Overview of Canine Heartworm Disease

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 artery (one of the big blood vessels from the heart), 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" long (15 - 36 cm) and 1/8" wide (3 mm). 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, females produce millions of offspring called microfilaria. These microfilariae live mainly in the small vessels of the bloodstream. 

What is the life cycle of the heartworm?

The life cycle of the heartworm is complicated; the parasite requires the mosquito as an intermediate host before it can complete its life cycle in the dog. The mosquito is needed for the life cycle of heartworm. As many as 30 species of mosquitoes can transmit heartworms.

"As many as 30 species of mosquitoes can transmit heartworms."

The life cycle begins when a female mosquito bites an infected dog and ingests the microfilariae during a blood meal. The microfilariae develop further for 10 to 14 days in the mosquito's gut and then enter its mouthparts. At this stage, they are infective larvae and can complete their maturation when they enter a dog. The infective larvae enter the dog's body when the mosquito bites the dog.

These infective larvae migrate into the bloodstream and move to the heart and adjacent blood vessels, maturing to adults, mating, and reproducing microfilariae within 6 to 7 months. 

Where is heartworm disease found?

Canine heartworm disease occurs all over the world. In the United States, the highest numbers of reported cases are still within the southeastern US, but it has been detected in all 50 states. In Canada, the disease is problematic in areas where mosquitoes are prevalent, such as along waterways and coastlines in many provinces. Affected regions include southern British Columbia, Manitoba, Ontario, Quebec, and New Brunswick.

Factors that affect the prevalence of heartworm infection include the species of mosquitoes, the climate, and the presence of reservoir animals. The risk of infection is greatest when mosquitoes are actively feeding. This typically requires temperatures over 50°F (10°C).

How is heartworm disease spread?

Since transmission requires the mosquito as an intermediate host, the disease is not spread directly from dog to dog. Spread of the disease therefore coincides with mosquito season, which can last year-round in many parts of the United States. The number of dogs infected and the length of the mosquito season are directly correlated with the incidence of heartworm disease in any given area.

What do heartworms do to the dog?

It usually takes several years before dogs show clinical signs of infection. Dogs of any age, breed or sex may be affected. The disease is rare in dogs less than one year of age, however, because the microfilariae take 5 to 7 months to mature into adult heartworms after infection. Unfortunately, by the time clinical signs are seen, the disease is usually well advanced.

"It usually takes several years before dogs show clinical signs of infection."

Adult heartworms cause heartworm disease through their effect on the heart and blood vessels that lead from the heart to the lungs (pulmonary arteries). They cause chronic inflammation that leads to scarring and narrowing of the pulmonary arteries and thickening (fibrosis) of the surrounding lung tissue. These irreversible changes cause pulmonary hypertension (increased blood pressure in the vessels of the lungs), which means the heart has to work harder to pump blood to the lungs for oxygen. 

Worms can also interfere with heart valves, further worsening workload on the heart. This will ultimately lead to heart failure. Heartworms may also affect the kidneys or the liver through damaging effects of the body’s reaction to them being in other vessels in the body (caudal vena cava). The signs of heartworm disease can depend on the location of the worms, the length of time the worms have been in the dog, and the degree of damage that has been sustained by the heart, lungs, liver, and kidneys. Even low numbers of worms can cause signs of severe heartworm disease.

The most obvious clinical signs of heartworm disease are a soft, dry cough, shortness of breath, weakness, listlessness, and loss of stamina. All of these signs are most noticeable following exercise, when some dogs may even faint or become disoriented. Your veterinarian may notice abnormal lung and heart sounds when listening to the chest with a stethoscope.

In advanced cases, congestive heart failure may cause the abdomen and legs to swell from fluid accumulation. There may also be evidence of weight loss, poor condition, and anemia. Severely infected dogs may die suddenly during exercise or excitement.

Microfilariae (immature heartworms) circulate throughout the body but remain primarily in the small blood vessels. Because microfilariae are about as wide as the small vessels, they may block blood flow in these vessels. The cells being supplied by these vessels are then deprived of the nutrients and oxygen normally supplied by the blood. Microfilariae primarily injure the lungs and liver. Destruction of lung tissue leads to coughing. Liver injury leads to cirrhosis of the liver, causing jaundice, anemia, and generalized weakness. The kidneys may also be affected and allow toxins to accumulate in the body.

How is heartworm disease diagnosed?

Dogs can be tested for heartworm disease by a simple blood test that can be run in 10 minutes at most veterinary clinics. Because of the life cycle of heartworms, puppies under 6 months should be started on heartworm prevention as soon as they are adopted. They receive their first heartworm test at 6 months of age. Adult dogs should be heartworm tested annually. If your dog tests positive for heartworm disease, a further workup including bloodwork to check their organ function and x-rays to evalute their heart and lungs may be recommended. 

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 they are diagnosed. This means that the heartworms have been present long enough to cause substantial damage to the heart, lungs, blood vessels, kidneys, and liver. Rarely, cases may be so advanced that it is safer to treat organ damage and keep the dog comfortable than it is to risk negative effects associated with killing the heartworms. Dogs in this condition are not likely to live more than a few weeks or months. Your veterinarian will advise you on the best treatment approach for dogs diagnosed with advanced heartworm disease.

How can I prevent my dog from getting heartworm disease?

You can prevent your dog from getting heartworms by using year-round heartworm preventive. When a dog has been successfully treated for heartworms, it is essential to begin a heartworm prevention program to prevent future recurrence. With the safe and affordable heartworm preventives available today, no pet should ever have to endure this dreaded disease. Here at Piedmont Communities Spay/Neuter and Wellness Clinic, we offer two different chewable tablets for Heartworm prevention. One is called Interceptor and the other is called Nexgard Plus which combined hearworm prevention with flea/tick prevention all in one pill. We would be happy to discuss Heartworm prevention options with you to decide which product is best for your pet.