Canine Heartworm Prevalence in Animal Shelters: A Statewide Study

Canine heartworm infection presents unique management challenges for animal shelters. To better understand these challenges, the authors of a new study set out to explore canine heartworm prevalence in this environment. Their objective? To determine the prevalence of Dirofilaria immitis microfilaremia and antigenemia in dogs from Mississippi animal shelters.

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“Not only do heartworm-infected dogs from these shelters enter the local community as pets,” the authors wrote, “they could potentially serve as reservoirs for infection throughout the country if not identified prior to transport.“

Study Design

The investigators collected 283 whole blood and 363 serum samples representing 363 dogs at 18 shelters in 17 Mississippi counties. The whole blood samples were tested with microfilaria tests, and the serum samples were tested with antigen tests. Microfilaria testing was performed using a modified Knott test along with a blood smear test.

Exposures of interest for both outcomes included age, body condition, weight, breed, source, sex, and length of stay at the shelter.

Prevalence of D immitis

Test results showed that the prevalence of D immitis antigenemia was 34.4%, and the prevalence of D immitis microfilaremia was 22.6%. “Prevalence of both microfilaremia and antigenemia was significantly higher in these sampled dogs compared with previous reports for the owned canine population in Mississippi,” the authors wrote.

Regarding risk factors for heartworm infection, the investigators found that weight, sex, source, breed, and length of shelter stay were not significantly associated with antigen presence, but age and body condition score were.

• Compared with dogs 6 months to 1 year of age, those between 1 and 3 years of age had 2.44 times increased odds of having heartworm antigen present and dogs over age 3 had 7.33 times increased odds of having heartworm antigen present.
• Overweight dogs were 2.26 times more likely to have heartworm antigen present than dogs with a normal body condition score.

With regard to the microfilaria tests, age was the only risk factor significantly associated with positive test results.

Conclusion

This study emphasizes better understanding of the different risk factors likely to affect canine heartworm disease prevalence, as well as the lesser acknowledged prevalence of heartworm infection in animal shelters.

The modified Knott test the investigators used for the study was able to identify 58% of antigen-positive dogs. The investigators believe this could potentially be used as a screening test in endemic areas—such as Mississippi—where the use of microfilaricidal heartworm preventives is unlikely and access to commercial antigen tests is limited.

“There is a need for local and regional risk assessments for specific populations in order to provide accurate and effective disease prevention, detection, management, and treatment protocols,” the authors wrote.