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"microscopic heartworm" refers to a very tiny worm that can infect and live in the hearts of animals, especially dogs. It is called "microscopic" because it is so small that it can only be seen under a microscope. These worms can cause harm to the animal and need to be treated by a veterinarian.
Full definition
Heartworm Disease is transmitted when a mosquito
carrying microscopic heartworm larvae bite your pet's skin, allowing the larvae to enter through the bite wound and into your pet's blood.
After being bitten by a mosquito
carrying microscopic heartworm larvae, the developing larvae will enter the bloodstream and migrate to the dog's heart.
Canine heartworm disease develops when a dog is bitten by a mosquito carrying
microscopic heartworm larvae (juvenile worms) of a parasite called Dirofilaria immitis.
Once the larvae mature, they begin to release immature,
microscopic heartworms, known as microfilaria.
Heartworm disease is transmitted to cats and dogs by a bite from a mosquito carrying
the microscopic heartworm larvae.
A microscopic heartworm larva is passed from the mosquito into the tissue of the pet when the mosquito draws a blood meal.