As part of an owner Q&A column, veterinarian Ernie Ward explains that the rare and mild adverse reactions to heartworm prevention treatment are no reason to avoid the potentially life-saving preventive medication. Responding to a question about whether an urban dog needs the prophylactic care, Dr. Ward says any dog that might be exposed to mosquitoes is at risk. "The treatment [for heartworm] is no fun and has the potential for side effects," Dr. Ward says. "And treatment is expensive. Prevention is best."