Are Heartworms Contagious?

Heartworms are not contagious from one pet to another or from pets to people through direct contact. The only way heartworm disease spreads is through the bite of an infected mosquito, which carries immature larvae from one animal to the next. That means your dog can’t catch heartworms by sharing a water bowl, playing at the park, or sleeping beside an infected housemate. Even so, a heartworm-positive pet in your neighborhood still raises the local risk, because mosquitoes can carry the parasite from that animal to others nearby. With warm, humid summers and plenty of standing water, the Wabash Valley offers ideal conditions for mosquitoes. At Brown Veterinary Hospital in Terre Haute, pet owners often ask us, “Are heartworms contagious?” In this article, we explain how heartworm disease really spreads, which pets are at risk, what signs to look for, and how year-round prevention protects your companion.

Are Heartworms Contagious Between Dogs, Cats, and People?

The short answer is no, but the full picture explains why every pet in the household still needs protection.

Heartworms (Dirofilaria immitis) are parasitic worms that live in the heart, lungs, and associated blood vessels of infected animals. Adult worms can grow up to 12 inches long and survive for five to seven years inside a dog. Since the parasite needs a mosquito to complete part of its life cycle, there is no direct path from one animal to another. So, are heartworms contagious through saliva, feces, or close contact? They are not.

What About Cats?

Cats can get heartworm disease too, although they are considered atypical hosts. Fewer worms usually survive to adulthood in cats, but even one or two can trigger serious lung inflammation known as heartworm-associated respiratory disease (HARD). Indoor cats are not immune, since mosquitoes easily find their way inside.

Can Humans Get Heartworms?

Human infections are extremely rare. In the uncommon cases where a mosquito transmits larvae to a person, the worms typically die before reaching maturity. Your family is not at risk from petting, cuddling, or caring for a heartworm-positive dog. If you’re bringing home a newly adopted pet and wondering, are heartworms contagious to the dog or cat you already have, the answer is no. Your veterinarian will still want to test the new arrival and start prevention so mosquitoes can’t carry the parasite between them.

How Do Mosquitoes Spread Heartworm Disease?

Understanding the heartworm life cycle makes it clear why mosquitoes are the key to transmission.

Pet owners asking “are heartworms contagious” are often surprised by how many steps it takes for the parasite to move from one animal to another. Here’s how the process works:

• Adult heartworms in an infected animal produce microscopic offspring called microfilariae, which circulate in the bloodstream.
• A mosquito bites that animal and picks up the microfilariae along with its blood meal.
• Over roughly 10 to 14 days, the microfilariae develop into infective larvae inside the mosquito.
• When the mosquito bites another dog or cat, those larvae enter the new host through the bite wound.
• Over about six to seven months, the larvae migrate through the body and mature into adult worms in the heart and lungs.

Wildlife and Untreated Pets Raise the Local Risk

Coyotes, foxes, and stray or unprotected dogs act as reservoirs that keep heartworm circulating in a community. Because one infected animal can supply larvae to many mosquitoes, heartworm cases tend to cluster in areas where prevention isn’t widely used. That’s why heartworms can feel contagious within a neighborhood, even though they never pass directly between pets. When people ask are heartworms contagious in a community sense, this reservoir effect is the real reason local risk rises.

Is There a Heartworm Season in Indiana?

Mosquito activity peaks during Indiana’s warm months, but temperatures can fluctuate dramatically from year to year. Mild winter stretches, garages, and indoor spaces can allow mosquitoes to survive longer than expected, which is why veterinarians don’t recommend skipping prevention during the colder months. Whether you’re wondering are heartworms contagious in January or July, the answer stays the same: any active mosquito can carry them.

What Are the Signs of Heartworm Disease?

Heartworm disease often develops quietly, and many pets show no symptoms in the early stages.

As the worms multiply and grow, they damage blood vessels and make it harder for the heart and lungs to function properly. Symptoms may not appear until months or even years after infection. By that point, the disease has frequently advanced. Active dogs and working breeds may show signs sooner, since exercise puts extra strain on an already stressed heart.

Many owners first start asking “are heartworms contagious” after a neighbor’s dog tests positive. Even if your own pet seems perfectly healthy, that news is a good reminder to confirm your pet is current on prevention and testing, because early infections rarely cause visible symptoms.

Signs of heartworm disease in dogs and cats may include:

• A mild, persistent cough
• Tiring quickly during walks or play
• Reduced appetite and weight loss
• Rapid or labored breathing
• A swollen belly caused by fluid buildup
• Vomiting, coughing, or wheezing episodes in cats, which can resemble asthma
• Sudden collapse, pale gums, or dark urine, which require emergency veterinary care

How Veterinarians Diagnose Heartworms

A simple blood test detects proteins released by adult female heartworms. Your veterinarian may also check for microfilariae and recommend chest X-rays or an ultrasound to assess the extent of heart and lung damage. Testing cats is more complex and may involve several types of tests.

What Treatment Involves

Dogs diagnosed with heartworms can be treated, but the process takes several months and requires strict exercise restriction to reduce the risk of complications as the worms die off. There is currently no approved treatment to kill heartworms in cats, so veterinarians focus on managing symptoms. These challenges make prevention far preferable to treatment. A heartworm-positive dog should also stay on a preventive during and after treatment, as directed by the veterinarian, to stop new infections from developing.

Heartworm Prevention: Your Pet’s Best Protection

Preventing heartworm disease is safe, simple, and far less expensive than treating it.

Once pet owners learn the answer to “are heartworms contagious,” the next question is usually how to keep their pets safe. The American Heartworm Society recommends year-round prevention for dogs and cats, along with annual heartworm testing for dogs. Testing every year confirms that prevention is working and catches infections early, since no product is effective if a dose is missed, given late, or spit out. Understanding the answer to are heartworms contagious helps explain why every pet in a multi-pet home needs its own protection: the mosquitoes, not the animals, are the link.

Your veterinarian can help you choose from several prevention options:

• Monthly chewable tablets: Flavored treats that many dogs take easily, with some products also covering intestinal parasites, fleas, or ticks.
• Topical treatments: Liquids applied to the skin once a month, available for both dogs and cats.
• Long-lasting injections: Veterinarian-administered injections for dogs that provide protection for six or 12 months, eliminating the need to remember monthly doses.

Why Prevention Requires a Prescription

Heartworm preventives require a veterinary prescription because dogs should be tested before starting or restarting prevention. Giving a preventive to a dog that already has adult heartworms can cause a serious reaction. Your veterinarian will make sure your pet is a good candidate and select the right product for their size, age, and health. The team at Brown Veterinary Hospital can also set up reminders so doses don’t slip through the cracks.

Heartworm Testing and Protection at Brown Veterinary Hospital

So, are heartworms contagious? Not directly, but a single mosquito bite can be enough to infect an unprotected pet, and treatment is long and difficult for dogs and limited for cats. The good news is that heartworm disease is almost entirely preventable. At Brown Veterinary Hospital in Terre Haute, IN, our team provides annual heartworm testing, personalized prevention recommendations for dogs and cats, and guidance on choosing a product that fits your household’s routine. If your pet has missed doses, has never been tested, or is starting prevention for the first time, we’re glad to help you get back on track. Call us at (812) 645-0715 to schedule a heartworm test or talk with our team about the best way to protect your pet all year long.