How often should you test for heartworm disease and is it necessary to give heartworm medication every month?

Original Question: I have multiple questions about heartworm disease. Do you feel it is necessary to use heartworm medication every month for 6 months of the year? Modern research seems to suggest that this is no longer the case. My cousin's vet does a heartworm test twice during the season and will treat only if there is a positive test. A friend's vet suggests every other month for the medication. Thank you! - Kim

How often should you test for heartworm disease and is it necessary to give heartworm medication every month? Mar 5, 2018

Hi Kim,

Thanks for the question.

I would still recommend preventing the disease with a monthly medication because there is always a risk and it is in the rise annually. Here is the info you need. Heartworm can exist if we have 2 consecutive weeks of temperatures above 14°C. With summers getting hotter sooner and staying hotter longer, it would suggest that the heartworm season is expanding, not contracting.

The medication works retroactively killing any parasites contracted in the preceding 30 days of giving it.  It would make no sense to skip months because that would your dog will go without protection for a few 30 day periods through the summer. It is simply wrong to skip months and it’s almost like not giving it at all.  This strategy may have arisen because the client only wants to pay for half the medication. If this were the case, and the client only wants 3 doses, I would suggest administering it at the end of the 3 hottest months in the middle of the summer, such as June 30th, July 30th and August 30th. Infection could still occur in May, September and October, but these will be less risky months because of the milder weather.

Only 1 heartworm test per season is necessary. Performing a second one in the same season is a waste of money, period and here’s why. It takes 5.5 months for a heartworm test to become positive so you have to wait until the winter is over to test again. If you test too early, and your dog has heartworm, the test will still come back negative.

Your last point is hard for me to understand. When you say that someone only treats if it’s positive, I don’t know if you’re talking about preventive medication or not. It’s best to prevent disease rather than wait until it occurs to treat it. The treatment is dangerous, costly, painful and sometimes ineffective.

Hopefully this answers most of your questions. The bottom line is that there is a very small risk of getting heartworm disease, but it exists. If you want to eliminate it, then it’s worth administering the medication. If you don’t mind accepting risk for your pet, then you don’t have to get the medication.

To me, the medication is safe so the only reason not to do it is the expense. This issue has to be a personal and individual decision for every pet owner.

Thanks and I hope this helps!

Dr. Clayton Greenway

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