Canine Heartworm Disease

Heartworms are the most life-threatening canine parasites. They reside in the dog's heart and pulmonary arteries, causing heart failure and eventually death.

Heartworms are transmitted by mosquitoes. When an infected mosquito bites a dog, it passes him the parasite’s larvae to the dog. These larvae migrate until they reach their final site (heart and pulmonary arteries) in about 3-4 months; here they grow to maturity (macrofilariae) within 3 months and start producing larvae (microfilariae) which can survive for about 2 years in the bloodstream. When a mosquito bites an infected dog, it picks up these larvae and can transmit the infection to other dogs.

Dogs are considered the definitive host for heartworms (Dirofilaria immitis). However, heartworms may infect more than 30 species of animals (e.g., coyotes, foxes, wolves and other wild canids, domestic cats and wild felids, ferrets, sea lions, etc.) and humans as well. When a mosquito carrying infective heartworm larvae bites a dog and transmits the infection, the larvae grow, develop, and migrate in the body over a period of several months to become sexually mature male and female worms. These reside in the heart, lungs, and associated blood vessels. Even as immature adults, the worms mate and the females release their offspring (microfilariae), into the blood stream. The time elapsed from when the larvae enter the dog until the offspring can be detected in the blood (pre-patent period) is about six to seven months. The male heartworms (four to six inches in length) and the females (10-12 inches) become fully grown about one year after infection, and their life span in dogs appears to be at least five to seven years.

Over time, the presence of the adult worms in heart and pulmonary arteries causes an inflammation and thickening of the walls of the blood vessels which leads to an increase in blood pressure and in the cardiac effort to push the blood in these vessels. As a consequence, the dog may develop heart failure, which can eventually cause death. Clinical symptoms usually appear only when the disease has reached a very severe stage. Initial signs include sporadic coughing, tiredness, and weight loss. With time, this cough becomes chronic and is accompanied by difficult respiration, particularly during and after exercise, and mild anemia. In advanced cases, the dog may collapse after even light physical exertion. Most dogs eventually develop congestive heart failure.

Heartworm disease may cause combinations of medical problems within the same dog including dysfunction of the lungs, heart, liver and kidneys. The disease may have an acute onset but usually begins with slow barely detectable signs resulting from a chronic infection with a combination of physiologic changes. Dogs with a low number of adult worms in the body that are not exercised strenuously may never have overt signs of heartworm disease. The heart and lungs are the major organs affected by heartworms in dogs with varying degrees of clinical signs.

The onset and severity of disease in the dog is mainly a reflection of the number of adult heartworms present, the age of the infection, and the level of activity of the dog. Dogs with higher numbers of worms are generally found to have more severe heart and lung disease changes. Until the number of mature heartworms exceeds 50 in a 55 pound dog, nearly all of the heartworms reside in the lower caudal pulmonary arteries (the arteries of the lower lung lobes). Higher numbers of heartworms will cause them to be found in the right chambers of the heart. In such infections, the most common early pathological changes caused by heartworms are due to inflammatory processes that occur in and around the arteries of the lower portion of the lungs in response to the presence of heartworms. Later, the heart may enlarge and become weakened due to an increased workload and congestive heart failure may occur. A very active dog (e.g., working dog) is more likely to develop severe disease with a relatively small number of heartworms than an inactive one.

The diagnosis of canine heartworm disease depends upon the following: an accurate patient history, the recognition of varied clinical signs, and the use of several diagnostic procedures that may include X-ray and ultrasound, blood (serological) testing, microfilarial detection and differentiation, clinical laboratory tests and possibly examination after death.

Most dogs infected with heartworm can be successfully treated. The goal of treatment is to kill all adult worms with an adulticide and all microfilariae with a microfilaricide. It is important to try to accomplish this goal with a minimum of harmful effects from drugs and a minimal degree of complications created by the dying heartworms. Heartworm infected dogs showing no signs or mild signs have a high success rate with treatment. Patients with evidence of more severe heartworm disease can be successfully treated, but the possibility of complications and mortality are greater. The presence of severe heartworm disease within a patient in addition to the presence of other life-threatening diseases may prevent treatment for heartworm infection.

In contrast to treatment, heartworm prevention is safe, easy and effective. Before prophylactic treatment is started, dogs should be tested for heartworm infection. Those found to be infected have to be treated against adults and microfilariae, before a prevention program is started. Preventatives are usually administered monthly on a year-round basis. Most heartworm prevention medications are also effective against all other common dog parasites (roundworm, whipworm, and hookworm) thus ensuring complete protection for your pet. There are monthly tablets (Heartguard®, Interceptor®, Sentinel®), and monthly topicals (Revolution®). All of these methods are extremely effective and when the drugs are administered properly on a timely schedule, heartworm infection can be completely prevented.

