Heartworm Disease (HWD) in Dogs

Overview of Canine Heartworm Disease

Canine heartworm disease (CHWD) is a common condition in many regions of the world. It is caused by the filarial (threadlike) worm Dirofilaria immitis that lives in the pulmonary arteries (blood vessels leading from the heart to the lungs) in most infected dogs. The name “heartworm” is a bit misleading because only in very heavy infections do the worms actually reside in the heart itself.

Nonetheless, the presence of these worms causes strain to the heart and an intense reaction in the blood vessels, resulting in problems for the dog. D. immitis is injected into the dog by infected mosquitoes. For this reason, CHWD is more common in tropical regions of the world, where the conditions of warmth and moisture are ideal for mosquitoes to live.

Predispositions for CHWD include living outdoors. Male dogs may be more likely to be infected than female dogs. The most important predisposing factor is failure to receive heartworm preventative medication. All dogs living in an area where heartworm disease exists are at risk, even if they live entirely indoors.

Transmission occurs when a mosquito bites an infected dog and ingests heartworm larvae (baby heartworms), which live in the bloodstream. When the insect bites another dog or cat, some of the larvae are injected under the skin. The larvae grow for 3 to 4 months and eventually make their way into the heart where they develop into adults, and the process is ready to repeat itself.

Impact on the pet is variable. Dogs can be without symptoms if the infection is light or has occurred recently. In some cases HWD can cause severe debilitation and eventually may be fatal. Because heartworm disease is easily preventable, it has become a less common diagnosis in pets that receive regular veterinary care.

What To Watch For

Symptoms of heartworm disease in dogs don’t usually develop until damage has already occurred to the heart. Dogs can have a wide range of symptoms, with some dogs being completely asymptomatic (no symptoms at all). Symptoms usually occur because of heart failure. These include:

Diagnosis of Heartworm Disease in Dogs

Two types of blood tests are used to establish the diagnosis:

Treatment of Heartworm Disease in Dogs

Killing of the adult heartworms (adulticide therapy)

There are two main agents administered by veterinarians for adulticide treatment:

Killing of the baby worms (microfilaricide)

Home Care

Preventative Care

Prevention of heartworm disease is easy and should be undertaken in all dogs. These include monthly and daily preventatives.

Monthly preventatives include:

In-depth Information on Heartworm Disease in Dogs

Canine heartworm disease is caused by infection with Dirofilaria immitis (D.immitis), a filarial worm living in the pulmonary arteries. Dogs who are not taking heartworm preventative medications become infected with heartworms when an infected mosquito carrying an immature form of D. Immitis (larvae) injects those larvae into the bloodstream of the dog. A portion of the larvae mature and migrate through the bloodstream and heart to arrive in the pulmonary (lung) arteries. In a period of 4 months after injection, the young form of the adult heartworm is present. It takes another few months before the worms are sexually mature and produce baby worms (microfilaria) that begin circulating in the bloodstream of infected dogs. Additional mosquitoes become infected by ingesting the microfilaria with the blood of the dog. The microfilaria are infectious to mosquitoes, not to other dogs. The microfilaria undergo a few changes in the body of the mosquito before they reach the larval form infective to the dog, completing the life cycle of the heartworm.

There are no known predispositions for heartworm disease other than living in a warm, moist environment favorable to the mosquitoes, spending the majority of the day and evening outside and the major predisposing factor of failing to receive heartworm preventative medication. Although some reports suggest male dogs are more likely to get heartworm disease, one study found spayed female dogs to be at higher risk. The modern heartworm preventative medications are highly effective and, if religiously administered, should prevent heartworm infection.

The severity of heartworm disease is variable depending on the number of heartworms present, the intensity of the reaction of the body to the presence of the heartworms, the duration of the infection, the age of the dog and the presence of other medical conditions. Heartworm disease is frequently detected in dogs without symptoms which are presented to the veterinarian for routine care. Left untreated, many dogs will display clinical signs. Clinical signs may include coughing, hemoptysis (the coughing up of blood), shortness of breath, unwillingness or inability to exercise or signs of right-sided congestive heart failure (RCHF). Signs of RCHF include accumulation of fluid in the belly (ascites), accumulation of fluid around the lungs (pleural effusion) and pulsation of the jugular veins located in the neck. Dogs with RCHF will appear fat or bloated, but are actually usually very thin and have difficulty breathing. Dogs who suffer a blood clot to the lung (pulmonary thromboembolism, PTE) will show signs of coughing, fast breathing and lethargy. In severe cases, PTE may be fatal.

