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Heat Wave = Heartworm Season

By Kim Everson, DVM

According to the calendar it is still technically winter. That seems impossible as we Wisconsinites enjoy another day of unseasonably warm 70-80 degree weather. Tank tops and flip flops were standard fare at the St. Patrick’s Day parade yesterday. Birds and frogs loudly chirp while we lie in our beds with windows open to balmy spring-like breezes. Last night while my kids played outside we were actually beset by mosquitoes. Whatever is this March madness?


Heartworm season in Wisconsin varies each year, but it has undeniably begun a bit early this year. Pesky mosquitoes–infectious vectors for so many different diseases worldwide in humans, horses, cattle and more–are a key player in heartworm disease, a serious and potentially fatal disease in dogs and cats.  Mosquitoes suck up heartworm larvae as they feed on infected dogs. Once daily temperatures reach the high-50s, the heartworm larvae mature in the mosquito getting ready for “injection” into a new host.

After a heartworm-bearing mosquito feeds on a dog or cat, the larvae continue to mature inside this new mammal host. It takes six to seven months for the larvae to develop into adult worms which live in the heart and large blood vessels entering the lungs. In dogs, the adult heartworms reproduce, creating the microfilaria that are picked up by mosquitoes completing the cycle. Cats are a “dead-end” host for heartworms, with the life cycle being somewhat different than in dogs.


It takes several years before dogs show signs of infection, and by then disease is usually quite advanced. Heart failure and respiratory disease occur as the adult heartworms clog the heart and arteries. Reduced blood flow can cause secondary problems in the kidneys and liver. Symptoms of disease in dogs include coughing, shortness of breath, weight loss and loss of energy. Cats with heartworm disease may have chronic vomiting or symptoms of asthma. Sudden death is possible in severe cases.

A simple blood test is used to diagnose heartworm disease in dogs. Blood tests are available for feline heartworm as well, but diagnosis is more complicated due to the nature of infection in cats. Heartworm positive animals require further diagnostics such as chest x-rays and comprehensive blood tests to determine the severity of infection.

Treatment for heartworm in dogs is costly and not without risk. After receiving a series of injections to kill adult heartworms, the dog must be strictly rested for at least one month. During this period the dead adult worms are decomposing inside the dog and can cause serious problems (even death) if the dog is too active. The dog also needs to be hospitalized while starting on a heartworm preventative to kill the microfilaria. Rapid death of the microfilaria can cause severe side effects in the dog (such as anaphylactic shock and death). This is why veterinarians require a heartworm test prior to starting a dog on heartworm preventative! Remember, it takes years for dogs to appear sick from heartworm disease. 

There is no approved treatment for killing adult heartworms in cats. Amazingly some cats will spontaneously resolve infection on their own! Cats that are sick from the infection may require treatment for their symptoms.

As with most things in life, prevention of heartworm infection is cheaper, easier and safer than the “cure”. There are many types of heartworm preventative available for dogs and cats. Most also treat certain intestinal worms. Some prevent flea infestations and certain kinds of mites. Ask your veterinarian for help deciding which medicine is best for your pet.

The American Heartworm Society offers good online resources on heartworm disease including detailed discussions on
Heartworm disease in dogs
Heartworm disease in cats

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