Dr. Katherine Andre, Bethany Animal Hospital
A mosquito becomes infected with the parasite by biting an infected dog (the one down the street that just moved here from Florida). The microfilaria (baby heartworms) are circulating in the dog's blood, and are consumed by the mosquito during a meal. The microfilaria go thru some changes within the mosquito, then enter a new dog (hopefully not yours!) when the mosquito feeds again. The larva stay in the dog's skin for awhile and go through more changes, then enter the blood stream and are carried to the heart. There they stay, developing eventually into long, spaghetti like adults. The adults it the heart produce microfilaria, which circulate in the bloodstream, providing more disease organisms for new mosquitoes, and the cycle continues. The adults in the heart take up space and end up causing heart disease and all the things that go along with that.
Heartworms are diagnosed by taking a small sample of blood from your pet, and testing it for the presence of antigens (proteins from the adult worms). If there are adult worms living in the heart, the test will be positive and treatment will have to be discussed. Treatment involves first doing more testing to determine how advanced the disease is (generally another blood test) to evaluate the liver, kidneys, and other organs, radiographs (x-rays) to evaluate the heart, and a urinalysis. Then a drug (Immiticide) is administered according to a schedule determined by the severity of the disease. Hospitalization is often required. Reactions to the drug are not uncommon. A period of STRICT confinement follows - usually at least 6 weeks - while the adult heartworms die and are disposed of slowly by the body. Exercise, excitement, and other activity may lead to pieces of the dead/dying worms breaking off and traveling thru the bloodstream causing clots in important places such as the lungs. The final step in treatment is killing the microfilaria which are circulating in the bloodstream, by giving another drug.
FORTUNATELY - Heartworm is much easier to prevent than it is to treat.
There are several monthly preventatives on the market which all work slightly differently, but in essence, they are all geared at killing the larval stages once they are injected by the mosquito into the dog. Killing the parasite at this stage is safe for the dog, and prevents the parasite from ever getting to the heart to cause damage. Most of the monthly heartworm preventatives also provide protection from intestinal parasites as well - doing double duty - and come with stickers for your calendar to remind owners when it is time for another dose. Some are chewable, making administration as easy as giving a treat.
Heartworm in dogs is a disease that has become clinically important during my lifetime and its prevalence has increased nationwide (if not worldwide) in the last 20 years. Other parts of the country learned their hard lessons years ago; dogs not on heartworm preventative were going to get heartworm, get sick, and probably die. Until fairly recently, Arizona and the southwest in general have had a much lower incidence of infected mosquitoes and heartworm-positive dogs. Many veterinarians felt the risks weren’t high enough to justify recommending testing and preventative for all their patients and asking owners to pay for it.
Now, however, things have changed. Nearly everyone is much more aware of Arizona's mosquito population, thanks to West Nile Virus. Nearly everyone can see that people (and their pets) are pouring into the valley from the rest of the country - where heartworm is prevalent - bringing the parasite with them. As more heartworm infected dogs move into Arizona, more of the mosquitoes here become infected and pass the disease along to other dogs. Veterinarians across the valley are seeing more and more cases of heartworm. A blood test every other year, and monthly safe, effective heartworm prevention provide inexpensive protection against this devastating and fatal disease.