Heartworm Disease in Dogs

Pet owners in the Northeast United States are unfortunately already familiar with Lyme Disease, but there is another vector-borne disease* that is growing in incidence in the area: Heartworm Disease. Whereas Lyme Disease is a bacteria transmitted by ticks, Heartworm Disease is caused by a parasite (Dirofilaria immitis) that is transmitted by mosquitoes. These worms will infect dogs and live in the heart and the blood vessels going from the heart to the lungs. This will cause heart and lung failure and damage other organs such as the liver. Around 1 million dogs in the United States are living with heartworm at any given time, with approximately 100,000 new cases being diagnosed each year. These numbers are likely underrepresented because they only include dogs who were tested for heartworm, meaning the true number is likely greater. While most of the cases occur in the Southeast of the U.S., heartworm positive dogs have been found in all 50 states. In addition to dogs, heartworms can live in cats, ferrets, wolves, coyotes, and fox, making it a growing problem in areas where mosquitoes are not active year round. 

The heartworm lifecycle relies on the mosquito for transmission – a mosquito will feed on an infected animal and ingest the young worms that are circulating in the blood stream. From there, the young worms will mature somewhat inside the mosquito over the course of 10-14 days, and then will be deposited into a new host when the mosquito bites another animal. Then the worms will travel to the blood vessels in the heart and lungs and continue to mature into adult worms. This whole process takes 6-7 months.  Adult heartworms are 6 to 12 inches long and can live for 7 years.

Signs of Heartworm Disease will not occur for at least several months to years after the initial infection, and when signs do occur, it often means the disease is quite advanced. Common signs include lethargy, decreased appetite, labored breathing, coughing, and weight loss. Heavy worm burdens and chronic cases will lead to life threatening conditions such as heart failure and blockage of the blood flow within the heart.

Heartworm Disease can be treated in dogs using several methods and medications. However, the process can be tedious, expensive, and dogs can develop life threatening reactions during treatment. Thankfully, Heartworm Disease is preventable using medication that kills the heartworms before they have a chance to infect. There are several products effective at preventing Heartworm Disease – most come in oral pills, topical liquid, or injections. These products are safer and less expensive than treating Heartworm Disease.

Cats are a different story that will be told in another article. Briefly – cats can become infected with heartworms, and it can be life threatening without any outward signs of the disease. Testing is tricky in cats and the infection can often be missed. Also, there is no treatment for Heartworm Disease in cats, making prevention the only way to avoid the disease.

While Heartworm Disease in the Northeast U.S. may not be as prevalent when compared to other parts of the country, it is a growing disease with more resident dogs being infected due to the number of heartworm positive wild animals. This means that routine testing and prevention are strongly advised to avoid life threatening and costly sickness.

*A vector-borne disease is one in which one living organism transmits a pathogen (i.e. virus, bacteria, parasites) to another living organism. In this article, the mosquito is the vector, transmitting a parasite (heartworms) from one animal to another. This means the disease cannot transmit directly between two animals without the use of the vector.