Are Heartworms Contagious?

Heartworms are not directly contagious between dogs and other pets.  

Heartworm disease has been diagnosed in all 50 states and is found globally in every region except Antarctica.  

Heartworms are most prevalent in places where there are lots of mosquitos. The bite of a mosquito can spread heartworms from one animal to the next.  

While heartworms can be fatal, preventing them can easily be done with a monthly chew.  

You may be wondering if heartworms are contagious. Let’s explore the answer to that question in depth. 

Are Heartworms Contagious?

Heartworms can be spread from an infected dog to another dog by the bite of a mosquito. This means that heartworm disease is an infectious disease. 

But heartworms are not spread from an infected dog playing with another dog or even sharing food or water bowls, so heartworm disease is not contagious in that sense. 

The human immune system can safely kill heartworm microfilaria (baby larvae) from the bloodstream if a person is bitten by a mosquito carrying heartworm disease. 

Humans cannot get heartworms from dogs. 

How Do Dogs Get Heartworms?

Dogs get heartworms when they are bitten by a mosquito that has fed from another animal infected with heartworms. 

A heartworm-positive animal’s blood has baby heartworms called microfilaria. After a mosquito feed on a heartworm-positive animal, the microfilaria grows into infectious larvae inside the mosquito over the next two weeks. 

When the mosquito takes another meal, the larvae are left on the second animal’s skin and enter the body through the mosquito bite. 

The larvae then travel from the tissue to the heart and lungs, maturing as they go along. As soon as six months after being bitten by an infected mosquito, a newly heartworm-positive dog will have infectious microfilaria in their bloodstream, and the cycle can be repeated. 

Mosquitos are very good at surviving and weathering many climates. They can readily sneak indoors when doors or windows are briefly opened and bite house pets. While they thrive in hot, wet environments, they can live indoors with only an overwatered houseplant to lay eggs in. It only takes one bite to become infected with heartworms. 

Even dogs who spend most of their day inside can get heartworm disease. 

Dogs at the highest risk for heartworm disease are those who spend a lot of time in a mosquito-rich environment and are not on regular heartworm prevention. 

What Are Heartworm Disease Symptoms?

Heartworm symptoms can include: 

• No symptoms at all 
• Mild, persistent cough 
• Exercise intolerance 
• Fatigue 
• Weight loss 
• Decreased appetite 
• Difficulty breathing 
• Advanced disease signs (collapse, swollen abdomen, pale gums) 

Can Other Pets or People Get Heartworms?

Remember, heartworms are spread by mosquitos. An infected dog cannot give heartworm disease to other pets or people in the household directly. 

People and other pets, like cats, can be infected with heartworms. Fortunately, their immune systems are much better at getting rid of the disease before the heartworms grow into adults.   

When a mosquito carrying microfilaria (baby heartworm larva) feeds on a cat, they can become infected with heartworms. The microfilaria travel through their body to the lungs. The juvenile heartworms can cause damage to the lungs during this movement that leads to asthma-like signs. 

This condition is called HARD, or heartworm associated respiratory disease. The cat’s immune system gets rid of the heartworms before they go any farther. 

In humans, microfilaria is almost always removed from the bloodstream by the immune system immediately after infection. 

People are considered “dead-end hosts” for heartworms. This means that if a heartworm baby enters our body, it’s almost always the end of the line for the heartworm. 

Rarely, people can get heartworm disease, called dirofilariasis. Most people infected with heartworms have no symptoms. 

How Is Heartworm Disease Diagnosed and Treated?

Heartworm testing is part of a dog’s annual wellness visit. Screening tests are done yearly by drawing a few drops of blood. 

There are several different types of heartworm tests used to diagnose heartworm disease, including: 

• Serum blood test: This is the most common type of heartworm screening test that is used routinely. This test looks for an antigen from an adult pregnant heartworm. The antigen can be found six months after the dog has been infected with heartworms. These tests are usually tableside, meaning that your vet can run them in their office and get results in 10–15 minutes. 
• Microfilaria testing: This test involves taking a drop of your dog's blood and smearing it on a microscope slide. The vet can then look at the edge of the blood smear for microfilaria. This test is often done as confirmation after the above screening test is positive to see if the baby heartworms are obviously moving in the blood. 
• Chest X-rays: Radiography (X-ray) is recommended in heartworm-positive dogs to look for any damage in the heart or lungs from the infection. 
• Echocardiography: An echo can be helpful in diagnosing caval syndrome, a life-threatening complication of severe heartworm disease where a bunch of worms plug up the vena cava (large veins) and the right side of the heart. 

