Heartworm facts

Does the idea of over two hundred spaghettilike foot-long worms living inside of you make your skin crawl?

That’s what happens to a dog that’s infected with heartworms, a potentially fatal disease caused by one thing: the bite of an infected mosquito. They’re called “heartworms” because they live and reproduce inside the animal’s heart, lungs, and associated blood vessels.

The life cycle of these worms is fairly straightforward. It starts when a mosquito bites a heartworm-infected dog. This infects the insect with what are called microfilariae, the offspring of an infected female heartworm. These microfilariae become infective larvae inside the mosquito; when that mosquito bites another dog, it spreads the larvae to that animal. In a few months, the larvae mature into adult heartworms, which mate. After mating, the females release their offspring into the dog’s bloodstream . . . and the deadly cycle is complete.

It’s important to note that heartworm disease is not contagious, and cannot spread from one dog to another, since the disease propagates only through the bit of an infected mosquito.

It’s extremely rare for humans to become infected: since 1941, fewer than 100 human cases have been reported.

Once inside a dog, a heartworm can live up to seven years. The average number of heartworms living inside an infected dog is 15, but that number can range up to 250 worms. The more worms there are and the longer the dog has been infected, the more severe the disease is.

You may not see any signs of the disease in its early stages, but as the number of worms increase, symptoms become obvious. They begin with an occasional cough and unusual tiredness after moderate activity, and progress to a persistent, heavy cough, trouble breathing and tiredness after just mild activity. It’s at this point that your veterinarian will be able to see changes in your dog’s heart and lungs on Xrays. In the last stage of the disease, blood flow to the heart is blocked by the worm mass. The only treatment at this point is surgery to remove the worms, but even then, most dogs who have progressed to this level will die. Even if your dog isn’t in that last phase, untreated heartworm disease will ravage the dog’s heart, lungs, liver and kidneys, eventually causing death.

While heartworm can be treated unless it’s in the advanced stage, it’s extremely hard on the dog, which must be kept as quiet as possible and not allowed to exercise for a full month following the final treatment. It’s also extremely expensive, requiring multiple vet visits, blood tests, x-rays, hospitalization, and a series of injections.

The disease began in the eastern U.S., but today it’s virtually endemic in most parts of the country, including Butte County. That means that as a responsible dog owner, you must take preventative measures. Although there is no vaccine against heartworm disease, there are several oral, topical, and injectable preventatives. These FDA-approved products are available by prescription, and are usually given monthly. It’s absolutely critical that you keep a strict regimen of monthly treatments, even in the winter. If you miss just one dose of a monthly medication — or give it late — it can allow immature larvae to molt into adult worms, which preventatives cannot kill. At that point, the only option is the arduous and costly treatment.

Finally, although heartworm medications are highly effective, no medication is 100% effective on all dogs and in all situations. That means it’s equally critical that your dog be tested yearly for heartworm disease.

Just remember the old adage: an ounce of prevention is worth a pound of cure!