Heartworm disease is often thought of as a condition that only affects dogs, but many other mammal species can suffer from it or act as carriers. Heartworms have been known to infect dogs, cats, ferrets, wolves, coyotes, sea lions, and in rare cases, even humans.
Severe lung disease, heart failure, and damage to other organs are caused by foot-long worms that colonize the heart, lungs, and blood vessels of infected animals.
Heartworm Disease in Dogs vs. Cats
In natural hosts such as dogs, heartworms are able to fully mature into adults, mate, and reproduce. Some dogs have been found with several hundred fully grown worms in their bodies. These parasites cause lasting damage and may affect a dog’s health for years, even after they are removed.
Cats are considered atypical hosts. While cats can become infected, the worms rarely survive long enough to reach adulthood and reproduce. This does not mean cats are safe from harm—immature worms can still cause significant damage. Cats often go undiagnosed because they do not show the same symptoms dogs do and may instead suffer from Heartworm-Associated Respiratory Disease (HARD). Medications used to treat heartworms in dogs cannot be used in cats.
How Are Pets Infected?
Heartworm disease is transmitted through mosquito bites.
When a mosquito bites an infected animal, it ingests microscopic worms called microfilaria. These mature inside the mosquito over 10–14 days into an infective stage. When that mosquito bites another animal, the larvae enter the bloodstream.
Inside the host:
- Larvae mature into adult worms within about 6 months
- Adult worms can live 5–7 years in dogs
- In cats, worms may survive 2–3 years if they reach maturity
Adult female worms produce more microfilaria, which circulate in the bloodstream until picked up by another mosquito.
Signs and Symptoms of Heartworm Disease
In Dogs
Dogs may show few or no symptoms in the early stages. As the disease progresses, signs may include:
- Mild, persistent cough
- Fatigue or lethargy
- Decreased appetite
- Weight loss
- Reduced interest in exercise or play
In advanced cases, fluid accumulation may cause a swollen abdomen. Dogs with heavy infestations can develop caval syndrome, a life-threatening condition caused by worms blocking blood flow in the heart.
Signs of caval syndrome include:
- Sudden labored breathing
- Pale gums
- Dark or bloody urine
Emergency surgery is required for survival.
In Cats
Heartworm disease in cats may cause either very subtle or very dramatic symptoms.
Some cats show no signs at all, while others may experience:
- Asthma-like breathing episodes
- Difficulty walking
- Fainting spells
- Seizures
In some cases, the first sign of heartworm disease in cats is sudden collapse or death.
What Tests and Treatments Will My Veterinarian Perform?
When symptoms suggest heartworm disease, veterinarians typically begin with an antigen blood test. If the result is positive, a second confirmatory test is recommended, as treatment is complex and costly.
Treatment for Dogs
If a dog tests positive, the first step is stabilization. Dogs with severe disease or underlying medical conditions may require months of therapy before heartworm treatment can begin.
Strict exercise restriction is critical, as physical exertion increases the damage caused by heartworms.
Because reduced activity can lead to boredom or depression:
- Keep your dog near you for companionship
- Speak to them calmly throughout the day
- Use veterinarian-approved long-lasting chew toys for enrichment
Once stabilized, treatment follows multi-step protocols established by the American Heartworm Society, which vary based on disease severity. Dogs with mild infections generally have better outcomes.
Treatment for Cats
Diagnosis in cats is more complex and may involve:
- Physical examination
- Blood tests
- X-rays
- Ultrasound
There is no approved drug treatment for heartworm disease in cats. In many cases, infections resolve on their own, though permanent damage may remain.
Treatment focuses on managing symptoms:
- Mild cases require stabilization and monitoring
- Severe cases may require hospitalization, antibiotics, IV fluids, heart and lung support medications, or surgical removal of worms
How Do I Protect My Pet From Heartworms?
Prevention is essential for both cats and dogs.
- Administer monthly heartworm preventatives year-round
- Preventatives are available as topical gels or oral medications
- Dogs that test positive require re-testing every 6 months until cleared
- Once cleared, pets must remain on preventatives for life
Some pet owners stop preventatives during winter, but this increases risk. Warm spells can allow mosquitoes to emerge unexpectedly, and missed doses in spring are common.
The safest approach is to choose a consistent day each month to administer heartworm prevention to ensure your pet remains protected year-round.
