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Year-Round Heartworm Prevention:
Reasons Why?

Heartworm disease is a potentially fatal, yet preventable disease. Treatment is a poor alternative to prevention. During treatment in dogs, the worms die, are driven further into the lungs, and slowly decay. Although this disease is completely preventable, about 500,000 dogs are treated for it each year in the United States. The original preventive products were given daily, and when recommendations were first made, seasonal treatment in “nonendemic areas” was recommended at that time, possibly because of the inconvenience of giving a daily medication when the risk of transmission was low.

With the current products, monthly dosing is recommended and there is no reason not to give preventive treatment year-round. Because these products were to be used monthly for the entire life of the pet, the Food and Drug Administration required testing that surpasses that for products designed for single treatment. During the approval process, the products were given to dogs throughout pregnancy (to make sure there were no detrimental effects to mother or puppies) and to breeding males (to make sure there were no detrimental effects to sperm numbers, motility or viability). Non-adherence with the preventive treatment is a real problem, perhaps in part because of confusion about the start and stop dates. Mosquitoes do not always sleep all winter, and it is difficult to predict when they will first appear in the spring. Despite the excellent preventive treatments available, heartworm disease has spread to all 50 states. We should be recommending year-round heartworm preventive treatment everywhere in the United States.

The 2014 American Heartworm Society guidelines recommend starting puppies on preventive treatment at 8 weeks of age. Although they recognize that transmission is seasonal in some parts of the country, they also recognize that lapses in chemoprophylaxis entail considerable risk and that, at least in some cases, adherence may be enhanced by year-round treatment. Because the disease is so devastating and prevention is so safe, year-round preventative is a good idea.