The males and females mate, producing offspring of microfilaria in the millions. (animals.mom.me)
As the mosquito stops by for a meal it also sucks up a number of circulating microfilariae in the blood. (vetiq.com)
That's due in part because ss many as 20 percent of dogs infected with heartworms continue to have circulating microfilariae for at least a year or longer when receiving monthly treatment. (thesprucepets.com)