Diagnosing Feline Heartworms
By Melanie T. Landis, DVM, MBA, TVMDL Microbiology Veterinary Diagnostician
We are all familiar with the saying that “cats are not small dogs”. This is true when it comes to feline heartworm disease. While dogs are the typical host for Dirofilaria immitis, cats do become infected with heartworms, but confirming the disease antemortem can be quite difficult. Since cats typically have such a small worm burden (e.g. 1-2 adult worms within the pulmonary arteries)1, the obstruction of blood flow is minimal. Thus, the principle clinical signs exhibited by cats, as well as histopathologic lesions, tend to be related to the respiratory system, and are associated with a hypersensitivity reaction to the larvae and dying worms. This hypersensitivity reaction tends to manifest as periodic coughing (or vomiting), or possibly as collapse and sudden death. Hypersensitivity reactions occur at two main stages of the heartworm life cycle – upon the arrival of the immature adults (previously termed L5 larvae) in the pulmonary arteries, and death of the adult worms.1
Several laboratory tests are commonly employed to help determine the presence of heartworms:
The use of tests that look for circulating microfilaria (e.g. direct smear, modified Knott’s test, Difil test), which are commonly used in dogs, are of little benefit in cats. Low numbers of adult worms or single sex infections in cats typically preclude the development or detection of circulating microfilaria (L1 or stage 1 larvae).
Another common testing method for heartworms involves the detection of antigens associated with gravid female worms2. Again, because worm burdens in cats tend to be very low, or are male-only infections, antigen tests are typically negative. However, a repeatable positive test, coupled with a positive antibody test, would indicate a current or recent infection. Additionally, heat treatment of the serum sample prior to testing may help to disrupt antigen-antibody complexes within the serum thus increasing the amount of available antigen and improving the likelihood of obtaining a positive antigen test.3 Heat treating serum samples is not currently done at TVMDL.
Use of antibody testing is also advantageous in trying to determine heartworm infections in cats. Cats start producing antibodies in response to migrating larvae (L3 and L4 stages)2. Since many of the immature adults die upon entering the heart4, a positive antibody test simply indicates that a cat has been exposed to larval stages, which may or may not have developed into adult heart worms. As stated previously, a positive antibody test in conjunction with a positive antigen test, provides a strong indication of current or recent heartworm infection.
Imaging diagnostics may also be employed to try to determine if heartworms are present. Cardiac ultrasound, while helpful in visualizing adult worms in canine heartworm infections, is typically of little help with the feline patient due to the low worm volume. Thoracic radiographs are beneficial in evaluating the size of the pulmonary arteries. Enlarged pulmonary arteries help support a diagnosis of heartworm disease.
Since feline heartworm disease significantly differs from canine heartworm disease, it is very difficult to make a definitive diagnosis without necropsy. Positive results using multiple diagnostic methods are therefore required to provide strong support for a diagnosis of feline heartworm disease. Given this diagnostic challenge, the prevalence of heartworm infection in cats is likely underreported1, and thus provides a solid argument for the use of heartworm preventatives in cats.
For more information about heartworms, contact Dr. Melanie Landis, veterinary diagnostician. To learn more about heartworm testing options, as well as other TVMDL test offerings, visit tvmdl.tamu.edu or call 1.888.646.5623.
1. American Heartworm Society. Current Feline Guidelines for the Prevention, Diagnosis and Management of Heartworm (Dirofilaria immitis) Infection in Cats (2014). https://www.heartwormsociety.org/veterinary-resources/american-heartworm-society-guidelines
2. Wortinger, A. Feline Heartworm Disease: Fact or Fiction. Today’s Veterinary Nurse, May/June 2017. Vol 2, Issue 3. https://todaysveterinarynurse.com/articles/feline-heartworm-disease-fact-or-fiction/
3. Little, S. et al. Heat treatment prior to testing allows detection of antigen of Dirofilaria immitisin feline serum. Parasites & Vectors2014, 7:1. http://www.parasitesandvectors.com/content/7/1/1
4. Lambert Vet Supply. Heartworm Disease in Cats – Heartworm Life Cycle. The Well Pet Post(3/27/2019). https://www.lambertvetsupply.com/wellpetpost-heartworm-disease-in-cats.html