The only way dogs can get heartworm is by being bitten by a mosquito that is carrying heartworm larvae. If an animal is infected with both a male and female, they can breed to produce tiny organisms known as microfilariae, which can only turn into larvae inside a mosquito.
When a mosquito bites an infected animal, the microfilariae are transferred to the mosquito inside the blood it draws out. However, the microfilariae must live within the mosquito for around two weeks before they can then be transferred to and infect another animal that the mosquito bites, such as a dog or cat.
After they enter the host, the worms slowly travel through the bloodstream to their final destination in the heart or lungs. This process takes approximately six months, and the worms can cause serious damage to blood vessels and organs as they pass through them.
Heartworm disease can lead to serious, often fatal, respiratory and circulatory problems, and it is not uncommon for one dog to have several hundred worms at a time, each of which can live for between five and seven years.
Heartworms occurs in all 50 states, and it is impossible to tell if a mosquito is carrying them. The only way to stop a dog from getting them is through preventative medication.