Feline Heartworm Disease As Great Imitator

Spunky the cat came in today with the owner’s son and a history of coughing twice each day for about two weeks. He was otherwise acting and feeling fine and eating well, he just had this nagging, gagging cough.

He even came in with a differential diagnosis list: “He’s coughing like he has a hairball. We think he has asthma.”

Actually, feline asthma is a fairly good starting point for a differential list for coughing cat, even if “hairball” isn’t.

You see, hairballs, also called tricholiths, are a gastrointestinal problem, not a respiratory tract problem. Coughing is a respiratory tract activity. Hairballs occur when a pet grooms himself, or another pet, or a hairy inanimate object, like a rug or a person’s coat. He ingests the hair and, if it is present in sufficient amounts, the hair becomes thatched inside the intestinal tract and begins to build up, resulting in an obstruction. That obstruction may completely stop the flow of ingesta through the gastrointestinal (GI) tract, or it may simply slow down the flow. In either case, when newly-ingested food arrives at the point of the blockage, the body’s attempts to move it down the intestinal tract result in a reverse action, causing the food, with or without the hairball, to be vomited up.

So, now you understand that your cat’s having a hairball doesn’t result in a cough.

Spunky’s physical examination revealed little in the way of abnormalities, other than a weight loss. We have been following a gradual weight loss for about a year, and seeing it continue added to our worries for Spunky.

Next, we formulated a list of diagnostics called a minimum database, which is a collection of the fewest tests we need to perform in order to determine, or at least begin to determine, a cause for a patient’s problems. In Spunky’s case, this was his minimum database:

Examination of Spunky’s chest X-ray immediately showed us enlarged, tortuous and blunted (shortened)
pulmonary arteries. This is a clear and pathognomonic indication of heartworm disease in cats and dogs alike. While there are other causes of pulmonary artery enlargement, nothing else makes them enlarged, tortuous and blunted, as Spunky’s are.

This radiographic finding made us want to add two more tests to Spunky’s labwork: heartworm antigen and heartworm antibody.

Antigen is a term the refers to any reactive protein. In this case the antigen heartworm test is looking for a specific protein from the uterus of the female heartworm. Researchers found long ago that testing for this particular protein prevented cross-reactions with other types of worms, giving excellent accuracy to the antigen or occult heartworm test.

The tricky aspect of heartworm testing in cats is that cats, being an aberrant host for heartworms, typically have very small numbers of heartworms, often only 1-3. If all of the heartworms are male, or if there are only one or two female heartworms, an antigen test may be negative because there is too little or no protein from the female heartworm’s uterus. Therefore, if the test is positive, we know for sure that there are heartworms present. However, if the test is negative we may experience what is called a false negative and there still be small numbers of heartworms there.

While the antigen test is looking for evidence of the worms themselves, the heartworm antibody test looks for the body’s response to the adult heartworms. Let’s look at an example.

Suppose you have a cold. If I take a blood sample from you in a month and test for your body’s response to the cold virus of primates, the test can give me a titer, or actual measurement of how much you reacted to the cold virus. You no longer have a cold, but the evidence that you did is still in your body.

It’s the same with the heartworm antibody test. If the test is positive we know that our feline patient has been exposed to heartworms at some time, although the test doesn’t tell us whether there are live heartworms in the patient right now.

Spunky’s owners administer heartworm preventive to him every month. While his case may be a simple matter of medication failure, our concern is also directed at the source of his heartworm preventive: an online “pharmacy.” What really is in that box of heartworm preventive they give every month? Or, have they inadvertently
 skipped doses here and there, leaving Spunky susceptible?

Clinically, feline asthma and feline heartworm disease have a long history of mimicking each other. In Spunky’s case, however, the chest radiograph is unequivocal. Spunky has heartworms.

The sad turn in this case is that Spunky’s dad called later in the day and asked that the laboratory tests be cancelled. He was convinced that Spunky has feline asthma and no heartworms, despite the unequivocal findings on the chest radiograph and despite my explaining our differential diagnosis list and plan for Spunky.

Just as importantly, we lost the opportunity to find out why Spunky is losing weight. If he has feline hyperthyroidism, damage to the liver, kidneys, heart and lungs will be ongoing. Treatable and preventible damage.

Sadly, Spunky’s family had done some reading on the Internet that is at least partially at fault for leading them astray. We must always remember that 62% of the medical information on the Internet is either outdated or outright wrong.Furthermore, your veterinarian is a doctor who is trained to understand the technicalities and the subtleties of body parts, their functions and their malfunctions. Just because an online description “looks” or “sounds” like what your pet is doing, doesn’t mean that is actually his correct diagnosis.

Poor Spunky got caught in the lurch of the knowledge barrier, unfortunately.

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