Heartworm in Dogs: Signs, Treatment & Prevention

Key Points

1. Heartworm disease in dogs is fatal when left untreated.
2. While heartworm disease can be treated, dogs may have residual damage and lifelong issues. Thus, it is critical to prevent heartworm from occurring.
3. Heartworm disease historically was confined to the Southern and Southeastern U.S. However, it can now be found throughout the continental U.S. and Hawaii, so any dog is potentially at risk.
4. Climate change, global warming, and mosquito migration to new areas increase the risk of spreading heartworm disease.

What is heartworm in dogs?

Heartworm disease in dogs is caused by a roundworm known as Dirofilaria immitis. Transmitted by the bite of a mosquito, this parasite can lead to blockage of the main artery in the lungs in dogs. It can cause disease in cats as well as in people. Though a dog or cat cannot directly transmit it to humans.

Causes of heartworm disease in dogs

Natural D. immitis hosts exist in nature. This life-threatening parasite naturally exists in domestic dogs and wild canids, e.g., foxes and wolves. A different subspecies (also found in people), D. tenius, can be found in raccoons but, thankfully, is not present in North America.

Heartworm, a global concern

Heartworm is found globally. Factors including climate, variable temperatures, weather patterns, and the presence of mosquitoes affect to what degree heartworms may be found in an area. In the U.S., heartworm was once confined to the southeastern and southern states. However, due to climate change, the movement of dogs nationwide via adoption programs, and other factors, heartworm disease can be found throughout the country.

Heartworm’s life cycle

Because dogs are definitive hosts, D. immitis can reproduce in this species. In dogs, the parasites multiply in the arteries feeding the lungs (pulmonary), releasing the young (microfilariae) into the bloodstream. As a result, when a mosquito takes a blood meal and ingests the microfilariae (MF), these MF continue to develop and then migrate to the mosquito’s proboscis (mouth part of the mosquito that takes the blood meal). The next time that mosquito takes a blood meal, it can infect the next species it bites.

While some species, like dogs, can continue the cycle, allowing the organism to reproduce, others, e.g., humans and cats, are considered accidental or dead-end hosts. While the larvae develop into adults, they cannot reproduce.

For a detailed review of the heartworm lifecycle and other information about the disease, feel free to visit the Companion Animal Parasite Council’s (CAPC) website. This independent, non-profit group aims to improve awareness of various parasites. Their vision is to have “Every Pet Tested and Protected.”

Juvenile Heartworms: Microfilariae

MF are produced in the millions during an adult heartworm’s lifetime (about 5 to 7 years). These young parasites start their life in the smaller blood vessels and can be found in the bloodstream, taking 5 to 7 months to fully develop. Thus, we generally do not see the disease manifest until pets are over 7-12 months of age.

How is heartworm infection transmitted

Since this parasite is transmitted by an intermediate host, the mosquito, dogs cannot directly pass it on to other dogs or humans. Thus, it isn’t contagious in the simple sense of the word. However, the more positive animals there are in an area, the more mosquitoes there are, and the more animals can become infected. Stopping the spread with prevention measures (mosquito control and heartworm preventatives for pets) minimizes exposure to other animals and lessens the transmission to mosquitoes.

Signs and Symptoms of heartworm in dogs

Many dogs are asymptomatic at diagnosis, meaning they show no signs. This occurs, thankfully, because many vets routinely screen dogs annually for this disease. This allows identification before clinical signs develop. However, if a pet does develop clinical signs, they can be mild to life-threatening.

Signs of heartworm in dogs

Signs of heartworm disease in dogs (often more evident after exertion) may include

• No signs (asymptomatic)
• Exercise intolerance (tire easily with exercise; decline to exercise)
• Collapse (fainting)
• Coughing (+/- with blood, often a short, dry cough)
• Difficulty breathing (shortness of breath)
• Weakness
• Weight loss (chronic disease)
• Failure to grow
• Nose bleeds
• Signs of congestive heart failure, including fluid in the lungs or belly
• Caval syndrome (this life-threatening condition occurs when large numbers of worms are present. They block the tricuspid valve’s function in the heart. This causes a type of anemia or low red blood cell count and leads to right-sided heart failure).
• Sudden death (while exercising or highly active)

Your pet may develop symptoms based on the location of the adult worms. In dogs, the worms generally reside in the pulmonary artery, nearby blood vessels, and within the heart. On rare occasions, the parasite may travel to distant locations.

Female worms may reach up to 14 inches in length, while male worms are about half the size. Dogs may have only a few worms or several hundred of them. The signs occur because the worms physically block various essential vessels and interfere with the heart’s ability to function. As a result, blood flow is decreased to multiple organs, including the liver, spleen, kidneys, and lungs. Furthermore, inflammation can develop as worms naturally die off.

Signs vary based on several factors.

• Location of the parasite
• Duration of infection
• Number of adult worms
• Level of damage done to various organs

Heartworm disease testing

Dogs can be tested for heartworm disease. Any pet who presents with signs consistent with heartworm disease, even if on preventative, should be screened. Various in-hospital and more comprehensive tests can be done at the major laboratories. A commercial lab should always confirm positive results found in-house to ensure accuracy.

The first screening test is a blood test. Our initial blood test detects adult parasites in dogs by identifying antigen (an organism protein). A separate test specifically checks for the babies (MF). If confirmed positive, radiographs of the chest are performed to see if there is radiographic evidence of damage caused by the parasites. If there is evidence of damage, this affects the stage of the disease and may affect treatment protocols. In patients with signs of inflammation or damage on X-ray, an echocardiogram (ultrasound of the heart) may be recommended to further stage the disease and assess for damage caused by the worms.

