Heartworm prevention recommendations vary by climate and mosquito prevalence, but year-round prevention is increasingly recommended globally as mosquito ranges expand.
The welfare case for year-round heartworm prevention rests on the increasing unpredictability of mosquito activity as climate patterns shift. A dog owner who relies on seasonal prevention, assuming mosquitoes are inactive during colder months, faces growing risk of assuming incorrect inactive seasons as ranges expand. Prevention failure through seasonal gaps allows disease establishment that causes months or years of slowly progressive welfare harm. The marginal cost of year-round versus seasonal prevention is small relative to the welfare cost of a preventable heartworm infection that causes prolonged suffering and expensive, risky treatment.