Heartworm Disease Varies by Location

This is the time of year in New Jersey that I start testing my patients for heartworm disease. I want to make sure they have a negative test before considering the start of oral heartworm prevention. The decision of whether to give heartworm prevention will need to be individualized for each pet, just like diet or vaccination schedule. I don't go by the "one size fits all" rule.

The map above gives an indication of the percentage of positive heartworm tests clinics will detect in 2017. There has been an increase in cases from 2013 to 2016, but I'd also like to know if we are just testing more dogs and therefore finding more positives through better education.

I am lucky enough to live in an area where the number of positive cases is small. But even the 25-mile distance between my two clinics shows a difference in percentage of positive cases. In the more rural area where our smaller office is located, we are more likely to find more positive cases. The county lies along the Delaware river and there are plenty of marshy, swampy areas. There are also many farm dogs and dogs that live outside year round that receive less care. With that said, if we diagnose 3 cases, that's a big year for us.

I do not recommend giving heartworm prevention chemicals all year round if the temperatures drop significantly in certain seasons. Heartworm larvae need to mature for two weeks in the salivary glands of the mosquito. If the ambient temperature drops below 57 degrees at any time during that period, the larvae stop maturing. So, until the temperature remains above 57 degrees for two weeks straight (including overnight), I don't worry about heartworms being spread. Tomorrow is June 1st and we are still getting temperatures below 57 at night, which means I may not start my dogs on preventative until June 15th or maybe even July 1st. I generally give the preventatives until two to four weeks after the temperature starts to stay consistently below 57 in the fall. (usually November)

Don't be swayed by pharmaceutical companies trying to sell more drugs. Treat your pets as needed, based on where you live. If I ever get to move south, I will treat my dogs differently when it comes to heartworm prevention. The number of positive cases in the very hot southern parts of our country are high. Unfortunately, there are also areas in the south where veterinary care is not high priority, which means the reservoir of positive dogs that mosquitoes can bite is much higher.

Before jumping on the chemical bandwagon, consider your pet's exposure to the disease. Give drugs when they are more likely to reap a benefit and cause little harm. Before you ask...I use plain milbemycin (Interceptor) for my dogs. I use a modified dose and schedule, which is spelled out in my book "From Needles to Natural".


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