How do dogs become infected with heartworms?

Dogs become infected with heartworms primarily through being bitten by an infected mosquito. This is the most common and effective route of transmission for the heartworm parasite, Dirofilaria immitis. When a mosquito bites an infected dog, it takes in microfilariae (the larval stage of the heartworm). Later, when that mosquito bites another dog, it injects the infective larvae into the new host's bloodstream.

The other options involve common misconceptions about the transmission of heartworms. Dogs do not become infected by ingesting contaminated soil, having contact with infected dogs, or drinking contaminated water, as the heartworm lifecycle is specifically dependent on the mosquito as the vector for transmission. Understanding this is key for effective prevention and management of heartworm disease in dogs.