Heartworm infections are increasing in number and geographic distribution, extending into traditionally low prevalence areas, such as California. Our team at Terra Linda Veterinary Hospital wants to help ensure your pet is protected from these dangerous parasites and the diseases they cause.

Heartworm transmission in pets

Heartworms can infect many mammal species, including dogs, cats, ferrets, wolves, coyotes, raccoons, and foxes. Dogs and wild canids are their natural hosts, which means the heartworms can complete their full life cycle while parasitizing these animals. As they feed, mosquitoes ingest baby heartworms (i.e., microfilariae) that circulate in an infected natural host’s bloodstream. If temperatures remain above 57 degrees, the microfilariae develop to an infective stage and travel to the mouth of the mosquito, which deposits them on a pet’s skin when they feed. The parasites enter the pet’s body through the mosquito’s bite wound and eventually migrate to the pet’s heart and lungs. In dogs, female heartworms can reach 10 to 12 inches inside their heart and pulmonary vasculature, and can mate and produce offspring about six to seven months after the initial infection.

Heartworm disease in pets

Dogs are natural heartworm hosts, while cats and ferrets are atypical hosts, which means that the disease process is different, depending on the affected pet. 

  • Dogs — In dogs, heartworms lodge in the vasculature supporting the lungs, causing inflammation and damaging the vessels and lung tissue. The vessels thicken in response to this inflammation, creating a high resistance area for the heart, which can no longer effectively pump blood through, and heart failure occurs. Most dogs don’t initially exhibit signs, but as their condition progresses, signs can include lethargy, a soft cough, exercise intolerance, and weight loss. Once heart failure develops, fluid may accumulate in the abdomen, causing a swollen belly. In addition, a large worm population in the heart can cause caval syndrome, which occurs when the worms form a heart blockage, resulting in collapse and sudden death.
  • Cats — Since cats are not natural heartworm hosts, their immune system responds strongly to the parasites when they reach the cat’s pulmonary vasculature, causing an extreme inflammatory response that permanently damages the cat’s lung tissue. This condition, called heartworm associated respiratory disease (HARD), can result in signs including coughing, wheezing, difficulty breathing, and vomiting, although many cats show no signs. Heartworms typically do not reach adulthood in cats. However, rarely, one or two worms may mature fully in the cat’s heart, causing significant turbulence because the heart is small, and producing a blood clot. If a blood clot lodges in the aorta where the vessel branches to supply the hindlimbs, one or both hindlimbs can become acutely paralyzed, a condition known as a saddle thrombosis. 
  • Ferrets — Heartworm disease affects ferrets similarly to cats and dogs. While ferrets are highly susceptible to the disease, and can carry a high parasite load, a single worm can have a devastating effect. Signs in ferrets are similar to those seen in dogs, but ferrets tend to develop signs more rapidly, since their heart is smaller. 

Diagnosing heartworm disease in pets

The few tests that are available to test for heartworm disease in pets each work differently to detect the parasites. Diagnostics include:

  • Antigen test — An antigen test, which is the most common screening test used in dogs, detects adult female heartworms present in the body.
  • Antibody test — Antigen tests have limited use in cats, because they rarely have adult female heartworms, but antibody tests are especially useful in diagnosing cats, because they detect the cat’s immune response to the parasites.
  • Imaging — Chest X-rays and ultrasound imaging of the heart can show changes indicating heartworm disease, and, in some cases, ultrasound can show worm movement. Imaging is typically used in conjunction with other tests to diagnose heartworm disease. 

Staging heartworm disease in pets

Heartworm disease can be staged in pets to help determine the best treatment strategy. The categories are as follows:

  • Class I — These pets typically don’t have signs and are usually diagnosed during a screening test. Other diagnostics are normal.
  • Class II — These pets exhibit signs such as mild fatigue and an occasional cough, and changes to the vessels around the heart can be seen on X-rays. 
  • Class III — These pets’ signs include difficulty breathing and weight loss, and severe damage to the vessels surrounding the heart is apparent on X-rays. 
  • Class IV — These pets are experiencing a heart blockage, and show signs including collapse, dark brown urine, and muddy mucous membranes. The worms must be promptly surgically removed to save these pets.

Treating heartworm disease in pets

Heartworm disease management depends on your pet, whose treatment will involve:

  • Dogs — Before treatment begins, your dog’s condition must be stabilized, because treatment can be dangerous, especially in pets class II and higher. Once your dog is stabilized, heartworm treatment involves painful injections to kill the parasites at every life stage, which requires medication administration in stages over several months. Your dog must be closely monitored to ensure no complications occur, and their exercise severely restricted, because activity can exacerbate the parasites’ damage.
  • Cats and ferrets — No safe treatment is approved for cats or ferrets—their disease can only be managed with close monitoring and supportive care. 

Heartworm disease is a potentially life-threatening condition, and treatment is not always successful. This makes year-round heartworm prevention medication critical to protect your pet from these dangerous parasites. If your pet is due for a heartworm test, or preventive refill, contact our team at Terra Linda Veterinary Hospital to schedule an appointment.