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What are Signs of Heartworm in Dogs?

Mosquitos are more than just annoying – they’re harmful. Besides inflicting nasty bites that itch a lot, these flying mini-vampires transmit heartworms. April is National Heartworm Awareness Month, but this isn’t just a disease of springtime – it’s something you need to know about year-round.

What is heartworm?
Heartworm is a blood borne parasite called Dirofilaria immitis. The adult is a whitish worm that grows up to 14 inches long and resembles angel hair pasta. Adult heartworms live in large blood vessels near the heart. Female worms mate with smaller male worms to produce babies called microfilariae.

Adult heartworms stay in the pulmonary artery, but the microfilariae venture into the circulatory system of their canine host. While in the bloodstream, microfilariae are picked up when a mosquito bites to take a blood meal.

The microfilariae mature inside the mosquito, molting from L1 to L2 to L3 larval stages until they outgrow their mosquito incubator and seek better living quarters.

When the hungry mosquito bites another dog, it deposits L3 larvae on the surface of the skin. These larvae crawl into the dog’s body through bite wound. The L3 larvae migrate through the new host’s body while further maturing. As grown heartworms, they locate the pulmonary artery, set up residence, find a suitable mate and have babies of their own. The heartworm life cycle is complete.

What are the signs of heartworm?
Heartworms are heartless, harmful parasites. They not only affect the heart and major blood vessels, they also damage the lungs, liver and kidneys.

Infected dogs may experience exercise intolerance, breathing difficulties, or loss of appetite. Heartworms can cause secondary fluid accumulation in the lungs or abdomen, and clog large blood vessels like the vena cava, making a dog gravely ill.

How is it treated?
To successfully clear a dog of heartworms, both the microfilariae and adults must be killed. Heartworm treatment is a lengthy, expensive process that includes an injectable drug to kill adult worms, plus oral medication to kill microfilariae. During treatment dogs must be kept very quiet, because the dead and dying worms can cause even more problems as they break up, leave the heart, and get into the bloodstream.

Keeping heartworms out with preventive medication
Heartworm disease can be prevented. There are oral or topical monthly medications, plus a six month injectable product, that can stop heartworm development. The key is to start young pups on heartworm medication and be consistent with it throughout their lives. The American Heartworm Society and the Companion Animal Parasite Council, two groups of veterinary thought-leaders, advocate giving heartworm preventives all year round even in “cold” states. Every state in the continental US has heartworms.

Heartworms can be detected with a simple blood test; however, due to the many stages in the life cycle, more than one test may be needed to confirm that a pet is free of heartworms. Follow your veterinarian’s advice on heartworm testing and prevention.


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