Heartworm Disease in the Cat

Filed Under: Cats, Parasites

Heartworms or Dirofilaria immitis are a spaghetti-type worm that will develop in the heart and lungs of an affected feline. Heartworm disease in cats was first reported in 1920’s both in Brazil and in the U.S. Domestic and exotic felines may all become infected. Heartworms affect male and female cats in equal numbers.

The disease is transmitted by the bite of a mosquito. Due to the insect vector, disease is more common in mild, moist climates The infections rate is only 5 to 20% the rate of infection in dogs. Still the prevalence of feline heartworm disease equals or exceeds the presence of Feline Leukemia or Feline Immunodeficiency Virus infections in comparable populations. The mosquito is partially responsible for the lower infection rate of cats. Some species of mosquito will feed on cats but most mosquitos prefer dogs. A mosquito infecting a cat must also have fed on a dog prior to a cat becoming infected.

Most infected cats are asymptomatic. Typical clinical signs include collapse, dyspnea (difficult breathing), coughing, vomiting and sudden death. Neurologic clinical signs such as ataxia (unbalanced), blindness, circling, dementia, and seizures may also occur usually due to aberrant worm migration. A study preformed at North Carolina State University revealed heartworm disease in 9% of all cats presented with cardiac or respiratory clinical signs.

As the immature worms develop in the body of a cat, they cause inflammation in the pulmonary arteries and lung tissue. This reaction typically occurs 70 to 80 days after being infected by a mosquito. This inflammation may produce an asthma-like response. Once the mature parasite develops, clinical signs of infection may be intermittent or absent because the parasite itself appears able to suppress the immune response of the host.

The second critical state for a cat is when an adult worm dies and degenerates within the host. An adult heartworm will typically have a life span of less than two years in the cat. The degeneration of the adult heartworm will cause more pulmonary inflammation, thromboembolism and anaphylaxis (exaggerated allergic reaction). Anaphylaxis is the most likely cause of sudden death in cats due to heartworms.

Microfilaria (immature baby heartworms) are rarely seen in infected cats. The cat is therefore considered to be a dead-end host because a mosquito cannot pick up microfilaria from heartworm positive cat and infect an additional cat by feeding on it.

Two types of blood tests are available to detect heartworms in the feline patient. A heartworm antigen test detects proteins shed from adult female worms. This test rarely causes a false positive test. False negative results are however common with antigen testing in the cat. Low numbers of adult heartworms are typically present in feline heartworm infections and may be a result of a single female worm or an only male worm infection. Depending on the antigen test used, false negative results may be obtained in over 50% of heartworm positive cases. Idexx Snap® Feline Heartworm Antigen test has recently come into the laboratory market with a reported increase in sensitivity of 15% over other conventional antigen tests which still allows for 35% of cases of feline heartworm disease to go undetected.

Tests detecting heartworm antibodies may detect infection as early as one to two months following exposure. Antibody tests may detect up to 70% of heartworm infections but they have a wider range of false positive results, up to 19%. An in-clinic feline heartworm antibody test is now available through Heska and is marketed as the Solo Step®. For routine screening the antibody test is preferred. Antigen and antibody testing are available at Animal Diagnostics Inc. of St. Louis, MO or from Antech Laboratories found in various larger US cities.

Another useful diagnostic test is a CBC or complete blood count. The number of eosinophils and basophils (types of white blood cells) is increased with a parasitic infection. Chest radiographs may show characteristic changes in the heart and lungs that may be relevant to heartworm infection. The most characteristic radiographic criterion occurs when the left caudal pulmonary artery is observed to be greater than six times the size of the ninth rib at the ninth intercostal space. Radiographic changes are seen in 53% of feline heartworm cases. Echocardiography may demonstrate adult heartworms in 78% of heartworm cases as well as their resulting pathology on the heart itself.

For best results, both antigen and antibody tests should be combined to achieve a diagnosis in conjunction with other laboratory data in the presence of characteristic clinical signs.

Heartworm treatment can be hazardous even to the mildly infected feline. Surgical retrieval of adult worms has been tried, but the rate of complications is high. Often there is failure in retrieval of the entire worm population and mortality levels of 25% or more are common.

Treatment is primarily supportive. Prednisone and bronchodilators may prove helpful especially in decreasing lung inflammation and coughing. Oxygen supplementation may be necessary when breathing is labored. Tetracycline use, for complicating bacteria called Wolbachia spp., is also advocated in the hope of decreasing inflammatory mediators. Aspirin use is controversial and should not be used concurrently with corticosteroids such as prednisone.

Prevention is the best way to avoid premature death when living in heartworm endemic areas. Outdoor cats are at greatest risk of infection, but indoor cats are actually more susceptible to heartworms since they do not mount an as aggressive immune response. Twenty-seven percent of cats naturally infected with heartworm disease were found to live exclusively indoors. A similar study by North Carolina State University demonstrated this number to be even higher, with 50% of cats diagnosed at NC State being housed solely indoors. Surveys of indoor only cats show that from 8 to 19% are antibody positive for heartworms.

Monthly heartworm preventatives are available for use in the cat and should be used regardless of whether a cat lives indoors or out. Testing for heartworms before using a feline heartworm product is unnecessary. The risk of adverse reaction between heartworm medication and dying microfilaria is small because most cats do not produce microfilaria, or have very low microfilaria burdens.

Cats with heartworm infection should also be prescribed a monthly preventative in conjunction with prednisone when respiratory clinical signs are present.

The medium survival rate of cats being diagnosed with heartworms is 1.5 years. The median survival rate for all heartworm infected cats that live beyond the first day of diagnosis is four years. Survival of the first day of diagnosis is critical. Many patients are brought in with critical cardiac and respiratory distress when a diagnosis of heartworm disease is first made.

References:

“Antech Diagnostics News.” March 2007.
Atkins, Clarke; “Understanding Feline Heartworm Disease”, Parasite Developments. Proceedings from the 2007 NAVC Conference and the 2007 Western Veterinary Conference. Supplement to Veterinary Forum Vol.24.No.3(A). March 2007.pp 2-7.
Dillon, Ray. “The Nature of Heartworm Disease in Cats: Different from Dogs”. Parasite Developments. Proceedings from the 2007 NAVC Conference and the 2007 Western Veterinary Conference. Supplement to Veterinary Forum. Vol.24 No.3 (A). March 2007.pp. 16-18.

Topics: heartworms, mosquitoes

Symptoms: blindness, circling, collapse, dementia, difficulty breathing, seizures

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