Rarity Of Microfilarial Reactions In Cats

Mrs. Jones, in the clinic for her cat Wilson’s semiannual examination, asked, “Dr. Randolph, when our dog Phred missed a dose of heartworm preventive last year, he had to have a heartworm test before he could take his preventive again. However, Wilson has been on heartworm preventive for all of his five years of life, and he’s never had a heartworm test. Why is that?”

All cats must be on heartworm preventive all year ’round.

All heartworm preventives have the ability to kill microfilaria, which are the offspring of adult heartworms. Microfilaria circulate in the blood of canine heartworm victims, although not all dogs with heartworms also have microfilaria.

Some dogs with microfilaria will have large numbers of them or “heavy burdens.” Other microfilaria-positive dogs may have only a few.

Microfilaria are important in the answer to this question because they cause the danger in reintroducing heartworm preventives to dogs who have missed doses. In addition to stopping heartworm larval stages from maturing into adult heartworms, all heartworm preventives also kill microfilaria. Some kill microfilaria quickly; some kill microfilaria slowly.

When microfilaria die suddenly and in large numbers there is frequently a violent reaction from the canine host body. Fluid rushes into the lungs where air is supposed to be. Histamine-carrying cells degranulate, releasing their inflammatory mediators. Small arteries all over the body close off. Kidneys stop working. Sudden death often results, and patients die even before they can be rushed to their veterinarian.

Therefore, a blood test for heartworms is required prior to restarting a dog’s heartworm preventive when he has missed one or more doses. It is literally a matter of life and death.

Even with a negative result, and dispensing six months of heartworm preventive, a retest six months later is mandatory.

Cats, as an aberrant host of heartworms, have a modified heartworm life cycle.  In addition to a different behavior in the maturation of adult heartworms, those heartworms living in cats almost never produce meaningful populations of microfilaria. Therefore, since the risk of reaction is directly related to and proportional to the number of microfilaria in the body, the likelihood of medical complications from restarting heartworm preventive in cats after missed doses is very, very low.

Which is why most veterinarians don’t test before beginning again heartworm preventive. However, if your veterinarian chooses to test first, I’m not going to criticize him.

Your big concern should be missing doses in your cat, which could lead to adult heartworms and sudden death.
Click here to read more about heartworm disease in cats.
To read about heartworm testing in cats suspected of being infected with heartworms, click here.


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