INTRODUCTION

Trypanosomiasis is a disease caused by the parasitic protozoa, trypanosomes of the genus Trypanosoma. They are microscopic elongated unicellular organisms that live and multiply in the blood and other body fluids of their host, causing sleeping sickness in humans and related diseases in domestic animals (ILRAD, 1991). They belong to five well differentiated subgenera which include all the ten less defined species of the genus Trypanosoma (Losos, 1986). Species of this parasite include; T. brucei, T. congolense, T. vivax, T. equiperdum, T. envansi, T. enquinum and T. cruzi. xxi Trypanosoma brucei, a distant relative of malaria parasite, is the principal scourge of Africa. It consists of three subspecies, two of which infect humans. T. brucei rhodesiense found in East Africa and southern Africa, T. brucei gambiense found in West Africa and T. brucei brucei which is not infectious to humans but in cattle causes a wasting disease called Nagana (Gull, 2005).This disease occurs in the northern part of Nigeria (Igwe and Onabanjo, 1989). The tsetse fly belts of sub-Sahara Africa are between 140 N and 200 Infection of the mammalian host starts with the bite by an infected tsetse fly (Glossina spp), which injects the metacyclic trypanomastigote form of the parasite in its saliva before taking its blood meal. The trypanosomes multiply locally at the site of the bite for a few days before entering the lymphatic system and the blood stream through which they reach other tissues and organs including the Central Nervous System (CNS) (WHO, 2001; Wellcome News, 2005). 

In the blood stream, the parasite can be attacked by the human immune system and get round this through a process of antigenic variation. In the mammalian host, the trypomastigote cell is completely covered by a dense monolayer of the identical glycoprotein that protect the parasite against direct lyses by complement (Borst and Fairlamb, 1998). Only when specific antibodies are present against the surface epitopes is the parasite destroyed. The antibodies kill most parasites, but small but sufficient fraction of the parasite population have switched coat and escaped the body’s onslaught (Borst, 2002) and proliferate until the new generation of specific antibodies, mainly of the immunoglobulin M (IgM) type are developed by the host. Up to 1,000 different genes encoding the variant surface glycoprotein are present in the T.brucei genome (Vanhamme et al., 2001). This phenomenon explains the fluctuating number of circulatory typanosomes in the patients blood (Ross and Thompson, 1910), which contributes to the limited sensitivity of parasite detection methods in clinical practice. xxiii Chemotherapy available for control and eradication of trypanosomiasis is very limited at the moment. Effective trypanocidal drugs available are beset with problems of drug resistance and toxicity (Pepin and Milford, 1994). In addition to emerging cases of drug resistance, drugs require lengthy, parenteral administration and almost all trypanodical drugs have severe side effects, thus, underscoring the urgent need to develop more effective and safer trypanocidal drugs. Several reports on evaluation of different chemicals/drugs for trypanocidal activity have appeared just as interesting reports on the anti-trypanosomal effects of plant extracts (Asuzu and Chineme, 1990).

 Some of these reports have indeed shown that some plants possess trypanocidal activities that caused be used in the management of African Trypanosomiasis. 1.2 LITERATURE REVIEW 1.2.1TRYPANOSOMES DESCRIPTION AND CLASSIFICATION Trypanosomes are unicellular protozoan (Fig 1) with a single flagellum that contains microtubules in the 9+2 arrangement typical of xxiv other flagellates. It belongs to the class Zoomastigophora and order Kinetoplastida, so-called because of the large DNA containing structure, the kinetoplast, found at the base of the flagellum which aids them in movement. The kinetoplast encloses a large amount of mitochondrial DNA and for the parasite to survive, the kinetoplast and its DNA contents must be faithfully copied at each cell division and segregated into two daughter cells (Gull, 2005). 

TRYPANOSOMIASIS

 Trypanosomiasis is a chronic disease caused by the protozoan blood parasite trypanosome of the genus Trypanosoma. In humans, it is referred to as sleeping sickness while in cattle and other animals which serve as a reservoir for the protozoa, the disease is called Nagana. The variation of the disease occurs in Central and Western Africa, both of them transmitted in the salivary glands of infected xxvii tsetse flies. T. brucei consist of three sub-species, two of which infect humans, these include T.brucei gambiense being the most common and a more local version T. brucei rhodesiense. The third subspecie which is not infectious to humans but in cattle is the causative agent of bovine trypanosomiasis (Nagana) also known as T. brucei brucei (Treoberg et al., 1999).

