Treatment for worms to prevent your dog’s heart Heartworms are the deadliest parasites for dogs. These parasites are capable of causing serious damage to vital organs like heart, lungs, and several other organs in the body. The adult heartworm parasites live in the infected dog’s bloodstream. When the mosquito bites an infected animal, it carries baby worms which develop within 10 to 15 days turn in the larvae that cause the infection. Once the infected mosquito bites another animal these larvae enter the new host’s bloodstream. They develop within 6-7 months and turn into adult heartworms.