Pet Wellness - Heartworm


Heartworm is a serious, life-threatening disease in dogs. It occurs in the presence of the adult stage of the parasite Dirofilaria immitis in the pulmonary arteries and right ventricle of a dog's heart. Until the early 1970s, heartworm in the United States was primarily confined to the southeastern part of the country. Today, it is found almost everywhere.

Transmission of heartworm depends upon the mosquito population of an area – the more mosquitoes in an area, the greater the chance of heartworm transmission to animals. About 70 species of mosquitoes are capable of transmitting the disease.

Heartworm disease occurs most commonly in dogs, but is also a major cause of heart disease in cats. Heartworm also infects wild animals, such as coyotes, wolves and foxes. When the wild animals in a particular area are infected, the disease is permanent.

adult heartworm

The adult heartworm is 6-14 inches in length. It is thread-like, white in color, and primarily found in the pulmonary arteries and right ventricle of a dog's heart. When adult male and female heartworms are present, mating occurs, and the female releases large amounts of small, microscopic "microfilaria" into the bloodstream.


Circulating microfilaria can live up to two years in a dog's bloodstream. In turn, several microfilaria are ingested by a mosquito when it bites a dog. The mosquito then serves as an intermediate host as well as vector (the transmitting agent) for the disease. The mosquito spreads the disease to another dog by injecting the microfilaria at the time of the bite.

In order for the microfilaria to become infectious, they must develop inside the body of the mosquito. This development occurs only under certain environmental conditions. Two weeks of temperature at or above 70 degrees F is required. As a result of this temperature requirement, transmission of the disease is limited to the warm months.

After the microfilaria have gone through their development, they are ready to infect a new victim. During a blood meal (mosquito bite), the mosquito injects the microfilaria into a new dog. These small, microscopic worms migrate under the skin and eventually enter the dog's blood stream. About 6 months after the initial mosquito bite, the microfilaria arrive at the heart. The final maturation and the mating of the heartworm occur in the pulmonary arteries. The adult worms live in the pulmonary arteries and right side of the heart, where they can survive for several years.

Adult heartworms cause inflammation and thickening of pulmonary arteries. As time passes, more arteries become inflamed and clots begin to appear. The blocked pulmonary vessels lead to increased blood pressure, which places a strain on the right ventricle of the heart. Eventually, heart failure occurs.


Clinical symptoms of heartworm disease develop very slowly. Often, symptoms are not noticeable until 3 years after the initial infection. Most of the early symptoms are associated with increased work load for the heart, including lack of energy and exercise intolerance. Chronic coughing and difficulty breathing are also common symptoms of heartworm disease. As the disease progresses, most dogs develop congestive heart failure and ascites, and may collapse in the final stage of the disease.

Not only is heartworm dangerous for your pet, but the treatment for heartworm disease is dangerous for your pet as well. Therefore, administration of preventive medication is the best method for keeping a dog free from heartworm disease.

Crossroads Mobile Vet uses and recommends Heartgard® Plus to help keep your pet healthy and free from heartworm.

Heartguard Plus

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