|Vector: an organism, typically a biting insect or tick, which transmits a disease or parasite from one animal or plant to another.|
The Protozoa are those members of the animal kingdom consisting of one cell only. A Protozoal Cell is different from a bacterial cell because it is bigger and possesses a nucleus. The protozoa are divided into four classes. Remembering that protozoa are the phylum and phyla are divided in classes.
THE FOUR CLASSES ARE
- Mastigophora: These are those protozoa that possess a flagellum. A flagellum is a whip-like appendage and is used in propelling the organism through a liquid medium.
- Rhizopodia: These are those protozoa that possess pseudopodia, and as the name suggests, pseudopodia are leg or limb-like structures. They are tentacle like projections that can change shape and position.
- Ciliophora: These organisms are characterised by their possession of numerous hairs (Cilia) around their perimeter.
- Sporozoa: These organisms are characterised by their particular type of life-cycle, which has a sexual and an asexual stage. They are intracellular parasites of animals and usually parasitise arthropods which show no symptoms.
In this lecture only three classes will be studied, omitting the class Cilliophora
|Enteric: relating to or occurring in the intestines.|
The most agriculturally important family of organisms in this class are the Trypanosomidae which includes the Genus Trypanosoma. Species of this genus are blood parasites of domestic stock, game and man. In man and domestic stock they produce a severe debilitating disease which in stock is called Nagana and in man sleeping sickness. The disease is transmitted by an insect vector called the Tsetse fly. When the Tsetse fly sucks blood from an animal it injects the
trypanosomes into the blood of the host animal. The organisms divide by binary fission as described in the previous lectures.
Wild life is not affected by the presence of trypanosomes in the blood and thus they serve as a natural reservoir of infection.
The organism causes anaemia by producing chemicals which prevent the body from making more red blood cells. Trypanosoma species are characcterised by having a flagellum that is attached to the main cell by a membrane called the undulating membrane. This structure is like a long fin down the length of the cell. See Figure 1 below.
There are a number of different species of Trypanosomes.
As mentioned earlier, these have processes called Pseudopodia which are used for engulfing prey and subsequently ingesting it. This is the characteristic of the Genus Amoeba species. Most amoeboid organisms are fairly harmless, however, some actually cause enteric disease (disease of the intestine and gut). See Figure 2.
These protozoan organisms are characterised by a particular type of life cycle which involves a sexual stage and an asexual stage. The former usually occurs on an arthropod host (e.g. an insect, tick of mite), while the latter occurs usually in a mammalian host or another vertebrate (animal with a backbone).
THE LIFE CYCLE IS AS FOLLOWS:
The vertebrate host becomes infected with a type of cyst, either in the blood, gut or tissues. After infection the Cyst ruptures and releases protozoal forms known as sporozoites. These sporozoites go on to invade a host cell and once inside it swells in size to form a trophozoite. The nucleus of the trophozoite divides repeatedly until a schizont is formed. This contains numerous merozoites and when ruptured are released. These can then re-infect some new host cells, or turn into either a male or female gamete. The gametes are then taken in by the insect or tick where they fuse to form a Zygote which then develops into a cyst and the whole cycle is repeated. See Figure 3.
Examples of sporozoan diseases are: Malaria, Babesiasis (Red water in Cattle), and Coccidiosis in Fowl.
In Malaria the host cells for the sporozoites are the human liver cells. The host cells for the Merozoites are red blood cells and the insect host for the fusion of the gametes is the Mosquito.
Babesiasis host cells are cattle red blood cells and the host for the sexual stage of the cycle is the blue tick.
Figure 1: A Trypanosome
Figure 2: An Amoeba
Figure 3: Lifecycle of a Sporozoan