Heartworm Prevention for Dogs: Symptoms and Treatment

Mosquitos are not just annoying insects that bug you while you’re out grilling or taking a hike. They’re carriers of a hidden threat that can harm your dog and could even be fatal: heartworm. The good news? Heartworm disease is preventable. Let’s explore its causes, treatment options and how to protect your dog.

What Is Heartworm Disease in Dogs?

Heartworm disease is caused by a parasitic roundworm (Dirofilaria immitis), which is transmitted through mosquito bites. Once infected, dogs serve as the natural host, allowing the heartworms to mature into adults and produce microfilaria (immature larvae) that circulate in the bloodstream. Adult worms live in the heart, pulmonary artery and surrounding large blood vessels of infected dogs. If left untreated, heartworm disease can lead to severe organ damage and is a potentially fatal disease. Heartworm infection has been diagnosed in all 50 states and around the globe.

How Do Dogs Get Heartworms?

Mosquitos play a vital role in the life cycle of heartworms. According to the American Heartworm Society, when a mosquito bites an infected animal, it ingests microscopic worms called microfilariae, which are produced by adult female heartworms in the animal’s bloodstream. When this infected mosquito bites another susceptible animal, it transmits infective heartworm larvae into the new host through its bite.

After entering a new host, heartworm larvae typically mature into adult worms within about six months. Once they reach adulthood, heartworms can live significantly longer in dogs, surviving for five to seven years, while in cats, they usually live for two to three years. This extended lifespan allows heartworms to accumulate in infected pets, meaning that with each mosquito season, the number of worms present can increase.

Heartworm disease is not contagious, meaning that a dog cannot catch the disease from being near an infected dog. Heartworm disease is only spread through the bite of a mosquito.

What Are the First Signs of Heartworms in Dogs?

In the early stages of heartworm disease, many dogs exhibit few or no symptoms. This is why it is important to have routine heartworm testing and regular veterinary checkups. As the heartworm infection continues, signs of heartworm disease will gradually start to appear.

Some of the first signs of heartworm infection you may see are an intolerance for exercise and a mild persistent cough. If your dog usually has a ton of energy and now seems unusually tired after mild activity or doesn’t want to play or exercise, that may be a sign of heartworm infection. These changes may be subtle at first, but over time they will become more noticeable.

If you start to see the early signs of heartworm disease and know that you live in an area where your dog may have been exposed to mosquito bites, consult your veterinarian. Early detection can help protect your dog from serious health issues.

Advanced Heartworm Symptoms

As the adult heartworms start to mature, the symptoms of the disease will be very noticeable. Infected dogs will continue to cough, be reluctant to exercise and have trouble breathing. Some dogs may cough up blood. Other symptoms may include pale gums, weight loss, fainting spells, swollen belly, decreased appetite, high blood pressure and/or rapid heartbeat.

As heartworm disease progresses, the symptoms will continue to deteriorate the health of the infected dog and there will be abnormal sounds in the dog’s heart and lungs as well as an enlarged liver.

In advanced heartworm disease, even with treatment, there is a high risk of long-term damage to the dog’s health, including damage to the heart and associated blood vessels, and, unfortunately, death.

Heartworm Tests to Confirm Infection

Blood tests, performed by a veterinarian, are used to determine if a dog has heartworms and diagnose heartworm disease. According to the U.S. Food and Drug Administration, an antigen test identifies whether specific proteins released by adult female heartworms are present in the dog’s blood. Typically, antigen tests reliably identify infections caused by one or more heartworms. The earliest at which these heartworm proteins can be detected in a dog’s blood is approximately five months after the dog has been bitten by an infected mosquito.

Another type of heartworm test looks for microfilariae in the dog’s bloodstream. The presence of microfilariae indicates that the dog has an adult heartworm infection, as only adult heartworms can reproduce and generate microfilariae. Microfilariae can first be detected in a dog’s bloodstream approximately six months after it has been bitten by an infected mosquito. This timeline reflects the period required for heartworms to mature from infective larvae into adult worms that reproduce and produce microfilariae.

What You Need to Know About Heartworm Treatment

It’s important to understand the difference between heartworm prevention and treatment. Prevention is given to healthy dogs to prevent heartworm infection, while treatment is required when a dog tests positive for heartworms and is sick.

Dogs showing no signs or only mild symptoms of heartworm disease, such as a cough or decreased exercise tolerance, typically respond well to treatment. Even dogs with more advanced disease can be treated successfully, though the risk of complications increases. Notably, the number of worms in a dog’s body doesn’t always match the severity of symptoms; some heavily infected dogs may show few signs early on.

As a pet parent, it’s important to understand that treatment, while often effective, does come with some potential risks and can be quite costly. The treatment process involves powerful prescription heartworm medications, and these drugs can place stress on your pet’s heart and lungs. During treatment, dogs must be strictly rested to reduce the risks of complications, as dying worms can restrict or block blood flow in the lungs.

Dog Life Expectancy after Heartworm Treatment

Dog life expectancy after heartworm disease treatment varies depending on the dog and the severity of the infection. If caught early, dogs can be successfully treated and go on to live a normal life expectancy. But if there has been damage to your dog during infection, then there is an increase in early death due to compromise on organ function, especially your dog’s lungs.

Overall, the life expectancy after treatment depends on many factors such as how long they were sick, how much damage was done, how the treatment was tolerated and the overall health of the dog.

Heartworm Prevention for Dogs

Prevention is a simple and effective way to protect your dog from heartworm infections. The Merck Veterinary Manual states that a prevention program in dogs is recommended beginning at 6 to 8 weeks of age and no pretesting is necessary. For starting treatments in dogs 7 months or older, it is recommended to have your dog tested to make sure the dog is not already infected and another test 6–7 months later.

Most heartworm preventative medication is given monthly, either as a liquid topical medication applied on the skin or as an oral tablet. Both chewable and non-chewable oral tablets are available. One product is injected under the skin every 6 or 12 months, and only a veterinarian can give the injection. Some heartworm preventives contain other ingredients that are effective against certain intestinal worms (such as roundworms and hookworms) and other parasites (such as fleas, ticks, and ear mites).

A year-round heartworm prevention program is recommended for all dog owners. The American Heartworm Society recommends following the “Think 12” approach: administer heartworm prevention to your dog every month and have them tested for heartworms annually. Talk to your veterinarian about the best plan to protect your dog from heartworm disease.