Heartworm in Cats & Dogs: Facts, Risks & Prevention

Heartworm disease is likely the most serious parasitic threat to our pets; not just dogs, but cats as well. Although most people believe cats cannot develop heartworm, they can, but the disease progresses differently in them. Be aware of heartworm facts, notice the signs of infection, and use prevention measures that will save your pet's life.

The FDA provides a helpful overview in its “Keep Worms Out of Your Pet’s Heart!” brochure.
 PetCareClub, a pet supply web page, also offers info and products for the prevention of heartworms.

Let's dig deeper.

What Is Heartworm Disease?

Heartworm disease is caused by infection with a parasite called Dirofilaria immitis. If an infected mosquito bites your pet, it leaves immature heartworm larvae (microfilariae) in the pet's blood. The larvae develop over several months, travel through blood vessels, and settle in the heart, lungs, and related vessels - destroying them, causing inflammation, congestion, and even heart failure in severe cases.

Differences: Dogs vs. Cats

  • Dogs are the natural hosts. In dogs, heartworms will mature, breed, and live for 5–7 years (longer in certain cases). Untreated, heavy infestations can lead to heart failure, lung disease, and death.

  • Cats are atypical hosts. Infections are not so frequent (about 1–5% as frequent as in endemic regions in contrast to dogs), and the parasite frequently does not achieve its life cycle. Cats typically carry a few worms with a short lifespan (2–3 years).
     Furthermore, cats react more intensely to larval death or migration than to the presence of adult worms. 

Due to such differences, diagnosis, treatment, and prevention of feline heartworm are particularly difficult. 

How Heartworm Spreads

  1. Mosquito bite: Infected host (wild animal, dog) is bitten by a mosquito that ingests microfilariae.

  2. Larval development in mosquitoes: Parasite develops inside the mosquito during ~10–14 days (temperature dependent).

  3. Transmission to pet: Infective larvae are transferred to another pet by being bitten by the infected mosquito.

  4. Larval migration: Within the animal, the larvae migrate via tissue and bloodstream. They eventually arrive at the heart and pulmonary arteries.

Due to the fact that mosquitoes are found in most climates, risk for heartworm exists in most areas throughout the year (particularly where temperature is conducive to mosquito survival and parasite development).

Signs & Symptoms of Heartworm Disease

In Dogs

Severity of infestation dictates signs. Initially, dogs are asymptomatic. Later, signs will be:

  • Chronic cough (particularly following exercise)

  • Inability to exercise, decreased stamina

  • Lassitude, lethargy

  • Weight loss

  • Shortness of breath

  • Distended belly (due to fluid accumulation, secondary to heart failure)

  • Collapse, syncope

  • In severe instances, caval syndrome (giant worm burden impedes blood flow) a medical emergency

In Cats

Cats tend to present with more insidious or acute presentations:

  • Recurrent coughing, wheezing, dyspnea

  • Vomiting

  • Lassitude, anorexia

  • Weight loss

  • Abrupt death or collapse (in severe instances)

  • Respiratory distress signs
    Since fewer worms infest cats, standard diagnostic tests (which identify circulating microfilariae or antigens) can fail to detect infection.

As such, veterinarians tend to utilize imaging (ultrasound, X-ray) or immune testing with or without clinical presentation for diagnosis of feline heartworm disease.

Diagnosis & Treatment

Diagnosis

  • Antigen test / antibody test: Detects heartworm proteins (more reliably in dogs).

  • Microfilariae test: Detects the microscopic worm larvae in the blood (less frequently in cats).

  • Imaging (X-ray, echocardiography): Useful to observe changes in heart, lungs, or even visualize worms.

  • Other lab work: To monitor organ function, search for complications, etc.

