That mosquito buzzing in your ear at night can drive you a little bit mad. But it can have far more serious consequences for your dog. Mosquitoes can carry heartworm larvae, and a bite from an infected insect could mean heartworm disease — and permanent damage to the heart, lungs, and blood vessels — for your dog down the line. A dog might not show any signs of infection, or he or she might cough, be lethargic, or lose weight inexplicably. Treatment usually involves a series of injections. The good news is that by following a heartworm prevention medicine regimen, it can be completely prevented.


Heartworm disease is a serious and potentially fatal condition that affects dogs, cats, and up to 30 other species of animals. It is caused by parasitic worms (heartworms) living in the major blood vessels of the lungs and, occasionally, in the heart. These worms are transmitted (as microscopic larvae) through the bite of an infected mosquito. The scientific name for the heartworm parasite is Dirofilaria immitis.

Heartworms are spread through the bite of a mosquito, and dogs increase the risk of infection for other dogs, cats and other animals. When a mosquito bites an infected dog it draws blood that contains immature heartworms (called microfilariae [pronounced micro-fill-air-ee-ay]). These microfilariae mature inside the mosquito to become infective larvae. When the mosquito eventually bites another dog or a cat, the larvae enter the new host. In dogs, these larvae often mature to become adult heartworms, which produce more microfilariae and continue the heartworm’s life cycle.

Heartworm disease can cause a variety of medical problems affecting the lungs, heart, liver, and/or kidneys. Severe complications can lead to death. Although a safe and effective treatment is available, it can be a costly and complicated process depending on how long the dog has been infected and how severe the infection is.

Despite the fact that heartworm disease is virtually 100 percent preventable, many dogs are diagnosed with it each year. The American Heartworm Society (AHS) estimates that one million dogs in the United States have heartworm disease today, and that this number may be rising.

Signs and Identification

Some dogs may show no signs of infection. However, depending on the number of worms and the duration of infection, dogs may begin to show the following clinical signs:

  • Persistent cough
  • Lethargy (tiredness)
  • Difficulty in exercising
  • Loss of appetite and weight loss

Apart from clinical signs, heartworm disease can be diagnosed using laboratory tests that check the dog’s blood for evidence of infection. Additional testing, including a CBC (complete blood cell count), blood chemistry panel, and urinalysis may be recommended for a heartworm positive pet to help assess the severity of the disease. Sometimes evidence of heartworms can be seen on ultrasound images or radiographs (X-rays) of the heart and lungs.

Affected Breeds

Dogs of any breed can become infected with heartworms.


If infection is detected early enough, heartworm disease can be treated before permanent damage to the heart, lungs, and/or blood vessels occurs. However, if the infection has been present for a long time or consists of a large number of worms, the risk of complications increases. In these cases, treatment can be more expensive and complicated and dogs may take many months to recover from the infection.

Standard treatment of heartworm disease in dogs involves administering a series of injections with a medication that kills the heartworms. Surgical removal of the worms may be recommended in some cases.

Unfortunately, untreated heartworm disease can be fatal.


Safe, easy-to-give, effective medications are available to prevent heartworm disease. Most heartworm preventive medications are administered as monthly oral or topical (spot-on) medications. There is also a product that can be administered as an injection every six months by your veterinarian. Whichever method of heartworm prevention is chosen, prevention is convenient and inexpensive compared to the dangers of the disease for dogs.

This article has been reviewed by a Veterinarian.