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 worms 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.
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.
Since symptoms of dog heartworm vary considerably in different animals, a veterinarian is the only person qualified to give a proper diagnosis. Many dogs show the first visible indications of infection only after the disease has progressed to the point where treatment is no longer feasible. Active dogs generally show a tendency to tire easily. Shortness of breath or repeated coughing are usually positive signs of heartworm infection. Hunting dogs are often no longer able to keep up the rapid pace of the chase and frequently drop from exhaustion. Jaundice, convulsions, and disturbances of vision may occur in extreme cases. Emaciation often precedes death. All dogs with adult heartworms should show some microfilarial activity in the peripheral blood. A blood test made by a veterinarian is the only way to determine whether or not your dog has heartworm disease. Since microfilariae generally appear in the blood within 9 months after an infection occurs, mild cases of the disease can and should be detected and treated long before any symptoms are evident.
Figure 3. Heavy infestations of dog heartworm migrate up the pulmonary artery and clog blood vessels of the lungs.
It was originally thought that long-haired dogs were less susceptible to heartworm infection because of the improbability that mosquitoes would be able to penetrate through the hair for a blood meal. It has since been shown that this line of thought is erroneous. Mosquitoes usually have difficulty penetrating even short hair. For this reason most feeding takes place on the underpart of the abdominal region. Since even long-haired dogs have relatively little hair on this area, mosquitoes have no difficulty feeding upon them. Limited feeding also occurs on the ears or muzzle area where the hair is matted down. Hunting breeds or breeds frequently run at field trials may often appear to be more susceptible to heartworm infection. Because these dogs spend a great deal of time in the field, they are probably fed upon by mosquitoes to a greater extent than are some other breeds. Also, hunting dogs are usually run in an area frequented by other canines. Such an area becomes a point for dispersion of the disease once the mosquitoes in the area become infected.
Although isolated human infections have been reported, dog heartworm is not presently recognized as a human health problem. The parasite, however, is by no means limited to the dog. The fox, wolf, and raccoon have been shown to be reservoirs, and evidence implies that the skunk, muskrat, and opossum may be infected as well. Although the household cat is capable of harboring Dirofilaria immitis, infected cats are rarely found. Evidence suggests that mature heartworm adults maybe rendered sterile in animals other than canines. If this proves to be true, blood tests on household cats would show no microfilarial activity even though the adult worms are present.
Infected canines apparently jeopardize only the health of other dogs. One infected dog serves as a focus from which other dogs are inoculated. An infected dog sleeping in an open kennel would be frequently fed upon by mosquitoes. These same mosquitoes might later return to feed upon the same dog, thus building up his own infection, or they might choose to feed upon a neighboring dog, thereby producing a new infection.
Figure 4. Here's the troublemaker. Remember that the dog heartworm parasite must pass through a developmental stage in a female mosquito before it can be transmitted to your dog.
Dog heartworm can be cured by chemical therapy if the disease is diagnosed early enough. Most chemical treatments are designed to kill and slowly disintegrate the adult worms over a period of time. If all of the worms were killed at one time, the dead bodies would flush into the lungs and asphyxiate the infected dog. Pet owners should realize that the chemicals used to combat heartworm infections are toxic to the dog as well as the parasite. The powerful chemicals needed to kill the parasite must be administered with utmost precaution.
Surgical correction of the disease is possible and may be feasible in many cases. Your veterinarian can help you decide what should be done.
Drugs have been developed that will prevent dogs from contracting heartworm. The drug attacks the immature stages of the parasite in the dog and prevents the worms from maturing to the adult stage. Therefore, a dog that is on medication can be repeatedly infected during the mosquito season and remain free of heartworm.
The medication available prior to 1983 was quickly excreted by the animal and had to be administered daily throughout the mosquito season. A new drug is now available in tablet form that provides protection for one month. Preventative medication can cause complications in animals with established heartworm infections. As a result, the drugs are available only through veterinarians and should be administered under their directions. The use of preventative medication does not exclude periodic blood tests. The combination has saved the lives of many dogs.
Protecting dogs from mosquito bites may be necessary in areas where mosquito populations are high. Screening the dog's sleeping quarters is essential to prevent repeated bites. Repellent sprays have only a limited effect when the dog is in the open. Preventative medication should be used, especially in areas of known infection.
Figure 5. Regular checkups by a veterinarian can help control dog heartworm.
A periodic blood test is the most dependable precautions measure because it will reveal an early infection in an apparently healthy animal. Because a pet appears healthy is no reason to believe that it does not have the disease. Your dog could be the carrier responsible for an entire local outbreak.
Mongrel or purebred, isn't your dog's well-being worth a visit to your veterinarian every year?