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Protozoan Parasite Life Cycles

Protozoa are single-celled organisms that cause the most sickness, death, mutilation, and debilitation in humans and animals. The protozoan parasite life cycle involves a sexual phase and an asexual phase, as well as an environmental infectious stage where it is out in the open between hosts. Each generation can develop and mature in as little as 12 to 14 hours.

  1. Sexual Phase

    • In the infected primary hosts, the protozoan parasite complete their reproductive stage. They develop into oocysts, which are passed into the environment in the feces of the primary hosts within five to 10 days.

    Environmental or Infectious Phase

    • Once in the environment, the oocysts develop spores within three to five days, at which point they become infectious. They live for a long time in the environment contaminating water, soil and plant life.

    Asexual Phase

    • Intermediate hosts, including birds, rodents and other animals, become infected after ingesting soil, water or plant material contaminated with oocysts. These oocysts transform into tachyzoites shortly after ingestion (within a week of infection). The tachyzoites invade neural and muscle tissue and develop into tissue cyst bradyzoites.

    Human Infection

    • Wild game and animals bred for human consumption become infected as well, which can lead to human infection. Also, contaminated water can lead to human infection. In the human host, they do not die, but they exist in a dormant state unless the immune system is weakened, in which case they can become active.

    Life Cycle Completion

    • Primary hosts become infected after consuming intermediate hosts harboring tissue cysts; for example, a cat eating a mouse that is infected. Primary hosts may also become infected directly by ingestion of sporulated oocysts; for example, by drinking contaminated water. When the primary host becomes contaminated, the life cycle starts over again with the reproductive sexual phase. The sexual phase takes one to five days where new oocysts are formed. It is not clear if the protozoan parasites die in the host or just keep transforming. They eventually die in the environment if they are not picked up by a new host, and can live in the environment for up to one year.

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