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Heartworm Disease

Canine Heartworm Disease: Current Treatment Recommendations
Alyssa Mourning, DVM, DACVIM

Heartworm disease, which was once only considered a disease in the Southeast, has now been diagnosed in all 50 states. There are three requirements for transmission of heartworm disease: a reservoir of infection, the mosquito and the right climate. The canine population not receiving preventive therapy, as well as stray and feral dogs (such as coyotes and other wild canids), makes up the reservoir. The mosquito serves as a vector for the disease-causing, third stage larvae of the Dirofilaria immitis to be transmitted during its blood meal. While studies indicate that a minimum temperature of 57°F is needed for the development of heartworm larvae into the infective stage, there are situations that even colder climates can have warmer environments within them that allow for transmission, even during the cooler months. It is because of this phenomenon that year round prophylaxis is recommended for the continental United States and Hawaii.

The clinical signs associated with heartworm disease vary and depend upon severity. Dogs with early infection will have no clinical signs. Those with mild disease may have a cough. Moderate disease will cause a cough, exercise intolerance and possible abnormal lung sounds. Patients with severe disease will have the most diverse clinical signs, which may include cough, exercise intolerance, dyspnea, syncope, ascites, abnormal heart and/or lung sounds, and sudden death. Caval syndrome results in sudden onset of severe lethargy along with hemoglobinemia and hemoglobinuria.

We all know that heartworm disease is preventable. The question is how to treat those patients who become infected. The treatment goals are to improve the clinical signs and to eliminate all stages of heartworm while preventing treatment complications. Worm burden and overall activity level of the dog are important considerations. For treatment of heartworm disease in both symptomatic and asymptomatic dogs, the American Heartworm Society recommends the use of doxycycline and a macrocyclic lactone (such as ivermectin, milbemycin, moxidectin, or selemectin) prior to the three-dose regimen of melarsomine (one injection of 2.5 mg/kg body weight followed at least one month later by two injections of the same dose 24 hours apart). Any method utilizing only macrocyclic lactones as a slow-kill adulticide is not recommended.

Melarsomine is the only adulticidal drug approved by the FDA. It is given as an intramuscular injection into the epaxial lumbar muscles. Exercise restriction (i.e. cage rest) during the recovery period is essential for minimizing cardiopulmonary complications (pulmonary thromboembolism). When melarsomine is used with the three-injection protocol, 98% of worms are killed as compared to the two-injection protocol where only 90% of worms are killed.

Because Dirofilaria immitis parasites often contain Wolbachia bacteria, doxycycline is also recommended in the treatment of heartworm disease. Studies have indicated that treatment with doxycycline (and a macrocycline lactone) before melarsomine injections has resulted in fewer gross pulmonary pathologic changes associated with death of the heartworm.

Melarsomine is not effective for killing heartworms less than 4 months old. To ensure successful elimination of younger worms, it is recommended to administer a macrocyclic lactone for 2 months before adulticide therapy is administered. Not only will this kill worms less than 2 months old, it will allow for maturation of worms between 2 and 4 months, making them susceptible to the melarsomine.

Due to the inherent inflammatory nature of this disease and its treatment, it is often recommended to use an anti-inflammatory medication (such as a tapering dose of prednisone at 0.5mg/kg BID) in addition to the three therapies listed above. Using this protocol, studies have shown that prednisone reduces lung parenchyma and arterial wall damage and help control clinical signs associated with pulmonary thromboembolism.