 In South America, another version of the disease known as chagas disease caused by T.cruzi which is transmitted by the triatomid bug of the family Rediviidae (Bailey et al., 2005). Generally, trypanosomiasis are present in three forms; i. Human African trypanosomiasis (HAT) or African sleeping sickness ii. Human American trypanosomiasis or chagas disease. iii. African Animal Trypanosomiasis (AAT) or Nagana. Human African trypanosomiasis (HAT) or sleeping sickness, is a disease caused by T. brucei gambiense or T. brucei rhodesiense. The two are transmitted by the genus Glossina (order Diptera) and are restricted to Sub-Saharan Africa. Both are fatal if left untreated. HAT is the prototype of a neglected disease, affecting the poorest people of the poorest continent (Trouiller et al., 2002). EPIDEMIOLOGY OF THE DISEASE It is estimated that 60 million people are exposed to HAT in nearly 200 separate active foci from 36 Sub-Saharan countries but only 4 million to 5 million are under surveillance (WHO, 1998). The location of endemic foci of HAT follows the patchy distribution of tsetse flies found in a belt that stretches south of the Sahara desert and north of the Kalahari Desert (Barrett et al., 2003). Despite the absence of reliable epidemiological figures, the World Health Organization (WHO,1998; WHO, 2001) reports that 300,000 to 500,000 people might be infected by the T.brucei gambiense form of the disease in Western and central Africa. 

The most severely affected countries are the Democratic Republic of Congo, Angola, Central African Republic and Southern Sudan, where HAT has remerged during the last decades mainly due to long-standing geopolitical instability and subsequent erosion or collapse of control programmes (Moore and Richer, 2001). T.b gambiense is transmitted by the bite of tsetse flies of the Glossina palpalis or G. fuscipes groups. Human vector contact occurs mostly in xxx forested rivers and shores but is also peridomestic when huts are built in or near plantations (Burri and Brun, 2003). T .b. rhodesiense HAT is a zoonosis present in Eastern and Southern Africa. Wild animals including game animals are usually affected, but epidemics occasionally occur in domestic animals and humans. The parasite is transmitted by the bite of tsetse flies of Glossina morsitans or G. fuscipes group. Human-vector contacts typically occur in savanna woodland but can be peridomestic during epidemics (Burri and Brun, 2003). The incidence of T. b. rhodesiense HAT is currently much lower than that of T. b. gambiense HAT, but large epidemics were observed in the past. Not more than 50 cases of HAT are diagnosed yearly outside Africa (Lejon et al., 2003). Visitors to some game parks in Eastern Africa are at particular risk for T. b. rhodesiense HAT (Jelinek et al., 2002).Migrants from countries where T. b. gambiense is highly endemic can have HAT that remain unrecognized for years (Sahlas et al., 2002). Studies conducted in Western and Central Africa have failed to find an increased risk of xxxi HAT among Human Immunodeficiency Virus (HIV) infected individuals, but no definite conclusion can be drawn from the available data (Louis et al., 1991).

During the course of infection, the trypanosomes are able to express many kind of surface proteins known as variable surface glycoprotein’s (VSG’S) (ILRAD, 1991). Thus, each trypanosome in the population expresses a particular surface variant antigen type (VAT) at any one point in time. Host antibodies produced in response to one VSG (=VAT) are ineffective against the antigenically different VSG’S, which appears in subsequent paratiaemic waves. Hence, each successive and antigenically distinct trypanosome population which arises, can evade the immune responses (i.e VAT –specific antibodies) of the host to earlier VSG’s. Antigenic variation is known to be antibody independent, non random and under genetic control; the number of possible VSG’s, but not finite. The role of each successive VAT- specific and antibody produced by the host is selective rather than inductive, at any one point in time during infection of small number of new (heterotype) VATS are expressed by some few individual trypanosomes among the predominant homotype population.The production of host antibodies in response to the numerous, successive homotypes results in their destruction, but allows the few new heterotypes to survive and proliferate in their place, thus producing a new parasitaemic wave. The greater the heterogeneity of VATS (i.e VSGS), sequentially expressed, the more likely is infection to persist, the response of an individual trypanosome to changing host conditions (antibodies) is far less important, if at all, than the cumulative response of the heterotype/homotype population as a whole. 

