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Elizabethton Veterinary Clinic

Article Written by:  Dr. Michael Brown - Elizabethton Veterinarian Clinic

Our Paws Newsletter by Elizabethton Veterinarian Clinic

Feline Heartworm Disease

October 2011

There really is a disease of cats that’s acronym is HARD – Heartworm Associated Respiratory Disease. Yes, cats do get heartworms and cats should be on medication to prevent them from getting heartworms. In the larger biological realm, a parasite is an organism that uses another organism for is livelihood and for its reproduction. The ‘used’ organism is called the “host”. The “host” can either be a definitive host or an accidental host. In the case of heartworms, the canid genus (dogs, wolves, coyotes, etc) is the definitive host and the felid genus (cats) is an accidental host. In the latter case, the heartworm can develop within the cat’s body but the cat is a dead end host because the heartworm rarely (if ever) reproduces itself in the cat.It is summertime again and time for your pets to be annoyed by fleas. As fleas are normally a much more significant problem than are ticks, this missive will be primarily instructive re the flea problem.

In the dog, a heartworm is a four to eight inch long thin worm that lives in the pulmonary artery and in the larger arteries of the lungs. It has a very involved life cycle that begins with a mosquito bite that gives the dog a microscopic sized lava. Over the next six months the larva goes through a staged developmental cycle that includes five stages. The mosquito gives the dog the L2 larva. Within 30 hours it develops into the L3 larva. Within a month it develops into the L4 larva. Within the next several months it develops into the L5 larva which resides in the tissue of the lung – still microscopic. From this position, within a few months it develops into the L6 stage – the adult worm. The dog’s body accommodates all of these developmental stages without immediate consequence.

The cat’s body has never developed a situation of immune tolerance to the heartworm larvae. Following a mosquito bite that gives the cat the L2 larva, the cat’s normal defense mechanisms of the immune system usually destroys that microscopic larva as it would do with almost any invading virus or bacterium. However, on occasion, the cat’s immune system does fail to capture and kill the L2 larva and the larva goes on through its multistage development. But, the cat’s body does not tolerate the L5 stage in the lungs as does the dog’s body. If the heartworm larva ever makes it to the L5 stage, the cat’s body reacts with a significant to severe immune response that, for years, has been called “feline asthma”.

In order to explain the fact that it is now believed that there is no condition of “feline asthma” but, rather, only HARD, I digress to explain the research. In short, a veterinarian at Auburn University’s School of Veterinary Medicine was researching the occurrence and condition and mortality of “feline asthma”. While doing the microscopic necropsy (autopsy) on hundreds of cats that did not survive an attack of “feline asthma” he noticed that in many instances he was finding an unidentified larva in the lungs. He was ultimately able to prove that the larva was actually a heartworm L5 larva. As a single microscopic L5 larva in a cat can produce the fatal “feline asthma” condition, it is easy to understand why the veterinarian did not find the larva in all necropsies – the sections of tissue that he was examining under the microscope simply did not always contain the L5 larva. After multiple years of effort, it was finally determined that the L5 heartworm larva was the stimulating cause of the cat’s respiratory condition that had, for years, been thought to be a true asthma.

In a few cases, the cat will survive the immune response to the L5 larva (“feline asthma”) and the larva will develop into the adult L6 worm that lives in the pulmonary artery – not the lungs. At this stage, the cat’s body tolerates the adult worm and there are no clinical signs of asthma nor heart disease. But, within six months to two years when the adult worm dies or moves from the pulmonary artery into the lungs, the result is sudden death of the host – the cat dies suddenly with almost no clinical signs of any disease being exhibited.

There are no reliable tests for heartworm disease in cats as there are in dogs. We cannot test your cat in any method that gives a definitive answer regarding the cat’s heartworm status. The only reliable method of diagnosis is a very detailed microscopic examination of the lungs of a deceased patient. There is no reliable antemortem (before death) test. Therefore, veterinary medicine has to rely upon the known facts that have resulted from research:

• Occasionally cats are an accidental host to Dirofilaria immitus – heartworms;

• The clinical response to heartworms in dogs and cats is vastly different;

• The asthma-like condition that occurs in cats is caused by the L5 heartworm larva;

• With treatment many cats will survive the “asthma” but the treatment can be long and the cat may have multiple occurrences of HARD (“feline asthma”);

• If a L5 larva does develop to the L6 larva, the cat will probably die very suddenly when the heartworm dies;

• A very detailed microscopic necropsy will be necessary to prove the cause of death;

• HARD can be prevented in cats by the monthly use of appropriate medication;

• HARD cannot be efficiently and effectively treated in most cases.

Because very few cats actually develop HARD, it is not a common disease and, therefore, very few people are aware of it. When informed of the disease, even fewer people are concerned enough about the disease to put their feline pet on preventive medication. As a result, at the Elizabethton Veterinary Clinic we routinely treat one to two cats per month for HARD.


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