Heartworms in Dogs

Canine heartworms were first reported by Dr. T. C. Osborne in Erie, Alabama (1847), who described a dog with a massive number of white worms in the heart and its vessels. This disease is caused by parasitic worm called Dirofilaria immitis, which lives in the lungs, heart and associated vasculature of an infected animal where they may cause serious damage and, in some cases, the animal’s death. In 2009, there were approximately 300,000 dogs that tested positive to heartworm disease in the United States (American Heartworm Society). It is present in in all 50 states, Puerto Rico, U.S. Virgin Islands, and Guam. Heartworm disease is most common along the Atlantic and Gulf coasts from the Gulf of Mexico to New Jersey and along the Mississippi River and its major tributaries, as well as, in the Caribbean islands. The dog is the definitive host but cats and ferrets are also susceptible to infection. Heartworms also live in other mammal species, including wolves, coyotes, foxes and sea lions. Heartworm disease is a serious disease that results in severe lung disease, heart failure, other organ damage, and death in pets, mainly dogs, cats, and ferrets. It is caused by a parasitic worm called Dirofilaria immitis. The worms are spread through the bite of a mosquito.

Many dogs show few or no symptoms in the early stages of the disease. Heavily infected and active animals usually show more pronounced clinical signs. The clinical signs of heartworm disease include:

• Mild persistent cough
• Reluctance to exercise
• Fatigue after moderate activity
• Decreased appetite
• Weight loss

Types

There are four classes, or stages, of heartworm disease:

• Class 1: No symptoms or mild symptoms (e.g. occasional cough).
• Class 2: Mild to moderate symptoms (e.g. occasional cough and fatigue moderate activity). Heart and lung changes are usually seen on chest x-rays.
• Class 3: General loss of body condition, a persistent cough, and fatigue after mild activity. Heart and lung changes are usually seen on chest x-rays.
• Class 4: A large mass of heartworms prevents blood from flowing back to the heart causing a cardiovascular collapse known as Caval syndrome.

Dogs with a severe infection may develop:

• Heart failure and the appearance of a swollen belly (due to excess of fluid in the abdominal cavity).
• Sudden blockages of blood flow within the heart (Caval syndrome), characterized by:
• Labored breathing
• Pale gums
• Dark bloody or coffee-colored urine.

Heartworm disease is a vector borne disease caused by a worm called Dirofilaria immitis and spread by mosquitoes. When a mosquito bites an infected dog, it becomes infected with the microfilariae (the offspring of adult worms). Microfilariae become infective larvae inside the mosquito and when the infected mosquito bites another dog transmission occur. Inside the infected dogs larvae become adults, reproduce and the offspring go to the blood. The worm’s lifespan is five to seven years and adult males measure 4 to 6 inches in length while females measure 10 to 12 inches in length.

There are two types of heartworm tests: the antigen detection test and the microfilariae detection test. Antigens are proteins released into the bloodstream by adult worms, thus the antigen detection test can only diagnose animals that are infected with one or more adult female worms (seven to eight months old. Similarly, the microfilariae detection test can diagnose animals infected with adult heartworms, which mate and produce offspring. Dogs should be tested for heartworm disease annually. The American Heartworm Society recommends the following testing schedule:

Puppies under 7 months of age

Heartworm prevention can be initiated without a heartworm test, and they should be tested 6 months after the initial treatment. These patients should be retested 6 months later and yearly thereafter.

Dogs over 7 months of age

The test should be conducted before starting heartworm prevention. These patients should be retested 6 months later and yearly thereafter.

If there has been a lapse in prevention

Dogs should be tested immediately, and then tested again six months later and annually after that.

• Ivermectin: this is the main ingredient of many heartworm preventive products and it is capable of killing microfilariae. Usually, infected dogs are administered a microfilariae-killing drug to prevent them from developing into adults and then an adult-killing drug is administered.
• Milbemycin: is also found on heartworm preventive products (e.g. Sentinel and Interceptor) and can kill the microfilariae much faster than ivermectin. This can be disadvantageous because large numbers of microfilariae dying at the same time can cause a circulatory shock.
• Melarsomine dihydrochloride (Immiticide® by Merial): is the only FDA approved drug to kill adult worms. Treatment consists of two or three deep injections into the back muscles. These injections are very painful so your dog may also need pain medication.

Dogs receiving heartworm treatment should be confined because physical activity can cause embolism-related problems. Heartworm treatment is potentially toxic, can cause serious complications and is expensive; therefore, prevention is the best option for your pet. There are various FDA-approved heartworm prevention products that can be prescribed by an veterinarian.