Feline Heartworm Infection

The veterinary profession’s understanding of feline heartworm infection is rapidly evolving. New research indicates that heartworm infection in cats causes pathology not only when the heartworm reaches an adult stage but also during the immature development stages. In response to this research, the American Heartworm Society has redefined feline heartworm infection as Heartworm Associated Respiratory Disease (H.A.R.D.).

Incidence

Feline heartworm incidence varies by region. Feline heartworm incidence is typically lower but closely parallels the incidence of heartworm infections in dogs. Here is a look at the U.S. incidence data of heartworms in dogs from the American Heartworm Society. This map provides a good indication of where heartworm disease in cats or dogs would be a concern:

© American Heartworm Society
	Average number of cases per reporting clinic
	<1 case/clinic 1–5 cases/clinic 6–25 cases/clinic		26–50 cases/clinic 50+ cases/clinic

Reproduced with permission of the American Heartworm Society. Available at: www.heartwormsociety.org

The severity of heartworm incidence as shown in this map is based on the average number of cases per reporting clinic. Some remote regions of the United States lack veterinary clinics, so there are no reported cases from those areas.

Transmission and Progression

Feline heartworm infection begins when an infected mosquito bites the cat. The larvae migrate away from the entry site, where many die in the subcutaneous tissue. Within 50–80 days of infection, the larvae become immature adults that migrate via blood to the main pulmonary artery, where final maturation and mating occur.

Cats with heartworm disease may present with coughing or respiratory distress. Presenting signs occur during one of two stages:

• Stage One coincides with the arrival of immature adult worms in the pulmonary arteries and arterioles. Immature worm death stimulates an inflammatory response. The clinical manifestation is often diagnosed as asthma or allergic bronchitis. New research suggests that many of these signs indicate H.A.R.D.1
• Stage Two coincides with the death of a mature worm or worms. Pulmonary inflammation and thromboembolism (blood clots) occur that can lead to fatal lung injury or death. The death of the worm releases numerous antigens and toxins into the pulmonary artery, resulting in damage. Stage Two has also been incorporated into the H.A.R.D. definition.1

Testing Guidelines from the American Heartworm Society (AHS):¹

• Healthy cats should be screened with concurrent antibody and antigen tests.
• Cats presenting clinical signs consistent with H.A.R.D. should undergo antigen and antibody tests, complemented with thoracic radiography and/or echocardiography.
• Cats diagnosed with heartworm infection should be tested to monitor the clinical course of the disease and to establish a baseline prior to initiation of chemoprophylaxis.

Managing a feline heartworm antigen positive result:

Even though the specificity of antigen testing is very high for feline heartworm, experts recommend confirming all antigen positives by retesting. This is best done with an ELISA at a reference laboratory. The following algorithm provides recommendations created with the help of members of the American Heartworm Society (AHS), the Companion Animal Parasitic Council (CAPC) and IDEXX Laboratories.

For a detailed overview of feline heartworm disease, including signs, diagnosis and management, consult the American Heartworm Society Guidelines on Heartworm Testing. (3.6 MB)

This algorithm was developed with input from members of the American Heartworm Society. Companion Animal Council and IDEXX Laboratories. Refer to the American Heartworm Society guidelines (www.heartwormsociety.org) and the Companion Animal Parasite guidelines (www.capcvet.org) for complete feline heartworm disease testing, management and prevention recommendations.

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