Heartworm (Dirofilaria immitis) is a well-known cause of disease of dogs in North America; however, it can also affect domestic cats, though with different clinical signs, prevalence, and treatment options.

Unlike dogs, cats are not the natural host for heartworms, however, they can still become infected. Heartworm in cats leads to two main syndromes: heartworm-associated respiratory disease (HARD) and feline adult heartworm disease (HWD). HARD occurs when immature worms die, causing lung-related issues like coughing and respiratory distress. Feline adult HWD is a result of the cat’s immune system overreacting to the inflammation and death of adult heartworms, which can lead to severe lung inflammation and death.

While heartworm-associated respiratory disease is increasingly recognized as a cause of disease in cats, there has been minimal investigation into the nationwide prevalence.

The purpose of this study was to investigate feline heartworm seroprevalence in apparently healthy pet cats in the USA. A total of 2,165 serum samples collected from cats across 47 states and Washington, District of Columbia were analyzed for D. immitis antibody and antigen with and without acid treatment of the samples.

Enrolled cats were from various parts of the USA and were being tested for rabies antibodies because of travel requirements, whether within the country or abroad. No samples were received from Montana, South Dakota, and North Dakota.

The authors recorded information about each cat’s sex, age, breed, and the geographic area where they lived.

They used these blood samples to check for the presence of antibodies to Dirofilaria immitis and the presence of the actual parasite itself (antigen). To do this, they used special tests. One test called the Feline Heartworm Antibody test, looked for antibodies in the blood, which would indicate if a cat had been exposed to the parasite. The other test called the DiroChek® Heartworm Antigen Test Kit, looked for the actual parasite’s presence.

The authors utilized acid pre-treatment of samples before antigen testing. This acid treatment was intended to increase the sensitivity of the test. Heska’s Feline Heartworm Antibody test identified an overall D. immitis antibody prevalence of 3.5% in cats from 26 of 47 states. The prevalence of D. immitis antibody in cats from the West region of the USA (5.4%) was significantly higher than that in cats from the South (3.8%), Midwest (2.7%) and Northeast (2.2%) regions.

The Domestic shorthair was the most represented breed among the antibody-positive samples (n = 44), followed by the Domestic longhair (7) and sphynx (4).

The DiroChek® Heartworm Antigen Test Kit detected antigen in 0.2% of the samples, with positive samples originating from Alabama, New York, and Tennessee. After acid treatment, antigen was detected in 0.1% of the samples (from Indiana and Florida). Acid treatment was inconsistent in its ability to detect increased numbers of cases. The distribution of the cases per year was 3.8% (13/343) in 2022, 3.9% (35/888) in 2021, 2.4% (14/574) in 2020, 4.0% (10/252) in 2019, 4.5% (3/67) in 2018 and 2.6% (1/38) in 2017.

Interestingly, no samples tested in this study were found to be both antigen and antibody-positive.

Traditionally, the number of cats testing positive for heartworm disease has been about 10% of the number of infected dogs in the same area. However, some studies have suggested that the number of infected cats might be even higher, up to 60% of the infected dogs in the same place. In addition, the Western US was shown to be over-represented in this study, as opposed to the South which has traditionally been expected to have more cases.

This study overall found that 3.5% of healthy cats tested had antibodies against heartworm disease. This suggests that cats are exposed to this disease more often than previously thought, even in seemingly healthy cats. Owners and veterinarians should be more aware of this disease in cats and consider using treatments to prevent it. Further research is needed to better understand and diagnose heartworm disease in this species.  ~MRK