Heartworm Infections

Heartworm infections are very serious, and potentially fatal for dogs. Heartworms are a type of roundworm known as filarids. They live in the arteries, lungs, and heart of an infected animal. Many mammals such as dogs, cats, foxes, and even people can contract heartworms.

Heartworm infestation is spread by infected mosquitoes. When an infected mosquito bites a dog it desposits heartworm larvae under the skin. The larvae eat through the skin and connective tissue for up to two months before reaching the blood stream. Once in the blood, heartworms are transported to the arteries and lungs where they mature into adults. At six months of age, heartworms will begin producing offspring. Heartworms may live for up to seven years, with males reaching eight inches and female worms growing to twelve inches.

In reaction to the infestation, the organs heartworms inhabit become inflamed. Because this inflammation interferes with blood flow, the most common causes of death from a heartworm infestation are blood clots in the lungs or congestive heart failure.

Because heartworms are spread by mosquito bites, dogs with short hair are more susceptible to infection than those with longer hair. While heartworm infection is found throughout the United States, infection rates are highest (approximately 50%) in areas within 150 miles of the Atlantic and Gulf Coasts.

The only way for your dog to become infected with heartworms is through a mosquito bite. Dogs can not be infected by eating or coming into contact with dog waste that contains the eggs of heartworms. An infected dog cannot spread heartworms to another dog.

Heartworms are a dangerous parasite and can permanently damage a dog’s health. Even once treated, the damage the worms cause may be permanent. Please—if you have not done so already—put your dog on a preventative medication!

Diagnosing Heartworm Infection

Your veterinarian may use both blood tests and physical tests to determine the severity of a heartworm infection.

Blood Tests

The following two blood tests are used to diagnose heartworm disease:

  • Microfilaria Test
  • Heartworm Antigen Test

The Microfilaria test detects pre-larval heartworms in the blood. When female heartworms produce microfilaria, they produce antigens that the heartworm antigen test detects. It may take up to seven months after the infection has occurred before either of these tests provides consistently accurate results.

Physical Tests

Diagnosing heartworm infection using a physical examination alone can be difficult to perform accurately. Mildly infected dogs may show no signs of infection. Therefore, your veterinarian will likely perform a blood test in addition to a physical examination.

Severely infected dogs may show signs of heart failure, fatigue, coughing, rapid heart beat, enlarged liver, loss of appetite, fainting, or jaundice. Occasionally, heartworms may also be present in the eyes, abdominal cavity, and even the spinal cord.

Physical tests used to identify the extent of heartworm disease and to develop a prognosis include x-rays and ultrasounds performed on the heart and lungs.

Heartworm Treatment

Unless the infection is severe, heartworm disease can be successfully treated in dogs. Treatment involves injecting a drug called adulticide into the muscle tissue of the dog to kill the heartworms. Adulticide is the only FDA drug approved for the treatment of heartworms. Treatment presents some risk: the blood vessels may become filled with dead worms, which may block blood flow, causing death.

Severe cases of heartworm disease may be extremely difficult to treat, and one dose of adulticide may be insufficient to kill all adult heartworms. Six months after the adulticide treatment has been completed, your dog will need an antigen test to ensure that all heartworms have been killed. If the test is positive for the antigen, your dog will likely need another treatment.

It is very important that your dog not be allowed to exercise during treatment as your dog’s entire pulmonary system will be under significant strain. So let your pooch take it easy with a gentle recovery period.

We Are Not Heartworm Experts

We hope this article helped to educate you about how heartworms affect dogs and how serious heartworm infection is. But recognize that we are not veterinarians or the definitive experts on heartworms. Please talk with your veterinarian about the best way to keep your dog safe from infection or if you believe your dog may be infected.


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