Heartworm in Dogs—the Big Picture

What is Heartworm?

Heartworm is a worm that is about the size of angel hair pasta that grow inside the right side of the heart and major blood vessels of the lungs.  Dogs, wild canids (fox, coyote, wolves), ferrets, cats, some other small mammals, sea lions and even very rarely people can become infected with heartworm.  Worms grow to adulthood most readily in dog and dog-like species.  Adult worms living in the heart will produce microfilaria (baby worms) that are transmitted via mosquito.  Adult worms can grow up to 12 inches long and live 5 – 7 years!

Heartworm disease refers to the illness caused by a heartworm infection.  Initially, an infected dog may have no signs of illness.  As heartworm progresses, damage to the lungs and heart will lead to cough, exercise intolerance, decreased appetite and weight loss, and ultimately heart failure.  Occasionally heartworm can contribute to kidney damage as well. 

The current recommendation of the American Heartworm Society, a veterinary organization that focuses on best way to contend with heartworm disease, is to keep pets on heartworm preventative all year around.  This recommendation is due to the many complex factors that contribute to the unpredictability and spread of this disease.  Prevention is safe, effective, and well tolerated.  

Important things to know:

1)  The mosquito is a necessary vector (carrier) for heartworm disease.  A very early stage of the heartworm development occurs in the mouthparts of the mosquito.

2)  Current tests can only detect adult heartworms living inside the heart. Following a bite from an infected mosquito, it takes 6 months for a heartworm (microscopic baby worm carried by the mosquito) to develop into an adult in the dog. 

3)  Heartworm has been diagnosed in all 50 states.  While transmission has not been documented in Alaska, there are brief times when this northern climate can support transfer of the disease. 

4)  Heartworm is on the move.  Some areas of the country have more heartworm than others.  The southeastern United States has consistently documented high infection rates.  Movement of dogs from these areas contributes to the spread of infection.

5)  “Heat islands” associated with buildings and parking lots of urban areas may extend active mosquito seasons and transmission of heartworm disease. 

6)  The course of treatment for heartworm disease takes many months.  In fact, dogs need to be restricted from activity for about 5 months during treatment due potentially severe lung embolism (clots) from dying worms. 

There are many great sources for pet owners about heartworm.  Here are some links to learn more:
 

The American Heartworm Society:    https://www.heartwormsociety.org/pet-owner-resources/heartworm-basics

The Companion Animal Parasite Council:  http://www.capcvet.org/capc-recommendations/canine-heartworm

The American Veterinary Medical Association:  https://www.avma.org/public/PetCare/Pages/Heartworm-Disease.aspx

Heartworm disease in cats manifests itself very differently and is deserving of a separate discussion. 

Sources:  The American Heartworm Society, the American Veterinary Medical Association, and the Companion Animal Parasite Council.

article written by:

Laura Rau-Holl, DVM

Wolf Merrick Animal Hospital, Kenosha, WI