Northwest Rankin Animal Clinic
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Pet Health : Parasites : Heartworms
HEARTWORMS

Heartworms are worms that reside in the canine and feline heart. Most preventable type of internal parasite. A mosquito bites an infected animal, usually a dog, and draws a small amount of blood in which tiny immature heartworm larvae - called microfilariae - start to circulate.
                         
Within two weeks, the heartworm larvae develope inside the mosquito.

The mosquito then bites an unifected animal, depositing these larvae on the skin.
 
The larvae migrate through the tissue and into the bloodstream of their new host, continuing to develope as they go.

Within 6 months, adult heartworms are present in pulmonary arteries! Let untreated, infection can lead to severe heart disease and even death.

Testing for heartworm disease and internal parasites is part of your preventative program Dr. Diffey has developed for your pet.

Dogs are considered the definitive host for heartworms (Dirofilaria immitis). However, heartworms may infect more than 30 species of animals (e.g., coyotes, foxes, wolves, domestic cats, ferrets, 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 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 (microfilaria) into the blood stream.

Offspring can be detected in the blood about six to seven months after the infective larvae from the mosquito enter the dog. The male heartworms and the females become fully grown about one year after infection, and their life span in dogs appears to average up to five to seven years. The number of worms infecting a dog can be very high, with the number of worms in dogs ranging from one to approximately 250.

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 and most significantly, the administration of heartworm preventives.

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.

Clinical Signs

Heartworm disease may cause a combination of medical problems in the same dog including dysfunction of the lungs, heart, liver and kidneys.

Diagnosis

The diagnosis of canine heartworm disease depends upon the following:
  • an accurate patient history
  • the recognition of varied clinical signs
  • the use of several diagnostic procedures that may include:
    • blood (serological) testing
    • microfilaria detection and differentiation
    • physical exam and clinical laboratory tests X-ray (radiology)

Preventives

While treatment of canine heartworm disease is usually successful, prevention of the disease is much safer and more economical. There are a variety of options for preventing heartworm infection, including monthly tablets, chewables and topicals. These products are extremely effective and when administered properly on a timely schedule, heartworm infection can be prevented.

The American Heartworm Society is now recommending year-round prevention, even in seasonal areas. One reason for this is compliance – to make sure the medicine has been given properly by the pet owner.

Before starting a preventive program, all dogs that could possibly be infected with mature heartworms should be tested.