Heartworm disease is caused by a parasite named Dirofilaria immitis. Heartworms are most commonly found in dogs, but now are known to also infect cats and other mammals. The adult heartworms live in the heart and major arteries of the lungs where they interfere with the function of the heart and lungs. The adult heartworm lives for 1-2 years.

Heartworms can only be transmitted from one animal to another by mosquitoes. Adult worms living in the heart produce offspring called “microfilaria,” which are found circulating in the blood of infected dogs. A mosquito must then feed on the infected dog and ingest some blood containing these “baby heartworms” if the heartworms are to develop into the stage where they can infect another pet. When the mosquito later feeds on another pet, the baby heartworms that have developed to the infective stage in the mosquito now escape from the mosquito into the second pet during the “blood meal” of the mosquito. Once these infective larvae pass through the pet’s skin, they begin migrating through the tissues, eventually making their home in the heart and lungs where they mature into adult heartworms and start the cycle over again. Heartworms can occur in cats kept totally indoors if an infected mosquito should enter the house.

Whereas dogs can live with quite a few heartworms in the heart, because of their smaller size heart, 2-3 adult heartworms can be fatal to the cat.

Clinical signs of heartworm disease in cats are quite variable. Many cats do not show any signs of heartworm disease until they die. In some acute cases, death may come so rapidly that there is insufficient time to make the diagnosis or provide any type treatment. There are cases reported where the cat can appear clinically normal one hour before death. However some cats will show such signs as vomiting, coughing, & difficult breathing. Coughing may be intermittent or occur in severe, sudden attacks that take place days apart.

Clinical signs of heartworm are very similar to several other cat diseases. The diagnosis is confirmed with a combination of radiographs, ultrasound, and blood testing.

Treatment of heartworms in cats is not nearly as successful as in dogs. Treatment is complicated by the fact that after even one heartworm is killed in the heart; it can lead to an arterial blockage that has the potential to be fatal. Blood vessels in the cat are so much smaller than larger dogs creating much more risk while the dead heartworm is being removed from the body. Another one of the major problems is that no clinical signs are observed in many cases until close to death.

Since treatment is not very successful, prevention is very important. As stated earlier, even cats kept totally indoors can become infected from a mosquito in the house. Fortunately, a heartworm preventive is available. The medication is given to the cat once/month.