Heartworm disease (dirofilariasis) is potentially fatal but highly preventable. The main predilection sites of the mosquito-borne causative agent, Dirofilaria immitis, are the pulmonary arteries of the animal hosts. Canines are considered natural hosts of heartworms, but felines can be affected too. Although cats are naturally more resistant, the prevalence of feline heartworm disease is between 5-20% of that in dogs in the same area.
Related: Feline Heartworm Disease: It’s More Common Than You Think
Feline heartworm disease
Feline heartworm disease is a lung disease rather than a heart disease, as it is in canines. Even with a low worm burden, cats can still develop more severe disease and even sudden death. Dirofilariasis infection in cats typically has a lower worm burden because most of the filariae die once they enter the pulmonary artery.
Affected cats are susceptible to the following conditions:
- Pulmonary thromboembolism
- Vascular and pulmonary inflammation
- Acute right-sided heart failure
- Anaphylaxis
Many cats affected by heartworms have no adult worms. While this means heartworm disease often goes undiagnosed in cats, it’s important to understand that even immature worms cause real damage in the form of a condition known as heartworm-associated respiratory disease (HARD).
Symptoms of feline dirofilariasis
Many heartworm infections in cats are subclinical, and those that manifest symptoms exhibit intermittent coughing, dyspnea, anorexia, lethargy, vomiting, and/or weight loss. The early signs are associated with an acute vascular and parenchymal inflammatory response to the newly arriving young heartworms and the subsequent death of many or all of these juveniles.
For most cats with heartworm disease, acute progressive respiratory distress brought about by acute right heart failure or pulmonary thromboembolism is the most common cause of death.
Prevalence of feline heartworm infection
The true prevalence of feline heartworm infection is likely underestimated. This is due to the limitations of current diagnostic techniques, inadequate testing protocols, and the high likelihood of cats exhibiting transient clinical signs or dying without confirmation of infection.
In a 2023 study regarded as the largest antibody-based nationwide serosurvey of feline heartworm in the US, antibodies to D. immitis were identified in 3.5% of cats from 26 states. There was significantly higher prevalence in cats from the westernmost US states (West region; 5.4%) compared to those from the South (3.8%), Midwest (2.7%) and Northeast regions (2.2%). Antigen from D. immitis was detected in 0.3% (6/2165) of cats. This survey was conducted on cats that were apparently healthy.
Results of the study reveal that cats in the US have a high risk of exposure to mosquitoes infected with D. immitis. The true prevalence may be significantly underestimated. There is a need for increased awareness as well as routine testing for heartworm infection in cats not only in known endemic areas but in non-endemic regions as well.
Incidence surveys
The American Heartworm Society (AHS) conducts an incidence survey every three years. Data from heartworm antigen tests voluntarily submitted by veterinary practices and shelters were used to establish prevalence. Based on the AHS 2022 Heartworm Incidence Map, the leading states for heartworm incidence continue to be those around the lower Mississippi Delta. This can be attributed to the presence of mosquito vectors. Also factoring in was a high population of untreated dogs as well as wildlife that can serve as viable reservoirs for infection.
The incidence map shows the following information:
- Mississippi, Louisiana, Texas, Alabama, and Arkansas have the highest density of diagnosed heartworm cases.
- Expansions of high-density areas are observed in Arkansas, Mississippi, Texas, Georgia, Florida, Tennessee, and the Carolinas.
- Significant increases in heartworm cases have been observed in Seattle, Boise, Bismarck, and Tucson.
- States that have had low rates of heartworm infections in the past show an increase in cases and the development of new “hot spot” areas within their borders. These include Washington, Oregon, Kansas, North Dakota, Massachusetts, and Connecticut.
Several factors have been attributed to the concerning increase in the prevalence of feline heartworm disease in the US.
- A greater influx of heartworm-positive animals from other states
- Higher testing rates
- Higher infection rates
- Human factors (influx of heartworm-positive pets, poor owner compliance with heartworm prevention)
- Changes in weather conditions resulting in a favorable increase in the mosquito population
Diagnostic challenges
In dogs, the cornerstones of heartworm diagnosis include the observation of microfilariae in the blood and the identification of circulating antigens from adult D. immitis females. However, these diagnostic procedures are less sensitive in cats. This can be attributed to the immune response of felines that leads to low numbers of adult heartworms. Despite these, antibody tests for heartworms are also licensed for use in felines.
Feline heartworm antibodies are detectable when there has at least been larval development in the tissues. Positive antibody tests cannot distinguish between current and previous larval infections. Thus, cats with positive antibody test results are considered currently or previously infected with D. immitis.
Prevention and treatment of feline heartworm disease
Prophylactic treatment remains the best way of protecting canines and felines from heartworm disease. It is especially important for felines because immiticides cannot be used for treatment. They are more prone to sudden death when infected.
Veterinarians play a very significant role in educating both new and long-time pet owners about the importance of heartworm prevention. Being proactive in prescribing preventatives can help ensure that cats and dogs won’t be compromised.
In high-prevalence areas, adequate resources should be allocated for heartworm screening and prevention. Improved compliance, that is, year-round use of preventatives that are given on time, as well as more compliant pet owners, can contribute to a decrease in heartworm incidence.
