How does heartworm spread in dogs?

Heartworm disease in dogs is primarily spread through mosquito bites. When a mosquito bites an infected dog, it picks up the microfilariae, which are the immature forms of the heartworm. These microfilariae then develop into infective larvae within the mosquito over a period of about two weeks. When the infected mosquito bites another dog, it can then transmit these larvae into the dog's bloodstream. Once inside a new host, the larvae migrate to the heart and pulmonary arteries, where they mature into adult heartworms, leading to serious health issues.

The other routes mentioned in the options are not applicable to the transmission of heartworms. Flea bites do not play a role in the lifecycle of heartworms, and direct contact with infected animals does not transmit the disease since heartworms cannot be spread from one dog to another directly. Similarly, contaminated food or water is not a transmission method for heartworms, as the disease requires the intermediate host of the mosquito for transmission to occur. Understanding this lifecycle is critical for prevention and treatment strategies in veterinary practice.