Most commonly affecting our canine patients, heartworm (Dirofilaria immitis) is a parasite that is carried from mosquitos. It is transferred via a bite into the bloodstream, where stage three larvae circulate and mature into adult worms that live in the right side of the heart and pulmonary artery. However, cats are not the definitive host, and therefore the parasite cannot complete its lifecycle, dying in the tissue or migrating to abnormal places.
The clinical signs are very vague and non-specific which may include lethargy, anorexia or weight loss. Some cats develop vomiting, coughing and dyspnea (difficulty breathing). Heart murmurs, fluid build-up in the chest or abdomen, seizures or other nervous system abnormality are seen in more serious cases. Sudden death can also occur in some instances.
The infection is difficult to diagnose; therefore the actual rate of infection cannot be determined. A variety of testing is done to help diagnose heartworm. Blood work, radiographs, and echocardiography (ultrasound of the heart) as well as an antibody test to detect early stages of infection. An antigen test may detect the presence of adult worms in progressed cases. A complete blood count may reveal eosinophilia (high eosinophil count) or high globulin level.
Medications to kill the adult heartworms given to dogs are not usually given to cats because of the serious and potentially fatal complications that can follow some treatment. The lives of adult heartworm are short-lived in the cat; infection is generally self-limiting and can resolve spontaneously. In this way, treatment is usually targeting the inflammation triggered by worms in the lungs, and bronchodilators to help the cat breathe. Hospitalization may be indicated for further care.
Preventatives are available for cats and should be used in areas at risk of infection. In Canada, most cases are reported in southern Ontario, southern Manitoba, and southern Quebec. The risk is higher when there is an average daily temperature of more than 18°C for 14 days consecutively. Please talk to your veterinarian about a preventative program for your cat!
Written by: Briarwood Animal Hospital
Life Learn handout, April 2019.