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Heartworm Prevention

Heartworm Disease: A Danger to Both Dogs and Cats


Heartworm disease is a serious and potentially fatal parasitic disease of pets that is caused by foot-long worms that live in the heart, lungs, and associated blood vessels of an affected pet. Heartworms can cause severe lung disease, heart failure, and damage to other organs in the body. While heartworm disease affects both dogs and cats, it can also live in other animals such as wolves, coyotes and foxes. Since these wild species can live in close proximity to people and their pets, they are considered important carriers of the disease.

Heartworm Disease in Dogs

Dogs are the natural host for heartworms which means that heartworms living inside a dog can mature into adults, mate and reproduce. If left untreated, the number of heartworms can increase significantly. Dogs have been known to harbor several hundred worms in their bodies. Heartworm disease causes lasting damage to the heart, lungs and arteries, and can affect the dog’s health and quality of life long after the parasites are gone. Infected dogs also serve as a source for infection of other dogs and cats in the area.

Heartworm Disease in Cats

Heartworm disease in cats is very different from heartworm disease in dogs. The cat is an atypical host for heartworms and most worms in cats do not survive to the adult stage. Cats with adult heartworms typically have just one to three worms and many cats affected by heartworms have no adult worms at all. Most heartworm disease in cats may go undiagnosed but even a few immature worms can cause severe damage to the cat. Cats with heartworms can develop heartworm-associated respiratory disease (HARD) which can be severe or even fatal. Unfortunately, the medication used to treat heartworm infections in dogs cannot be used in cats, so prevention is the only way of protecting cats from the effects of heartworm disease.

Drawing of the exterior of a dog's heart
Interior view of a dog's heart containing numerous small, spaghetti-like worms.

Heartworm Transmission


Heartworm disease is transmitted by mosquitos. Adult female heartworms living in an infected dog, fox, coyote, or wolf produce microscopic baby worms called microfilaria that circulate in the bloodstream.


When a mosquito bites and takes a blood meal from an infected animal, it picks up these baby worms, which develop and mature into “infective stage” larvae inside the mosquito over a period of ten to fourteen days. When the infected mosquito bites another dog, cat, or susceptible wild animal, the infective larvae are deposited onto the surface of the animal's skin and they enter the new host through the mosquito’s bite wound.


Once inside a new host, it takes approximately six months for the larvae to develop into sexually mature adult heartworms. Once they are full mature, heartworms can live for five to seven years inside a dog and up to two or three years inside a cat. The long lifespan of these worms means that each new mosquito season can lead to an increasing number of worms in an infected pet.

Signs of Heartworm Disease

Signs of Heartworm Disease in the Dog

In the early stages of the disease, many dogs show few symptoms or no symptoms at all. The longer the infection persists, the more damage is done to the circulatory system and the more likely symptoms are to develop. Very active dogs, dogs heavily infected with heartworms, or those with other health problems may show clinical signs sooner than an “ordinary” dog.


Signs of heartworm disease may include a mild persistent cough, reluctance to exercise, fatigue after moderate activity, decreased appetite, and weight loss. As heartworm disease progresses, pets may develop heart failure and a swollen belly due to excess fluid in the abdomen. Dogs with large numbers of heartworms can develop a sudden blockages of blood flow within the heart leading to a life-threatening form of cardiovascular collapse called caval syndrome. Caval syndrome is marked by a sudden onset of labored breathing, pale gums, and dark bloody or coffee-colored urine. Without prompt surgical removal of the blockage, few dogs survive.

Signs of Heartworm Disease in Cats

Signs of heartworm disease in cats can be very subtle or very dramatic.  Cats may show signs such as coughing, asthma-like attacks, periodic vomiting, lack of appetite, or weight loss. Occasionally an affected cat may have difficulty walking, faint or have seizures, or suffer from fluid accumulation in the abdomen. Unfortunately, the first sign in some cases is sudden collapse or sudden death.

A Pomeranian pants in front of a chest x-ray displayed on a screen
2010 American Heartworm Society Heartworm Incidence Map shows 1-5 cases per clinic in Oswego County

How High is the Risk?


Many factors need to be considered when assessing your pet's risk of developing heartworm disease. Even though heartworm may not seem to be a big problem in our area, our community may have a greater incidence of heartworm disease than we realize. You could also unknowingly travel with your pet to an area where heartworms are more common.


More importantly, heartworm disease is constantly spreading to new regions of the country each year. The relocation of infected pets through human movement, adoption and rehoming can contribute to increasing cases in areas that are normally less at risk. In Oswego County, we see a number of heartworm positive dogs each year who have been brought from the south through adoption programs. Stray dogs, neighbors dogs who are not on prevention and wildlife can all act as a potential source of infection for your pets.

Heartworm disease has been diagnosed in all 50 states, Multiple variables, from climate variations to the presence of wildlife carriers, cause rates of infections to vary dramatically from year to year—even within a single community. Since infected mosquitoes can come inside, both outdoor and indoor pets are at risk. Mosquitos may also over-winter in homes, posing a risk of infection even in the cooler months. 

2019 American Heartworm Society Heartworm Incidence Map shows 6-25 cases per clinic in Oswego County

Recommendations for Heartworm Prevention


For all the reasons listed above, the American Heartworm Society recommends year-round heartworm prevention for all dogs and cats.


At Lake Ontario Veterinary Clinic, we recommend year round heartworm prevention for all dogs, regardless of lifestyle. We also strongly encourage cat owners to consider the risk heartworms pose to cats when they select their preventatives, especially when their cats spend time outdoors. 

Recommendations for Heartworm Testing:

For more information on heartworm testing recommendations, please see:

Hand feeds a small heartworm chew to a curly haired dog
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