What is dog heartworm?
Dog heartworm is a disease common in canines throughout most of the United States. It is caused by a nematode (roundworm) that belongs to the family Filariidae. The worm, discovered in 1856 by the American parasitologist Joseph Leidy, has been given the scientific name Dirofilaria immitis. The adult worms live primarily in the heart and large vessels of the lungs. Adult female Dirofilaria measure from 9 to 16 inches in length. The males are a little more than half as long and are characterized by the cork screw turns of the posterior end often referred to as the "pigtail."
Both male and female live within the chambers of' the heart, usually extending through the valves. The worms prohibit proper valve closure as blood is pumped from one chamber to the next, seriously impairing the operation of the heart. In heavy infestations the wo
rms migrate up the pulmonary artery and clog the blood vessels of the lungs. The results are loss of body weight, dropsy, chronic cough, shortness of breath, muscular weakness, disturbances of vision, chronic heart failure, and eventual death. (See Figure 1.)
Figure 1. Tangled mass of adult worms in the right ventricle of an infected dog's heart.
What is the role of the mosquito?
Adult worms living within the heart produce minute organisms known as microfilariae that circulate in the blood stream of the dog. Microfilariae, so named because they are microscopic, are actually larval heartworms that are incapable of reaching the adult stage without first passing through a developmental stage in the mosquito.
Mosquitoes feeding upon an infected dog take up a number of these microfilariae with the blood meal. The freshly acquired microfilariae migrate from the digestive tract of the mosquito to the abdominal region where they undergo a transformation. Within 2 or 3 weeks they reach the infective stage. At this time the larvae are actually miniature adults that are small enough to live within a mosquito.
Next, they break into the body cavity of the mosquito and migrate to the mouthparts. By this time the mosquito is ready for its second blood meal. As the mosquito feeds upon its host, the infective larvae are deposited on the skin. These small worms burrow into the host animal and lodge in the tissue where they remain for several months. Worms that enter hosts other than canines generally die within several days.
After an increase in size, the worms leave the tissue and enter the blood stream through the wall of a small vein. Then they travel through the blood stream and eventually lodge in the chambers of the right side of the heart where they develop into mature heartworms. The entire cycle takes approximately 9 months. (See Figure 2.)
Remember that the microfilariae cannot complete their life cycle without first passing through a mosquito. Of the millions of microfilariae that are produced by the adult worms, only a very few ever get a chance to pass through a mosquito. The remainder circulate through the blood of a dog where they eventually die and disintegrate; however, they are continually being replaced with fresh microfilariae supplied by the breeding adults living within the dog's heart.
Figure 2 Dog Heartworm Life Cycle.