Visiting Veterinarian : Affairs of the heart(worm)
By Michelle Gerhard Jasny V.M.D.
Published: April 10, 2008
In springtime clients often call inquiring about when I am having a heartworm clinic.
"I'm not," I respond. "But doesn't my dog need a test every spring?" they persist, having been well trained in decades past. Well, that depends.
Reviewing the life cycle of Dirofilaria immitis, the heartworm, will help you understand. Heartworm (HW) is a parasite carried by mosquitos. An infected mosquito takes a blood meal from your dog - let's call her Robin (your dog, that is, not the skeeter). While poking his nasty needle nose into Robin, the mosquito injects microscopic larvae, which migrate around until they find a blood vessel. Molting through several stages, they eventually arrive at the heart, where they settle happily, grow to adulthood, then start laying baby worms, called microfilaria, into the bloodstream. When the next skeeter stops by for lunch, he ingests microfilaria, which mature into the infectious stage inside the bug, and the cycle begins again.
In the old days, we used diethylcarbamazine (DEC) as a preventative. DEC did not affect adult HWs, but prevented infection by killing a specific larval molting stage. In our temperate climate, Robin needed daily DEC as soon as mosquitos appeared in spring until two months after they were gone, usually April through December. There was no reason to give DEC in the dead of winter. Come spring, it was time to start again, but there was a problem. Maybe you forgot to give some days. Maybe you had an elephant's memory but Robin barfed her pill in the backyard occasionally.
Even with perfect compliance, DEC wasn't 100 percent effective. There was a small chance Robin could have gotten infected. Imagine one winter, unbeknownst to you, Robin had HWs maturing into adults in her heart and making babies. You're not giving DEC because it's winter. The microfilaria build up in her blood. Spring arrives. You decide not to bother with that silly heartworm test your veterinarian advises just to pad her pockets. You pop Robin a DEC. All the microfilaria start dying at once. Robin has a severe allergic reaction, maybe even dies. You wish you had gone to the heartworm clinic.
So how come I'm not running a spring clinic? Because no one uses DEC anymore. The current once-a-month HW medications, called macrolides, are far more effective than DEC, and I recommend year-round administration. Why medicate Robin in winter when there are no mosquitos? For many reasons. First, macrolides have the added benefit of controlling many intestinal worms Robin might pick up gamboling through the forest or eating poop at the park. Second, macrolides have a property known as the "reach-back effect." If you use them faithfully, year-round, Robin will still be protected in the event that you forget once in a while. (With DEC even a few days lapse could put a dog at risk.) This reach-back effect cannot be relied upon if you discontinue treatment all winter. Isn't it bad for Robin to get medication all year? I don't think so. The macrolides are not long-acting drugs. They work as a quick monthly "housecleaning" and are rapidly metabolized and excreted. Third, it's easier to remember if you keep up your routine every month than if you start and stop.
Okay. You take my advice and give Robin year-round monthly macrolides. Does she need an annual test? There are arguments both for and against. Not even the macrolides are 100 percent effective. The American Heartworm Society still supports yearly testing. If you live in a high-risk area, like Florida, or frequently forget to give Robin her meds, I agree. If you stop preventative treatment for more than a month or two in the winter, I agree. On the other hand, if you are consistent with giving the pills, and live in an area like ours of relatively low incidence, in my opinion, testing every other year is adequate.
What about the timing of the test? Does it have to be spring? Well, yes and no. Early detection is always best. It takes five to nine months from the time Robin gets infected until our test will show positive. Do the math. It is true that in our neck of the woods the most effective screening time for early diagnosis of the majority of infections is spring. Maybe I'm getting lazy. Maybe motherhood has worn me out. Or maybe I just find so few cases of heartworm here that I question the effort and expense of testing every mutt every spring, especially if they get macrolide preventative year-round.
What if we have an unseasonably warm winter? Recent studies show that the average daily temperature has to be at least 57 degrees for the larvae in the skeeter to mature to the infective stage. Despite global warming, that ain't happening here - at least not yet. What if you forget for a couple of months while you hibernated through January and February? If your lapse was brief, and you've given pills reliably otherwise, the reach-back effect has Robin covered. What if by some weird confluence of circumstances, Robin did get infected and you didn't know? Could restarting medication without a test hurt her? Unlikely. Macrolides don't tend to create the same problems when given to heartworm-positive dogs as DEC did. The major down side to biennial testing is the chance of missing that early diagnosis in an infected individual.
Now I am not telling you to be lax about either testing or prevention. Heartworm infection is a serious, life-threatening disease. The treatment is no picnic either. It is up to you to assess Robin's risk. If you travel to the Caribbean, or Florida, or other locales with a high prevalence of heartworm, all bets are off. You should give Robin preventative year-round, test annually, and talk with a veterinarian about preventative for your southbound cats, too. Feline heartworm is uncommon but increasing in incidence. But if you have complied with your monthly preventative like clockwork, if you don't travel, and Robin comes annually for a physical examination, then I would recommend HW testing every other year at her routine visit. Your veterinarian may disagree.
Many experts still advise an annual spring screening and there is a reasonable case to be made for erring on the side of caution. So, I'm happy to run a spring test if you want to make sure Robin didn't get that worm. I'm just gonna do it one dog at a time during regular office hours.
