Morphology and cell structure

African trypanosomes are unicellular parasites that live in body fluids such as blood, lymph or cerebrospinal fluid. Their size is about 20 µm long and 5 µm wide, and their general structure and organelles are shown Figure 1. Trypanosomes have two DNA-containing organelles, the nucleus and the kinetoplast, the latter being the characteristic organelle of the Kinetoplastida order. Glycosomes (another organelle specific to the kinetoplast organisms) are considered to have evolved from peroxisomes and contain the first seven steps of the glycolytic pathway among other

biochemical pathways [2]. The parasites harbour a single unit of several eukaryotic organelles, such as the Golgi apparatus and the mitochondrion.

Trypanosomes are characterized by the presence of a single flagellum composed of the conventional 9+2 axonemes. The flagellum is attached at the basal body and exits the cellular membrane at the level of the flagellar pocket (which is important also for the interaction with the environment). The flagellum plays a role in the mobility of the cell as well as in mechanically sweeping surface material. Flagellar movement can induce endocytosis of antibodies bound to the cell surface [2, 16].

Figure 1 General structure of an African trypanosome (Image taken from [2]) and Trypanosomes among blood cells (Image taken from [12]).

A very dense coat of glycoproteins (the so-called Variant Surface Glycoproteins;

VSGs) covers the surface of trypanosomes (thickness: 15nm). These glycoproteins (or VSGs) are the predominant antigen present at the surface of the parasite. VSGs are very immunogenic and the immune response is usually focused on these antigens.

However, by the time the immune response is ready to attack the trypanosomes carrying the specific glycoproteins, some of them will already have changed the type of VSGs at its surface, thus escaping the response of the immune system [12, 17, 18].

This is possible because the T. brucei genome contains up to 2000 different genes and pseudogenes coding for VSGs, but only one type is expressed at a time [12, 19].

In addition, it has been observed that even more variants of VSG genes are obtained by exchange of small region between two genes, thus creating new chimeric variants.

The VSG coat is specific to the bloodstream form of the parasite, and its expression needs to be activated already in the salivary gland of the tsetse fly before injection into

the human host [20]. In the insect, another type of proteins, called procyclins, composes the coat.

Finally, the presence of transport proteins, called permeases, in the plasma membrane allows the parasite to access all the nutrients from the host needed for surviving [21, 22].

1.1.3. Vector

T. brucei are transmitted by the bite of a tsetse fly belonging to the genus Glossina which is part of the Glossinidae family of the Diptera order. Both female and male of this genus are haematophagous and possible vectors of African trypanosomiasis. The Glossina genus has been divided into 3 subgenera (Nemorhina, Glossina s. str. and Austenina) all together divided into 31 species and subspecies. Their classification is based on external characteristics, the shape of genitals and the geographical distribution. Depending on the infected tsetse fly species, it shows different abilities of transmitting T. b. gambiense or T. b. rhodesiense diseases [12]. Their distribution, with an area of 10 million km², is concentrated in Sub-Saharan African countries [2, 23].

Figure 2 shows the picture of two main tsetse fly subspecies, Glossina palpalis (G.

palpalis) and Glossina morsitans morsitans (G. morsitans morsitans).

Figure 2 Tsetse fly. (A) Female Riverine (G. palpalis). (B) Female Savannah Tsetse fly (G. morsitans morsitans).

(C) Bloodfed female Savannah Tsetse fly (G. morsitans morsitans). Pictures taken from[24]

Subgenus Nemorhina (“riverine Tsetse”)

Species of this subgenus are of small (6-8 mm) to medium (8-10 mm) size and are found in West and Central Africa in vegetation located close to water. Moreover, evidence of peri-urban transmission in medium and large towns has been observed largely extending the tsetse fly distribution. The subspecies G. palpalis and G. fuscipes are considered to be the main vectors of both HAT and are more and more found to

be responsible for AAT due to demographic growth and their ability to adapt to high density population regions [2, 25].

Subgenus Glossina s. str. (“savannah tsetse”)

Species of this subgenus are of medium size (8-11 mm) and are found principally in savannah woodland. They are the main vector of AAT as they are found in region with a high density of wild animals. Some species of this subgenus, principally G. morsitans, G. pallidipes and G. swynnertoni, are also considered as important vectors for rhodesiense HAT [2].

Subgenus Austenina

Species of this subgenus are of large size (11-16 mm) and are mainly found in forest belts. They could be good vector for AAT but only few animals live in the region where these tsetse flies are found [2].

Due to environmental changes throughout the year (rainy/dry and warm/cold seasons) the density of tsetse flies varies extensively. Tsetse flies can only survive in tropical areas because they need between 50-60% of humidity for the savannah subspecies and between 65-85% for the riverine and forest species. Moreover, the temperature necessary for their survival is between 16 and 36 °C and higher temperature can be lethal for both pupae and adult flies. During the short life of the tsetse fly the female produces one at the time about 3 to 5 larvae only. For this reason, the growth of a tsetse population is very limited and small interferences can easily affect the population density. For example, the risk of being killed during the blood meal could represent a real threat to the population. To avoid being killed, the tsetse fly tends to bite where it is protected from possible defensive movements of the host [2].

During a blood meal the tsetse fly first injects some saliva into the skin which provokes a vasodilatory effect and also prevents blood clotting. It is at this stage where an infected fly transmits matures trypanosomes to the new host. Since an infected tsetse fly will remain a carrier of trypanosomes its whole lifetime, control and eradication of the vector is very important to reduce the risk of HAT and AAT transmission.