A less common syndrome seen with HWD is “caval syndrome.” This syndrome occurs when the heartworms are in such large numbers that they are present in the right heart chambers and jugular veins. In this position, the worms form a mesh that causes some of the red blood cells in the heart to break up (hemolysis). The end result of the hemolysis is anemia (low red blood cell count) and jaundice (yellow color to the eye whites and gums caused by byproducts released by red blood cell breakdown in the bloodstream). Caval syndrome is very serious and requires mechanical removal of the heartworms from the heart using special instruments that are advanced into the heart through the jugular vein in the neck. This removal process is dangerous and sometimes results in death of the patient.

Disease which might mimic signs of heartworm disease would be other diseases resulting in coughing (bronchitis, tracheal collapse, pneumonia), other diseases resulting in right-sided congestive heart failure such as:

Diagnostic Tests for Heartworm Disease in Dogs

To establish the diagnosis, there are two type of tests used.

Causes of Occult Canine Heartworm Disease

Tests To Stage Heartworm Disease in Dogs

Treatment of Heartworm Disease in Dogs

After evaluation, treatment is generally recommended.

Adulticide Therapy

Killing of the adult heartworms. One hopes to accomplish complete death of the heartworms, but slowly enough to allow the body time to break down and absorb the dying heartworms.

Microfilaricide Treatment

Killing the circulating baby worms. This is unnecessary in dogs with occult heartworm infections. Microfilaria can be killed in a few ways:

Follow-up Care for Dogs with Heartworm Disease

Optimal treatment for your dog requires a combination of home and professional veterinary care. Follow-up can be critical.

Administer all prescribed medications as directed. Alert your veterinarian if you are experiencing problems treating your dog.

STRICT EXERCISE RESTRICTION is the most important aspect of care at home after treatment for heartworm disease. This is imperative for the first four weeks after treatment. During this time, exercise or activity may result in an increased likelihood of pulmonary thromboembolism (PTE). PTE is interruption of blood flow to a portion of the lung due to blockage of a pulmonary artery with dead or dying worms, and associated blood clot and inflammation. The body can take care of the dying worms by breaking them down and absorbing them, but this takes time. Although PTE can occur in a dog that is being quiet, it is much more likely to occur in a dog who is exercising. The PTE episodes can be very severe, even fatal.

Owners need to be very alert for the signs of PTE: coughing, fast or heavy breathing or fever. If these signs are noted, the dog should be presented for examination as soon as possible. If PTE is identified, it is usually treated with the administration of the anti-inflammatory drug prednisone. Oxygen may need to be administered until the dog is breathing more easily.

Some very mild coughing is to be expected after heartworm treatment. However, if the coughing is occurring frequently during the day, or if the dog coughs up blood, the dog should be examined. Any coughing should be reported to your veterinarian.

The heartworm tests need to be repeated after treatment, to ensure the treatment has been successful in eliminating the heartworms. The filarial test is done one month after the microfilaricide is given. If it is positive, microfilaricide is repeated. Cases with occult heartworm infection have repeat serology three months after treatment. If the serology is still positive, it should be repeated three months later. At that time, if it is still positive, the dog should be re-evaluated for repeat treatment. Luckily, adulticide treatment is usually successful the first time!

All dogs with heartworm disease should be started on heartworm prevention immediately after the adulticide for occult infections, and after successfully killing the microfilaria in the other cases.

In dogs determined to be medically unsuitable for heartworm treatment, an appropriate heartworm preventative will be prescribed by the veterinarian to prevent the dog from acquiring additional heartworms. Some of the heartworm preventatives available are dangerous to administer in dogs already infected with heartworms.

The American Heartworm Society (AHS) recommends that all dogs in areas endemic for heartworms should take a year-round preventative. Annual testing will ensure that an infection is caught in plenty of time to effectively manage it. Testing is also recommended when a pet owner switches between preventative medications.