From Janet Tobiassen Crosby, DVM,
Your Guide to Veterinary Medicine.
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Most pet owners have heard of heartworms. But what do heartworms look like and how are they transmitted from animal to animal? Learn the answers to these questions and more in this FAQ.

A. Most pet owners have heard of heartworms. But what do heartworms look like and how are they transmitted from animal to animal? Learn the answers to these questions and more in this FAQ. heart heartworm coughing vomiting worm dirofiliaria larva larvae worms mosquito mosquitoes heartgard medicine vet veterinarian anaphylaxis lungs cough anorexia lethargy treatment disease dogs cats horses monthy pill preventive preventative interceptor What are Heartworms, and can I catch them from my dog? Heartworms are large worms that live in the hearts of dogs and cats. They are also found in other species; including ferrets, foxes, wolves, sea lions, and horses. Dogs are the common host for this parasite. Humans are not a natural host for heartworm, but there have been a few rare reports of human cases. In humans, the heartworm is usually found as a single worm in the lung versus the heart. Heartworm is also known as Dirofilaria Immitis. It is a long, spaghetti-like worm that can be anywhere from 6 to 14 inches in length (~17 - 27 cm). Female worms are longer than the male heartworms.

Heartworms cannot be spread animal-to-animal; heartworms need a mosquito to complete their life cycle.

• A mosquito bites a heartworm-infected animal.
• The mosquito is then carrying microscopic versions of the heartworm, called microfilariae.
• When the mosquito bites another dog or cat, that animal is now infected with the heartworm microfilariae.
• Within 70 to 90 days, the microfilariae have made it through the tissues to the animal's heart, where they reproduce (providing both male and female worms are present) and live for several years. If both sexes of worms are present, they will be producing their own little microfilariae within 6 - 7 months after that mosquito bite.
• The cycle continues.

The best defense against heartworms is prevention. If you live in an area that has heartworm, a once a month chewable tablet will protect your pet from heartworm and other parasites. Please see your veterinarian to discuss what heartworm preventive is best for your pet.

Photos of heartworms
(warning some photos are graphic - not for the squeamish)

Dirofilaria immitis (canine or dog heartworm)
Heartworms in a dog (cadaver)
Heart with large number of heartworms (specimen)
Feline Heartworm Disease
Specimen photos, radiographs, ultrasound images and heartworm incidence maps (US).