False negatives

We may see dogs that are negative one month and positive a month later, even if they were started on preventatives the day of the initial test. Commonly, veterinarians will test a newly adopted dog, which is positive only for owners to be astonished. Often owners say, “I just adopted the dog about a month ago from the South. He tested negative a month ago and is on heartworm prevention. How can he be positive now?”

Because of the parasite’s life cycle, it takes 6.5-7 months for adults to develop, thus, for the test to detect adult worms. If a dog is tested the day it is adopted, it may have been infected with the parasite by a mosquito bite days to months before. If no adult worms are present, the test will remain negative until adults are detectable in the blood. Preventatives address the larval stages of the parasite. If just one of those larval stages develops into an adult, we can see negative tests become positive. It takes time for the MF to die off; if even one matures into an adult, the test will be positive.

The treatment for canine heartworm disease

Treating heartworm in dogs is feasible, though not without risks. The best thing any owner can do is prevent the disease from occurring. As Benjamin Franklin’s famous proverb notes, “an ounce of prevention is worth a pound of cure.” Though Franklin was talking about preventing fires, it works here too. We can easily prevent this disease, preventing damage to the blood vessels, lungs, and heart and eliminating the risk of death from this infectious cause.

If heartworm disease does develop in dogs, treatment includes

1. Immiticide® (melarsomine) — This drug kills adult heartworms (adulticide). While there are a few different protocols out there, most dogs get an injection deep in their back muscle, then 30 days later get 2 injections 24 hours apart. Side effects include pain at the injection site. As worms die off, they can create an inflammatory cascade causing coughing, trouble breathing, worsening exercise intolerance, and more.
2. Strict exercise restriction during therapy and 1-2 months following the last adulticide injection. This is critical. As the worms die off, they can cause irritation and inflammation to various organs and blood vessels until the body clears them. With significant worm burdens, this can worsen clinical signs and even lead to death. Exercise restriction minimizes the work needed by the heart and lungs and lessens the chance of complications.
3. Doxycycline — This antibiotic is prescribed at diagnosis and given for 30 days. Heartworms commonly carry another parasite called Wolbachia. Treating as we address the heartworms allows for a better outcome with overall therapy, lessening the chance of complications and potentially preventing further development of adult worms.
4. Treatment to kill the larval stages (MF) — Various monthly preventatives are used to treat this parasite stage. Talk with your veterinarian about available options. This generally starts during treatment or upon a positive diagnosis, even before treatment begins. By killing the earlier stages, we prevent adults from developing. Since the adults cause the damage and trigger clinical signs, we aim to prevent the bug from maturing to adulthood.
5. Additional medications — Further treatment may be needed depending on the severity of the disease, clinical signs, and underlying conditions.

Additional therapies may include pain medications, steroids, and medications for heart disease or heart failure. The injection used to kill adult heartworm is painful, and pain medications are used for the first few days following injection. Due to the severity of the infection, stage of disease, and risk of an inflammatory response as the worms die, some dogs may need steroid therapy. If animals have developed heart failure, treating this failure will be required and likely lifelong.

Prognosis

For patients who respond well to therapy, the prognosis is usually good. Some dogs recover entirely, never looking back, while others may have residual exercise intolerance or underlying heart disease. For dogs who showed clinical signs, when recovered (2-4 months after therapy), patients often act like puppies again as they feel better and have newfound energy. Despite treatment, some patients succumb to heartworm disease; without therapy, it will eventually be fatal.

How to prevent heartworm in dogs

Veterinarians recommend all-year-round prevention of heartworm disease regardless of where one lives in the U.S. Why? Because the D. immitis’ life cycle is not as straightforward as other roundworms. Besides the fact that with global warming and climate change, mosquitoes are migrating further north, and winters are warmer, the entire lifecycle takes 7 months. Therefore, parasites could still develop if a month of prevention is missed. Just because you don’t see mosquitoes doesn’t mean heartworm couldn’t already have developed in your pet.

Heartworm prevention has come a long way over the past few decades. Products are generally safe and effective in the prevention of disease. Prevention is recommended for all dogs, regardless of where they live, all year round. Even for owners who claim their dogs never go outside, you still can have mosquitoes in your home, and they, too, should be on prevention. Prevention may be monthly, or a newer option is an annual injection.

Sometimes we may see disease breakthroughs occur. This can happen as a result of poor owner compliance. Either an owner fails to give the medication on time every month or intermittently gives it occasionally.

Unfortunately, we also see some drug resistance developing (like with antibiotics and bacteria) in areas near the Mississippi River.

Monthly prevention saves lives

By giving your pet a monthly heartworm preventative or injectable preventative annually, you protect your pup from a potentially life-threatening disease. If your dog were to become positive, your pet would become a reservoir for mosquitoes to transmit the disease to others, even humans. It is better to prevent this disease than treat it once identified, and symptoms have arisen. The treatment itself can have side effects, and the dying of the worms can trigger massive inflammation and even be life-ending.

Never think you cannot have heartworm in your area. Heartworm disease can hit anywhere with increasing average temperatures, warmer winters, and an ever-migrating mosquito population. Take precautions to protect your pet and prevent unnecessary health risks and suffering. Understand the signs and seek help if at any time you suspect your dog may have heartworm disease. The sooner one gets therapy, the better the outcome.