Matthew Krecic, DVM, MS, MBA, DACVIM (SAIM) and Adrienne Abel, CVT
In the May/June 2017 issue of Today’s Veterinary Nurse, we were pleased to come across the article “Feline Heartworm Disease: Fact or Fiction” by Ms. Ann Wortinger and agree that feline heartworm disease is indeed fact.1–3 We likewise agree that confirming heartworm disease in cats is complex, and the author discusses the advantages and limitations of several modalities, including ELISA antibody testing.4 However, the author did not mention another antibody detection method called immunochromatography. An in-clinic test using immunochromatography is available for detection of heartworm antibody within the serum, plasma, or anticoagulated whole blood of cats (Solo Step FH® Heartworm Test, Heska). Although veterinarians and veterinary parasitologists may debate the accuracy of each antibody test method, we believe informing your readers that such an in-clinic test exists is important. Similarly, a variety of ELISA and immunochromatography tests are available for in-clinic use to detect feline heartworm antigen. These may be the same standalone heartworm antigen tests used for dogs. Often, a combination of antigen and antibody testing is necessary in an attempt to detect an infection in cats.1–3
The author also commented that “most cats with heartworm disease are antigen negative because of low titers.”4 We believe the use of the word “titers” here is inaccurate. A titer refers to the highest dilution of serum at which a test is able to identify antigen (or antibody) within that serum.5 Yet, to our knowledge, such serum dilution studies have not been performed with these heartworm antigen tests. Instead, studies have been published regarding the relative sensitivities—or rather, insensitivities—of the in-clinic heartworm antigen tests at identifying infected dogs with low adult heartworm burdens.6,7 Infected cats will have zero or low numbers of adult heartworms.1,2
Accompanying the article is the well-written client handout “Heartworm Disease in Cats” that Vetstreet personnel authored and editors of Today’s Veterinary Nurse reviewed.8 We wanted to also inform your readers that, in addition to the mentioned SNAP® test (IDEXX) for feline heartworm antigen testing, other in-clinic heartworm antigen tests, including one from Zoetis, are available. Specifying only one test may erroneously lead cat owners and your readers to believe that such test is the only available in-clinic heartworm antigen test for use with cats.
With respect to the information in the handout, we also wish to note that many veterinarians likely do not begin evaluating cats with clinical signs compatible with heartworm disease with any in-clinic antigen test because of the relative insensitivity for antigen detection in any infected cat.1–3 In an effort to increase the success of confirming a suspected infection, veterinarians may instead choose to send samples to a reference laboratory for both antigen and antibody testing, because an in-clinic combination antigen and antibody test for identifying heartworm-infected cats is not currently available. Alternatively, veterinarians may not proactively test any cat—healthy or ill—specifically for heartworm disease, because, among other reasons, they may believe asthma or bronchitis is instead the cause of clinical signs (which heartworm disease often mimics) or because no specific treatment is available for heartworm-infected cats (even if heartworm disease is confirmed).3 They may instead administer and prescribe empirical therapies to prevent, lessen, or resolve any clinical signs.
We hope that the above is not dissuasive for heartworm testing of cats. We strongly believe testing remains important for several reasons: (1) client peace of mind for having a diagnosis; (2) impetus for monthly heartworm prevention for both asymptomatic and symptomatic, infected cats in order to prevent maturation of any susceptible tissue-stage larvae; and (3) impetus to ask about any other pets within the household that should be tested and/or receive preventive. For example, an infected dog within the household may be the source of heartworm and therefore should be tested for heartworm antigen and microfilariae. Also, other cats within the household may also benefit from monthly heartworm preventive, given that they are subject to the same environment as the infected cat.
We are glad that the editors of Today’s Veterinary Nurse published this article and the accompanying client handout on a difficult-to-diagnose condition that likely affects more cats than veterinarians have been able to confirm.1–3 Veterinary technicians can be positive influences to the veterinarians with whom they work and can feel that they are empowered to encourage their veterinarians to consider heartworm disease and test their feline patients after reading both the article and the client handout.
Disclosure: Both authors are employees of Zoetis, manufacturer of heartworm preventives and heartworm tests for both dogs and cats.
Do you have a story you’d like to share? Write me at LJohnson@navc.com.