Visiting Veterinarian : Rewind
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.