Heartworm Disease

Heartworm Disease in Dogs

The first description of heartworm disease was in an article published in 1847. Since then, much more has been learned about this deadly parasite. Heartworms can infect more than 30 species of animals including domestic dogs and cats, ferrets, coyotes, foxes, wolves, wild cats, and even sea lions. HeartwormHeartworms can also infect humans, although they do not typically result in disease (most often they are an incidental finding at the time of autopsy). Infection in dogs is most common.

Heartworms are transmitted by mosquitoes. The mosquito becomes infected while taking a blood meal from an infected animal. It then carries and transmits the infective heartworm larvae (immature worms) to another dog. The larvae migrate through the body and bloodstream and eventually develop into adult heartworms that live in the heart, lungs, and associated blood vessels. The heartworms start to cause inflammation and other complications in these tissues. Mild disease may only cause a cough and/or some exercise intolerance. In advanced cases, however, the heart may enlarge and become weakened due to the presence of the worms. Unfortunately, death is a possible consequence of infection and can occur suddenly or from developing heart failure and/or lung disease.

The onset and severity of disease is a reflection of several factors. They include the number of adult worms present, the length of time the infection has been present, and the activity level of the dog. The number of worms infecting a dog can range from one to approximately 250 and their lifespan in the dog’s body is estimated to be at least five to seven years. In general, the more active a dog is, the more significant his or her symptoms. With this in mind, it can be expected that a very active patient with many worms that have been present for a long time would have more severe heartworm disease than a quiet dog with few worms and an infection which was detected very early.

It is important to note that all dogs in Michigan (regardless of age, breed, or environment) are susceptible to heartworm infection. We therefore recommend that all dogs receive monthly heartworm preventative with annual blood testing for heartworm disease.

Why test? We can only treat heartworm disease if we can find it, and annual testing allows us to identify positive dogs as early as possible, which generally results in a better prognosis. Even if you feel your dog is low risk because he or she already receives monthly heartworm preventative, testing is still important. Have you considered the consequences of a late or missed dose? Other factors may decrease the protective effects of the medication as well, including poor absorption from vomiting or diarrhea, and so on. So please don’t take any chances.

In addition to continuing with annual heartworm testing, the doctors here at the Goodison Veterinary Center want to remind you to continue your pet's monthly heartworm preventative once monthly all year round. This recommendation is backed by the American Heartworm Society, an independent organization dedicated to leading the veterinary profession and the public in the understanding of heartworm disease. Their latest research continues to confirm that warm weather seasonal use of heartworm preventative is no longer enough!

In Michigan, our variable weather means that we occasionally have warm fall and winter days when mosquitoes are active. It can also take a while for any mosquitoes trapped in the house to die off, so threats can remain both indoors and out. Remember, it only takes one single bite from one single mosquito to transmit potentially fatal heartworms to your pet.

Furthermore, the treatment for heartworm disease in dogs is to use an arsenic type drug called Immiticide to kill the adult heartworms. Immiticide is the only drug approved by the FDA for this purpose but it is currently NOT readily available. This means that treating your dog will be much more difficult, should he or she become heartworm positive. Prevention is a MUCH better option.

An added benefit of using these products year round is that many heartworm preventatives also protect against a range of intestinal parasites. These can be acquired from exposure to droppings from other pets or wild animals, hunting/eating wild animals, by licking grass, dirt, or snow, or even by brief contact with contaminated areas. These parasites are present all year and have the potential to be transmitted to humans. Please protect yourselves and your pets with year-round use of preventive medication.

These products are greater than 99% effective at preventing heartworm disease when used as directed. They are also safe and easy to use. We routinely stock Heartgard Plus, Interceptor, and Revolution and would be happy to discuss them with you. Remember, this fatal disease is preventable!

Heartworm Disease in Cats

Heartworm disease in cats is still fairly new to our clients, but over the past ten to fifteen years it has been increasingly recognized by the veterinary profession as a serious health threat. Heartworm disease in the cat is transmitted by the mosquito (the same way it is transmitted to the dog). The mosquito carries the larvae (immature heartworms) and injects them into the cat during its blood meal. The adult heartworms that develop then reside in the heart, lungs, and associated blood vessels.

In cats, the worms can also migrate to other areas of the body which does not commonly occur in dogs. Another difference between heartworm disease in dogs and cats is that cats are relatively resistant to heartworm infection. The odds of heartworm larvae developing into adults is low in cats (less than 25% of worms transmitted reach maturity) compared to dogs (40-90% of worms transmitted reach maturity). The number of adult heartworms infecting cats is usually low (fewer than six) and their lifespan is also shorter (typically two to three years). Microfilariae (infective heartworm larvae) are less common in the cat. This means that they are a poor reservoir of infection because it is less likely that they will spread the infection to feeding mosquitoes who will then pass the infection on to other animals. Unlike dogs, some cats even appear able to rid themselves of the infection spontaneously due to a strong immune response to worms.

The clinical signs of heartworm infection in cats can be very non-specific and may mimic many other feline diseases. Most cats with heartworm disease are free of any specific signs, but others may exhibit coughing, intermittent vomiting, asthma-like signs, gagging, or weight loss. Occasionally, infected cats die suddenly without warning.

The diagnosis of heartworm disease in cats is complex and multi-factorial. It is made based on both clinical signs and various tests which may include blood work, radiographs, and/or ultrasonography. This is in stark contrast to diagnosing heartworm disease in the dog which can be done with a simple blood test.

Although outdoor cats are at greatest risk of being infected, a relatively high percentage of indoor cats also become infected. Sadie, an indoor cat owned by Dr. Degner, became infected and spontaneously cleared her infection without treatment. Not all of these stories have happy endings, however, and because there is no heartworm treatment for cats currently approved in the United States, prevention is key.

For cats that are infected with heartworms but are not demonstrating clinical signs, no treatment may be recommended. Instead, close monitoring may be advised to allow the cat time to fight the infection and potentially self-cure. For infected cats with signs of heartworm disease, supportive therapy using steroids and other medications can be provided. Other medical treatments have also been investigated, though as mentioned above, none are currently approved. In general, their success rates have been low and risk of complications has been high. It is hoped that as more research is undertaken, promising options for treatment will surface. In the mean time, preventive medications like once-monthly Heartgard for Cats and Revolution could save your cat’s life.