Heartworm Disease

Heartworm disease. While most of us have heard of this in reference to a shelter pet or a stray, this disease is much more prevalent in the family pet setting than it would seem. The more that people understand about the disease process and ways to prevent the disease, the fewer pets will have to struggle with it. While most people know that this is a problem occurring in dogs, not as many know that it can affect feline pets as well. There are many common ideas about how heartworms are spread and how we can test for them that are incorrect. Most importantly of which is you can’t purchase heartworm prevention over the counter at a pet store, and even indoor pets can get heartworms.


Heartworms, as the name suggests are parasitic worms that live in the heart of the host animal; usually our domesticated dogs and cats, wolves, foxes, and coyotes. Adult heartworms can reach up to 14 inches in length! Mosquitoes are the way that these heartworms are transmitted from animal to animal. During an active infection in an animal, the adult heartworms live in the blood vessel that takes freshly oxygenated blood from the lungs and distributes it through the body. They do not travel through the body. However, the microscopic immature worms (called microfilaria) do travel through the blood stream. When a mosquito feeds on an infected animal, they can actually intake some of these immature heartworms. The next time this mosquito feeds, these microfilaria will be injected into the animal during the feeding process. From the time of infection, it takes about 6 months for the microfilaria to become the large adult worms. The only way to check for a heartworm infection is a blood test that checks for antibodies the body has made in response to female heartworm antigens that are secreted into the blood stream. Since it is checking for this antibody, if there are only the microfilaria present, the test will come up a false negative.


There are parts of the United States (primarily out West where it is dry and warm) where heartworm prevention is considered ‘unnecessary’ as the populations of mosquitos is so minimal the chance for infection isn’t considered a risk. Naturally the populations of mosquitos are highest in the Southern states where there are a lot of the boggy areas where mosquitos breed. However, according to the American Heartworm Society, heartworms have been diagnosed in pets in every one of the 50 states. They recommend a “think 12” mindset: pets, especially canines, need to be tested every 12 months and kept on heartworm prevention all 12 months of the year. Even though the chance of infection is lower during the winter months, it only takes one mosquito to cause this deadly disease, and if one gets in the house this could be enough to cause a problem. The American Heartworm Society takes averages for every part of the country every three years to see how heartworm positive cases are trending in each state. In 2001, Indiana veterinary clinics saw on average 26-50 heartworm positive cases per year. Since 2013, this has increased to 51-99 heartworm positive cases per clinic per year. This change can be based on a few criteria such as how many heartworm positive hosts are in the environment and how many pets are on heartworm prevention. Lower income areas will be less likely to keep their pets on preventions and more secluded areas will have more coyotes and foxes that can act as natural vectors for heartworm disease.


As far as signs of infection go, pets can have absolutely no signs while initial infection and damage is being caused. In essence, heartworm disease causes heart failure signs in canine patients. This makes sense as the heartworms are in the heart and surrounding vessels, which in turn makes the heart have to work harder to pump blood around the partial blockages caused by these worms. Once the number of worms has started having a negative impact on the function of the heart, an owner may notice signs at home such as breathing hard, and inability to run or play like they used to be able to. As the disease progresses, the clinical signs become more similar to heart failure as that is technically what is being caused. This can include coughing, a distended and fluid filled abdomen, lethargy, and inappetance. Cats usually have a very different range of symptoms including but not limited to asthma-like symptoms, vomiting, difficulty walking, fainting, seizures, and sudden death. Treatment in our dog cases is the only way to prevent even more cardiac and organ damage.


As far as treatment for a heartworm infection goes, there is only one medication that is currently approved to be used in dogs. This medication, however, is really more of an insecticide and comes with it’s own set of risks and side effects. Most protocols require two or three injections usually spanning a month. When the heartworms die, they can cause clots in the lungs and through the body. Treatment for heartworms is required to prevent severe damage to the heart and other organs but it is not to be taken lightly. Unfortunately, there is no treatment for our feline pets, which is why prevention is so important. The average cost of treatment for a 60-pound dog is $1,300-1,500. At the average online pharmacy, 12 YEARS of Revolution, which is an all-in-one heartworm, flea, and intestinal parasite control, is $2,100. Heartgard, basic heartworm prevention only costs $1,200 for a 12-year supply.


Heartworm disease is a completely preventable problem that is very difficult to treat. For the cost of treatment, an owner can purchase years worth of prevention. Yearly testing is also very important, as it can be dangerous to give heartworm prevention to a pet that is heartworm positive. As members of our family, we try to do everything possible for our pets. Heartworm prevention is a simple thing we can do to keep our pets healthy and comfortable.