Comparison of Life Cycles and Taxonomy of Protozoa, Fungi, and Unicellular Parasites

Arthropod-Borne vs Sexually Transmitted Parasites

1. Compare the life cycle of one arthropod-borne parasite with the life cycle of one sexually transmitted parasite, using named examples of medically important protozoa.  

2. You should consider differences between mode of transmission, infective stages, developmental stages in the human body and types of parasite reproduction.

3. Taxonomy and characteristics of selected medically important fungi and unicellular parasites.

1. Some species are always pathogenic

2. Other fungal species are opportunistic

3. They cause disease through three major mechanisms:

A. Hypersensitivity (allergic) responses

B. Mycotoxin production

C. Mycoses

D. Schizogony in Plasmodium falciparum

E. In a schizont, nuclei repeatedly divide by mitosis to form daughter cells called merozoites

F. Murder by mushroom

G. Poisonous and psychedelic mushrooms

H. Amanita muscaria

I. A hallucinogenic and poisonous mushroom (Basidiomycota) contains ibotenic acid and muscimol

J. Psilocybe semilanceata

K. ‘Magic’ mushrooms contains psilocybin.

L. Supergroup: Chromalveolates

M. Clade: Alveolates

In a schizont, nuclei repeatedly divide by mitosis to form daughter cells called merozoites.

Previously 20 million people were infected, so there has been progress with control of this disease through killing the kissing bugs (vector control management), blood and organ screening and increase in public awareness and anti-trypanosome drugs.  American trypanosomiasis is still difficult to control because it infects more than150 species of domestic and wild animals, they act as reservoirs of the disease. Also, most cases are asymptomatic, people don’t know that they have the disease so they act as a source of infected blood for kissing bugs.

The Excavata supergroup contains the clades of protozoa with flagella, the Euglenozoa, Parabasalids and Fornicata.

Trypanosoma cruzi has an indirect life cycle involving a human or animal and a triatomine bug (don’t worry about learning the species names of the bugs). The infective stages of Trypanosoma cruzi are called metacyclic trypomastigotes live in the rectum of the bug.  When infected kissing bugs take a blood meal, they defaecate the trypomastigotes onto the skin. When people get bitten, remember this is at night, the natural reaction is to rub the skin. Therefore trypomastigotes get rubbed into the bite wound. The immune response to entry of parasites causes a swelling called a chagoma. If the trypomastigotes are rubbed into a mucous membrane such as the conjunctiva of the eye, swelling of the eye occurs and this is known as a Romaña sign.

Metacyclic trypomastigotes invade cells at the bite site, fusing with lysosomes in host cells and they become enclosed in a vacuole from which they escape. They transform into spherical amastigotes that replicate by asexual binary fission. Amastigotes do not have a flagellum. When the host cell is swollen with amastigotes, the parasites transform back into flagellated trypomastigotes and escape the host cell by lysis. Trypomastigotes spread throughout the body in the blood and lymphatics. They invade a variety of cells, but particularly cardiac, muscle and neural cells where they undergo further cycles of multiplication as amastigotes.  This invasion of cells causes pathology.

Triatome bugs ingest circulating trypomastigotes during a blood meal on an infected host. The trypomastigotes transform into epimastigotes within the stomach of the bug. Epimastigotes multiply by binary fission and migrate to the rectum. Here they undergo final transformation into infective metacyclic  trypomastigotes and are defaecated onto the skin by bugs.