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Heartworms And Your Dog

July 26, 2020

Heartworm disease in dogs is serious and can be fatal. The worms are spread through the bite of a mosquito. It is caused by a parasitic worm, Dirofilaria immitis, and once they find their way into your dog, they mature into adults, mate, and produce offspring. The worms are called “heartworms” because they live in the heart, lungs, and associated blood vessels of an infected animal. 

Lifestyle of heartworm infection

There is a distinct lifestyle of the heartworm. In an infected dog, adult female heartworms release their offspring, called microfilariae, into the dog’s bloodstream.  When a mosquito bites the infected dog, the mosquito becomes infected with the microfilariae.  Over the next two weeks, the microfilariae may become infective larvae while living inside the mosquito.  Microfilariae must pass through a mosquito to become infective larvae.  When the infected mosquito bites another dog, the mosquito spreads the infective larvae to the dog through the bite.  In the newly infected dog, it takes about six to seven months for the infective larvae to mature into adult heartworms.  The adult heartworms mate and the females release their offspring into the dog’s bloodstream, completing the lifecycle.

A heartworm’s lifespan in a dog is 5 to 7 years.  Adult heartworms look like strands of cooked spaghetti, with males reaching about 4 to 6 inches in length and females reaching about 10 to 12 inches in length.  The number of worms living inside an infected dog is called the worm burden.  The average worm burden in dogs is 15 worms, but that number can range from 1 to 250 worms. 

Tests to detect heartworms

A veterinarian will use one of two types of blood tests to check a dog for heartworms.  One is an antigen test which will detect specific heartworm proteins, called antigens, which are released by adult female heartworms into the dog’s bloodstream. The earliest this test will detect the proteins is five months after the dog is bitten by an infected mosquito. The other blood test can detect microfilariae in a dog’s bloodstream.  Microfilariae in the bloodstream indicate that the dog is infected with adult heartworms. The earliest that microfilariae can be detected in a dog’s bloodstream is about 6 months after it is bitten by an infected mosquito.

Frequency and timing of heartworm testing:

  • All dogs that are 7 months of age and older should be tested before being started on heartworm prevention and then annually.
  • If the owner forgot to give heartworm prevention and for how long.
  • If the dog is switched from one type of heartworm prevention to another.
  • The length of the heartworm season in the region where the dog lives.

Annual testing is important, even when dogs are on heartworm prevention year-round, to ensure that the prevention program is working. Heartworm medications are highly effective, but dogs can still become infected. If you miss just one dose of a monthly medication, or give it late, it can leave your dog unprotected.

If a dog is not tested before starting a preventive, and is heartworm positive, the dog will remain infected with adult heartworms until it gets sick enough to show symptoms. Heartworm preventives do not kill adult heartworms. Also, giving a preventive to a dog infected with adult heartworms may be very harmful.  If microfilariae are in the dog’s bloodstream, the preventive may cause the microfilariae to suddenly die, triggering shock in the dog and possibly death. 

There are four stage of heartworm disease:

  • Class 1:  No symptoms or mild symptoms such as an occasional cough.
  • Class 2:  Mild to moderate symptoms such as an occasional cough and tiredness after moderate activity.
  • Class 3:  More severe symptoms develop such as a persistent cough, and tiredness after mild activity.  Trouble breathing and signs of heart failure such as a swollen belly. For class 2 and 3 heartworm disease, heart and lung changes are usually seen on chest x-rays.
  • Class 4:  Can develop caval syndrome evidenced by labored breathing, pale gums, and dark bloody or coffee-colored urine. Without prompt surgical removal of the heartworm blockage, few dogs will survive.

Treatment of heartworm disease

Melarsomine dihydrochloride is an arsenic-containing drug used to kill adult heartworms in dogs. It’s given by deep injection to treat dogs with stabilized class 1, 2, and 3 heartworm disease. The treatment is potentially toxic to the dog and can cause serious complications. It is also very expensive.

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Prevention of heartworm disease

Preventative heartworm medications can be given as a pill, a topical medication or as an injection They all work by eliminating the immature (larval) stages of the heartworm parasite. This includes the infective heartworm larvae deposited by the mosquito as well as the following larval stage that develops inside the animal. Unfortunately, in as little as 51 days, immature heartworm larvae can grow into an adult stage, which cannot be eliminated by a preventive. Because heartworms must be eliminated before they reach this adult stage, it is extremely important that heartworm preventives be administered strictly on schedule (monthly for oral and topical products and every 6 months for the injectable).

By following strict preventative measures and having your dog tested annually for heartworm disease, you can keep your dog healthy and not infected with heartworms. For further guidance and information, please refer to the American Heartworm Society.

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