Harlan Animal Hospital

Heartworms In Dogs:

Dogs are considered the definitive host for heartworms. However, heartworms may infect more than 30 species of animals (e.g., coyotes, foxes, wolves and other wild canids, domestic cats and wild felids, ferrets, sea lions, etc.) and humans as well. When a mosquito carrying infective heartworm larvae bites a dog and transmits the infection, the larvae grow, develop and migrate in the body over a period of several months to become sexually mature male and female worms. These reside in the heart, lungs and associated blood vessels. As mature adults, the worms mate and the females release their offspring (microfilariae), pronounced: (micro-fil-ar-ee-a), into the blood stream.

 

Offspring can be detected in the blood (pre-patent period) about six to seven months after the infective larvae from the mosquito enter the dog. The male heartworms (four to six inches in length) and the females (10-12 inches) become fully grown about one year after infection, and their life span in dogs appears to average up to five to seven years.

 

In experimentally induced infections of heartworms in dogs, the percentage of infective larvae developing to adults is high (40% to 90%). However, the percentage of experimentally infected dogs from which adult worms are recovered is virtually 100%. The number of worms infecting a dog can be very high, with the number of worms in dogs ranging from one to approximately 250.

 

Microfilaremia, the presence of heartworm offspring in the blood of infected dogs, is relatively common. However, not all heartworm infections result in such offspring circulating in the blood. These are known as occult heartworm infections and may be the result of a number of factors such as single sex heartworm infections, host immune responses affecting the presence of circulating offspring (microfilariae) and most significantly, the administration of heartworm preventives.

 

The onset and severity of disease in the dog is mainly a reflection of the number of adult heartworms present, the age of the infection and the level of activity of the dog. Dogs with higher numbers of worms are generally found to have more severe heart and lung disease changes. Until the number of mature heartworms exceeds 50 in a 25-kg dog (approximately 55 pounds), nearly all of the heartworms reside in the lower caudal pulmonary arteries (the arteries of the lower lung lobes). Higher numbers of heartworms result in their presence in the right chambers of the heart. In such infections, the most common early pathological changes caused by heartworms are due to inflammatory processes that occur in and around the arteries of the lower portion of the lungs in response to the presence of heartworms.

 

Later, the heart may enlarge and become weakened due to an increased workload and congestive heart failure may occur. A very active dog (e.g., working dog) is more likely to develop severe disease with a relatively small number of heartworms than an inactive one (e.g., a lap dog or couch potato). Occasionally, a dog with a large number of heartworms may not only have worms in the heart, but also in the caudal vena cava (large primary vein of the lower body) between the liver and the heart. If the heartworms are not removed surgically, this syndrome causes sudden collapse and death within two to three days.Canine heartworm infection is widely distributed throughout the United States.

 

Heartworm infection has been found in dogs native to all 50 states. All dogs, regardless of their age, sex, or habitat, are susceptible to heartworm infection. The highest infection rates (up to 45%) in dogs (not maintained on heartworm preventive) are observed within 150 miles of the Atlantic and Gulf coasts from the Gulf of Mexico to New Jersey and along the Mississippi River and its major tributaries. Other areas of the United States may have lower incidence rates (5% or less) of canine heartworm disease, while some regions have environmental, mosquito population and dog population factors that allow a higher local incidence of heartworm infection. Regions where heartworm disease is common have diagnosed infections in dogs as young as one year of age, with most areas diagnosing infections primarily between the ages of three and eight years. Although there are differences in frequency of infection for various groups of dogs, all dogs in all regions should be considered at risk, placed on prevention programs and frequently examined by a veterinarian.