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Heartworm Disease Basics

Heartworm disease is caused by infection with Dirofilaria immitis worms. These worms live within the cardiovascular system, particularly the heart and the pulmonary arteries leading to the lungs. The presence of these worms in the circulation causes severe inflammation of the lung and heart tissue. In severe cases, heartworm disease can cause heart failure.


Heartworms are transmitted via mosquitoes. When the mosquito bites an infected animal, the mosquito ingests the heartworm larvae from the animal’s blood. The larvae in the mosquito require temperatures of at least 57 degrees Fahrenheit to develop into the infective stage. After the infected mosquito bites a pet, the larvae travel from the mosquito to the animal. The larvae, called microfilariae, grow for the next 6-7 months, when they reach sexual maturity. Development of the microfilariae within the animal depends on the presence of Wolbachia species bacteria, which acts symbiotically with the worms. Once the female worms reach sexual maturity, the proteins they produce can be detected by antigen tests, which are the most common in-hospital heartworm tests. This means that prior to the 6-month maturation period, your pet may test negative despite being infected due to the lack of mature heartworm proteins in the blood. Just because the infection can’t be detected yet doesn’t mean it’s not causing damage to your pet’s health. Damage to the heart and lungs can occur before the worms mature, meaning the damage can occur before we have a chance to detect it with a routine test! Without treatment, the worms can survive for up to 4 years in cats and 7 years in dogs, damaging the heart and lungs the entire time. Cats are atypical hosts, so their immune systems react differently to heartworm infections than dogs. They can develop severe respiratory disease from larva. Due to their small size and physiology, even one adult worm can be deadly for cats!


After a dog is initially infected, it can take over 6 months for the infection to be detected on routine antigen tests. It's important to note that the most common in-house tests only detect the presence of mature female worms. If an infection only consists of male worms, or immature females, the antigen test will not detect them. These antigen tests also do not produce consistent results in cats because of the low number of worms they harbor. Because of this, cats are not routinely tested for heartworm disease unless there is suspicion of an infection, at which point an antibody test can be performed to determine if the cat has been exposed to heartworms and developed antibodies against them.

To learn more, visit the American Heartworm Society website.


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