Heartworms: Diagnosis, Treatment, and Prevention

As a veterinarian, I am always speaking with my clients about different things they can do to keep their pets healthy. One of the most frequent and important conversations I have is regarding heartworms. Heartworm disease is a potentially fatal disease found not only in the United States but across the globe. It is caused by long worms that live in the right side of a dog or cat’s heart, associated blood vessels and lungs of a pet with an active heartworm infection.

These heartworms, left untreated, can cause a number of serious problems for an infected animal such as heart failure, lung disease, or multi-organ damage that can lead to death. Dogs, cats, and ferrets are the main species that can contract heartworm disease but heartworms can also live in other species like coyotes, foxes, and wolves. These wild species usually live close to urban areas which make them likely carriers of the disease.

The dog is the normal host for a heartworm. This means that the heartworm can mature in the dog to the adult stage and produce the baby worms to complete the life cycle. Cats are not the normal host and most heartworms do not live to the adult stage. The main difference between dogs and cats getting heartworms is the number of worms that the animal has living inside the body.

An infected dog can have hundreds of worms whereas cats will usually only have 1 to 3 worms. Heartworm disease can cause permanent damage to the animal’s heart, lungs, and associated blood vessels. Even if the animal is treated early in the disease, their health and quality of life can still be affected even after the worms are gone. Prevention of heartworms is the key to ensuring your dog or cat stays heartworm free.

There seems to be a myth that only pets that live along the Gulf Coast and Atlantic region are at risk for contracting heartworm disease but this is false. Heartworms have been found in all 50 states. Yes, there are certain factors such as temperature and humidity that make heartworms more prevalent in certain areas of the country. Areas of the country that have a higher average daily temperature and higher humidity tend to have more mosquitos which is how heartworms are transmitted. No matter where you live, your pet is at risk.

Heartworm Life Cycle

The figure above represents the heartworm life cycle and how transmission occurs. The adult female heartworms that are living inside an infected dog begin to release the baby worms, called microfilariae, into the blood stream. When a mosquito lands on the infected dog to get a blood meal, it ingests the baby worms at the same time. Over the next 10-14 days the baby worms will reach an infective life stage inside the mosquito’s mouth. The baby worms cannot become infective without passing through the mosquito first. Development into the infective life stage inside the mosquito is dependent on temperature. Development will stop if the temperature drops below 57 degrees Fahrenheit and will resume once the temperature rises above 57 degrees Fahrenheit.

When an infected mosquito bites another dog, the mosquito will deposit the baby worms onto the skin of the dog. The baby worms will migrate through the bite wound into the tissues under the skin and will molt into the next life stage over the next 1-3 days. It takes between 6-7 months for the baby worms to mature into adults. The adult worms will mate and release their baby worms into the dog’s blood stream which will complete the life cycle. Mature adult female heartworms can reach lengths of 10-12 inches and mature adult male worms can reach lengths of 5-6 inches.

Clinical Stages of Heartworm Disease

Stage 1: Most animals in stage one will show little to know symptoms. The dog may have an occasional cough.

Stage 2: At this stage the dog may start to show signs of lethargic behavior, be more tired after periods of activity such as walks, and may have the occasional cough.

Stage 3: The cough is now more persistent, the dog begins showing signs of weight loss, lethargic behavior, exercise intolerance, difficulty breathing, and distended abdomen.

Stage 4: This stage is life threatening. All of the above symptoms can be seen as well as dark, coffee colored urine. Surgical removal of the worms is often needed at this stage.


Different testing that might be recommended to aid in the diagnosis could include:

  • In-house or external heartworm test

  • Complete blood count

  • Chemistry analysis

  • Urinalysis

  • X-rays

  • Echocardiogram


If your dog tests positive, the Veterinarian may want to run an additional test to confirm a positive diagnosis and ensure that treatment is absolutely necessary. Dogs that are in Stage 1, 2, or 3 are usually candidates for heartworm treatment if their condition is stable. If the dog’s condition is not stable, be prepared for heartworm treatment to be postponed until the pet’s condition is more stable.

Currently there are two drugs that are approved by the FDA for treatment of heartworms but one is not manufactured in the United States. The drug commonly available for treatment in the United States is called Immiticide. This medication is given by injection deep into the muscles of the back. There are different guidelines for treatment which your pet’s Veterinarian can review with you once your pet is found to be stable enough to receive treatment. After treatment has been completed, it is recommended to repeat a heartworm test 6 months later to ensure the treatment was successful.


A potential heartworm infection is prevented by giving your pet monthly heartworm preventative. Heartworm preventatives can be given orally, by injection, or by applying a topical product directly onto the pet’s skin. The risk of puppies and kittens being infected with heartworms is the same as adult dogs and cats. It is recommended that puppies and kittens be started on heartworm prevention no later than 8 weeks of age.

Monthly heartworm preventatives have more benefits than just preventing heartworms. Depending on the product being used, most monthly preventatives offer protection against intestinal parasites. Some of the intestinal parasites include roundworms, hookworms, whipworms, and tapeworms. Some heartworm preventions are also effective against fleas, ticks, and ear mites. There are certain products that are not safe to give if a pet has a certain medical condition. You should always make sure your Veterinarian knows your pet’s entire medical history so the appropriate product can be selected.

We all live crazy, busy lives. We recommend that you keep a schedule that is visible and have the dates marked clearly when your pet is due to receive their heartworm prevention every month. The prevention needs to be given every 30 days. 

Published by Dr. Mason Romero, DVM


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