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Heartworm disease

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Heartworms are long, thin, white worms that spend their adult lives in the pulmonary artery near the heart, where they feed off nutrients in the blood. Heartworms interfere with normal blood flow and make the heart work harder. Initially, infected dogs show no signs of illness, but over time they develop heart disease associated with coughing, fatigue, and build up of fluid in the legs or abdomen.

Heartworm infection is most common in dogs, but cats can also be infected. Female heartworms produce immature forms called microfilariae, which circulate in your pet's blood until they are ingested by a mosquito, in which they continue to develop. When the mosquito bites another dog or cat, it infects the new dog or cat with these immature forms, which migrate to the pulmonary artery and develop into adult heartworms that start the cycle all over again.

Heartworms can cause severe disease such as congestive heart failure, and treating your pet for heartworms has additional risks. Treatment includes intravenous administration of an arsenic compound to kill the heartworms. However, arsenic can also be toxic to your pet, and your veterinarian will recommend blood and serum tests for your pet to check its kidney and liver function before beginning treatment. In addition, the arsenic must be given slowly (over a few weeks) so that large numbers of heartworms don't suddenly die and block the blood vessels. Your pet will also need to be kept quiet for several weeks to prevent smaller pieces of worms from falling off, traveling through the bloodstream, and lodging in the lungs.

Clearly, when it comes to heartworm disease, prevention is the way to go. Fortunately, prevention is very easy and effective. First, your dog should have a blood test every spring to check for heartworm disease. Then (assuming the test is negative), it should be placed on heartworm preventive during mosquito season. If you live in a hot or more tropical area where mosquitoes are present year round, then your pet will need preventive medication year round as well. Several highly effective preventive medications can be used to provide monthly protection. Your vet can help you select a preventive medication for your pet. Several common flea treatments also repel mosquitoes, further reducing your pet's risk.

 

Q&A

What are heartworms and how can they harm my pet?

Heartworms are worms that spend their adult lives in the pulmonary artery near the heart, where they feed off nutrients in the blood.  Heartworms interfere with normal blood flow and make the heart work harder, eventually causing serious heart disease.  

Can cats get heartworm?

Heartworm infection is most common in dogs, but cats can sometimes be infected.  

How do pets become infected?

Female heartworms produce immature forms called microfilariae, which circulate in the blood until they are ingested by a mosquito.  When the mosquito bites another dog or cat, it infects the new pet with these immature forms, which the develop into adult heartworms that start the cycle all over again.

How is heartworm infection treated?

Treatment includes intravenous administration of an arsenic compound over a series of days to weeks to kill the adult heartworms.  Arsenic can be toxic to your pet, so your veterinarian will recommend blood tests to check your pet’s kidney and liver function before beginning treatment.  

How can I protect my dog?

All dogs should be tested every spring, and negative dogs should be placed on monthly heartworm preventive at least during mosquito season (year round in warmer climates).  Several common flea treatments also repel mosquitoes, further reducing your pet’s risk.

 

Credit: Written and reviewed by John A. Bukowski, DVM, MPH, PhDand Susan E. Aiello, DVM, ELS
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