Heartworms pose a significant risk for your pet and can lead to life-threatening complications. Our Island Animal Hospital team knows you want to protect your four-legged family member from these dangerous parasites, so we answer your frequently asked questions about heartworm disease.

Question: How are heartworms transmitted to pets?

Answer: Mosquitoes transmit heartworms. Wild canids, such as coyotes, wolves, foxes, and raccoons, as well as domestic dogs, are natural heartworm hosts, and a mosquito that feeds from an infected natural host ingests baby heartworms (i.e.,  microfilariae). The baby parasites mature inside the mosquito for several weeks and are then deposited on a vulnerable pet when the mosquito takes their next blood meal. The parasites swim through the mosquito’s saliva to infect your pet.

Q: Do cats get heartworm disease?

A: Cats do get heartworm disease, but they are atypical parasite hosts, so the disease affects them differently than dogs. 

  • Heartworm disease in dogs — In dogs, the disease mostly causes vascular problems. The parasites lodge in the distal pulmonary artery, creating inflammation that causes the vessels to fibrose and thicken. This prevents the heart from efficiently pumping blood through the body and eventually leads to right-sided congestive heart failure. A single dog can be parasitized by large numbers of worms that can back up into the right heart chamber, leading to a condition called caval syndrome that obstructs blood flow through the heart.
  • Heartworm disease in cats — The cat’s immune system reacts strongly to the presence of the baby heartworms, and many don’t survive to adulthood, but the intense immune reaction results in severe lung inflammation, called heartworm-associated respiratory disease (HARD). When worms do survive to adulthood, only one or two can cause caval syndrome because of the cat’s small heart. 

Q: What are heartworm disease signs in pets?

A: Most heartworm-infected pets don’t exhibit signs, especially in the early stages. When signs are present, they differ in dogs and cats:

  • Dogs — Heartworm disease signs in dogs include a soft, persistent cough, exercise intolerance, weight loss, and fluid accumulation in their abdomen.
  • Cats — Heartworm disease signs in cats include coughing, wheezing, open mouth breathing, increased respiratory effort, and sudden death.

Close-up of long, thin parasitic worms on a metallic surface often examined by a vet. The light-colored worms appear tangled, with one prominently curved in the foreground. The reflective sheen of the surface highlights details that could intrigue any veterinarian's scrutiny.

Q: Are indoor pets safe from heartworm disease?

A: Pets who are allowed outdoors are at higher risk for heartworm disease, but indoor pets can also be exposed to mosquitoes. According to the Mosquito Control Association, mosquitoes frequently enter homes through screens, open doors and windows, exhaust vents, and attic soffits. In addition, indoor pets can escape when you’re not looking and be exposed to mosquitoes and other outdoor critters. All pets should receive heartworm preventives to protect them from this dangerous disease.

Q: Are pets in every state at risk for heartworm disease?

A: Since heartworms are transmitted by mosquitoes, pets in warm, humid areas are at higher risk, but heartworm cases have been reported in all 50 states. Climate changes are increasing the risk in historically low prevalence states, and pets in every state should regularly receive a heartworm prevention product.

Q: Can I stop my pet’s heartworm medication in the winter?

A: Mosquitoes are most active in the warmer months, but they can emerge unexpectedly during a sudden warm spell. The American Heartworm Society (AHS) recommends a year-round heartworm prevention regimen to protect your pet from these parasites.

Q: How often should my dog be tested for heartworms?

A: Heartworm testing recommendations depend on your dog’s age and circumstances:

  • Puppies — Puppies younger than 7 months can be started on a heartworm prevention plan without a heartworm test, but they should be tested six months after their initial visit, retested six months later, and then yearly.
  • Adult dogs — Dogs older than 7 months who have not been on a heartworm prevention regimen should be tested before starting a preventive, retested six months later, and then every year.

Q: What does heartworm testing in pets entail?

A: Several tests are available to help diagnose heartworm disease in pets using only a few drops of blood. Options include:

  • Microfilariae testing — Testing can detect circulating microfilariae in the pet’s bloodstream.
  • Antigen testing — These tests identify proteins produced by the adult female heartworm. The AHS currently recommends annual testing dogs in using antigen and microfilariae tests. 
  • Antibody testing — Antibody tests, which detect the pet’s immune response to heartworms, are most helpful in cats who don’t typically have microfilariae or adult female antigens circulating in their blood.
  • X-rays — Chest X-rays can help diagnose heartworm disease by revealing enlarged pulmonary vessels and lung inflammation.
  • Ultrasound — In some cases, ultrasounding the heart can reveal live heartworms. 

Q: Why does my dog need yearly testing if they are on a heartworm preventive?

A: While heartworm preventives are highly effective, no medication is 100%. In addition, one missed dose can leave your pet unprotected, and some pets are sneaky and spit out their dose when you aren’t looking.

Q: What heartworm prevention is best for my pet?

A: Several heartworm prevention options that can ensure your pet receives the protection they need are available. Choices include:

  • Chewables — Heartworm prevention can be administered once a month as a tasty chewable treat.
  • Spot-on preventives — Heartworm medications are available as monthly spot-on treatments.
  • Injectables — A heartworm prevention injection can be administered by a veterinary professional every 6 or 12 months.

Our veterinary team can recommend an appropriate heartworm prevention product based on your pet’s individual circumstances and lifestyle.

This information should encourage you to provide year-round heartworm prevention for your pet. If you would like your pet screened for heartworms, or need advice on the best product for your pet, contact our Island Animal Hospital team. We can ensure your pet’s heart is in tip top shape.