INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

PROTISTS - PROTISTA

PROTOZOA

FLAGELLATES

PHYLUM EUGLENOZOA

CLASS KINETOPLASTIDEA

The Class comprises predominantly parasitic forms, although free-living ones also exist, such as the genus Bodo, species of which inhabit freshwater bodies. There are approximately one thousand known species of kinetoplastids.

A characteristic feature of kinetoplastids, which gives them their name, is the presence near the kinetosome of a kinetoplast—a modified region of a single giant mitochondrion with a high DNA content that provides energy for the flagellum. Morphologically, the kinetoplast integrates the mitochondrion and the basal structures of the flagellum.

Structure. Kinetoplastids are colorless, heterotrophic flagellates with a single Nucleus and one or two flagella emerging from the bottom of a flagellar pocket. The flagella vary in length, and one of them is often reduced. Free-living species (order Bodonida) possess two long flagella; one is directed forward and beats actively, while the second trails backward and acts as a rudder (Fig. 9). Parasitic trypanosomes and leishmanias (order Trypanosomatida) have only a single flagellum. Many parasitic species feature an undulating membrane—a thin plasma fold connecting the flagellum to The Cell surface, which facilitates the parasite's locomotion in a viscous medium, such as the Blood of vertebrates (Fig. 10).

Fig. 9. Diagram of The structure of kinetoplastids of the genus Bodo:

1 - nucleus; 2 - kinetoplast; 3 - anterior flagellum; 4 - contractile vacuole; 5 - steering flagellum

Fig. 10. Structural diagram (a) and ultrastructure (b) of Trypanosoma brucei (adapted from Hausmann, with modifications):

1 - anterior end of the body; 2 - flagellum; 3 - undulating membrane; 4 - nucleus; 5 - kinetosome; 6 - kinetoplast; 7 - mitochondrion

Many parasitic species exhibit alternation of hosts and a corresponding shift in morphological forms that differ in THE POSITION OF the flagellum (Fig. 11). In the trypanosomal (trypomastigote) form, the flagellum originates near the posterior end of the cell, and the undulating membrane is elongated; in the epimastigote form, the flagellum emerges from the middle of the cell, and the undulating membrane is short; in the promastigote form, the flagellum is located at the anterior end of the cell, and the undulating membrane is absent; the amastigote form lacks a flagellum.

Fig. 11. Morphological forms of trypanosomatids

1 - amastigote; 2 - promastigote; 3 - epimastigote; 4 — trypomastigote

The pellicle consists of The Plasma Membrane underlain by microtubules running along the length of the cell, which together form a unified complex.

Nutrition in free-living kinetoplastids occurs via a cell Mouth, or cytostome, complicated by a narrow canal known as the cytopharynx. The Pharynx is reinforced by a system of microtubules that enables them to ingest large particles and Bacteria. In parasitic forms, the cytostome is reduced, and liquid nourishment is acquired through pinocytosis within the region of the flagellar pocket.

A contractile vacuole is present in both free-living and parasitic forms, where it performs an excretory function by discharging its contents into the flagellar pocket.

Reproduction in kinetoplastids occurs via division; a sexual process has not been reliably documented in this group.

Various species of the genus Trypanosoma parasitize the Blood Plasma of vertebrates, including humans. In Equatorial Africa, a severe human vector-borne disease known as sleeping sickness is widespread, caused by Trypanosoma gambiense and T. rhodesiense. Diseases caused by animal agents are termed parasitic infections (or infestations), whereas those caused by Viruses, bacteria, or Fungi are referred to as infectious diseases.

In the wild, the reservoir hosts of trypanosomes are antelopes and certain other animals that suffer little from these parasites yet serve as their carriers. The causative agents are transmitted to vertebrates and humans through the bite of blood-sucking tsetse flies (genus Glossina). Within the vertebrate host, trypanosomes live in the blood and multiply by longitudinal binary fission, whereas in the vector (the tsetse fly), the trypanosomatids (after Hausmann) initially multiply in the gut, then migrate to the Salivary Glands where they multiply further and accumulate in the salivary ducts. During blood-feeding, the trypanosomes enter the blood plasma, and subsequently the Lymph and CEREBROSPINAL FLUID, attacking the Central Nervous system. The disease is accompanied by fever, lymph node enlargement, weakness, mental disorders, and lethargy. If left untreated, the acute form of the disease results in death within six to nine months, while the chronic form proves fatal over the course of several years.

Diseases transmitted by blood-sucking arthropod vectors are known as vector-borne diseases. Sleeping sickness is a prime example of a naturally focal disease. A natural focus of a vector-borne disease is a specific geographical area where the pathogen, its vector, and the animal reservoir can persist indefinitely under natural conditions, completely independently of human settlements and domestic livestock.

In Latin America, another species—Trypanosoma cruzi—causes Chagas disease, a severe human illness. The pathogen is transmitted by blood-sucking bugs of the family Reduviidae.

A disease affecting cattle, small ruminants, horses, and camels known as "nagana" is caused by T. brucei, T. vivax, and T. congolense. It is widespread in Africa, where it is transmitted by various species of blood-sucking tsetse flies, and in tropical Asia, where horseflies serve as vectors. The natural reservoirs for this disease are populations of wild ungulates.

Close relatives of trypanosomes are leishmanias (genus Leishmania), which are intracellular parasites of humans and vertebrate animals. Their vectors are blood-sucking dipterans—sandflies (family Phlebotomidae). Leishmanias inhabiting the Cells of vertebrate hosts are spherical and lack a flagellum (amastigote form); however, in the gut and salivary Glands of the vector, the parasite develops an elongated shape with a flagellum at the anterior end (promastigote form) (Fig. 12).

Fig. 12. Leishmanias (after Polyansky): a — Leishmania tropica within a cell from an ulcer; b — Skin ulcers caused by L. tropica parasitism;

c — L. donovani from culture; d — the sandfly Phlebotomus papatasii, the vector of leishmaniasis pathogens

Leishmanias lack an undulating membrane. In humans, L. tropica causes a skin condition known as cutaneous leishmaniasis, Borovsky's disease, or oriental sore (also known as Pendine ulcer).

The natural reservoir for leishmanias consists of various rodents, predominantly large colonial gerbils. L. donovani causes visceral leishmaniasis, or kala-azar, which is prevalent in Central Asia, among other regions. In this case, the parasites infect Cells of the Liver, Spleen, Lymph Nodes, and intestinal walls. The disease is characterized by fever, enlargement of the affected Organs, and anemia. The natural reservoir of the parasite primarily includes dogs and jackals.

Species of the genus Phytomonas parasitize plants, inhabiting laticiferous vessels, intercellular spaces, and cell vacuoles, where they often multiply in massive numbers. Their vectors are herbivorous insects with piercing-sucking mouthparts, particularly certain true bugs. These parasites cause significant damage to coffee plantations, especially in South America.