See that little worm-like figure?
Thats microfilaria in a SINGLE drop of blood from a dog heavily infected with Dirofilaria immitis (Canine Heartworm).
Once the female nematode, located in the pulmonary artery/left ventricle of the heart matures (adult) and mates with a mature male, she will produce many microfilaria (live larvae). The larvae will circulate in the dog's blood until a mosquito bites/feeds on the infected dog and takes the microfilaria with it.
The mosquito then becomes the vector for D. immitis and is capable of spreading the canine heartworm to other susceptible dogs.
Prophylactic treatment requires administration of Macrocyclic Lactones.
Curative treatment of established canine heartworm requires surgical removal in addition to administration of Melarsomine dihydrochloride.