Tired and sick dog

Heartworm disease is a condition affecting the lungs and heart of infected dogs, and is common in certain areas of the United States. It has been reported in all of the contiguous 48 states, although high incidence of the disease in the U.S. occurs in the Southeast, the Mississippi River Valley, and the Mississippi Delta region.

Not all dogs infected with heartworms develop the disease, but some dogs become very ill and can even die as a result of their infection. The disease has also been documented in coyotes, wolves and foxes. These wildlife species can serve as a reservoir for infection, since they are not treated for the disease. The best approach for protecting your dog against heartworms starts with understanding the disease, transmission, treatment and prevention.

Cause of Heartworm Infection

Heartworm disease is caused by a parasite called Dirofilaria immitis. The adult worms live and reproduce in dogs, coyote, fox and wolves. Larval (baby) heartworms circulate through the blood of an infected individual and are picked up by a mosquito during feeding.

The heartworms begin the next stage of their life cycle by developing into the infective form in the mosquito. This takes about 10 to 14 days. The mosquito then bites another dog and passes the worms through the mosquito bite and into the new host.

Then, the baby heartworms travel to the pulmonary artery and develop into mature adults, a process that takes about six months. The first five to six weeks of maturation in the new host is a critical time period for prevention, as explained below.

Symptoms of Heartworm Disease

Heartworm disease occurs when the worm burden is sufficient to cause symptoms. In the early stages of disease, symptoms may not be present. As the worms multiply, the dog may develop a cough, exercise intolerance, and weight loss.

With continued, untreated infection, heart failure can occur, with fluid accumulating in the abdomen. In some cases, the worms can block blood flow within the heart, causing collapse and necessitating immediate surgical removal of the worms.

The good news is, infection with heartworms does not necessarily ensure that a dog will develop heartworm disease. Plus, if caught early, the disease is treatable with a series of injections (described below).

Testing for Heartworm Infection

There are two types of tests available to determine if a dog has heartworms. The only one that is accurate is an antigen test, which accurately detects the presence of female heartworms in the body.

Some clinics still check a drop of blood for microfilaria (the larval stage circulating in the blood). The presence of microfilaria indicates infection. However, the absence of them does not ensure negative heartworm status. Yearly antigen testing is recommended, since prevention failure can occur and early treatment of infection minimizes damage.

Treatment for Heartworms

Only one treatment method is approved for use in dogs: Melarsomine, an arsenic compound, can effectively kill heartworms via a series of injections. Dogs generally tolerate treatment well but must be kept confined and quiet for 2 months after the injections.

As the worms die, circulating worm pieces can cause respiratory damage, but restricting activity during recovery can minimize this. No other treatment is approved or effective for heartworm disease, although some people have advocated a “slow-kill” method. This involves simply keeping the dog on prevention in hopes that the worms will be weakened and the dog’s own immune system will eliminate them. This is not recommended as it may take years, and irreversible damage to the dog’s heart and lungs will continue to occur during this time period.

Prevention of Heartworm Infection

The simplest way to address heartworm disease is with monthly preventatives or a 6-month injection. These prescription medications work by killing the infective larval stage after it enters the blood stream from the mosquito bite.

They are only effective during the first 5 to 6 weeks of infection, which is why you must administer them monthly. These medications are inexpensive and are nearly 100% effective if administered as labeled. Monthly preventatives are recommended year-round in much of the United States.

If your dog has not been tested and is not current on heartworm preventative administration, check with your veterinarian about the safety of starting prevention.

Further, it is recommended that preventatives be purchased directly from a veterinarian instead of from online sources. All of the manufacturers of these products only sell directly to veterinarians. Purchasing from your vet ensures that the product is pure and will be backed by the manufacturer in case of product failure.

Heartworm disease is a very serious condition that can lead to chronic illness and even death in untreated dogs. It is so important in the veterinary world that a task force for studying and disseminating information about the disease was formed back in 1974. The American Heartworm Society has the most up-to-date information about the disease (prevalence, distribution, recommendations) on its website.

Prevention is the easiest way to address this disease, is inexpensive and is very effective. Talk with your veterinarian about the best strategy for your dog.

Products You May Be Interested In:

Dr. Catharine Hennessy