Heartworms in cats: new insights (Proceedings)


Heartworms in cats: new insights (Proceedings)

Aug 01, 2011

"If you're not seeing heartworm-positive cats in your clinic, rest assured that heartworm-positive cats are seeing you."
Ray Dillon, DVM, DACVIM

A new study by Dillon and Blagburn

A new study performed at Auburn University's College of Veterinary Medicine is radically changing our understanding of how heartworms affect the cat. In this study three groups of cats were studied for 8 and 16 months with antibody and antigen tests, radiographs of the heart and lungs, and necropsy examinations of the heart and lungs. Group 1 looked at the effects when heartworm larvae were deposited under the skin and allowed to mature without interference. Group 2 also received heartworm larvae under the skin; however, three months later a drug (ivermectin) was given to kill the larvae before they became adults. Group 3 was put on selamectin about one month after being infected with the larvae. This study allowed us to understand the disease caused by adult heartworms (Group 1),the disease caused by juvenile (immature) adult heartworms (Group 2), and the effectiveness of the heartworm prevention product selamectin (Group 3).

Life cycle

     1. Mosquitoes bite cats about as frequently as they bite dogs. Some cats are allergic to the bite and show a characteristic dermatitis on the nose and pinnae. The majority do not have this reaction, as the majority of dogs bitten by fleas do not have flea bite dermatitis.
     2. When a mosquito bites a cat, third-stage larvae (L3) are deposited on the cat's skin. Within minutes they enter the subcutaneous tissue through the bite wound.
     3. The L3 molt within a couple of days to fourth-stage larvae (L4). L4 migrate subcutaneously in fat and muscle for 2 months then molt to become a juvenile worm. (Juvenile worms have been incorrectly called L5.)
     4. Juvenile worms enter circulation via a peripheral vein. This occurs about 60 days after infection.
     5. An antibody response begins about this time; some cats test antibody positive.
     6. Within the next 15 to 30 days (75 to 90 days post infection) the juvenile worms arrive in the caudal pulmonary arteries.
     7. By about 100 days post infection there are 2 inch long juvenile heartworms in the caudal pulmonary arteries.
     8. The vast majority of the juvenile worms die causing an intense inflammatory response affecting the pulmonary arterioles, bronchi, and alveoli.
     9. Antibodies begin to wane as the juvenile worms die.
     10. If the juvenile worms mature to adults, the adult worms suppress the immune system causing antibodies to wane. Most antibody tests turn negative about 4 months later as long as new infections do not occur. In locales with long mosquito seasons, antibody tests will remain positive due to formation of new juvenile worms.


     1. Heartworm Associated Respiratory Disease (HARD) is unique to the cat.
     2. It is defined as vascular, airway, and interstitial lung lesions caused by the death of juvenile worms. The inflammation may last up to 8 months.
     3. If we administer 100 infective larvae to a dog, 75 will mature to adults.
     4. If we administer 100 infective larvae to a cat, many will become juvenile heartworms, but only 3-4 will mature to adults.
     5. A very large number of juvenile worms develop but never make it to adulthood due to the effects of the cat's immune system. They are the cause of HARD.
     6. Juvenile worms are, according to Ray Dillon, "hit and run drivers." "They get into the cat, cause significant lung damage, and leave no evidence they were there." Severe lung lesions are present, but:
          a. There will be no adult worms on necropsy.
          b. The juvenile worms disintegrate within the lung tissue.
          c. Antibody disappears very quickly.
          d. Antigen tests will be negative because there have been no adults.
     7. Radiographically these cats look similar to cats with allergic bronchitis. Interstitial or bronchial patterns may be present. In addition, the caudal pulmonary arteries may be enlarged and blunted. In some cats, apparent enlargement may be due to periarterial inflammation.
     8. Clinically these cats look like a cat with allergic bronchitis. Coughing is common and is usually relieved with corticosteroids.
     9. The lung damage caused by the juvenile worms may not show up until later in life as chronic interstitial lung disease, chronic bronchitis, or COPD.
     10. Many of these cats are antibody positive and antigen negative. However, many of these cats test negative on both antigen and antibody tests making differentiation from cats with allergic bronchitis virtually impossible.
     11. In colder states, HARD occurs 4 to 6 months after the mosquito season. In warmer states, HARD is a year round problem but may have higher incidence in winter and spring. Allergic bronchitis is also most common in winter and spring.
     12. HARD occurs even if an adult heartworm never develops because it is not caused by adult heartworms.