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Heartworm Disease in Cats

By: Dr. Arnold Plotnick

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Diagnosis In-depth

When clinical signs of heartworm disease are detected, a data base will need to be generated. There are a number of tests that can help make the diagnosis. Some are better than others. The following tests have been suggested.

  • Complete Blood Count and chemistry panel. A complete blood count can be helpful when trying to make a diagnosis. In approximately one third of cats with heartworm disease, there will be an increased number of a type of white blood cell called an eosinophil. The increased numbers of eosinophils is a transient event. It is most likely to be seen 4 to 7 months after infection. Sometimes, there is an increased number of another type of cell called a basophil. Increased basophils is much less common, however, it is more strongly suggestive of heartworm disease. Serum chemistry panels are usually normal. Occasionally, infected cats will have a higher level of proteins called globulins in the bloodstream, but this is not consistent or predictable.

  • Thoracic radiographs (chest X-rays). Radiographs can be an important diagnostic tool, especially in early cases, as radiographic changes suggestive of heartworm disease can be seen as soon as 120 days after infection. It takes a fair amount of expertise to interpret the X-rays correctly and many veterinarians send the films out for a second opinion by a board certified veterinary radiologist. The most common changes seen on X-rays are enlarged pulmonary arteries and changes affecting the lung tissue itself. This may result in patterns called "perivascular interstitial" or "alveolar" changes.

  • Serum antibody tests. When heartworms infect a cat, they release microscopic molecules called antigens. Antigens trigger the immune system to produce antibodies. This antibody test is a very sensitive test. It can detect infection in 80 percent of infected cats just two months after getting infected. If you do the test 3 months after cats are infected, it will detect it in 97 percent of them. By four months after infection, it detects nearly 100 percent. A positive test does not necessarily mean that the cat has adult heartworms in the heart and lungs, however. This is the main problem with this test. The most valuable aspect of the antibody test is that a negative test almost certainly means the cat is NOT infected with heartworms. In other words, the antibody test is a good screening test. A negative test virtually excludes heartworm disease as a cause of the cat's clinical symptoms. A positive test means that heartworm disease is "an issue" for this cat and further testing needs to be performed, such as an antigen test.

  • Serum antigen tests. Antigens are molecules given off by invading organisms, such as heartworms. The serum antigen test detects these antigens. Unfortunately, the antigen it detects is a molecule produced by mature adult female worms only. The test in dogs is very reliable; a positive test means there are heartworms in the heart and lungs. A negative test means there are no heartworms. In cats, a positive test means that there is at least one mature female heartworm in the cat. A negative test, however, does not mean that the cat isn't infected. Since the tests do not detect immature worms (worms less than seven months old), cats recently infected may go undetected. Also, since the test detects antigens given off by mature female worms, all male infections would be missed by the test. In a study of 108 naturally heartworm infected cats, 18 percent had all male infections. Finally, exactly how many female worms must be present before the test can detect them is debatable. In the past, antigen tests could not detect two or fewer worms. The newer tests claim that they can detect infections containing at least one female worm.

    To reiterate, a negative antigen can mean:
  • The cat is not infected
  • The cat is infected with immature worms (less than 7 months old)
  • The cat has an all male infection
  • The cat has one or two female worms, but this particular test is not sensitive enough to detect it

    A positive test means that the cat most likely has at least one mature adult female worm in his body.

  • Microfilaria test. A definitive diagnosis of heartworm infection can be made by detecting microfilariae in the circulation. Adult heartworms mate and produce microfilariae approximately 8 months after infection. Circulating microfilariae are detectable for only about a month, however, before the cat's immune system eliminates them from the bloodstream. (This doesn't happen in dogs; microfilariae can circulate for years in dogs). As a result, about 80 percent of cats with heartworm infection do not have circulating microfilariae, so there are many false negatives with this test. If you do see them, however, the cat is definitely infected.

  • Echocardiography (cardiac ultrasound). This test is more sensitive in cats than in dogs for detecting heartworm infection, especially if you're specifically looking for worms, and if the test is performed by an experienced cardiologist.

  • Angiography. This test requires injecting a special dye in the jugular vein and then take X-rays a few seconds later. This test can demonstrate enlarged pulmonary arteries, other characteristic vessel changes and, occasionally, the heartworms themselves. Performance of the test and interpretation of the angiograms usually requires referral to a specialist.

  • Transtracheal wash. This test involves putting a small amount of sterile fluid down the trachea into the lungs and then retrieving some of the fluid and analyzing it. The retrieved fluid usually contains cells that can help make a diagnosis. Similar to the results of a complete blood count, the findings of many eosinophils on a transtracheal wash is supportive of feline heartworm disease. It is also supportive of feline asthma and/or lung parasites, so other tests should be performed to rule in heartworm disease and rule out other diseases.

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