Heartworm Disease

By Dr. Karen Burgess

What is heartworm?
Heartworm disease is caused by Dirofilaria immitus, an up to foot long spaghetti-like worm that grows to maturity in the heart and pulmonary artery (blood vessel connecting the heart to the lungs). Dogs become infected with heartworm disease when they are bitten by an infected mosquito that acts as an intermediary host.

What are symptoms of heartworm disease?
Dogs with heartworm disease range in showing no symptoms to experiencing heart failure. More common symptoms include exercise intolerance or a cough. The adult heartworm can clog blood vessels and cause dangerous inflammation within the lungs. Symptoms often do not develop for several years after initial infection, but by this time extensive and permanent damage to the lungs has often already developed.

What is the life cycle of the heartworm?
Heartworms develop through five different life stages. The adult heartworm resides in an infected dog’s pulmonary artery (blood vessel connecting the heart to the lungs). After mating, adult female heartworms produce microfilaria, or microscopic larva that travels through the blood stream. When a mosquito bites an infected dog, they ingest microfilaria along with blood. The heartworm larva goes through two life stages in the mosquito over a several week timespan before being infectious to another dog via mosquito bite. After a dog is infected, the heartworm must then go through three more lifestages before becoming an adult in the heart. Ultimately it takes approximately four months from time of mosquito bite to when a mature heartworm is found in the heart.

How is heartworm disease diagnosed?
There are several blood tests readily available to screen for heartworm disease. The biggest limitation being that most tests are only able to detect more than three adult female heartworms. Since it takes four months from time of infection to having an adult heartworm, some dogs may test falsely negative for heartworm disease initially.

How is heartworm disease typically treated?
There are several components to successful treatment for heartworm disease.

The microfiliaria (baby heartworms, L1) and recently injected and developing heartworms (L3 and L4) are killed by ivermectin based heartworm preventatives (Heartgard Plus). This prevents other dogs from being infected and recently injected larva from growing into adult heartworms in an exposed dog. Injected heartworms (L3) remain in the skin for three months developing to the larval stage that ultimately migrates to the heart (L5). Developing larval heartworms (L3, L4) are killed by ivermectin based (Heartgard Plus) heartworm preventatives.

Treatment for the adult heartworm (L5) involves a very specific medication called immiticide, a derivative of arsenic. Prior to treatment, dogs should have their overall health and stage of disease determined. Blood tests and thoracic (chest) radiographs are recommended at a minimum. If there is evidence of lung or heart disease on radiographs, a modified treatment regimen or further testing may be necessary in an effort to prevent complications. Current recommendations for treatment involve an initial one month course of doxycycline. Research has shown that doxycycline kills a specific bacteria associated with the adult heartworm thus making it more susceptible to immiticide and potentially decreasing side effects of treatment. After the course of doxycycline, there is a month delay prior to treatment with immiticide. This allows time for immature heartworms to either be killed by preventative or mature to adulthood allowing immiticide to kill them. A confirmatory heartworm test should be performed after this eight week delay and prior to immiticide therapy.

Immiticide is administered as an injection in the back muscle. One month after this first injection, the same injection is given again twice over a two day period. Each injection of immiticide kills a certain percentage of heartworms. By dividing the treatment over a two month period, the total number of dead worms a dog is exposed to at any one time is limited.

What is follow-up care after treatment for heartworms?
Dying adult heartworms and the ensuing inflammation they cause in the lungs is a potentially life-threatening problem for dogs undergoing heartworm treatment. There is no way to completely prevent complications secondary to heartworm treatment. Anti-inflammatories are often administered immediately after treatment to assist with any local pain at the injection site which can be severe enough to cause a dog to bite. An estimated 30% of dogs will experience some sort of reaction secondary to the injection. Strict rest is required for four weeks after each injection; this means short leash walk to go to the bathroom in the yard, no running, walks, playing, or excessive excitement. The dying adult heartworm can be very dangerous and the more the heart is rested the safer it is for the dog. It is better to error on the side of caution (crating a pet) than risk a possible complication. A heartworm test should be performed again six months after treatment.

What are signs of complications from treatment?
Cough (often closely resembling vomiting but with no food present), lethargy, vomiting, difficulty breathing, loss of appetite, or collapse are all possible complications and warrant immediate medical attention.

Can heartworm be prevented?
There are several different products available to prevent heartworm disease. These are typically a pill that is administered once monthly year round. Care should be taken to purchase a reliable product from a trusted brand as several product lines have experienced recalls over the years.

Can a dog get heartworm disease while on preventative?
Heartworm treatment is nearly 100% effective. Reasons for breakthrough include failure of pet to ingest pill, poor owner compliance in giving pill monthly, resistant strains of heartworm (not a current problem in this region), and an animal that had immature worms that were not detected on a previous heartworm test.

Why do dogs need to be tested for heartworm disease prior to giving preventative?
Heartworm preventatives quickly kill off microfilaria. In rare instances this can lead to a potentially fatal anaphylactic reaction in a dog. Heartworm positive dogs with microfilaria should be monitored in a hospital setting when first being given preventative.