What causes heartworm disease?
Heartworm disease, or dirofilariasis, is a serious and potentially fatal disease. It is caused by a blood-borne parasite known as Dirofilaria immitis.
Adult heartworms are found in the heart, pulmonary arteries (the blood vessels that carry blood from the heart to the lungs), and adjacent large blood vessels of infected dogs. Rarely, worms may be found in other parts of the circulatory system.
Female adult heartworms are 6–14 inches (15–36 cm) long and approximately 1/8 inch (3 mm) wide. Males are about half the size of females. One dog may have as many as 300 worms present when diagnosed.
Adult heartworms may live for five to seven years. During this time, female worms produce millions of offspring called microfilariae, which live mainly in the small vessels of the bloodstream.
How is heartworm disease spread?
Heartworm disease requires mosquitoes to spread from one dog to another. When a mosquito bites an infected dog, it ingests microfilariae during its blood meal. These immature heartworms develop inside the mosquito for 10 to 14 days, and become infective larvae, and enter the mosquito’s mouthparts.
Then, when the mosquito bites another dog, the infective larvae enter the dog's body. The larvae migrate through the dog’s bloodstream to the heart and adjacent blood vessels, where they mature into adult heartworms. Within six to seven months, they begin mating and producing new microfilariae.
Transmission requires the mosquito as an intermediate host, so heartworm disease is not spread directly from dog to dog. Disease spread coincides with mosquito season, which can last year-round in many parts of the United States. The risk of infection is greatest when mosquitoes are actively feeding, which typically requires temperatures above 50°F (10°C).
Where is heartworm disease found?
Canine heartworm disease occurs all over the world. In the United States, the highest numbers of reported cases are found in the southeast, although heartworm disease has been detected in all 50 states. In Canada, the disease is most common in areas where mosquitoes are prevalent, including southern British Columbia, Manitoba, Ontario, Quebec, and New Brunswick.
How does heartworm disease affect dogs?
It usually takes several years before dogs show clinical signs of infection. Dogs of any age, breed or sex may be affected, although the disease is rare in dogs less than one year of age because heartworms take five to seven months to mature. Unfortunately, by the time clinical signs are noticed, the disease is usually well advanced.
Damage to the Heart and Lungs: Adult heartworms cause heartworm disease through their effects on the heart and pulmonary arteries. They cause chronic inflammation that leads to scarring and narrowing of the pulmonary arteries and fibrosis (thickening) of the surrounding lung tissue.
These irreversible changes cause pulmonary hypertension (increased blood pressure in the vessels of the lungs), which means the heart must work harder to pump blood to the lungs for oxygen. Heartworms can also interfere with heart valves, further increasing the workload on the heart and eventually leading to heart failure.
Effects on Other Organs: Heartworms may also affect the kidneys and liver. The severity of disease depends on the number of worms present, how long they have been present, and the amount of damage sustained by the heart, lungs, liver, and kidneys. Even relatively low numbers of worms can cause severe heartworm disease.
Microfilariae circulate throughout the body but remain primarily in the small blood vessels. These immature heartworms can block blood flow, depriving tissues of oxygen and the nutrients. The lungs, liver, and kidneys may all be affected. Destruction of lung tissue leads to coughing. Liver injury leads to cirrhosis of the liver, causing jaundice, anemia, and generalized weakness. Kidney damage can allow toxins to accumulate in the body.
What are the signs of heartworm disease?
Early stages of infection often do not cause obvious signs. As the disease progresses, dogs may develop:
- A soft, dry cough
- Shortness of breath especially after exercise
- Weakness, listlessness, and loss of stamina (exercise intolerance)
- Fainting or becoming disoriented
Your veterinarian may also detect abnormal heart or lung sounds during a physical examination. As heartworm disease becomes more advanced, congestive heart failure may occur. Signs may include:
- Swelling of the abdomen and legs due to fluid accumulation
- Difficulty breathing due to fluid build-up in the chest
- Pale or bluish gums (cyanosis)
- Weight loss
- Poor body condition
- Anemia
Severely infected dogs may die suddenly during exercise or excitement.
How is heartworm disease diagnosed?
In most cases, one or more simple blood tests will diagnose heartworm disease. Further diagnostic tests are often required in heartworm-positive dogs to determine whether the dog can safely undergo treatment. Recommended tests may include:
- A serological (blood) test for heartworm antigens (ELISA)
- Chest X-rays (radiographs) to assess the extent of heart and lung damage
- Echocardiography (heart ultrasound) to evaluate heart size and function, and determine whether worms are present
- Bloodwork (complete blood cell count and serum biochemistry) to assess for heartworm-associated organ damage
How is heartworm disease treated?
