Jennifer writes: My cat had blood work done at her last visit and I was told yesterday that she tested positive for heartworm. She did not test positive in November, 2009, when given the same test so she had to have been bitten within the last year. The veterinarian told me there was nothing I could do besides monitor my cats for breathing problems. I have another cat that is not due for his annual exam until June so he will not be tested for heartworms until then. The veterinarian told me that there was no way for my infected cat to give the heartworms to my other cat but I was wondering if this was true. Should I have him tested to make sure? Also, my veterinarian put both cats on Revolution. I gave them the first dose yesterday. However, as I read more information about heartworms in cats, no information says to use a preventive after the cat has been diagnosed with heartworms. The information on the medicine also states to test the animal before administering. My question is: should I continue using the Revolution on my cat that was diagnosed with heartworms or is it harmful for her?
Jennifer’s questions are good ones and deserve a complex answer.
Yes, and no.
We must make some assumptions here because there are unknowns. First, we will assume that the test Jennifer’s cat had was an antibody heartworm test. An antibody test shows the host’s response to a heartworm infestation that may be active now, or may have occurred in the past.
An analogy: Someone with a cold accidentally sneezes in your face today. You have blood drawn at that moment to test for your immune system’s response to the human cold virus. The test is negative because your body has not yet had time to respond. You are sick for two weeks. A month later, the test is repeated. You have been totally well for two whole weeks, yet the test is positive. The positive test reflects the body’s fight against the virus. That evidence will linger for a long time, even though the infection is gone.
However, an antigen test made specifically for the cold virus will be negative, because the cold virus is long gone.
A similar process may occur in cats exposed to heartworm-carrying mosquitos, even cats on heartworm preventive.
If infective heartworm larvae are injected into your cat today, his immune system begins responding today. That response won’t be measurable for some time, so an antibody test may take a couple of months to turn positive.
During that time, your cat’s immune system and his heartworm preventive may kill the larval stages before they become adult heartworms. If the heartworms are unable to reach maturity they cannot trigger a positive antigen test. Furthermore, male heartworms cannot trigger a positive antigen test because the test is checking for the presence of certain proteins in the femalefemale heartworm’s uterus. Beyond that, at least three adult female heartworms must be present for the antigen test to turn positive. Many, many cats have fewer than three heartworms. Still, the antibody test will be positive.
As you see, then, a positive heartworm antibody test is a reason for concern and watchfulness. If a kitty develops respiratory tract complications it is often an emergency situation.
A positive heartworm antigen test in a cat, on the other hand, means that actual adult heartworms are present and the cat-owner’s attentiveness to possible complications must be much greater.
As we have said many, many times, “All cats need to be on heartworm preventive year-round. Feline heartworm disease is easily prevented, there is no treatment and the disease often takes the life of the cat.”
Jennifer should, with the approval of her local veterinarian, continue her cats’ Revolution every month. The risk of failing to do so is much, much greater than the risk of complications from heartworm preventive.
She should also have the other cat tested.
Do not apply any of what you read on this page to dog heartworm disease. Cats are not small dogs and heartworm disease in dogs is a totally different picture.MMHTWF