Heartworm Disease

Its that time of year again. The birds are chirping, the flowers are starting to bloom, and a veterinarian’s thoughts turn to--heartworm prevention. I have the feeling that in warmer climates many dog owners could give the heartworm speech as well as their veterinarians, (if you just say the words "Florida" or "Texas" too loudly your dog may get heartworm disease) but here in Colorado I find that there are many people who have heard that heartworm prevention is an important thing for their dogs, but they tend to be a little foggy on the reasons why. So here is the tale of a parasite that goes overcomes much adversity before reaching its final destination inside the heart of a dog where it hopes to live happily ever after, and how we can prevent it from ever getting there.

This story starts inside the guts of a mosquito who has just bitten a dog that is infected with heartworms. Along with blood the mosquito has ingested a tiny worm that is only a little longer than the diameter of a red blood cell. That worm makes a cozy home inside the mosquito, where it must mature to a form that can be injected into a new dog when the mosquito bites again. This metamorphosis is very temperature dependent for the worm, and if the temperature drops below 56 degrees F the development of the worm is suspended until it warms up again. For this reason there is less heartworm disease in cooler climates like Colorado than in warmer areas because the mosquito’s life span may end before the worm has had enough warm time to mature to the point it can be injected by the mosquito into another host. Don’t worry for the heartworms, however. There are still plenty of warm days out here for some of them to get through.

When a worm is ready it makes its way into the salivary glands of the mosquito and is deposited under the skin when the insect bites its next victim. If that victim happens to be a horse or a person or a Preble’s Jumping Mouse the unfortunate worm gets lost and confused and just dies where it is. If the next victim is a dog, however, the lucky worm will feel right at home and will immediately start on the next leg of its journey.

Over the course of about five months the worm migrates through the dog’s body, going through various stages of development, until as a mature adult it reaches about three inches in length and sets up permanent housekeeping in the heart. Once established in the heart the worm settles in and starts pumping out gazillions of microscopic larvae that float around in the dog’s bloodstream hoping to be picked up by a hungry mosquito where they can start a new life of their own.

It is not difficult to imagine that it wouldn’t take too many adult worms living inside your dog’s heart before that heart might not be working so well. In this modern age of medicine there are quite a few safe, effective, and easily dosed products that can be given to dogs on a monthly basis to prevent the immature forms of the heartworm larva from surviving to adulthood and damaging the heart. Unfortunately, none of these products has much effect on adult heartworms that are already established in the heart, and those adult heartworms can cause enormous amounts of damage before any clinical signs are seen. For this reason is we do heartworm tests in association with heartworm prevention. This sensitive blood test can detect the presence of adult heartworms long before any clinical signs develop. Although it is always better to catch it before a dog is sick, even when a dog is showing signs of heart disease as a result of heartworm infestation there is a special treatment that will kill the adult worms. Unfortunately the treatment carries a high risk for complications that may be life threatening, and even after the worms are gone we are still left with the damage that has been done to the heart.

In spite of the fact that the chance that your dog will develop heartworm disease in Colorado Springs is considerably lower here than in much of the rest of the country, we still recommend heartworm prevention because it is better to be safe than sorry. If someone developed a tasty pill with no significant side effects that I only had to take once a month which would virtually guarantee that I would never have a stroke I would probably consider that a good investment in preventative medicine, even if I wasn’t in a high risk category for having a stroke. The same principle applies to heartworm prevention. There is no reason why any dog should ever have to die from heartworm disease again, yet thousands do in spite of the many options we have now to eliminate it as a threat to our dogs. Don’t let your dog be one of the unlucky ones.