Disease Screening: Canine Heartworm Infection

A survey of 504 shelters in the Southeast found that heartworm testing of adult dogs was performed by 41% of open-admission municipal shelters, 80% of limited-admission nonprofit shelters, and 98% of rescue/foster groups. Only 35% of the municipal shelters administered heartworm preventive medication to the dogs in their care, compared to >80% of private nonprofit shelters and rescue/foster groups. Cost was cited as the primary reason shelters did not perform routine testing or provide heartworm preventives.

A more recent survey of 244 shelters across the US found that most (82%) of the shelters tested dogs >6 months old for heartworm infection, most (81%) provided monthly heartworm prevention to dogs, and most (87%) had written protocols for canine heartworm diagnosis and management. Compared to private nonprofit shelters and rescue groups, municipal shelters were less likely to administer heartworm preventive or have canine heartworm protocols. Cost was the biggest factor governing shelter heartworm policies and practices.

Only a limited portion of shelter budgets is allocated for medical and surgical care of the animals, including vaccination, parasite treatment, well care, treatment of sick and injured animals, and spay/neuter surgery. Many times, little money is available for screening for infections such as FeLV/FIV in cats and heartworm in dogs. You have already learned how cost-to-benefit analyses are driving a shift in shelter policies for retroviral testing. What about heartworm testing of adult dogs?

Review the Primary Diagnostic Screening section and the Heartworm Prevention section in the 2024 American Heartworm Society Canine Heartworm Guidelines and Guideline Highlights.

Dogs with a negative heartworm antigen test and positive microfilaria test should be re-tested using heat-treated serum samples. For these cases, heat-treating serum samples prior to antigen testing increases the sensitivity of heartworm detection by releasing antigen bound in immune complexes., and the heartworm antigen test converts to a positive result. This reinforces the importance of routine microfilaria testing when screening dogs for heartworm infection.

The 2024 ASV Heartworm Management Position Statement for shelters supports the AHS guidelines for canine heartworm management. However, the ASV recognizes that shelters may not always be able to implement all the AHS recommendations for testing.  Many shelters test all adult dogs on admission or prior to adoption for heartworm infection, but other shelters cannot afford to test all dogs. These shelters prioritize heartworm testing for dogs with potential adopters, dogs transferred to rescue groups, and dogs being re-located to other states. For microfilaria testing, shelters should at least perform a microscopic exam of a drop of blood mixed with a drop of saline.

The AHS Guidelines and ASV Heartworm Management position statement recommend year-round administration of heartworm-preventive drugs to all dogs regardless of geographical location. This includes dogs in shelters.  Shelters that cannot test all age-appropriate dogs should still provide year-round prevention to these dogs. The FDA-approved heartworm preventives currently marketed (ivermectin, milbemycin oxime, moxidectin, and selamectin) belong to the macrocyclic lactone (ML) class of drugs. These drugs are highly protective against infection and safe when given according to label instructions.