Dog And Cat Heartworm Disease

Heartworm University (HU) is a function of the American Heartworm Society, based in Wilmington, Delaware. It is a new feature of the Society and its program was presented six times in calendar 2009.

As a member and director of the Louisiana Academy of Veterinary Practice I was fortunate to be able to attend HU this week and I have some fascinating facts to share with you.

HU is designed as a team-teaching program, with one speaker a specialist in parasitology and the other a veterinarian who is a board-certified veterinary internist (internal medicine specialist). The Academy was fortunate to be blessed with Dr. Dwight Bowman, a parasitologist from Cornell University, and Dr. Clarke Atkins, a board-certified internist with a sub-specialty in cardiology. Dr. Atkins teaches at the North Carolina State University College of Veterinary Medicine.

A heart from a dog who died from severe heartworm infestation
Heartworm Disease is a completely preventible condition.

Heartworm disease is, of course, a preventible condition. Still, over 250,000 (that’s right, over a quarter of a million) dogs are treated for heartworms every year.

 Heartworms are believed to have been in North America for about 500 years, and probably came over harbored in the dogs of the Spanish conquistadores. Europe was probably the unfortunate recipient of Asian heartworms, where the parasite is believed to have originated.

Heartworms are naturally hosted in canids, including (in North America) dogs, wolves, coyotes and foxes. Unusual or aberrant hosts include cats and people. Heartworms commonly infect ferrets. All dogs, cats and ferrets, regardless of geographical location should be on a monthly heartworm preventive. The American Heartworm Society recommends that all pets take their heartworm preventive year-round.

Heartworms are transmitted by mosquitos, and a female mosquito must feed on blood before she can lay eggs. Male mosquitos don’t feed, they are useful only for reproduction. Mosquitos can live as long as a year, and adults will find protected places (including inside your house) to overwinter and begin a new population the next spring. Mosquitos ingest microfilaria, the offspring of the adult heartworm, and the microfilaria molt or undergo maturation through recognized stages within the mosquito. At the third larval stage, or L3, they are ready to complete the cycle and will enter a victim’s body from the insect’s proboscis to the saliva pool on the skin, then into the wound created by the mosquito bite. More maturation takes place within the host’s body, with maturation to full adulthood requiring about six months in the natural hosts, canids.

Adult heartworms find their way to the right ventricle of the heart, where blood flow pushes them into the pulmonary arteries, which lead from the right side of the heart to the lungs. When an infected host dies, and blood flow stops, some heartworms will simply “slip” back into the heart. Heartworms are so adept at finding their way that immature or young heartworms can be transplanted to other parts of the body and most will find their way again to the pulmonary arteries.

Aberrant hosts, however, might have heartworms almost anywhere. I once was performing an ovariohysterectomy surgery on a young cat and had a male heartworm crawl up at me through the incision, much like a viper transfixed by an Indian’s flute.

Male heartworms average 4-6 inches in length, females 10-12 inches. These are impressive parasites.

Weather greatly affects the time required for maturation of young heartworms. Once a microfilaria enters a mosquito, it can become an infective L3 in as little as one week at an ambient temperature of 89 degrees. At 68 degrees it can take as long as a month.

Microfilaria can live inside the host for as long as two years. Adult heartworms may live a natural lifespan of up to five years. It was once thought that they lived shorter lifespans in cats, but four year old cat heartworms have been documented.

Cats diagnosed with feline asthma may actually be undiagnosed feline heartworm hosts. Diagnosis of heartworms in dogs is straightforward in most cases. Definitive diagnosis of heartworms in cats is difficult in all but a very few cases.

The median survival time for cats diagnosed with heartworms is 1½ years. However, for those cats who survive the first 24 hours after diagnosis, the median survival time is four years.

Pulmonary hypertension, or increased blood pressure in the lung circulation, is a common finding in dogs with heartworms. Exercise is known to worsen this problem, which is one of several reasons heartworm treatment patients must be kept inactive. Working and hunting dogs with heartworms are known to have worse pulmonary hypertension and, if they die, microscopic study of their pulmonary arteries proves that they have more arterial damage than sedate dogs with an approximately equal heartworm burden. In cats, however, pulmonary hypertension is infrequently a problem. Why? Dr. Atkins thinks that at least one reason is because cats don’t exercise. Makes sense, doesn’t it?

As Paul Harvey used to say, “Now I’m about to tell you more than I know.” Wolbachia are bacteria that live in the adult heartworms and in their microfilaria. Wolbachia and Dirofilaria immitis, the scientific name of the heartworm parasite, seem to have a symbiotic relationship. Wolbachia need a place to live, and heartworms are made stronger and more difficult to kill when Wolbachia live within them. We know that Wolbachia can play a part in kidney damage, treatment failure, inability to remove microfilaria from patients and more, yet there is still much to be learned about the creature inside the creature.

In heartworm treatment, male heartworms are easy to kill. 100% will be killed by the first injection of adult-killing medication. However, as much as 85% of the female heartworms may survive the first injection, to be killed by the second or third. American Heartworm Society recommends the three-injection treatment for all treatable dogs, regardless of the severity of their heartworm disease.

We’ve come a long way from the heartworm treatments of the beginning of my career, when success occurred in about 85% of patients, 25% had complications and 2-5% died. Now, success approaches 98% of cases, 10% have complications and the fatality rate is below 1%.

That’s progress we can all applaud.

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