What is Heartworm in Dogs and How Do You Prevent It

What is Heartworm in Dogs and How Do You Prevent It

Being a dog parent means you'll spend a few hours outside throughout your dog's lifetime. From daily walks to dog parks, your dog thrives in the great outdoors.

The unfortunate reality is that pesky bugs, like mosquitoes and ticks, also live outdoors. Mosquitoes are most prevalent in areas with warm, humid climates.

They can be found all over the United States throughout the year — similar to flea season. Dogs that spend a lot of time outdoors where mosquitoes thrive may be at risk of heartworm infection.

Heartworm disease can cause serious health problems for your pet. So, it's important to know how it is transmitted and what you can do to prevent it.

In this article, we'll explore:

What is a heartworm

Heartworms are long, parasitic worms that live in the blood vessels of animals. They usually reside near the heart, lungs, and related blood vessels. They are most commonly transmitted to dogs by an infected mosquito.

Fully-grown female heartworms can reach up to 14 inches long, while male heartworms reach about half the size. An infected animal can inhabit more than 300 worms at one time. This large number and the size of heartworms in your pet's heart and lungs can turn fatal very quickly.

What are the life stages of heartworm

The heartworm life cycle has three distinct stages from larvae to adult worms:

The cycle typically begins when an infected mosquito bites your dog. An infected female mosquito bites a dog, cat, or wild animal and transmits immature heartworms, called microfilariae, into the bloodstream.

Larvae (L1)

After an infected mosquito bites your dog, the infective larvae enter through the bite wound and begin to develop. The larvae travel through the bloodstream to different organs where they develop into adult worms and mate.

Infective larvae are the most dangerous stage because they migrate through the blood vessels into your dog's lungs. After about four weeks, infective larvae develop into immature heartworms (L2) before moving to the right side of your dog's heart.

Immature heartworms (L2, microfilariae)

Left untreated, infective heartworm larvae will eventually reach the immature heartworm (L2 or microfilariae) stage. The microfilariae travel to the heart and lungs where they mature into adults.

Immature worms develop for several weeks before reaching adulthood. After that, they mature into foot-long adult heartworms that live in the right heart chamber and pulmonary arteries.

Adult heartworms

The time it takes for these parasites to develop into adult heartworms depends on the age of your dog. Younger, smaller dogs can take as little as three months to become infected. Larger breeds may take up to six or seven months.

When adult heartworms are present, they can cause:

Adult female heartworms deposit their eggs in the host’s blood supply. The adult worms' eggs travel back to their original site in the body. The eggs may then be ingested by mosquitoes when they bite an infected animal for a blood meal. This ensures the life cycle continues again and again.

Where and how do dogs get heartworm disease

How Do Dogs Get Heartworm Disease

Heartworm disease is a serious infection in dogs and cats. The mosquito is the primary host of heartworm, and they're found all over the world. If you live in an area where mosquitoes are common, your dog's risk of becoming infected with heartworms is high.

The most common high-traffic states are located mostly in the south. These include:

Heartworm disease affects dogs of all qualifications. This includes puppies and adult dogs, active dogs and sedentary pups, and all breeds and genders.

Pets or infected animals

The parasites that cause heartworm disease depend on the blood of animals. When mosquitoes bite and carry an infective heartworm from another animal, that mosquito bite could transmit disease to your pet.

Heartworm is most commonly spread through direct contact with infected animal (or infected cats') urine or feces. Heartworm infection can also happen by direct exposure to another infected dog. Or, by eating one that has already passed away from symptoms caused by heartworm disease.

The American Heartworm Society states that heartworm infection can affect dogs, cats, and ferrets, but also:

Blood transfusions

While rare, if donated blood or organs come from a dog with heartworm infection your dog could get infected. However, adult worms typically die long before a transfusion. Plus, dogs are tested prior to being chosen for blood transfusions or organ donations.


Dogs cannot get heartworm infections from humans. Canine heartworm disease occurs when a specific parasitic worm is transmitted to dogs. Humans do not carry this type of worm and cannot transmit it to dogs or other animals.

What is heartworm disease

Heartworm disease is a serious infection caused by heartworms. It is very common in dogs and cats, but it can also affect humans. Heartworm disease in cats often looks very similar to heartworm disease in dogs.

Heartworm disease occurs from an infestation of a worm called Dirofilaria immitis. This parasite is transmitted through infected mosquitoes. The parasites then travel through the bloodstream to their heart and lungs.

Untreated, heartworms can stay in your pet's heart and lungs for up to five years. Once in the body, the worms live on the right side of your dog’s heart and pulmonary arteries. These are the major blood vessels leading to your dog's lungs.

When these worms die, they cause inflammation of the pulmonary arteries and surrounding tissues. As heartworm disease progresses, the arteries become plugged with the worms' eggs and debris. This can lead to serious complications, particularly blood flow restriction.

The only way to get rid of these dangerous creatures is with medication prescribed by your vet. In severe cases, heartworm blockages may need surgical removal. Otherwise severely infected dogs may face serious and potentially fatal complications.

