Heartworms and Cats – Treating the Problem

cat with heartworms

Heartworms are 9-11” long worms that live in a cat’s heart or in the arteries leading to the lungs, known as the pulmonary arteries. Although heartworms are common in dogs, they are not typically considered a problem for cats. However, recent studies on cats with heart and respiratory diseases have found an incidence of heartworms that is far greater than previously thought.

How Are Heartworms Transmitted to a Cat?

Heartworms are transmitted by mosquitoes. When an infected mosquito bites a cat, it deposits baby heartworms (larvae). The larvae migrate and mature for several months, eventually ending up in the right side of the heart and the pulmonary arteries. They mature into adult heartworms about six months after entering the cat. Shortly thereafter, they begin to release immature heartworms, known as microfilariae, which live in the cat’s blood for about one month. Mosquitoes feeding on the cat then ingest the microfilariae. However, most mosquitoes acquire microfilariae by feeding on heartworm-infected dogs. Because of their life cycle, it is necessary for a cat to be bitten by a mosquito to become infected with heartworms. Heartworms are not transmitted directly from one cat to another or from a dog directly to a cat.

How Are Heartworms Diagnosed?

There are several methods used to diagnose heartworms, but unfortunately, none are 100% reliable. A combination of tests is often required. The diagnostic process usually progresses as follows:

Clinical Signs

One of the challenges in diagnosing heartworms is the lack of consistent clinical signs. The most common signs are coughing and rapid breathing, but both can be caused by several other diseases. Other common clinical signs include weight loss and vomiting, which are also seen in other conditions. Some cats seem normal, only to die suddenly. This happens due to a reaction within the lungs to the young heartworms or when dead or live heartworms enter the pulmonary arteries and obstruct blood flow to the lungs.

Blood Tests

1. There are two relatively new tests that have proven helpful in diagnosing heartworms. The heartworm antibody test determines if the cat’s immune system has been exposed to heartworms. A positive result may indicate an active infection. However, cats who have had heartworms but whose heartworms have died will also have antibodies for an unknown period of time. It is believed that these antibodies persist for 2-4 months after the heartworms have died. Cats with late-stage larvae (that are not yet adults) or cats with adult heartworms in areas other than the heart may also test positive. This test is highly sensitive and is used first. If it is positive, the next test is performed.
The second test is the heartworm antigen test, which detects the presence of adult female heartworms. It is very specific but not as sensitive. A positive result indicates that heartworms are present, but a negative result does not necessarily mean they are absent. The test requires at least two adult female worms to be positive, so a negative result could mean the cat has only a small number of worms or that all the worms present are male.
In summary, a diagnosis of heartworms is confirmed if both the antibody and antigen tests are positive. Most veterinarians are able to perform an in-hospital test to detect heartworm antigen in dogs. However, the canine heartworm antigen test is not as sensitive as the one for cats, which can result in more false negative results.

Blood can also be tested for the presence of microfilariae, but less than 10% of cats with heartworms have microfilariae in their blood, and microfilariae are only present for 1-4 weeks. Therefore, a negative test does not provide much information.

Cats suspected of having heartworms can also be tested for eosinophils, a type of white blood cell that increases in number when certain parasites are present. Eosinophils are elevated in the presence of heartworms, but this elevation only occurs for a few months. Cats with intestinal parasites or allergies may also have increased eosinophil counts.

Radiographs

Radiographs (x-rays) can show the size and shape of the heart and measure the diameter of the pulmonary arteries. Many cats with heartworms have an increase in the size of these arteries, which may appear to stop abruptly (blunted) on their way to the lungs due to worms obstructing them. However, many cats with heartworms show no abnormal findings on their radiographs, especially in the early stages of the infection.
An angiogram is an x-ray procedure in which contrast material (dye) is injected into the heart or veins to help visualize the pulmonary arteries. Although this test illuminates the arteries, there is some risk involved, so it is not used frequently.

Ultrasound

An ultrasound machine produces images of internal organs and structures without using radiation. It is becoming more common in veterinary practices. Ultrasound allows us to view the heart and pulmonary arteries. In some cats, the actual heartworms can be seen, which confirms their presence. However, in many cats, the worms are not visible.

Can Heartworms Be Treated?

There is no drug approved for treating heartworms in cats. One of the drugs used for treating dogs has been tried in cats, but it has potential side effects. Another concern is that when heartworms die, they pass through the pulmonary arteries to the lungs, which can result in sudden death. Therefore, we face a dilemma when a cat is diagnosed with heartworms. Two choices must be made:

  1. Treat with the drug designed for dogs, which has been shown to have side effects in cats, including acute pulmonary (lung) failure and death in a small percentage of cases.
  2. Treat the symptoms of heartworm disease and hope the cat outlives the worms. Since heartworms live in a cat for about two years, several months of treatment are required. When cats are in crisis, they are treated with oxygen and corticosteroids (“cortisone”) to relieve reactions in the pulmonary arteries and lungs. If needed, diuretics are given to remove fluid from the lungs. Once stable, the cat is treated continuously or periodically with corticosteroids. However, the risk of an acute crisis or sudden death remains.

Is There a Way to Prevent Heartworms?

It is strongly recommended that all dogs receive year-round monthly heartworm preventatives. Even in very cold climates, it is accepted that dogs should be on heartworm prevention for part of the year. There are now excellent heartworm preventatives available for cats. Prevention is safe, easy, and should be considered for your cat for the following reasons:

  1. Diagnostic Difficulty: Diagnosing heartworms in cats is not as simple as in dogs. A reliable in-hospital blood test is not yet available, and the tests that are most reliable must be sent to an outside laboratory. Radiographs or ultrasound studies are often needed to confirm the diagnosis. Many cats are diagnosed only after sudden death.
  2. Unknown Incidence: While heartworms are less common in cats than in dogs, they are probably more common than we realize. As we search more aggressively with improved tests, we expect to find that the incidence is greater than previously thought.
  3. No Good Treatment: There is no effective treatment for heartworm-infected cats. There are no drugs available that work well, and cats that seem to be doing well may die suddenly. Treating heartworm infections in cats is risky, and not treating them is just as risky. If a cat is cured, it takes about two years.
  4. Prevention Is Easy: Cats given heartworm prevention drugs have shown no signs of toxicity. There is a wide margin of safety, even in kittens as young as six weeks old.
  5. Indoor Cats Get Heartworms, Too: Exposure to mosquitoes is required for transmission. Cats do not need to be exposed to other cats or dogs with heartworms. While outdoor cats are more likely to be exposed, about 25% of cats diagnosed with heartworms are reported to be indoor-only. This means that mosquitoes entering the house are just as dangerous as those outdoors.

About the Author: This article was supplied by Elizabeth Hodgkins DVM – All About Cats Health Center.
View website: https://www.yourdiabeticcat.com

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