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Heartworm Disease in Cats - Testing

What is heartworm disease? How does a cat become infected?

Heartworm disease is a parasitic disease that typically affects dogs but can occasionally occur in cats. Heartworm parasites, known by the scientific name of Dirofilaria immitis, are long thin worms that live in the right side of the heart and the pulmonary arteries, the large blood vessels that carry blood from the right side of the heart to the lungs. The disease is transmitted by mosquitoes carrying immature worms called larvae. When an infected mosquito bites a cat, the larvae pass from the mosquito into the cat.


Is heartworm disease the same in cats as it is in dogs? heartworm_disease_in_cats-1

The dog is the normal host for heartworm, which means heartworms have adapted to parasitize dogs and related species. In the dog, the worms are able to complete their life cycle, and eventually migrate to the right side of the heart, where they become adults, mate, and reproduce; the 'baby' worms, calledmicrofilariae, will circulate in the bloodstream of an infected dog. If the dog is bitten by another mosquito, these microfilariae pass from the dog into the mosquito, which starts the cycle all over again.

"The cat is not the normal host for heartworms..."

However, the cat is not the normal host for heartworms, and the worms typically are not able to complete their life cycle. They either die in the tissues, especially the lungs, or get 'lost' and end up in abnormal places like the brain, the skin, or body cavities. A small number of worms are able to mature in the heart, but there are usually too few adults to mate and produce microfilariae. On the rare occasions that microfilariae are produced, they are typically present in the blood stream for only a brief period (less than one month).


Is heartworm infection common in cats?  

Heartworm infection in cats is much less common than in dogs. Studies have estimated that for every 100 dogs that develop heartworm disease, only 5 to 15 cats are infected.

"Heartworm infection in the cat is difficult to diagnose, and the true rate of infection may be higher than initially estimated."

Approximately 1/3 of infected cats live indoors only. However, heartworm infection in the cat is difficult to diagnose, and the true rate of infection may be higher than initially estimated.


Where is heartworm infection most common?  

Heartworm disease is widespread in the United States and is particularly common along the eastern and gulf coasts and in the Mississippi River valley. In Canada, heartworm infection is much less common, with most cases reported in southern Ontario, southern Manitoba, and southern Quebec.

The risk of infection is greatest when mosquitoes are actively feeding. This typically requires an average daily temperature of more than 64°F (18°C) for a month. In areas that experience killing frosts, the risk of contracting heartworm disease is limited to the warmer months from late spring to early autumn, even though mosquitoes may appear in the early spring. By comparison, in subtropical United States heartworm infection can be a year round risk.


Can infection be spread directly between cats, or from cats to people? 

Cats can only get heartworm from an infected mosquito. There is no direct transmission of infection from cat to cat, cat to dog, or from cat to people. 


What are the clinical signs of heartworm disease in the cat?

Many infected cats have no noticeable signs of illness, while others may show vague or non-specific signs such as lethargy, lack of appetite, or weight loss. Some cats develop obvious signs of illness, typically difficult breathing, coughing, and vomiting. Some cats develop heart murmurs or have fluid build-up in the chest or abdomen; seizures and other abnormalities of the nervous system have been reported. A small number of cats die suddenly without warning.


How is heartworm disease diagnosed in the cat?

Heartworm disease in the cat is difficult to diagnose and a variety of testing methods are usually needed to confirm infection. These include blood tests, radiography (X-rays), and echocardiography (ultrasound of the heart).

Routine blood tests (see CBC and Biochemistry Profile) may show non-specific changes such as high eosinophil count or high globulin level. Blood tests that are more useful include the antibody test, which is able to detect early stages of heartworm infection, and the antigen test, which may detect the presence of adult worms later in the course of disease. The antibody test is often the first test done, followed by the antigen test if the first test is positive. A test for microfilariae may also be done, although it will be negative in most cases.


What do positive test results mean?heartworm_disease_in_cats_-_testing-2

A positive antibody test indicates there has been exposure to heartworm larvae, which increases the probability that there is underlying heartworm infection. A positive antigen test confirms heartworm infection and indicates that adult heartworms are present. However, neither the antibody test nor the antigen test is 100% reliable, and the two tests may give contradictory results or be inconsistent with clinical signs. A positive microfilariae test confirms that at least one male and one female adult worm are present. Further testing often includes radiography of the chest and ultrasound of the heart. These tests reveal changes in the heart, lungs, and major blood vessels that support a diagnosis of heartworm infection.


Is there treatment for heartworm disease in cats?  

Routine use of drugs to kill adult heartworms in the cat is usually not recommended due to serious and potentially fatal complications that may develop. Since adult heartworms are relatively short-lived in the cat, infection is often self-limiting and frequently resolves spontaneously. Treatment is usually limited to controlling inflammation triggered by the presence of the worms. In rare situations, treatment to eliminate adult heartworms may be used.


Can heartworm disease in cats be prevented?

Heartworm preventatives are available for cats and should be used in areas where there is a high rate of infection among dogs and therefore an increased risk of infection for cats. Your veterinarian is the best person to advise you on whether a preventative program is appropriate for your cat, which product would be best, and what type of dosage schedule you should follow.

Additional information about heartworm disease in cats can be found in our feline series handout "Heartworm".

This client information sheet is based on material written by: Kristiina Ruotsalo, DVM, DVSc, Dip ACVP & Margo S. Tant, BSc, DVM, DVSc

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