Don't tell anyone, but I have a tape deck in my head. If I were younger, it would be an iPod, but what I've got is a tape deck.
Every time I vaccinate a cat, I hit the button labeled Injection-Related Fibrosarcoma, and words spew automatically from my mouth: "There's a form of cancer in cats that appears to be related to injections in genetically susceptible animals...." Then there's a button marked Fleas: "The adult flea spends 95 percent of its time on the animal," I drone.
I can tell it's springtime because I am now repeatedly hitting the Heartworm button: I hope reviewing the life cycle of the heartworm will help you understand the whys and wherefores of testing and preventative medications. Hit Play.
Heartworm (HW) is a parasite carried by mosquitoes. When an infected mosquito bites Rover, it injects microscopic larvae into the tissue. These migrate around, molting several times as they head toward Rover's heart. It is during this stage of tissue migration that preventative medications work by killing developing larvae.
Once the larvae complete these early molts, find a blood vessel, and enter the circulatory system, preventative medications are no longer effective. Remember this. It will be important later when we talk about prevention protocols.
Once larvae arrive at the heart and pulmonary vessels, they settle in happily and set up housekeeping. Adult females produce legions of baby worms, called microfilaria, that circulate in Rover's blood. It takes six to nine months from the time the mosquito first bites Rover until microfilaria appear in his blood. The next mosquito bites Rover and ingests a snootful of microfilaria, which then molt inside the insect, and once again reach the larval stage that can infect Fido. Mosquito bites Fido. The cycle begins again.
In the old days, we used a heartworm preventative called diethylcarbamazine (DEC), given on a daily basis throughout mosquito season. DEC does not affect adult heartworms, but if given properly it can prevent infection by killing a very specific larval molting stage during the initial tissue migration. In our temperate climate, this requires daily DEC administration April through December. Most dog owners would stop giving DEC around Christmas. Come spring, it was time to start again, but before doing so, veterinarians advised that Rover be tested to make sure he did not have an active heartworm infection.
"My dog doesn't need a test," clients would object. "I gave his DEC every day." Well, maybe you did, but no one's perfect. Even the best owner sometimes forgets.
Even if you never forgot, no medication is 100 percent effective. Now imagine that somehow Rover got infected, only you didn't realize it. There are no clinical signs of illness during the tissue migration stage. Rover seems fine all winter. By spring, worms have reached the heart, matured, and begun making babies, thousands of babies. You decide since it's April you should start his DEC again. You pop him a dose, causing a massive die-off of thousands of microfilaria leading to a potentially life-threatening allergic reaction. To avoid this scenario, we recommended a HW test every spring prior to starting DEC daily preventative.
Nowadays, we use once-a-month preventatives, called macrolides, such as Heartgard and Interceptor. Far more effective than DEC, macrolides are not long-acting medications. They work like a once-a-month housecleaning, killing any larvae that may have entered Rover's body over the preceding 30 days, then are rapidly metabolized and excreted.
Although it is still unlikely that a dog in our neck of the woods could contract heartworm in the winter, there are advantages to giving macrolides year round. For one thing, macrolides have activity against intestinal parasites, some of which can infect people as well as pets. They also have a property known as the "reach-back effect." Given monthly, they will still provide a modicum of protection for Rover, even in the event you forget a pill now and again. This reach-back effect cannot be relied upon if you discontinue treatment all winter. Finally, most folks find it easier to remember a continual monthly routine than one that stops and starts.
So does Rover need an annual heartworm test if he is on year-round macrolides? What if you forgot a pill? What if you forgot two pills? What if you were a week late most months?
Luckily, giving a macrolide to a heartworm-positive dog, even one with circulating microfilaria, has less dire consequences than in the old days with DEC. The American Heartworm Society advises that if there has been a lapse of three months or longer with macrolides, Rover should be retested prior to starting preventative again, then again four to nine months later.
Why? If you were paying attention, you know it takes six months or more from the time of initial infection until Rover has mature worms in his heart. Our standard test looks for antigen from adult worms and will not diagnose Rover's infection in the early stages. The retest assures that we won't miss diagnosing any infections, and will allow us to treat right away should Rover test positive for heartworm.
What if you are an exemplary owner who never misses a month? The American Heartworm Society still supports annual testing, the timing of which may take into account regional climate, your history of compliance, and the heartworm life cycle. You vacationed in Florida? That's a high-risk heartworm-endemic part of the country. Get in here for a test. You only missed one pill in January, Rover never left Martha's Vineyard, and you are a serious Type A personality who pays attention to detail? In my opinion, you can go ahead and pop that pill. No worries. We'll test Rover at his annual physical exam. When in doubt about what to do, ask your regular veterinarian.
What about kitties? Cats are considered an atypical host and are more resistant than dogs to heartworm infection. Some cats spontaneously self-cure. When cats do get infected, they have fewer and smaller worms, and the worms do not live as long. Studies done at shelters indicate the rate of feline HW infections to average five to fifteen percent of the regional rate of canine infections. In our area, that's a pretty low incidence. However, feline heartworm infection may cause very serious disease, so if your veterinarian advises, cats can also be given a monthly macrolide preventative.
That's it. Hit Rewind. "Heartworm is a parasite carried by...."
