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As the snow melts, temperatures rise, and we see our neighbors outside again, we will soon see the mosquitoes too. While mosquitoes may seem to be nothing more than an annoyance to our enjoyment of the great outdoors, they can pose a serious threat to our pets.

Heartworm disease, a deadly disease that affects both dogs and cats, is spread by hungry mosquitoes when they feed on our pets. The life cycle of heartworm is complicated:

  • It all begins when a mosquito bites an infected animal (e.g., dog, cat, fox, coyote) and ingests microfilariae (“baby heartworms”) during its blood meal.
  • The microfilariae develop for 10-30 days in the mosquito’s gut, maturing into larvae.
  • The larvae travel to the mosquito’s mouthparts from where they are passed to the next animal (e.g., a dog or cat) when the mosquito bites again.
  • The larvae grow in the body tissues and then migrate into the bloodstream to enter the heart, pulmonary artery (the large blood vessel that carries blood from the heart to the lungs) and other adjacent blood vessels.
  • They then mature into adults, mate, and produce microfilariae within 6 to 7 months.

Over time, adult heartworms will clog the heart and major blood vessels, reducing the blood supply to the body’s organs, which can lead to organ malfunction. Microfilariae circulate in the blood and can cause problems as well. They will clog the small blood vessels, blocking the flow of blood to tissues, depriving them of nutrients and oxygen. Microfilariae primarily damage the tissues of the liver, causing liver disease (cirrhosis) and lung disease (with coughing).

Severely infected dogs, with a high heartworm “load” (number of worms in the heart), may die suddenly during exercise or excitement. Even a small heartworm load in cats can lead to death.

Treatment of heartworm disease is costly, potentially life-threatening, and does not always have positive outcomes.

While the life cycle of the heartworm is complicated, prevention is not! Prevention starts with a simple screening test that only takes a few minutes. Even pets on year-round preventives should be screened once a year – you never know if your pet might have spit out or rubbed off a dose, leaving him or her vulnerable to exposure and infection. 

Various heartworm preventives are available for dogs and cats, including monthly oral tablets/chews, monthly topicals applied to the skin, and an injectable product that offers protection for 6 months.

With climatic and environmental changes expanding the range of “heartworm areas” and an increase in the relocation of dogs with rescue efforts, heartworm is spreading into areas not previously impacted.

Indoor cats (and dogs) are no exception! Mosquitoes can easily enter your home as you enter or leave. Even a small hole in a window screen can let mosquitoes get in.

Your pet’s best line of defense is annual testing and preventives. Book an appointment with your veterinarian today to protect your pet from heartworm disease with a simple and effective preventive medication that is best suited for your pet.

LifeLearn News

Note: This article, written by LifeLearn Animal Health (LifeLearn Inc.) is licensed to this practice for the personal use of our clients. Any copying, printing or further distribution is prohibited without the express written permission of Lifelearn. Please note that the news information presented here is NOT a substitute for a proper consultation and/or clinical examination of your pet by a veterinarian.


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Animal Hospital of Seminole


11375 Park Boulevard Seminole, FL 33772

Clinic Hours

Monday: 8:00am – 5:30pm, Tuesday: 8:00am – 7:00pm, Wednesday: 8:00am – 5:30pm, Thursday: 8:00am – 7:00pm, Friday: 8:00am – 5:30pm
Saturday 8:00am – 12:00pm, Sunday: Closed