There is some risk involved in treating dogs with heartworms, although fatalities are rare. Many dogs have advanced heartworm disease at the time of diagnosis, meaning substantial damage may already be present in the heart, lungs, blood vessels, kidneys, and liver. In rare cases, disease may be so advanced that managing complications and maintaining comfort is safer than attempting heartworm elimination. Your veterinarian will recommend the most appropriate treatment approach based on your dog’s condition.
Adult Heartworm Treatment: Treatment to kill adult heartworms requires an injectable medication called melarsomine (brand name Immiticide® or Diroban®). Melarsomine kills adult heartworms in the heart and adjacent vessels and is given as a series of injections. Most dogs receive an initial injection, followed by a 30-day rest period and then two additional injections given 24 hours apart. Your veterinarian will determine the specific injection schedule according to your dog's condition.
Additional Medications: An antibiotic called doxycycline is commonly prescribed because heartworms may contain symbiotic bacteria called Wolbachia that help them to survive and reproduce. Prednisone is often prescribed to help reduce inflammation associated with dying worms.
Microfilariae Treatment: Your dog will also receive a drug to kill the microfilariae. Most common heartworm preventives will kill microfilariae. Your veterinarian will determine the safest product to use. Your dog may need to stay in the hospital for observation on the day this medication is administered, often before starting injections for adult heartworms. Following treatment, your dog will be continued on a heartworm preventative.
Why Exercise Restriction Is Critical: Exercise restriction is one of the most important parts of heartworm treatment. Complete rest is essential after treatment. Adult heartworms die within a few days of treatment and begin to break down. As they decompose, fragments are carried to the lungs, where they become lodged in small blood vessels before eventually being reabsorbed by the body. This resorption can take several weeks to months, and most post-treatment complications are caused by these fragments of dead heartworms. For this reason, your dog must be kept as quiet as possible and should not exercise for one month following the final melarsomine injection. The first week after the injections is particularly important because this is when the worms are dying. A cough may persist for seven to eight weeks after treatment in heavily infected dogs. If the cough becomes severe, contact your veterinarian.
If your dog has a significant reaction in the weeks after the initial treatment, prompt treatment is essential. Treatment with anti-inflammatory medications, antibiotics, cage rest, supportive care, and intravenous fluids is usually effective if complications occur. Notify your veterinarian if your dog shows:
- Loss of appetite
- Shortness of breath
- Severe coughing or coughing up blood
- Fever
- Depression
Follow-Up Testing: Your dog will be tested for microfilariae approximately one month after the final melarsomine injection. About nine months after treatment, your dog will be retested for adult heartworms. If either test is positive, the treatment plan will be modified.
The “Slow-Kill” Treatment: The “slow-kill” heartworm treatment protocol uses a continuous monthly heartworm preventive together with doxycycline to gradually eliminate heartworms. This approach is generally not recommended because it may take a year or longer to eliminate all heartworms, allowing additional damage to occur. It also requires strict exercise restriction for a much longer period. However, this method may be considered in certain situations when standard treatment is not appropriate (e.g., if the risk of rapid-kill methods outweigh the benefits, if the dog is likely to die from an unrelated disease, or if melarsomine is not available).
Other Treatments: Dogs with severe heartworm disease may require antibiotics, anti-inflammatory medications, pain relief medications, special diets, diuretics to remove fluid accumulation, and medications to improve heart function. Even after heartworms have been eliminated, some dogs may require lifetime treatment for heart failure, which may include diuretics, ACE-inhibitors, beta-blockers, cardiac glycosides, and low-salt diets.
What is the prognosis after treatment?
Many owners are surprised at the improvement they see after treatment. Dogs that previously showed clinical signs often experience increased energy and vitality, improved appetite, and weight gain.
How can I prevent my dog from getting heartworm disease?
Heartworm disease can be prevented through the regular use of FDA- or Health Canada-approved heartworm preventives. In the United States, year-round prevention is generally recommended. In addition to preventive medication, reducing mosquito exposure is important. Measures include:
- Using FDA-approved or EPA-approved mosquito repellents or insecticides designed for dogs
- Eliminating standing water where mosquitoes breed
- Limiting outdoor activity during peak mosquito activity at dawn and dusk
Dogs that have been successfully treated for heartworms can become infected again if they are not protected. With the safe and effective preventives available today, heartworm disease is largely preventable. Talk with your veterinarian about the best heartworm prevention program for your dog.
