Southern Ontario is cited as a high-risk area for heartworm disease, and if your dog, cat, or ferret has a pulse, they are at risk for the potentially deadly parasite infection. Our Animal Hospital of Stoney Creek team has a concerted interest in your pet’s heart health, so we are providing information about heartworm disease and simple ways you can protect your four-legged family member.

Heartworm basics

While heartworms can parasitize many mammals, including dogs, cats, ferrets, wolves, coyotes, foxes, bears, seals, and sea lions, dogs and wild canids are their natural hosts, which means that the heartworm can complete its life cycle, including mating and producing offspring, while living in these animals. Wild canids and unprotected domestic dogs serve as a parasite reservoir, although they can’t pass heartworms directly to your pet. Another parasite, the pesky mosquito, is needed for transmission. Let’s look at the heartworm life cycle:

  • A mosquito bites an infected dog or wild canid and ingests baby heartworms (i.e., microfilariae).
  • Over the next two to three weeks, the microfilariae develop inside the mosquito to an infective larval stage.
  • The mosquito bites your pet and deposits the heartworm larvae in their saliva in your pet. The larvae swim through the bite wound to infect your pet.
  • The larvae develop in your pet’s tissues for three to four days and then migrate to their heart, lungs, and associated vasculature.
  • The larvae mature, and after about six months, the female heartworms produce microfilariae.

Heartworms in pets

Heartworms affect dogs, cats, and ferrets differently.

  • Dogs — Since dogs are natural heartworm hosts, the parasites thrive and mature, grow to up to 12 inches long, and can live in high numbers in a dog’s heart. The dog’s immune system recognizes these invaders as foreign, and creates inflammation that involves the pulmonary vasculature and adjacent lung tissue, and the vessels thicken and enlarge. As the inflammation progresses, the vessels fibrose, causing a high-resistance area that inhibits the heart’s ability to effectively pump blood throughout the body, which eventually leads to congestive heart failure (CHF). Initially, dogs don’t typically exhibit signs, but as their condition worsens, signs include a soft, persistent cough, lethargy, exercise intolerance, weight loss, and fluid accumulation in the abdomen. A dog who has a large parasite load can develop caval syndrome, in which the heartworms block blood flow through the heart. Affected dogs typically collapse and will die if the worms aren’t surgically removed.
  • Cats — Cats are an atypical host for heartworms, and have an immune system that can effectively clear the parasites, so most heartworms that parasitize a cat do not grow to adulthood. However, the young heartworms are quite capable of causing significant damage. The cat’s immune system strongly responds when the parasites reach their lung vasculature, resulting in heartworm-associated respiratory disease (HARD) that has similar signs to feline asthma, including coughing, wheezing, increased respiration rate and effort, and vomiting. In some feline cases, the first disease sign is sudden collapse and death. While most heartworms don’t survive to adulthood in the cat, only two or three adult heartworms can cause blockage in the cat’s small heart, leading to caval syndrome.

Heartworm diagnosis in pets

Diagnostic methods to detect heartworms include:

  • Microfilariae tests — In some cases, microfilariae can be identified in the pet’s blood, indicating heartworm infection. However, microfilariae may not be present if the pet has:
  • Early infection — Adult female heartworms don’t produce microfilariae until six months after the initial infection.
  • Single worm infection — A single adult worm cannot mate and produce microfilariae. Microfilariae also cannot be produced if the parasite population is all males or all females.
  • Strong immune response — The cat’s strong immune response typically clears microfilariae.
  • Heartworm prevention — Heartworm prevention medications kill circulating microfilariae, but not adult heartworms.
  • Antigen tests — Antigen tests can detect a protein produced by the adult female heartworm. False negatives can occur if only immature parasites or only adult male worms are present.
  • Antibody tests — Antibody tests assess the pet’s immune response to heartworm infection. These tests are especially important for cats, who frequently don’t have circulating microfilariae or adult female worms.
  • Imaging — Imaging techniques, such as X-rays and ultrasound, are also frequently used for heartworm disease diagnosis to assess heart damage and potentially visualize the worms.

The current recommendation for dogs is annual testing using microfilariae and antigen tests.

Heartworm treatment in pets

Currently, no safe medications are available to treat heartworm disease in cats or ferrets, and supportive care to stabilize the pet’s condition is the focus. Treatment for dogs is risky and prolonged and involves:

  • Strict exercise restriction — Physical exertion can exacerbate the heartworms’ damage, and your pet must be strictly confined.
  • Stabilization — Your dog’s condition must be stabilized to ensure they are strong enough to undergo heartworm treatment.
  • Medications — Medications are administered over several months to kill the parasites at every life stage.
  • Monitoring — Your pet must be closely monitored for serious, potentially life-threatening side effects.

Heartworm prevention in pets

Year-round heartworm prevention medications are the best way to prevent heartworms in all pets, and the only way to protect your cat and ferret, since no approved treatments are available. These medications can be administered monthly as a chewable treat or spot-on application, or every 6 to 12 months as an injection. 

Protect your four-legged friend from this deadly parasite by providing year-round heartworm prevention. Contact our Animal Hospital of Stoney Creek team to discuss the best heartworm prevention product for your cat, dog, or ferret.