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 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
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."