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Canine heartworm infection is widely distributed throughout the United States. Heartworm infection has been found in dogs native to all 50 states. All dogs regardless of their age, sex, or habitat are susceptible to heartworm infection. The highest infection rates (up to 45%) in dogs (not maintained on heartworm preventive) are observed within 150 miles of the Atlantic and Gulf coasts from the Gulf of Mexico to New Jersey and along the Mississippi River and its major tributaries. Regions where heartworm disease is common have infections diagnosed in dogs as young as one year of age, with most areas diagnosing infections primarily between the ages of three and eight years. Although there are differences in frequency of infection for dogs in different regions of the U.S., all dogs should be considered at risk, placed on prevention programs and frequently examined by a veterinarian

Feline Heartworm Disease

Heartworm infection takes place when a mosquito carrying microscopic-size heartworm larvae , obtains blood from an infected animal and draws into its system the microfilariae which grow into the infective larval stage. These heartworms are injected into the new host as larvae and develop further as they travel through the cat's body over a period of eight months. As they become sexually mature male and female worms (Dirofilaria immitis) , they usually settle into the heart and lungs, and their arteries and blood vessels. Even as immature adults, the heartworms can mate and the females can then release their offspring (microfilariae) into the blood stream.

Cats, like dogs, are in danger of serious health issues when infected by mosquitoes with heartworm larvae. Fortunately, felines are resistant hosts even when injected by an infective mosquito. The percentage of microfilariae in cats developing into the adult stage is low (0% to 25%) compared to dogs (40% to 90%). The percentage of infected cats from which adult heartworms are recovered is generally high (61% to 90%), but still lower than in dogs, where they are recovered in virtually 100% of those infected. The number of adult heartworms in infected cats is usually low, fewer than six, although there have been documented cases with as many as 45 in one male cat and 65 in another. Single sex and ectopic (adult worms in sites other than the heart, lungs, and associated vessels) heartworm infections are also more common in cats than in dogs. Although heartworms reach a substantial size in cats, they are smaller than those in dogs. The life span of heartworms in cats is approximately two to three years, compared to five to seven years in dogs.

The average time from when the microscopic parasites enter the host until the females develop into mature worms, producing microfilariae is approximately eight months and is referred to as the prepatent period. This is about one month longer than in dogs

Microfilaremia (the presence of heartworm offspring in the blood of the host animal) is uncommon in cats (usually less than 20% of cases). When present, it is inconsistent and short-lived. This indicates that cats are poor reservoirs of infection. There is no predictable age in cats for becoming infected with heartworms. Cases have been reported in cats from nine months to 17 years of age, the average being four years at diagnosis or death.

Some cats appear to be able to rid themselves of the infection spontaneously. It is assumed that such cats may have developed a strong immune response to the heartworms, which causes the death of the parasites. These heartworms may die as a result of an inability to thrive within a given cat's body.

Clinical Signs
The clinical signs of heartworm infection in cats can be very non-specific, and may mimic many other feline diseases. Diagnosis by clinical signs alone is nearly impossible. As with dogs, most heartworm infected cats are free of any clinical signs of disease and may exhibit chronic clinical signs, such as vomiting intermittently, coughing, asthma-like signs (intermittent difficulty in breathing, panting, open-mouthed breathing), gagging, rapid breathing, lethargy, anorexia, or weight loss. Some cats exhibit acute clinical signs, with disease often related to the organs where the adult heartworms are thriving.

Diagnosis
Actual cases of feline heartworm infection have been difficult to estimate because diagnosis has been based mainly on clinical signs and tests developed for use in canines. The diagnostic plan for heartworm disease in cats can include a physical examination, X-ray, ultrasound, complete blood count (CBC), serologic testing (antigen and antibody study), microfilaria testing, and necropsy. Unfortunately, these highly specific tests lack the sensitivity to identify many infections in cats. They fail to detect infections of immature heartworms, infections with only male heartworms and some infections with only one or two adult female worms.

Treatments
Currently, there are no products in the United States approved for the treatment of feline heartworm infection. If there is evidence of disease in the lungs and their blood vessels consistent with feline heartworm infection, such cases can be monitored with chest X-rays every six to twelve months, as needed. Supportive therapy with small, gradually decreasing doses of prednisone is recommended for cats with radiographic or clinical evidence of lung disease.

Cats with severe manifestations of feline heartworm disease may require additional supportive therapy, and may benefit from intravenous fluids, oxygen therapy, cage confinement, bronchodilators (which expand the air passages of the lungs), cardiovascular drugs, antibiotics and nursing care.

Heartworm extraction with various surgical devices has been performed in cats in which the worms can be visualized with ultrasound at the tricuspid valve or in the right atrium (of the heart), and especially in those rare instances of caval syndrome (obstruction of blood flow affecting the heart and the liver.

Preventives
It is generally recommended that all cats be tested for antigens and antibodies prior to administration of a heartworm preventive. There are three products approved by the FDA for use in cats, Heartgard® for Cats, RevolutionTM and Interceptor®. All of these products are considered effective in preventing the development of adult heartworms when administered properly on a monthly basis relative to the period of transmission.

For more information about heartworm disease, please contact the American Heartworm Society at www.heartwormsociety.org