Heartworm Disease

Frequently Asked Questions (FAQs) About Heartworm

Is heartworm found across the US?

Yes. Heartworm can be found in all 50 US states. It is more prevalent in the south and in areas with larger populations of mosquitos.

How is heartworm transmitted?

Heartworm is spread by mosquitos. It is not spread directly between mammals. For example, a human cannot get heartworm from a dog.

Which domestic animals are most susceptible to heartworm?

Dogs and ferrets are the most at risk; however, cats can become infected as well. Especially in the case of ferrets, prevention is very important as it can be hard to treat once infected.

Can Humans become infected?

Yes! Heartworm is a serious illness that can kill your pet. Proper preventative measures are essential to keeping your animal safe.

Series of person feeding pet dog with preventive heartworms chewable

Heartworm disease in pets is a severe and sometimes fatal condition in the United States and many other countries around the world. Dogs, ferrets, and to a lesser degree, cats are susceptible to heartworm disease, which is caused by foot-long worms that reside in the lungs, heart, and arteries. These worms also find hosts in wolves, foxes, coyotes, sea lions, and in a few instances, humans. Heartworms can cause heart failure, severe lung disease, and other species-specific health consequences:


Canines serve as natural hosts for heartworms, fostering the worm's maturation into an adult worm. After that, the parasite will reproduce and produce offspring. There have been reports of dogs harboring hundreds of worms, and if left untreated, this number can increase exponentially. Heartworm disease causes irreversible damage to a dog's heart, lungs, and arteries, impacting its health and quality of life even after the parasites have been eliminated. This makes heartworm prevention very important for dogs. If your pet becomes ill, treatment is required, and it should start as soon as possible as the worms can multiply exponentially.


While cats can become infected, it is less common. Cats infected with heartworms usually have no more than one to three adult worms in their bodies and sometimes have no adult worms at all. Heartworm sickness in cats is frequently misdiagnosed, but it's important to know that even juvenile worms can harm cats. The juvenile worm causing heartworm linked respiratory disease (HARD). Cats can only be protected from the effects of heartworm disease through prevention because the medication used to treat heartworm infections in dogs is not safe for cats.


Ferrets are susceptible to heartworm, but treatment options are less available. Like cats, ferrets' smaller heart size makes them susceptible to fatal damage from even a tiny number of worms, perhaps as few as one. Heartworm disease in ferrets is difficult to diagnose, and there is no reliable cure. For ferrets prevention using a heartworm prophylactic medicine is highly recommended.

What Are the Symptoms of Heartworm Disease?

While there may not be any symptoms in the early stages of infection, symptoms of disease will start to show as the worms mature and multiply, intensifying the severity of symptoms as they spread. Here are the four classes of heartworm infection:

  • Class One. The first class is asymptomatic or manifests as a mild cough.
  • Class Two. The second class is characterized by a persistent cough with mild exercise intolerance.
  • Class Three. The third class displays greater exercise intolerance, weak pulse, fainting, abnormal lung sounds, loss of appetite, weight loss, and swollen stomach.
  • Class Four. The fourth class, also known as Caval syndrome, is marked by cardiovascular collapse which can be recognized by pale gums, dark urine, and labored breathing that subsequently result in organ failure and death.

How Are Heartworms Transmitted?

Only mosquitoes can spread heartworms from one animal to another. Microfilariae, which are immature heartworms, enter the mosquito's system when it bites an infected animal. Within two weeks, the microfilariae inside the mosquito transform into infectious larvae, which transfer to other mammals as the mosquito feeds on them. Contrary to dogs, infected cats do not frequently have microfilariae circulating in their blood, and so are less likely to spread the heartworm infection to another mosquito.

Within six months, the infectious larvae develop into adult heartworms. The larvae travel through the animal's body during the first three months before arriving at the lungs' blood arteries. The young worms spend the last three months maturing with females reaching lengths of up to 14 inches. Severe lung and heart illness are caused by the damage adult worms cause to the blood arteries and to the heart's capacity to pump blood. By the time the animal starts to exhibit the signs of adult heartworm infection, it already has heartworm disease.

When both sexes of adult worms are present, they will procreate new microfilariae within five to seven months after infection. The immune system of the animal may mount a response to the microfilariae that may harm other organs in the process. When a mosquito attacks an infected animal and picks up the microfilariae, the life cycle is recommenced. Infectious heartworm larvae can infect other mammals 10 to 2 weeks after developing from microfilariae to infective larvae inside the mosquito. In dogs, adult heartworms can live for five to seven years, while in cats, they can live for months or even years.

How Common Are Heartworms?

According to a Stanford University study, the first case of canine heartworms in the United States was found in the Southeast in 1856. The American Heartworm Society warns that heartworms are spreading across the nation and around the world, despite it earlier being more prevalent in the Atlantic and Gulf coast states. While the Southern U.S. has historically been linked to heartworm disease, pets in all regions of the country are at risk for heartworm disease at any time of year. All fifty states have reported cases of the sickness, according to the U.S. Food & Drug Administration.

Even dogs who spend most of their time inside are not completely protected from mosquito bites in areas with high numbers of stray or wild animals. Moving pets from one state to another may bring heartworms into areas where it has not previously been an issue. This is particularly evident with dogs from heartworm-endemic areas that are brought into shelters outside their home region during disaster relief operations. For instance, during Hurricane Katrina in 2005, over a quarter of a million animals were relocated from the New Orleans region to new residences across the nation. Some of these canines brought heartworms with them. According to AHS, there are more than a million heartworm-afflicted pets in the United States.

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