Dogs and cats housed outdoors are at the greatest risk for contracting heartworm, but even indoor animals contract heartworm. Heartworm disease is caused by Dirofilaria immitus, a parasite in the nematode family. Dirofilaria immitus is transmitted by mosquitoes, which also serve as an intermediate host. An intermediate host is a host in which the parasite develops, but does not reach sexual maturity. The primary host is where the parasite reaches adulthood and is able to reproduce, passing the new organisms to other hosts. According to Merck Veterinary Manual, another interesting fact about heartworm is that it can be transmitted to humans, though there are no known cases of infection. Usually the larvae die in the lungs.
Life Cycle (Merck and Co., Inc, 2008)
- Approximately 70 different species of mosquitoes can transmit heartworm to pets.
- The cycle starts when a mosquito bites an infected host and acquires the L1 stage heartworm larvae also called microfilaria.
- The larvae develop in the mosquito to stages L2 and L3. This can take 1-4 weeks depending on the climate. The larvae will develop faster in warmer climates, especially those over 86 degrees.
- The mosquito bites a new host, and the larvae migrate to the new host and remain in the subcutaneous tissue for approximately two months, molting in to the L4 stage and finally becoming young adults (L5 stage).
- The young worms migrate through the tissues to the pulmonary arteries, which brings blood from the right ventricle of the heart to the lungs. When the worm burden is high enough, the worms will migrate into the right ventricle.
- 6-7 months after the new host has been infected, the female heartworm will start to produce microfilaria.
How Dirofilaria immitus Affects Pets (Merck and Co., Inc, 2008)
The adult heartworms irritate the walls of the pulmonary artery, suppress the immune system, and cause immune reactions, and lung disease. This results in lesions and scarring in the lungs, increased by physical exertion, and may even lead to congestive heart failure. The immune reaction causes the veins, arteries, and bronchi to constrict; resulting in lung inflammation, fluid in the lungs and damage to the cells that line the blood vessels and heart.
In cats and ferrets, the arteries also develop severe hypertrophy, or abnormally large growth. In addition to this a condition called thrombosis develops, which is characterized by blood clots and worms lodged in the arteries.
Classes of Heartworm in Dogs (Merck and Co., Inc, 2008)
Heartworm infection in dogs is broken down into 4 classes, based on symptoms and severity of the infection.
- Asymptomatic or mild: no clinical signs or radiographic changes, no lab abnormalities. May see a loss of body condition, decreased exercise tolerance and an occasional cough
- Moderate: occasional cough and mild to moderate exercise intolerance, slight loss of body condition, increased lungs sounds (crackles and pops), mild to moderate radiographic changes such as an enlarged right ventricle. Lab work will show: anemia, and protein in the animal’s urine.
- Severe: anemia, weight loss, exercise intolerance, tachypnea at rest, severe and persistent cough, difficulty breathing, fainting episodes, sneezing of blood or bloody mucus, and fluid in the chest cavity. Radiographs show an enlarged right ventricle and pulmonary artery, and changes in the lungs. Lab work shows anemia, a decrease in platelets, and protein the urine.
- Caval Syndrome: sudden onset with collapse, bloody urine and difficulty breathing. This class is usually fatal without immediate surgery.
For more information about the symptoms and treatment of heartworm read: How to Protect Your Pet From Heartworm Disease.
Work Cited
Merck and Co., Inc (2008). Heartworm Disease: Introduction. Retrieved November 3, 2008 from The Merck Veterinary Manual. www.merckveterinarymanual.com.
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