Hemoflagellates

Excerpt

The family Trypanosomatidae consists of many parasitic flagellate protozoans. Two genera, Trypanosoma and Leishmania, include important pathogens of humans and domestic animals. The diseases caused by these protozoa are endemic or enzootic in different parts of the world and constitute serious medical and economic problems. Because these protozoans require hematin obtained from blood hemoglobin for aerobic respiration, they are called hemoflagellates. The digenetic (two-host) life cycles of both genera involve an insect and a vertebrate. The family also includes the digenetic genus Phytomonas, which infects plants, and some monogenetic (one-host) species which infect only invertebrate hosts.

The hemoflagellates have up to eight life cycle stages which differ in the placement and origin of the flagellum. Two stages—the amastigote and the trypomastigote—may occur in vertebrate hosts, and three stages,—the promastigote, paramastigote, and epimastigote—in invertebrate hosts (Fig. 82-1).

Besides the nucleus and the flagellum, a trypanosomatic cell has a unique organelle called the kinetoplast. The kinetoplast appears to be a special part of the mitochondrion and is rich in DNA. Two types of DNA molecules, maxicircles which encode mainly certain important mitochondrial enzymes, and minicircles which serve a function in the processof RNA editing, have been found in the kinetoplast; when Giemsa stained, the kinetoplast is reddish purple and darker than the nucleus, contrasting with the pale blue cytoplasm.

Monogenetic trypanosomatids are more primitive than the digenetic species and grow easily in synthetic culture media. Some digenetic species can be cultivated in complex synthetic media. The medium most commonly used is NNN medium, which has a solid phase of rabbit blood agar and a liquid phase of a physiologic salt solution. Liquid media are also available. Only the invertebrate stages appear in such media, and they may or may not be infectious for the vertebrate hosts, depending on the species.

Replication of trypanosomatids occurs by single or multiple fission, involving first the kinetoplast, then the nucleus, and finally the cytoplasm. However, evidence for sexual reproduction has been presented.