Treatment in Dogs

Treatment is more established in dogs. It typically involves:

  • Stabilization (if the dog is unwell)

  • Eliminating adult worms with adulticide treatment (e.g. melarsomine) after checking heart, liver, and kidney function

  • Severely limiting exercise before and after treatment (to reduce risk of dead worm pieces becoming lodged in lungs)

  • Follow-up after treatment, monitoring, and microfilariae clearance

Treatment is risky; status of dog, worm burden, and overall health affect protocol and outcome.

Treatment in Cats

Because cats are "atypical hosts," and because adulticide (arsenic-containing) therapy carries greater risk, treatment of adult worms is avoided. Cats are usually treated with:

  • Supportive therapy corticosteroids, oxygen, symptomatic treatment

  • Surgical removal (in some cases)

  • Monthly heartworm prophylaxis to kill newly arrived larvae and prevent further growth

Prognosis in cats is poorer than in dogs; most cats recover with supportive therapy, but damage (especially to lungs) may persist.

Prevention Is Key: How to Protect Your Pet

As it is challenging and sometimes risky to treat heartworm (especially late-stage ones), prevention is far better than cure.

Following are prevention techniques including those recommended by PetCareClub:

1. Year-round heartworm prevention

The majority of recommendations (including the one quoted by FDA) recommend keeping pets on a heartworm preventive year-round, especially where mosquitoes overwinter or there is warm climate.

2. Use approved preventive medications

Some of the commonly used preventive options are:

When selecting a preventive, always inquire of your veterinarian what the correct product, dose, time, and if your pet has contraindications (age, illness, co-medications, etc.).

3. Test before starting preventive (most especially for dogs)

Before starting heartworm preventive treatment, most veterinarians recommend testing your dog to ensure that it is free from infection. Giving a preventive to an infected animal may result in issues.

4. Regular dosing schedule

Missing doses or delaying preventive medicine decreases protection. Most preventives have to be administered monthly, and it is crucial to remain on schedule.

5. Mosquito control & environmental management

  • Remove or decrease standing water near your house (mosquito breeding areas).

  • Utilize screening, netting, indoor protection to decrease mosquito contact.

  • Curtail outdoor activity at dawn and dusk when mosquitoes are most prevalent (particularly in high-risk areas).

6. Annual checkups & monitoring

Even on preventive, annual testing may be indicated (especially in dogs) to identify "breakthrough" infections early.
Periodic visits by the veterinarian also help to screen for other health issues that can complicate heartworm disease.

Common Misconceptions & FAQs

  1. “Only outdoor pets need protection.”
     Many pet owners believe indoor pets are safe from heartworm, but mosquitoes can easily enter homes. Even indoor cats and dogs are at risk, so year-round prevention is important.

  2. “Cats don’t get heartworm, so no need to prevent.”
     Cats can and do get heartworm, though less frequently than dogs. Since treatment options for cats are limited, prevention is the safest approach.

  3. “Once a pet tests negative, no need to continue prevention.”
     A negative test result does not protect pets from future infections. Heartworm prevention should continue consistently, as pets can become infected at any time.

  4. “Over-the-counter products are fine.”
     Some over-the-counter products are not effective or safe. Using veterinarian-approved heartworm preventives ensures proper dosing, safety, and protection for your pet.


Conclusion

Heartworm disease is a serious, potentially life-threatening illness in cats and dogs. Dogs are more commonly infected and have more treatment options available, but cats pose unique challenges and usually require more conservative treatment. Prevention is obviously the best, most effective method.

Using effective, veterinarian-prescribed preventives (many of which are available through organizations like PetCareClub) on a regular regimen; in addition to mosquito control, testing, and veterinary monitoring, gives your pet the best chance to be free from heartworms.

Pet owner takeaway:

  • Speak with your veterinarian to start or resume year-round heartworm prevention.

  • Use proven medications (e.g., Heartgard, Simparica Trio, Interceptor, Revolution) as prescribed.

  • Keep preventive schedule and testing intact.

  • Limit mosquito exposure in and around your residence.

  • Keep a close eye on your animal for illness symptoms and see your vet as soon as they appear.
Location: United States

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