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What is Heartworm?

Every dog owner has probably heard about heartworm, in fact, veterinarians might already have your dog on a monthly prevention plan for it. But what is heartworm and how can your dog get it?

Heartworm is a parasitic organism found in blood vessels, the heart, and the lungs. These parasites can cause fatal heart and lung problems. Heartworm comes from an infected mosquito.

What is the Life Cycle of a Heartworm? 

The heartworm has a complicated life cycle; the parasite requires the mosquito as an intermediate host before it can complete its life cycle in the dog. The mosquito is an essential part of the heartworm life cycle. Up to 30 different mosquito species can transmit heartworms.

The life cycle begins when a female mosquito bites an infected dog and consumes the microfilariae during a blood meal. The microfilariae develop further in the mosquito’s gut for 10 to 30 days before entering its mouthparts. They are infective larvae at this stage and can complete their maturation when they enter a dog. The infective larvae enter the dog’s body when the mosquito bites the dog.

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

How Common is Heartworm?

Canine heartworm disease occurs worldwide. It was once restricted to the south and southeast of the United States. Most reported cases to continue to be within 150 miles of the Gulf of Mexico and Atlantic Ocean coastlines, along the Mississippi River and its tributaries. The disease, however, is spreading and can now be found in most parts of the United States, including California, Oregon, and Washington.

In Canada, the disease is a problem in mosquito-infested areas along waterways and coastlines in many provinces. The southern Great Lakes have the highest number of Canadian cases.

The prevalence of heartworm infection is affected by mosquito species, climate, and the presence of reservoir animals.

When mosquitoes are actively feeding, the risk of infection is most significant. This usually necessitates temperatures above 50°F (10°C).

How Does Heartworm Spread? 

The disease is not transmitted directly from dog to dog because transmission requires the mosquito as an intermediate host. The disease’s spread thus coincides with mosquito season, which can last all year in many parts of the United States. The number of infected dogs and the length of the mosquito season are directly related to the prevalence of heartworm disease in any given area.

What Does Heartworm Do to Dogs? 

It typically takes several years for dogs to exhibit clinical signs of infection. As a result, the disease is mostly diagnosed in dogs aged two to eight years. Because microfilariae take 5 to 7 months to mature into adult heartworms after infection, the condition is uncommon in dogs under one year old. Unfortunately, the disease has usually progressed significantly by the time clinical symptoms appear.

What are the Different Stages of Heartworms?

There are two distinct stages of heartworms, the adult stage and the microfilariae, but what is the difference between the two?

Adult Heartworms

Adult heartworms cause disease by clogging the heart and major blood vessels that branch from it, including the pulmonary artery. They also interfere with the function of heart valves. Blood supply to other body organs is reduced when the main blood vessels become clogged, particularly blood flow to the lungs, liver, and kidneys. These organs can malfunction due to decreased blood flow and oxygen delivery.

The severity of heartworm disease is determined by the number of adult worms present, their location, the length of time the worms have been in the dog, and the degree of damage to the heart, lungs, liver, and kidneys.

A soft, dry cough, shortness of breath, weakness, listlessness, and loss of stamina are the most apparent clinical signs of heartworm disease. These symptoms are most visible after exercise, when some dogs may faint or become disoriented. When listening to the chest with a stethoscope, your veterinarian may notice abnormal lung and heart sounds.

In advanced cases, congestive heart failure can cause the abdomen and legs to swell due to fluid accumulation. Weight loss, poor health, and anemia may also be present. Dogs with severe infections may die suddenly during exercise or excitement.


Microfilariae or 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 Can Heartworm be Treated? 

Though fatalities are uncommon, there is some risk in treating dogs for heartworms. A new drug with fewer side effects is now available, allowing more than 95% of dogs with heartworms to be successfully treated.

Previously, the drug used to treat heartworms contained high levels of arsenic, and toxic side effects were common. A new drug with fewer side effects is now available, allowing more than 95% of dogs with heartworms to be successfully treated.

Many dogs are already infected with heartworms when they are diagnosed. This indicates that the heartworms have been present long enough to cause significant damage to the heart, lungs, blood vessels, kidneys, and liver. Rarely, cases may be so advanced that treating organ damage and keeping the dog comfortable is preferable to risking the adverse effects of killing the heartworms. Dogs in this condition will likely die within a few weeks or months. Your veterinarian will advise you on the best treatment for dogs with advanced heartworm disease.

Adult heartworms are killed with this treatment. Melarsomine, an injectable drug, is used to kill adult heartworms. Melarsomine kills adult heartworms in the heart and its surrounding vessels. This medication is given in a series of injections. The specific injection schedule will be determined by your veterinarian based on your dog’s condition. Most dogs are given an initial injection, then a 30-day rest period, and then two more injections 24 hours apart.

Many dogs will also be given an antibiotic like doxycycline)to prevent infection with the bacteria that live in the heartworm.

Rest is Very Important

Following treatment, complete rest is required. Adult worms die within a few days and begin to decompose. They are carried to the lungs as they break up, where they lodge in the small blood vessels and are eventually reabsorbed by the body. This resorption can take several weeks to months, and these fragments of dead heartworms are responsible for the majority of post-treatment complications.

This can be a difficult time, so the dog must be kept as quiet as possible and not allowed to exercise for one month after the final injection of heartworm treatment. The first week after the injections is critical because the worms die during this time. Many heavily infected dogs have a cough for seven to eight weeks after treatment. If your cough is severe, contact your veterinarian to discuss treatment options.

If the dog has a significant reaction in the weeks following the initial treatment, prompt treatment is critical, though such reactions are uncommon. Notify your veterinarian if your dog exhibits any of the following symptoms: loss of appetite, shortness of breath, severe coughing, blood coughing, fever, or depression. In these cases, anti-inflammatories, antibiotics, cage rest, supportive care, and intravenous fluids are usually adequate.

Treatment for Microfilaria

Your dog will be given a drug to kill microfilariae in addition to the drug used to kill adult heartworms or heartworm larvae. Your dog may need to stay in the hospital for observation on the day this medication is given, which could be before or after the adult heartworm injections. After treatment, your dog will be put on a heartworm preventative.

To kill the microfilariae, newer heartworm treatment protocols employ a variety of drugs. Your veterinarian will select the correct drug and administration time based on your dog’s condition.

Are There Additional Treatments Required? 

Prior to heartworm treatment, dogs with severe heartworm disease may require antibiotics, pain relievers, special diets, diuretics to remove fluid accumulation in the lungs, or drugs to improve heart function. Even after the heartworms are eradicated, some dogs may require ongoing treatment for heart failure. Diuretics, heart medications such as ACE inhibitors, beta blockers, or cardiac glycosides, and special low-salt diets are examples of such treatments.

What is the Treatment Response and Post-Treatment Prognosis?

Dog owners are frequently surprised by their dog’s improvement after heartworm treatment, especially if the dog had previously displayed clinical signs of heartworm disease. Many dogs exhibit renewed vigor and vitality, as well as increased appetite and weight gain.

How to Prevent Getting Heartworms?

You can keep your dog’s heartworms at bay by using a heartworm preventive. After a dog has been successfully treated for heartworms, starting a heartworm prevention program is critical to prevent a recurrence. With today’s safe and affordable heartworm preventives, no pet should ever suffer from this dreaded disease. Consult your veterinarian to determine the best heartworm prevention program for your dog.

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