Heartworm disease is a life-threatening but preventable condition that affects more than a million dogs and cats in the United States every year. To help ensure you know how to protect your pet from this infectious disease, our Town & Country Animal Hospital team believes you must understand the facts about heartworm disease, your four-legged friend’s risk, prevention basics, and treatment. Read our Heartworm 101 guide for everything you need to know to help prevent your pet from contracting this devastating disease. 

How do pets contract heartworm disease? 

The parasitic worm Dirofilaria immitis causes heartworm disease. When a mosquito bites an animal infected with heartworm, the pest ingests larvae (i.e. microfilariae) living in the host’s bloodstream. The larvae become infectious over the next two weeks, and when the mosquito bites your pet, the bug transmits the infectious microfilariae to your furry pal. 

Which pets have the highest heartworm disease risk?

While more heartworm disease cases occur in warm, humid climates, pets nationwide are at risk for contracting this life-threatening condition, with the disease having been diagnosed in every state. Dogs have the highest heartworm disease risk, but any mammal—including cats and ferrets—can contract this infection. Cats tend to be more resistant to infection than dogs, but because no safe drug therapy treatment options exist to treat feline heartworm, prevention is the best way to keep them safe.

Many pet owners assume their indoor-only pets are safe from infection. However, mosquitoes can get into your home through the tiniest window screen hole or an open door, or even by hitching a ride on your clothing or your pet’s fur. Once this opportunistic pest is in your home, every household pet is at risk for heartworm disease. 

How does heartworm disease affect pets?

Inside a pet’s vasculature, juvenile heartworms slowly migrate from the tissues to the lungs and heart. If the pet is not on a heartworm preventive, the microfilariae mature into adult worms that can grow 12 to 14 inches in length. Adult heartworms cause severe inflammation within a pet’s heart, lungs, and large blood vessels, leading to severe lung disease, heart failure, permanent organ damage, and death. Heartworms mature in about six months, reproduce, and begin causing visible disease signs. 

What are pets’ heartworm disease signs?

Many pets do not show heartworm disease signs until the condition has progressed, and they are severely affected. Heartworm disease presents differently in cats and dogs and may include the following signs:

  • Dogs — Canine heartworm disease progresses through four stages:
    • Stage 1 — No signs or mild signs, such as an occasional cough
    • Stage 2 — Mild to moderate signs, such as an occasional cough, and fatigue after moderate activity
    • Stage 3 — More severe signs, such as a persistent cough, fatigue after moderate activity, and an unhealthy appearance, with breathing difficulty and heart failure also being common
    • Stage 4 — Onset of caval syndrome, in which the worm load has become so high that they block blood flow back to the heart. Caval syndrome is deadly unless the worms are manually removed using a high-risk surgical procedure.
  • Cats — Many cats never show heartworm disease signs, and may die suddenly. Some cats show nonspecific signs such as vomiting, decreased appetite, and weight loss. Cats may also develop heartworm-associated respiratory disease (HARD), which occurs when the microfilariae die, causing a severe inflammatory response in the lungs. Signs include: 
    • Coughing or wheezing
    • Asthma-like respiratory distress
    • Intermittent vomiting
    • Incoordination
    • Seizures
    • Sudden death
  • Ferrets — Ferrets’ heartworm disease signs are similar to those of cats. Ferrets’s signs include decreased activity, cough, breathing difficulty, and weakness.

How often should pets be tested for heartworms?

Although your pet takes year-round preventives, your veterinarian should test them annually for heartworm infection. Canine heartworm screening is a simple blood test that detects proteins adult female worms produce. In cats, heartworms are more difficult to detect. The feline test detects antibodies produced by a cat’s immune system in response to the parasites. If your pet’s heartworm test is positive, your veterinarian may do additional blood work, take X-rays, and perform diagnostic screenings to determine disease severity and assess your pet’s overall health before beginning treatment.

How can heartworm disease be prevented in pets?

Treating heartworm disease in dogs can be expensive and challenging, and no heartworm treatment is available for cats, making prevention the only means of protecting your feline friend from this life-threatening disease. Every dog and cat should receive year-round preventives to ensure they are fully protected. Preventives come in many forms, including:

  • Monthly oral medications
  • Monthly topical medications
  • Six-or 12-month injectable medications administered by our veterinarians 

Now that you’ve completed Heartworm 101, you can help ensure that your pet never has to experience the devastating effects of this disease. When your pet is due for their annual heartworm test or preventive refill, contact our Town & Country Animal Hospital team.