Heartworm treatment is a multi-step process spread out over three months. 

For most infections, your vet will start your dog on a medication regimen right after diagnosis. Your dog will likely be prescribed doxycycline and prednisone. 

The doxycycline works to kill a bacterium called Wolbachia that lives inside heartworms. These bacteria have a symbiotic relationship with the heartworms, so killing the Wolbachia weakens the heartworms and makes it easier to treat the disease. 

The prednisone works to reduce inflammation (swelling) and your dog's risk of having an anaphylactic-type reaction to the dying worms. 

After 30 days, your dog will be started on a slow-kill heartworm prevention product, like Interceptor Plus, to kill the microfilaria in their blood. 

This will need to be continued every 30 days long term. 

The following month, they will be given their first melarsomine injection. This injection kills the adult heartworms and is part of a three-injection series. 

Your dog will need to be on very strict activity restriction after starting the adulticide treatment. 

Leash walk only while taking them outside and use a crate to prevent running, jumping, and playing indoors. 

Try to keep them from getting their heart rate up during this mandatory rest period of two months after starting treatment. 

While the adult heartworms are dying, dogs undergoing treatment are at an increased risk of having a worm embolus. 

This happens when a worm or large piece of a dying worm gets stuck in an important blood vessel. This can be dangerous and lead to trouble breathing or sudden death. Activity restriction during treatment and for eight weeks afterward minimizes this risk. 

The adulticide injection is repeated the following month in a two-day series. 

Six months after treatment is complete, your dog can be tested for heartworms again to ensure they are cleared of the disease. 

Continue monthly heartworm prevention long term and get testing annually.   

How Does Heartworm Medicine Prevent Infection?

Monthly heartworm prevention products work by killing the heartworm larvae before they’re able to grow into juvenile and adult worms. 

It takes as little as 51 days for the baby heartworms to grow into juvenile adults. 

Heartworm prevention medicine will not work after the microfilaria have grown this big. 

Classic prevention heartworm medicine works by killing any larvae that have infected your pet the previous 30 days. 

By clearing any microfilaria from the bloodstream monthly, you prevent the heartworms from ever maturing and causing disease. This is why it is very important to not skip a dose. 

There are licensed products on the market now that have slow-release technology. Injectable heartworm prevention medicine given by a vet, like ProHeart 6 or ProHeart 12, is available at some veterinary clinics. 

These injections have encapsulated the medication into slow-dissolving biodegradable beads. This allows for small amounts of the medication to be continually released over time. This option can be helpful for pet parents who struggle to remember a monthly treatment.   

Heartworm prevention should be continued year-round. Mosquitos are very hardy and can even survive cold temperatures indoors by living in damp plant soil or drip trays. 

Treating heartworm disease is very costly and has risks. 

It’s much safer and cheaper to prevent heartworm disease than to treat it if your dog becomes infected. 

Can Heartworm Disease Be Prevented?

Heartworm disease can be prevented. Monthly heartworm prevention is a safe, cost-effective way to protect your pet from heartworms. 

If you struggle to get your dog to eat a monthly chew, there are long-acting injectable products that your vet can give your dog to keep them safe from heartworms. 

Annual testing for heartworms is recommended to catch any infections before they lead to severe disease. Screening ensures that your dog’s prevention is working well and that your dog is still heartworm free. 

While heartworm prevention medications work very well, no product has 100% success. Standard heartworm prevention is over 99% effective at preventing heartworm disease. Because of how well these products work, many heartworm prevention companies offer guarantees. 

This means that if your dog gets heartworms and you can prove that you’ve bought a year's supply and have been giving their prevention regularly, the company may pay for associated treatment costs. 

Reducing mosquito populations in your yard can also reduce your pet’s risk of getting heartworms. Consider pet-safe yard treatments. Local pest control companies are often trained in which products are fine to use around pet areas and how long pets should stay off the lawn after treatment. 

Avoid having any standing water in your yard as it can serve as a breeding ground for mosquitos. 

Heartworm prevention medicine is given by prescription. Be sure to keep up to date on your dog’s annual veterinary visits and testing so there are no gaps in your vet’s ability to prescribe regular heartworm prevention products.