Your Guide to Heartworm Disease in Dogs

What is heartworm disease?

Heartworm disease is a serious medical condition that often results in severe heart and respiratory illness in affected dogs. It is caused by a helminth parasite Dirofilaria immitis, more commonly known as heartworm. Unlike other internal parasites which mainly invade the gastrointestinal system, heartworms stay inside a dog’s blood vessels and eventually find their way to the animal’s heart where it will lodge itself and grow.

Heartworm infection in dogs causes serious heart and lung problems and can be potentially fatal if not diagnosed and treated early. Proper diagnosis is essential in planning a treatment approach for dogs affected with heartworm disease. This is also one of the few diseases that can be prevented with prophylactic medications.

How can dogs become infected with heartworm?

Heartworm is a vector-borne parasite, and dogs get infected when bitten by a mosquito carrying microfilaria, the immature and infective form of heartworms. The microfilaria will then migrate along the dog’s blood vessels and circulatory system as it matures and becomes adult heartworm, which takes about 6 to 7 months. It will then find its way to the large blood vessels and into the heart where it can stay for up to 5 years and can grow as long as 12 inches.

Female and male adult heartworms mate inside the dog’s body and produce offspring that are released into the host’s bloodstream. These larvae will need an intermediate host to reach the next larval stage. When a mosquito gets its blood meal from an infected dog, they then become infected with the circulating microfilaria. This tiny heartworm turns into an infective larva and can be passed on to another dog through the infected mosquito’s bite, completing the heartworm’s life cycle.

Strictly speaking, heartworm disease is not contagious. An infected dog cannot directly transmit the infection to other dogs and animals. The presence of the mosquito, the heartworm’s intermediate host, is essential in the infection and spread of the parasite among canine patients.

Symptoms of Heartworm Infection in Dogs

The severity of clinical signs associated with heartworm disease in dogs depends highly on how many worms are inside the animal and how long the parasites have been staying in the host’s body. Since the condition affects the dog’s heart, their activity levels also play a role in the development of symptoms.

Canine heartworm disease has four stages, depending on the clinical signs the affected animal is showing. The higher the class or stage of heartworm disease, the more severe the symptoms are:

• Class 1: No observable clinical signs or mild symptoms such as occasional cough.
• Class 2: Mild to moderate clinical signs - occasional to more persistent cough and mild exercise intolerance observed in affected animals.
• Class 3: Severe clinical signs - persistent cough, moderate to severe exercise intolerance, breathing difficulties. Observable gross changes in the appearance of the heart and lungs can also be seen on chest radiographs (x-rays).
• Class 4: Also known as caval syndrome. This is characterized by a severely compromised blood flow to the heart and major blood vessels due to the heavy blockage caused by adult heartworms. This is often considered fatal if not treated immediately and aggressively.

How is heartworm disease diagnosed?

Heartworm infection in dogs is diagnosed through different blood tests. Since the parasite is mainly a blood infection, the presence of heartworm in suspected dogs can easily be confirmed through their blood.

The most commonly used test to diagnose heartworm disease in dogs is the heartworm antigen test kit. A heartworm antigen test kit detects the presence of heartworm antigens, a protein released by female adult heartworms, in the dog’s blood. This test can accurately detect the presence of one or more female adult heartworms. However, the earliest that heartworm antigens can be detected is around 5-6 months after the initial infection.

Another test that veterinarians commonly use to diagnose heartworm disease in dogs is a microfilaria examination. This involves collecting a blood sample from a suspected dog and examining it under the microscope for the presence of microfilaria. The presence of microfilaria in the dog’s blood indicates the presence of adult heartworms. The earliest microfilaria can be detected is around 6 months after the initial infection.

Treatment and Prevention of Heartworm Disease in Dogs

The success of treatment is dependent on the severity of the symptoms, how heavy the worm burden is, and how long the dog has been infected. The goal of the treatment is to eliminate the adult heartworms in the dog’s system and control any complications or side effects that may arise from the treatment or the death of the parasite.

In treating heartworm disease, your vet will need to get a detailed medical history and run baseline tests before starting treatment to reduce the risks of complications. To date, the only approved and recommended treatment for heartworm infection is Melarsomine dihydrochloride, an injectable medication formulated to kill adult worms in infected animals.

Dogs being treated for heartworm with Melarsomine will need close monitoring because side effects are common, especially in animals with heavy worm loads. In some cases, confinement in the veterinary hospital may be necessary during treatment. Your vet may also put your dog on other medications to reduce the risks of complications during treatment.

Thankfully, heartworm disease in dogs is preventable. A regular monthly dose of ivermectin-based preventive or other heartworm-specific medication is enough to prevent any immature microfilaria from becoming adult worms and causing health problems. Heartworm preventive medications come in different brands and are readily available in most veterinary clinics.

Another method, called slow-kill, has been developed to treat mild heartworm infection in dogs. This involves giving ivermectin-based medications, usually seen in heartworm preventive medications, weekly for at least a year. Ivermectin kills microfilaria but doesn’t really affect adult heartworms. It has been theorized that regular ivermectin administration can slowly weaken and kill adult worms and can be used as a safer alternative in treating heartworm infections. However, this has been debunked by the American Heartworm Society because of several studies showing dogs that still test positive for heartworms despite years of treatment.

If you think your dog has heartworm, or has been diagnosed with it, it’s best to discuss treatment options thoroughly with your vete so you can be made aware of what to expect and what to look out for during the course of the treatment.

Read more:

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Pet Medication 101: Ivermectin

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