Module 2: Preventive health care and health surveillance

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 2020 update of the American Heartworm Society Canine Heartworm Guidelines.

Key Takeaways for Canine Heartworm Diagnostic Testing

  • All adult dogs >6 months old should be tested with BOTH a heartworm antigen detection test AND a microfilaria test.
  • The heartworm antigen POC tests are highly sensitive and specific and are suitable for routine screening.
  • The modified Knotts test is recommended for microfilaria testing, but microscopic exam of a drop of blood mixed with a drop of saline may be suitable when the Knott’s test is not available
  • Dogs with a negative heartworm antigen test and positive microfilaria test should be re-tested using heat-treated serum samples. Reference labs offer the heat treatment testing.
  • Heat treatment is not part of routine screening with heartworm antigen tests. This is reserved for cases that are microfilaria-positive and antigen-negative, or antigen-negative dogs that have compatible clinical signs for heartworm disease.

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.

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

The AHS Guidelines recommend year-round administration of heartworm-preventive drugs to all dogs. This includes dogs in shelters. 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 when given according to label instructions.

Shelters in high heartworm prevalence areas should prioritize allocation of resources for heartworm screening and prevention. Heartworm testing protocols should stipulate selection of dogs for testing, what tests should be used, and when the tests are performed (at admission or at adoption/transfer). Heartworm prevention protocols should stipulate what dogs to treat, what preventive to use, and the dose formulations available.


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