1.2.6 CLINICAL AND PATHOLOGICAL MANIFESTATION OF THE DISEASE The clinical manifestations that characterize sleeping sickness are classified to the clinical progression of the disease that is the haematolymphatic or first stage and the meningoencephalitic or second stage (WHO, 1998). The clinical signs are generally unspecific and their frequency varies between individuals and disease foci. (A) First stage signs and symptoms Clinical manifestation of trypanosomiasis is hard and painful. At the site of inoculation, a painless skin lesion, the trypanosome chancre develops (Molyneux, 1984; WHO, 1998). (ii) Lymphadenopathy: lymph glands swell as the parasites multiply within; in Gambian sleeping sickness swollen neck glands are useful sign. Lymph glads can be a useful source of parasites for diagnosis (WHO, 1998; Pentreath and Kennedy, 2004). (iii) Fever: Waves of parasiteamia follow with fever; the pathological effect of these invasive waves on the composition of the blood (WHO, 1998; Pentreath and Kennedy, 2004). (iv) Headache: Headache is the most frequent complaint and is typically severe and persistent (Dumas and Girard, 1978; WHO, 1998; Pentreath and Kennedy, 2004). (v) Hepatomegaly and splenomegaly: Enlargement of the liver and spleen may be revealed by examination of abdomen (Dumas and Girard, 1978; WHO, 1998; Pentreath and Kennedy, 2004). (vi) Anaemia: Anaemia is frequent and may be severe causing cardiac failure. It is often detected by examination of the conjunctive (Molyneux, 1984; WHO, 1998). (vii) Skin rash: Skin rash known as trypanids occur as ring like patches with polycyclic contours of 1-10cm diameter. These patches are not easily recognizable on dark skin (WHO, 1998). 

(viii) Musculoskeletal pains: Muscle aches and joints pains are also very frequent but are common in most febrile illness (WHO, 1998). (B) Second stage signs and symptoms (i) Neurological signs: Neurological signs and symptoms are specific to the second stage. These include cranial nerve dysfunction, abdomal reflexes, neuronegative disorders and deterioration of consciousness which may lead to coma (Legon et al., 2003). (ii) Sleep disturbance: The circadian rhythm of sleep and wakefulness disappears. Periods of sleep and wakefulness may occur at any time of the day and night; the periodicity varies according to the severity of the disease (WHO, 1998). (iii) Alteration of mental state: Mental confusion and temporospatial disorientation may occur. Psychiatric disorders varied may include personality disorders, behavioural changes and alteration of mood such as euphoria or depression (Molyneux, 1984; WHO, 1998; Pentreath and Kennedy, 2004). (iv) Tone disorders: Hypertonia (extrapyramidal origin) or hypotonia (cerebellar orgin due to sensory disorders) may occur (WHO, 1998).Other neurological disorders include: Convulsion, cranial nerve dysfunction, deterioration of consciousness, which may lead to coma Molyneux, 1984; WHO, 1998; Pentreath and Kennedy, 2004). 

1.2.7 BIOCHEMISTRY OF DRUG TARGETS IN TRYPANOSOMES The biochemistry of trypanosome is centred on the survival in the host and selective elimination. The design of drugs based on different metabolic pathways and possible points for drugs target(Antitrypanosomal targets), which covers purine and pyrimidine metabolism; glycolysis; trypanothione and trypanothione reductase; tryparedoxins; polyamine and ornithine decarboxylase; mitochondrial DNA, antigenic variation in African trypanosome; protein farneylation, and cysteine proteases (Mansour,2002). (1) Nucleic acid as a target for antitrypanosomal Purine and pyrimidine metabolism are target for antitrypanosomal by the enzyme, ribonucleotide reductase. xl Ribonucleotide reductase (RNR) is the enzyme which catalyses the rate limiting step in the de novo synthesis of DNA precursor molecules (Thelander and Graslund,1994). The enzyme catalyses the reduction of ribonucleotides to deoxyribonucleotides, the precursor for DNA synthesis.A reation assisted by a protein bound 5- deoxyadenosyl cobalamin- derived free radical. There are three different classes of RNRs, where most of the eukaryotic and some prokaryotic organisms belong to class I. Class I RNR is a heterodimeric enzyme of α2β2 type. The large R1 subunit binds substrates and allosteric effectors, while the small R2 subunit contains a tyrosyl radical (Hofer et al., 1997; Hofer et al., 1998), generated by a binuclear ferric iron center.