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Beware of Mosquitoes and Heartworm DiseaseWhen dog owners hear the word heartworms, it always evokes emotion. If nothing else, it means spending money on a test (usually $40-50) and preventative medication ($60-125/year) for something you can't see. If that doesn't make the hair stand up on the back of your neck then nothing will.

The question we hear every day is "Is it important enough to warrant the expense?" The answer is clearly yes. Here's why.

Canine Heartworms is a parasitic disease that, in most cases, is fatal if left untreated. The cause is a worm with a very complicated life cycle (birth to adult) that involves not only your dog, but another member of the dog family (coyote, raccoon, etc.) and the lowly mosquito.

It is not directly contagious from one dog to another as we will see next. I think once you see how easily YOUR dog can get heartworms, you'll understand why it is so important to prevent at all costs.

Heartworm Lifecyle:  Adult -> Microfilaria -> Larvae -> Adult

So let's follow the life of the typical heartworm and her babies.

  1. The adult heartworms look like spaghetti. They are 7-11 inches long, and live in one of the heart's chambers. There are both male and female heartworms.

  2. The females give birth to thousands upon thousands of very tiny babies after mating. The babies, called microfilaria, are pumped out of the heart into the circulation. They are so small that they fit into the tiny capillaries in every part of a dog's body. They never grow up into more adults, but are sucked out of the blood by mosquitoes when they bite a dog. They stay in the mouth of the mosquito for about 10 days, molting (changing form) during that time.

  3. When the mosquito then bites another dog, the new form of the microfilaria are left on the skin, where they then migrate (crawl) into the hole left behind by the mosquito bite. Now the fun starts. These tiny babies, now called larvae, begin to migrate toward the heart of your dog. That trip takes 6 months.

  4. On the way, they molt several times until they reach their destination, then find their way into the heart where they complete development into adults, and the whole cycle begins again. Try to imagine 100 or more of these creepy things migrating through your dog! Perish the thought.

Symptoms of Heartworm Disease

The disease that then develops by having all of these worms in the heart is slow to develop. Over the next year or two, the heart will enlarge and become a less efficient pump, causing the signs of congestive heart failure, which include:

  • coughing
  • weight loss
  • lethargy
  • abnormal breathing

They affect the circulation so badly that the kidneys and liver can become compromised as well, adding to the symptoms. Death occurs due to heart, kidney, or liver failure over the next few months to a year.


Treatment is not only difficult for your veterinarian, but hard on the patient as well. Most dogs survive the harsh treatment, but some are never the same afterwards. Before treatment, total medical evaluation is necessary to determine what harm has been done. This will include x-rays, blood tests, and urine tests. Since these dogs are ill, other diseases are often present at the same time, making life and treatment just that more difficult for your dog. Without getting into details, suffice it to say that treatment is expensive.

So what should you do? There re only two simple rules.

  1. Keep your dog on heartworm prevention once a month, all 12 months of the year (heartworm medication also helps prevent intestinal parasites).

  2. Have your pet tested for heartworm every year to be sure that the disease did not sneak in despite your best efforts of prevention.

Never believe that any prevention is 100%, although the heartworm program is pretty close.

If your dog does show positive for heartworms, don't panic. Have him evaluated immediately and begin treatment as soon as possible. Unless the disease is in advanced stages, the outcome will most likely be a good one.