is not at risk," Dr. Bowman says. "You don't want to wait until you see a lot of heartworm disease."


1. Bowman DD. The case for year-round parasite control. Companion Animal Parasite Council. The members speak out page. Available at: www.capcvet.org/default.asp?p=MembersSpeakOut_Article02&h=0&s=6. Accessed May 20, 2008.

2. Nelson T. What's new in heartworm disease (American Heartworm Society Heartworm prevalence 1980 map). Western Veterinary Conference, Las Vegas, Nev., Februrary 2008.

3. American Heartworm Society. Download-able graphics page. Available at: www.heartwormsociety.org/download/incidencemap.jpg. Accessed June 23, 2008.

4. Sacks BN, Caswell-Chen EP. Reconstructing the spread of dirofilaria immitis in California coyotes. J Parasitol 2003;89:319-323.

5. Bowman DD, Torre CJ, Mannella C. Survey of 11 western states for heartworm (Dirofilaria immitis) infection, heartworm diagnostic and prevention protocols, and fecal examination protocols for gastrointestinal parasites. Vet Ther 2007;8:293-304.

From the expert

1. Why talk about heartworm disease out West?

In 2002 and 2005, the AHS conducted surveys of clinics throughout the United States on the prevalence of canine heartworm disease. Western clinics that responded to surveys in both years reported a substantial increase in heartworm positive cases in 2005 from 2002. Nevada's cases almost doubled and Washington's almost tripled. Utah's case increased by about 20%, California's by 8%, and Oregon's increased by 5%.a Overall, the reported heartworm-positive cases in the West are comparatively low; however, what's crucial is the rate at which the numbers are increasing.

Tom Nelson, dvm a Companion Animal Parasite Council Board Member and the past President for the American Heartworm Society (AHS), explains why some practitioners in the West are taking a closer look at heartworm disease.

2. How is the arid West a suitable environment for heartworms?

People create a suitable environment as they change it. Salt Lake City went from almost no incidence of heartworm disease to having a problem when the city planted trees to improve the urban environment. Over time, knotholes developed, which created a breeding ground for the Aedes sierrensis. A common misconception is that all mosquitoes need a lot of standing water to breed, but the Aedes sierrensis needs only a knothole filled with water for a breeding ground.1-3

If our society and environment were static, heartworms would only be in the southeast—but they're not. Mosquitoes migrate and transform an area that was not endemic for heartworms to an endemic area—it just takes a little bit of time. Aedes sierrensis can be found throughout California, Oregon, Washington, Idaho, Nevada, and Utah.

3. How has the relocation of heartworm-positive animals from Hurricane Katrina affected the West?

Initially, some veterinarians believed the relocation would create an increase in the incidence of heartworm disease in the West. But, what it has done is increased the awareness of heartworm disease in the West while possibly speeding up a process that has been occurring for quite some time. As heartworm-positive dogs continually move into low endemic areas, the infection rate in untreated domestic dogs and the wild canid population is expected to increase. And once heartworms are established in the local wildlife reservoirs, eradication is virtually impossible. This has been best demonstrated by looking at prevalence of heartworn in wild canids over the past several years. One study detected pockets of heartworm infection as high as 91% in northern California coyotes.4

4. How aware are veterinarians of the existence of heartworm disease within the West?

Many practicing veterinarians started their careers when there were few, if any, reports of heartworm disease out West. Because of this, I think some western veterinarians don't view it as a potential threat to or as an established issue in their areas I heard an internal medicine expert say that Los Angeles did not have many cases of heartworm disease, and most cases diagnosed were in dogs that have moved from a more endemic area. However, the L.A. County West Vector Control District website states that Aedes sierrensis are common in L.A. County, which indicates that heartworm disease can be locally transmitted.

Historically, heartworm disease in the United States was found in the Southeast. Now it's spread across the country, including areas that aren't testing for it or recommending use of heartworm preventives. If testing and preventive use in those areas doesn't increase, the incidence of heartworm disease will reach a critical mass.


* 1. Scoles GA, Dickson SL, New foci of canine heartworm associated introductions of new vector species: Aedes sierrrensis in Utah, in Proceedings. Heartworm Symposium 1995;25-35.

2. Mar Vista Animal Medical Center. "The heartworm situation in L.A." Available at: www.marvistavet.net/html/body_the_heartworm_situation_in_la.html. Accessed July 23, 2008.

3. Davis County Mosquito Abatement. "Western treehole mosquito." Available at: www.davismosquito.org/mosquitoes/mosquitoes.keyhtm#treehole. Accessed July 23, 2008.

4. Sacks BN. Increasing prevalence of canine heartworm in coyotes from California. J Wildl Dis 1998;34:386-389.

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