Heartworms are relentless, life-threatening parasites that affect wild mammals, dogs, ferrets, and cats—including those who live exclusively indoors. If you believe your indoor cat is low-risk for heartworm disease, consider the tiny, resilient, and ubiquitous nature of the mosquito—the primary vector (i.e., carrier) for heartworm disease. If you’ve ever marveled—or screamed—at a mosquito buzzing inside your tent, car, or home, you know these pesky parasites can sneak past every barrier and blockade to reach their next victim—who could be your cat. 

Protect your indoor-only feline with this guide to heartworm disease in cats from Tamberly Animal Hospital.

Heartworm disease transmission and pathology in pets

Heartworms (Dirofilaria immitis) begin as immature microscopic blood-borne parasites (i.e., microfilariae) that rely on the mosquito for development and transmission. When an infected mosquito bites a pet, the microfilariae are injected under the skin, and begin their slow migration toward the pet’s lungs and heart. If the pet is not on a heartworm preventive, the microfilariae reach their destination and mature into large adult worms that can grow 12 to 14 inches long. Adult heartworms cause severe inflammation in the pet’s heart, lungs, and large blood vessels that can lead to severe lung disease, heart failure, permanent organ damage, and death. Heartworms mature in about six months, begin to reproduce, and then cause visible disease signs.

Indoor cats—an unnatural host and an easy target

Unlike dogs, cats are not natural heartworm hosts, which means that their bodies are not a sustainable environment for heartworm development and survival. This makes heartworm disease manifestation different—but equally harmful—for cats.

Cats can have a range of responses to immature and adult heartworms, including:

  • Spontaneous resolution — Some cats may clear microfilariae infection on their own, because of a suspected immune response.
  • Low adult heartworm population — Because the cat is an inhospitable host, mature infections may contain no adult heartworms, or as few as two to three, compared with affected dogs, who can harbor more than 100 circulating adult worms. Yet, heartworm disease can still be dangerous and life-threatening for cats, despite their seemingly light worm load.
  • Vague or nonexistent clinical signs — Feline heartworm disease signs can vary from nonexistent to severe. Immature heartworms can cause heartworm-associated respiratory disease (HARD), which often is mistaken for asthma or bronchitis. Some cats may display neurological signs, a swollen abdomen, or general poor health (e.g., weight loss, vomiting, diarrhea), while others will show no signs before they suddenly die—often from a worm-induced blockage (i.e., embolism).

Unfortunately, many indoor-only cat owners are unfamiliar with feline heartworm disease, and countless indoor cats do not receive regular heartworm prevention medication, which is the only proven protection against this heartbreaking disease.

Heartworm disease’s impact on feline health

Heartworm-positive cats may begin experiencing internal injury between 60 and 100 days after the infection—long before the heartworms reach their adult size. The migrating microfilariae enter the small arteries and vessels that flow in and out of the heart and lungs, and set off a pronounced inflammatory response that damages the vasculature and the lungs’ small airways, so the cat has difficulty breathing.

Significant inflammatory reactions also occur as the cat’s immune system responds to the infection, and when adult heartworms die and break down. Deteriorating adult worms can form embolisms as they are carried out of circulation.

Recognizing heartworm disease in cats

Because heartworm disease in cats has a vague, inconsistent presentation, the disease can be challenging to identify and accurately diagnose without a specially designed blood test. However, you should be able to recognize some of the most common clinical signs, so you can identify the disease in some cats and provide supportive care. The range of signs includes:

  • Persistent cough
  • Gagging
  • Difficulty breathing
  • Rapid breathing
  • Intermittent vomiting and diarrhea, which may be bloody
  • Weight loss
  • Fainting or seizures
  • Loss of coordination
  • Enlarged abdomen
  • Collapse

Any suspected heartworm disease sign in your cat merits a trip to Tamberly Animal Hospital for a complete physical examination and heartworm testing. Although no effective feline heartworm disease treatment is available, you can take steps to control heartworm-related signs and provide supportive care.

Heartworm testing and prognosis in cats

At Tamberly Animal Hospital, we use a small blood sample to test your cat for heartworm disease. The test detects heartworm-specific proteins in the blood, which can indicate an active infection. If your cat tests positive for heartworm disease, your Tamberly Animal Hospital veterinarian will order additional testing and formulate a supportive care plan.

Because no treatment can safely eliminate adult heartworm infection in cats, care is focused on managing clinical signs, reducing inflammation, and providing comfort. Some cats will succumb to complications from heartworm disease, but some will possibly outlive the adult worms, which can live for two to three years.

Heartworm disease prevention for cats

Heartworm prevention is simple, convenient, and the only effective way to protect your indoor cat from heartworm disease. Feline preventives are available in oral and topical form, and many products also provide additional parasite protection (e.g., gastrointestinal parasites, ear mites, fleas). Heartworm prevention medication must be given year-round—including the winter—to ensure your cat is completely protected. 

Minimize your indoor cat’s risk for deadly heartworm disease—schedule an appointment for all their wellness care needs, including heartworm disease testing and prevention, at Tamberly Animal Hospital.