Hot, humid South Carolina summers breed hordes of blood-thirsty mosquitoes. Their itchy bites are annoying, but did you know that mosquitoes can transmit dangerous heartworms to your furry friend? According to the Companion Animal Parasite Council, South Carolina veterinarians diagnosed more than 5,000 heartworm cases in 2018. With another muggy summer in store, this dangerous disease will threaten many more pets. Do you know how to keep your beloved companion safe?


Heartworms are parasitic worms that live and reproduce in a dog’s heart and lungs. The worms are microscopic at birth, but can grow to 12 inches in six months as they mature to adults, and clog major blood vessels. Heartworms cause significant damage and even death to pets they infect.

Heartworm disease

Heartworms are introduced into a dog’s body as microscopic, larval worms when bitten by a mosquito that has previously bitten an animal infected with heartworm. The worms multiply as they reach adulthood and can quickly accumulate to a significant threat. Adult worms cause damage in several ways:

  • They impede blood flow through the heart and lungs.
  • They trigger inflammation within the heart and lung vessels.
  • They make breathing difficult when inflammation and congested blood flow cause fluid to accumulate in the lungs.

Untreated heartworm disease in dogs progresses to congestive heart failure and death.

Cats can also develop heartworm disease, although they are not the intended host. The larval worms transmitted to a cat may develop to adults, but cannot reproduce. However,  although the worms can’t multiply to numbers that will cause an infestation, just a few worms can cause significant inflammation in a cat’s lungs and heart.

Heartworm transmission

Mosquitoes transmit heartworms to dogs and cats when they bite an infected dog and pick up a few microscopic worms, or microfilariae. The heartworms develop into infective larvae within the mosquito, which  transmits them to the next animal bitten. The larvae simply die in most animals, but if transmitted to a dog or cat, they can survive and wreak havoc on the unsuspecting host.

Dogs spend more time outside than our feline friends, so they are more likely to be infected with heartworms. However, some cats do venture outdoors, and mosquitoes often find their way inside a home, so heartworm disease is a risk to cats, as well.

Heartworm diagnosis

The best way to know if your pet has dangerous heartworms is to have a heartworm test performed as part of her annual wellness visit. The test requires only a few drops of blood and can often be performed in our veterinary office.

You should also be aware of the symptoms of heartworm disease. Monitor your pet for signs such as:

  • Decreased appetite
  • Coughing
  • Lethargy
  • Vomiting
  • Difficulty breathing
  • Collapsing suddenly

Infected cats often show no symptoms, but can suddenly collapse or die without warning.

If heartworm disease is diagnosed in your pet, additional testing is performed to assess the disease severity and the extent of heart and lung damage before starting treatment. Testing may include:

  • Blood work to evaluate organ function
  • X-rays
  • Echocardiogram (an ultrasound of the heart)

Heartworm disease treatment

A positive heartworm test means your pet has adult heartworms. Treatment will address several aspects of the disease:

  • Medication to kill the adult heartworms will be injected into your pet’s back muscles in several doses spaced about a month apart. During treatment, pieces of dead worms can lodge in blood vessels, blocking blood flow and causing sudden death. The risk increases with severity of infection. Dogs must be kept calm and restricted to a crate with short leash walks only, to minimize the chance of this deadly complication.
  • Additional medication will be administered to kill circulating microfilariae before they mature into adults.
  • Antibiotics will be administered to kill the bacteria the heartworms carry that cause inflammation.
  • Anti-inflammatories will be used to decrease inflammation caused by the heartworms.

Heartworm prevention

Heartworms cause a dangerous, life-threatening disease no pet should suffer. Fortunately, prevention is simple and much less costly than treatment. Many preventive products are available, from flavored chewables given monthly to injections administered biannually. Our veterinary team can recommend a product that will best fit your pet’s needs.

Mosquitoes are active at temperatures above 50 degrees, which includes the winter months in South Carolina. All dogs and cats should receive year-round protection, whether or not they go outdoors.

If your pet is not currently receiving heartworm preventive, we will perform a heartworm test  to ensure she is heartworm-free. Once she tests negative, we will administer a preventive.

If you have questions about heartworm disease, call our office. A team member will be happy to help.