Eukaryotic parasites typically
fall into one of two categories; either protozoa or helminthes. Protozoa are
one-celled eukaryotes and can be transmitted by a vector or by water, as they
have difficulty surviving in the environment since they are susceptible to desiccation.
Transmission by the fecal-oral route is common, as in Giardia, which causes
‘Beaver fever.’ Protozoa typically damage the host cell by replication,
and do not release any toxins, and can inhabit the body both intracellular and
extracellular. At some point in the life cycle of protozoa, a cyst form and
tropozoite form normally exist. The cyst is more resistant to the environment
and is normally infectious, while the tropozoite is the form that inhabits the
body. Protozoa include Plasmodium falciparum, which causes malaria.
Helminths are multicellular worms and include both flatworms and roundworms, which invade the body extracellularly. They commonly have very complex life cycles, and pathogenesis is directly related to parasitic burden, as helminthes do not produce toxins. Helminths include Ascaris lumbricoides and Trichinella spiralis.
© 2003, J.Graf, for comments please contact Joerg.Graf@uconn.edu