How to tell if your dog has heartworm

It can be difficult to diagnose heartworm disease in dogs. This is because many clinical signs of heartworm disease are similar to general pet wellness issues.

The following clinical signs can indicate that your dog has heartworm disease:

Heartworms live in your dog's heart and associated blood vessels. This can cause damage to your dog's cardiovascular system which can lead to heart failure.

Inflammation of the pulmonary arteries and surrounding lung tissue leads to other mild symptoms. Common signs include weight loss and fluid accumulation around the lungs.

More severe signs of heartworm disease in dogs include:

Can your dog test positive for heartworm infection

Heartworm Tests for Dogs

If your dog has shown signs of heartworm disease, you may wonder, "can my dog get tested for canine heartworm disease?" The answer depends on your vet and the heartworm test methods they offer.

According to the American Heartworm Society, dogs should receive heartworm tests every year to detect the presence of heartworm. Dogs with heartworm disease should start heartworm disease treatment and then get tested again in six months.

Blood tests are the only type of heartworm test that confirms if your dog has heartworm disease. But, there are other tests that can help determine whether or not your pet has been infected with heartworms. The two most common types of blood tests are microfilariae tests and antigen tests.

Antigen Tests

An antigen test is a type of blood test that detects whether heartworm proteins are present in your dog's body. An antigen test usually starts with a urine sample from a dog which will show evidence of antibodies. These are produced by the immune system in response to an antigenic stimulus such as a heartworm.

Microfilariae Tests

This type of blood test detects if there are microfilariae (immature adults) in the bloodstream. If so, your dog is likely infected with adult heartworms. Microfilariae blood tests are challenging because it takes six months to detect them in your dog's bloodstream.

Testing for heartworm is pretty routine. The results of these tests will help determine whether or not further testing should be performed prior to starting treatment options. If you suspect that your canine friend may have been exposed, schedule a dog check-up for annual testing right away.

Types of heartworm disease prevention

Heartworm disease is a serious condition that requires regular testing and treatment. With preventative care, it’s easy to protect your pet against this serious disease. But there are some key facts you should know about heartworm prevention.

  1. Your dog should receive a heartworm test every year. Heartworm tests are usually a standard part of an annual dog check-up. But if you want a reminder, use the "Think 12" method from the American Heartworm Society. This rule encourages pet parents test to their dogs every 12 months and protect their pets all 12 months.
  2. Heartworm medication should be given year-round. Even when temperatures are cold or rainy. You may need to give your dog more frequent doses if they've been exposed to animals with heartworm infections. Some examples of exposure include swimming in a pond, hiking, and camping.
  3. You need to schedule a visit with your veterinarian. All types of heartworm prevention require a heartworm test prior to starting treatment and a prescription. Before exploring options, schedule a vet visit and discuss options for your dog.
  4. Heartworm prevention doesn't kill adult heartworms. But don't worry, the medication will still be effective in helping protect your pet. Preventives actually treat the microfilariae that are currently in the bloodstream before they have a chance to develop into adult heartworms.

Heartworm preventives are available in a variety of forms:

Oral Heartworm Prevention


The most common heartworm preventives are chewable tablets that can be given with food. They can usually be mixed with food or placed inside a treat to make them more palatable. Heartworm prevention chews need to be given once a month.

Vet Tip

Dogs with heartworm disease will need special care. A doctor of veterinary medicine (DVM) will treat your dog with a high concentration of ivermectin. This medication helps kill adult heartworms in your dog's body. Your vet may also suggest surgical removal or prescribe heartworm preventive(s) to keep your dog protected.

Topical Heartworm Treatment

Topical solutions

Topical heartworm preventives are spot-on treatments applied to your pet’s skin and hair follicles. The active ingredient, moxidectin, helps kill heartworm and ticks within 24 hours. Topicals are usually very affordable and are a good alternative if your dog doesn't enjoy their oral heartworm preventive treatment.


Injections are a convenient option for treating heartworm disease in dogs who are at least six months old. This type of heartworm treatment is administered by a vet and lasts for about six months.

In some cases, heartworm injections may be accompanied by other treatments. These include pain medication like carprofen to help manage any discomfort or inflammation when the adult worms die.

Surgical removal

Heartworm surgery is not a replacement for traditional heartworm treatment. It is typically only used in cases where the standard treatment has been unsuccessful or is not an option. If you are considering surgery for your dog's heartworm infection, be sure to discuss the risks and benefits with your vet.

The best way to prevent heartworm disease in dogs

Heartworm disease in cats and dogs is serious and potentially fatal. It's best to keep your pet on a heartworm prevention program. Especially if you live in an area where mosquitoes are common.

Starting heartworm prevention when your puppy is old enough is highly recommended to:

Heartworm infection and disease are dangerous and costly. That's why heartworm prevention is well worth the investment for your pet's health and happiness.

Looking for ways to keep your pet protected this flea & tick season? You’ll love Wags Advance for Dogs!

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