Heartworm 101
by Dr. Jennifer K. Stone
CCHS Staff Veterinarian

 Heartworm season is here, and pet owners should start preparing for the inevitable onslaught of the dreaded mosquito! Heartworm--a parasite transmitted by these little blood-sucking fiends--should be taken seriously by all dog and cat owners. It can be deadly, yet it is very easy to prevent. There are a variety of options for preventing heartworm infection, including daily and monthly tablets and chewables and monthly topicals. It is so easy to prevent and yet every year we get several dogs who have not received this preventative and have to be treated for the disease.

Many of our volunteers probably remember Nico, a rottweiler/lab mix with a very sweet temperament. We are very happy to report that Nico finally went to her forever home recently, but it took her a long time to get there. When Nico was relinquished to our shelter, she was obese and also heartworm positive. Luckily for her, we had the resources to treat her, but if she had been given the preventative that all dogs should receive then she would never have had to have the treatment and it wouldn’t have taken her nearly as long to get home.

The heartworm parasite is an actual worm that lives in the pulmonary vessels of the heart (the vessels that carry blood to the lungs to pick up oxygen). If the infestation becomes severe enough then the worms can actually live in the heart itself. As they increase in number the worms begin to take up space in the heart and pulmonary vessels causing high blood pressure, difficulty in breathing, and eventually death due to heart failure.

Heartworm is spread from animal to animal by mosquitoes. When the mosquito feeds on the blood of an infected animal, immature worms (called microfilaria) enter the mosquito. The microfilaria must then undergo an incubation period inside the mosquito during which the temperature outside must be constantly warm for several weeks. Because of this developmental period, danger of infection for pets begins in the spring after a few weeks of warm weather.

After the heartworm has reached the infective stage inside the mosquito, it is passed to a new host animal when the mosquito bites again. After entering the new host, microfilaria must spend up to five months maturing before they migrate to the pulmonary arteries. During this developmental period the worms cannot be detected, so veterinarians recommend testing for heartworm every spring in case infection occurred the previous summer. (This is also why we do not test any puppies for heartworm that are under 6 months of age. Their tests will always be negative, but puppies should be started on the heartworm preventative just like the adults to be sure that they do not develop the disease.)

Once the worms reach the pulmonary arteries, they grow and reproduce, releasing more microfilaria into the bloodstream. The next mosquito that bites this host animal then carries the heartworm microfilaria to its next victim, starting the cycle of infection anew.

Signs of heartworm include coughing, fainting, fatigue, and difficulty breathing. Animals may have heartworm for several years before showing any sign of the disease, and when signs finally occur, the heart and pulmonary arteries are often so full of worms that treatment becomes very risky.

Treatment for this disease can be just as taxing to the animal as the infection. The primary treatment available for heartworms is a form of arsenic administered at doses designed to kill the worms but not the dog. Although this treatment is safer today than in the past, there is still a risk that the animal may suffer complications, especially in those dogs that have large numbers of worms. The worst complication of the treatment is that as these worms are dying, pieces of them can break off and flow downstream in to the blood vessels in the lungs causing blockages and oxygen deprivation to a section of the lungs. This is a medical emergency that can be very difficult to treat, and it is more likely to happen with increased activity and increased blood pressure. This is why we usually send dogs that have been treated for heartworm into foster care. Occasionally we cannot do this and so from time to time you may see a sign that says that a dog has been heartworm treated and therefore cannot go out for walks. It is very important for all volunteers to follow these instructions for the safety of our patient!

In Nico’s case we had a wonderful foster who kept her for a whole month. Nico was a model patient and not only weathered the treatment without any complications, but as we understand it was one of the best house guests our foster has ever encountered!

Be sure that as you are doing your spring cleaning and getting ready for some summer fun that you think of getting your preventative and your yearly heartworm test done.  Your pooch will thank you!