Parasitic Life Cycle and Vectors

Parasitic Life Cycle

• A vector is a carrier, especially an animal, that transmits a disease organism from one host to another.
• An intermediate host is one that alternates with the definitive host.
• A definitive host is one in which the terminal (frequently sexual) stage of the parasite occurs.
• A reservoir host is a definitive host in which the parasite passes through partial development, but not to sexual maturity.

Phylum Euglenozoa

• Possess flagella.
• Formerly under Phylum Sarcomastigophora.
• Subphylum Kinetoplasta has a unique organelle, the kinetoplast (modified mitochondrion).
• All are parasites.

Trypanosoma

• Causes sleeping sickness and Chagas disease.
• Has a morphological form called amastigote, which is spherical in shape, has no free flagellum, and is non-motile.
• Another form is the promastigote, which has a free flagellum and is motile.
• African sleeping sickness is a chronic disease that lasts for years, transmitted by the Tsetse fly (Glossina), and affects 60 million people in 36 nations.

Leishmaniasis

• Caused by Leishmania, which is transmitted by the sand fly (Phlebotomus).
• Affects 12 million people.

Phylum Retortamonada

• Lack mitochondria and Golgi bodies.
• Class Diplomonadea includes Giardia, which causes giardiasis, also known as backpackers' disease or beaver fever.
• Giardia inhabits the intestines of several animals and humans, and is acquired by drinking water with cysts.

Phylum Apicomplexa

• Parasites of animals and some cause serious human diseases.
• Apex contains complex organelles specialized for penetrating host cells and tissues.
• Locomotory structures are less obvious.
• Includes Toxoplasma gondii, which causes toxoplasmosis, and can infect cats, humans, mammals, and birds.
• Malaria is characterized by chills, fever, and in most cases, coma leading to death, transmitted by the mosquito genus Anopheles (60 spp.).
• Protozoan parasites include Plasmodium falciparum, P.vivax, P.malariae, and P.ovale.
• Infectious stage is sporozoites.

Malaria

• Sporozoites from a mosquito bite enter the liver and undergo schizogony to make merozoites.
• Merozoites enter RBCs and undergo schizogony to make more merozoites.
• Merozoites rupture RBCs to invade other RBCs, causing the sudden chills and fever typical of malaria.

The Amoebas

• Formerly under Phylum Sarcodina (under Phylum Sarcomastigophora).
• Shape of pseudopodia is used for classification.
• Found in aquatic and moist environments.
• Few are parasitic.

Cercozoans

• Have thread-like pseudopodia.
• A newly recognized clade, Cercozoa, contains a diversity of species that are among the organisms referred to as amoebas.

Amoebozoans

• Have lobe-shaped, rather than thread-like, pseudopodia.
• Include gymnamoebas, entamoebas, and slime molds.

Gymnamoebas

• Common unicellular amoebozoans in soil, freshwater, and marine environments.
• Most gymnamoebas are heterotrophic.
• Actively seek and consume bacteria, other protists.

Entamoebas

• Parasites of vertebrates and some invertebrates.
• Entamoeba histolytica causes amebic dysentery in humans.
• Life-threatening manifestations are hemorrhage, perforation, appendicitis, and tumor-like growths (amebomas).