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Heartworms 101: Danger, Prevention, and Treatment

sick dog at vet

Heartworms are parasites that infect both dogs and cats. If left untreated, these parasites cause major health issues for pets and even result in death. When a mosquito bites an animal, it can transmit heartworm larvae into a pet’s bloodstream through capillaries in the skin. While blood flows through the heart, the larvae settle there and become adult heartworms.

What Happens When a Pet Gets Heartworms?

Adult heartworms cause havoc in a pet’s body. When heartworms live in a pet’s heart, the heart becomes filled with worms. These worms reproduce and release larvae into the bloodstream, continuing the parasites’ lifecycle. When the heart is full of parasites, it needs to work harder to pump blood, causing a thickening of muscle tissue and enlarging the heart.

Soon the heart has to work so hard to pump blood that it will start to fail. Once a pet enters heart failure from heartworms, it’s difficult for them to recover. Heartworm heart damage often leads to congestive heart failure, which builds up fluid in the heart, lungs, and abdomen. The animal’s body simply cannot move fluid properly anymore. The kidneys begin to become damaged, and eventually, the heart fails. 

How Do You Treat Heartworms?

Heartworm infection is very dangerous, but it can be treated if caught early.

The most effective treatment is called the “fast kill” method, which involves injections into the deep muscles around the spinal area. These injections are followed by 30- to 90-days of mandatory rest. A veterinarian may also prescribe steroids or antibiotics to supplement treatment. If the situation is severe, it may require hospitalization.

There is also a “slow kill” method that kills larvae and prevents heartworms from reproducing. Eventually, adult heartworms die off without adding any new worms. The American Heartworm Society does not recommend this method because while heartworms live in the heart, they can continue to cause damage.

Heartworm Symptoms and Testing

The most common symptoms of heartworms are coughing (especially with exertion) and tiring quickly after normal activities like walking or playing. You might also see a bloated appearance in a pet’s abdomen.

You will want to get a test done before starting any prevention and then again in six months. It takes six months for a test to show positive once infected. If your pet has never been on heartworm prevention or has not in a while and develops a cough, it is best to rule out heartworms with a visit to the veterinarian as soon as possible. If your pet is on a heartworm prevention medication, veterinarians recommend a heartworm test once a year.


How Can I Prevent My Pet From Getting Heartworms?

Both dogs and cats can become infected with heartworms, though it is less likely for a cat. This is thought to be due to cats’ sensitive skin, which reacts quicker to touch, and can shake off a mosquito before it bites.

There are no over-the-counter medications that can prevent heartworms safely. Your veterinarian will need to test your pet and then prescribe medication. There are three types of heartworm prevention available for dogs and cats: pills, topical, and injections.  

Heartworm Prevention Pills

The most common way to prevent heartworms in dogs is to use oral medication, typically a monthly pill. These pills may contain a variety of preventatives, including flea, tick, and intestinal parasite control. Oral medication is often available as easy-to-give flavored tablets and is usually cost-effective. Remember, it’s vital to stay on your pet’s heartworm prevention schedule.

If your pet has a sensitivity or a reaction to a certain heartworm prevention pill, several other oral medications options are available.

Heartworm Prevention Topicals

If you have a pet who cannot take a pill, there are some limited options for heartworm prevention in the form of a topical medication.

Revolution is a leading topical brand that prevents fleas, heartworm, and some intestinal parasites. Both dogs and cats can use this monthly medication even if they get wet or go outside. Refrain from bathing your pet for 48-72 hours before and after application to ensure the topical medication can become fully absorbed. Once absorbed into the skin, it is absorbed into the bloodstream. 

Heartworm Prevention Injections

A newer heartworm prevention medication is an injectable called ProHeart. There are two injections available: one lasts for six months, and the other lasts for twelve months. These are a veterinary favorite in heartworm prevention as they are usually well-tolerated and remove the need to remember monthly medications.

This injection protects against heartworms and hookworms, but it does not protect against fleas, ticks, or other parasites. Because it is newer, there is also less parasite resistance to it.

ProHeart is only for dogs who are six months or older. Dosage is weight-based, and since it is in the bloodstream for an extended time, the pet must be kept at a consistent weight.

Remember, there are no over-the-counter medications that can safely prevent heartworms. There may be products that can help repel mosquitos, but like humans who use insect repellent, it is not 100% guaranteed to prevent bites and may not work for some dogs.

Where Do Heartworms Occur?

Heartworms can happen anywhere in the United States, although there are areas more affected than others. Southern and southwestern states are the most affected.


If you live in an area where heartworms do not seem to exist, you can talk to a veterinarian about your best next steps to keep your pet protected from mosquito bites.

It is safer to prevent than to take a chance, but speaking with a veterinarian when making this decision is best. Hello Ralphie veterinarians can offer guidance for pest prevention, including heartworms.

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