Heartworm becoming resistant to preventives

Dr. Jody Kaufman

We are always finding out new things about medications, diseases, and treatments. It can be tricky to sift through all the available information, even without being confused by the clutter and misinformation that crowds the Internet.

Evidence of heartworm resistance to the common heartworm preventives (including Heartgard, Triheart, Interceptor, Sentinel, and Revolution) has been filtering through the veterinary literature for the last couple of years. The bulk of cases have clustered around the Mississippi River Valley, where heartworm disease is rampant. Not all dogs on the preventive develop heartworm infection, but it has become clear that the medications may not always be 100% effective.

How do we known that it isn't just a matter of people not giving the medication as directed? Drug companies and universities have conducted studies in which dogs are given various preventives and then injected with a precise number of infective larvae. The control dogs are not given any preventive. After the incubation period, the number of adult worms living in the dogs' hearts are counted. In the study groups, between zero and five worms may show up for every 100 larvae injected, as opposed to 80-90 in the control group. So it's not that the preventive is totally ineffective, but it isn't completely reliable either.

What does this mean for the dog? Since the changes in the lungs and heart are due to the patient's immune response to the worm, even one or two worms can cause significant disease. What does this mean for dogs in our area where heartworm disease is relatively uncommon? I would not be very concerned were it not for two things. First, some people travel south for the winter or go on road trips with their pets. Traveling to areas where heartworm is endemic may pose a risk for dogs on the oral preventives. Second, rescue organizations that ship dogs up from the South have proliferated, and some of those dogs are infected. Heartworm is spread by mosquitoes that feed on heartworm positive dogs. The mosquitoes suck up microfilaria, the microscopic offspring of adults that circulate in a dog's blood. The microfilaria develop in the mosquito and are then injected into a new victim. If the microfilaria come from one of the resistant strains, it could spread to dogs who live in the area.

The most recent studies have indicated that the topical Advantage Multi seems to be 100% effective as a heartworm preventive. It is also used to prevent fleas (a timely topic for another discussion) and some intestinal parasites.

Should everyone switch over to Advantage Multi? I don't know for sure yet, but I'm open to the idea. I would certainly recommend it for pets that travel to the south. And I'm nervous about the migration of heartworm-positive dogs to our area. I'm also aware that we need to constantly evaluate new information, so I may feel more or less strongly next year. Or maybe there will be a new preventive that's even more broad spectrum. Stay tuned.