At a Glance

Heartworm (Dirofilaria immitis) infection in dogs has been diagnosed around the globe, including all 50 states within the U.S. Heartworm can only be spread by the bite of an infected mosquito. When a dog or cat is bitten by an infected mosquito, a number of baby larvae (microfilaria) are deposited on the skin next to the bite. These larvae migrate through the fresh bite wound and enter the blood stream eventually reaching the heart and blood vessels of the lung.  The microfilaria mature into adults (10-12 inches) in 6-7 months. The adults are sexually mature and females begin producing microfilariae of their own which circulate in the blood stream and are available to other mosquitoes that bite the pet. The microfilaria incubate in the mosquito for 10-14 days and become infective and are passed to the next pet bitten by the mosquito.  Because heartworms can live for 5-7 years in a dog and 2-3 years in a cat, each mosquito season can lead to an ever-increasing number of worms in unprotected pets. Heartworm does not infect people.


Heartworm infection is truly a silent disease. In the early stages there may not be any signs, but as the heartworms slowly cause damage to the blood vessels of the lungs, signs may include mild cough, reluctance to exercise, fatigue after activity, decreased appetite, and weight loss. In cats, signs of migrating larvae are usually misdiagnosed as “asthma” or “bronchitis” as the cat may cough and have some respiratory difficulty.  The acronym HARD (Heartworm Associated Respiratory Disease) has been given to this sign in cats.


Your veterinarian can test your dog’s blood for the presence of adult female heartworms. Since it takes 6-7 months for the microfilaria to mature into adults, puppies can’t be tested until they are at least 7 months old. So, if you adopt a young puppy, it is important to have your puppy tested when he/she is 7 months old by your private veterinarian. If a test is positive, a confirmation test and other diagnostics such as radiographs (X-rays) or ultrasound imaging may be recommended.  Heartworm testing should be done every year.


Heartworm can be treated in dogs, there is not treatment for cats. Without treatment, heartworm disease worsens and can lead to many serious problems including death. While treatment is not without risks, a thorough physical exam and pre-treatment testing can help insure a good outcome. Sadly, there is no treatment for infected cats and diagnosis can also be difficult.  Unfortunately cats can suddenly die from heartworm infection.


Fortunately, heartworm preventives are very effective when used properly. It is important to understand that the medications are eliminated quickly from the body and don’t protect the pet from future infection.  Instead, they eliminate infections acquired since the last dose …. the “month after pill”, so to speak. All heartworm preventives are highly effective, safe, easy to use, relatively inexpensive, and often give protection for other parasites. Treatment is expensive and can be dangerous so here is a perfect example of “an ounce of prevention is worth a pound of cure.” Remember, an inside pet is not immune from heartworm as mosquitoes have no barriers. Cats must be on prevention as well since there is no treatment for heartworm in cats. The American Heartworm Society recommends year round prevention for both dogs and cats… even in the winter.  Mosquitoes winter over in crawl spaces and basements and can transmit heartworm at any time. Please talk to your veterinarian about the appropriate preventive medication for your pet.

Written by: Dr. Sheldon Rubin, Immediate Past President,  American Heartworm Society



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