Final answer:
Heartworms are not transmitted by direct dog-to-dog contact but rather through infected mosquitoes, making the statement false. Vector transmission is the process by which heartworm larvae are spread from host to host, usually through bites.
Step-by-step explanation:
The statement 'Heartworms are transmitted by direct dog-to-dog contact' is False. Heartworms, which are a type of roundworm known as Dirofilaria, are transmitted through vector transmission, specifically by mosquitoes. An infected mosquito bites a healthy dog and transmits the larvae (L3 stage) to the dog's subcutaneous tissues. These larvae then develop into adult heartworms that live in the heart and pulmonary arteries, causing harm to the host animal. Direct dog-to-dog contact does not spread heartworms. Instead, prevention and control involve safeguarding pets against mosquito bites and administering preventative treatments.