Epidemiology

There are 31 known species of tsetse fly under the genus Glossina; speciation of flies has been facilitated by a recently produced computer program called 'Glossina expert'. These species can be grouped into three main groups, which have different ecological preferences. The Glossina morsitans group preferentially occupy savanna woodland but have recently adapted to vegetation in the face of extensive deforestation. The Glossina palpalis group inhabit areas of secondary forest and vegetation associated with surface water (e.g. mangrove forest and riverine forest). The Glossina fusca group is not thought to transmit human African trypanosomiasis; it inhabits primary forest. Changes in the distribution of some species can occur with alterations in the environment. Detailed information regarding the distribution of the tsetse fly may be obtained from maps produced under the auspices of the Inter-African Bureau for Animal Resources of the Organisation of African Unity (OAU/IBAR).

Fig. 14a.4 Genetic mechanisms for antigenic variation. (A) In situ activation. (B) DNA recombination by (i) gene conversion and (ii) reciprocal recombination. White and grey boxes represent transcribed and silent genes, respectively. White and black circles represent 70 bp repeats and telomeric repeats, respectively. Redrawn after Pays et al., 1998

Fig. 14a.4 Genetic mechanisms for antigenic variation. (A) In situ activation. (B) DNA recombination by (i) gene conversion and (ii) reciprocal recombination. White and grey boxes represent transcribed and silent genes, respectively. White and black circles represent 70 bp repeats and telomeric repeats, respectively. Redrawn after Pays et al., 1998

Glossina spp. have an ability to spread rapidly, being able to fly about 6 km/day. They can also be dispersed passively by animals, floating vegetation and vehicles. Not all tsetse flies have the same vectorial capacity (i.e. capacity to transmit trypanosomes). The factors affecting vectorial capacity remain unclear, but a large number of environmental factors are implicated.

Human trypanosomiasis is characterised by periods of long-term endemicity in specific foci, interspersed with short-term epidemics.

In endemic foci, transmission of African trypanosomiasis to humans is influenced by several factors related to tsetse flies and the animal and human reservoir. The factors relevant to Glossina species are vectorial capacity, degree of anthropophily, longevity and dispersal. The density of the fly population is also of relevance in East African trypanosomiasis. The factors pertaining to the human reservoir that affect transmission include place of residence and behaviour, particularly agricultural and water-related activities, and the nature of the animal reservoir. Data from studies based on molecular and biological strain characterisation suggest that each focus may be associated with a particular trypanosome strain that is responsible for the long-term stability of that area as a disease focus (Hide, 1999). There is also evidence from population studies using minisatellite markers that frequent genetic exchange occurs in the field, and that a significant proportion of tsetse flies and mammalian hosts are infected with a mixture of trypanosome genotypes (Macleod et al., 1999).

Factors influencing transmission at epidemic level include sudden environmental alterations (e.g. deforestation), variations in human behaviour (produced by ethnic diversity) and massive population movements. Studies based on restriction fragment length polymorphism (RFLP) analysis of trypanosomal repetitive DNA sequences have shown that the strains that are harboured during periods of endemicity are similar to those that circulate during an epidemic peak (Hide et al., 1998).

Trypanosomiasis is found exclusively in sub-Saharan Africa between latitudes 14°N and 29°S.

East African trypanosomiasis

West African trypanosomiasis

Fig. 14a.5 Distribution of East and West African trypanosomiasis. Redrawn after Kirchhoff, 1990

vectors inhabit forests and wooded areas along rivers, where favourable conditions of temperature, moisture and darkness are combined with the availability of mammalian blood. This distribution of the vectors restricts the occurrence of human infection to the tropical rain forests of Central and West Africa. Transmission is related to the site, intensity and frequency of contact between humans and the tsetse fly and occurs mainly in the following situations:

• Savannah and forest galleries—places that humans visit during their daily domestic schedule (e.g. for washing, fetching water), work (e.g. fishing) or while walking by or across rivers.

• Forest habitats—areas of human activity attract tsetse flies.

• Mangrove swamps—transmission mainly occurs in areas of human activity (e.g. encampments).

Other transmission sites include mango plantations and patches of forest around villages. Peridomestic transmission is relatively rare, occurring mainly when there are few zoonotic hosts or when the ecological environment around human habitations is unfavourable for the tsetse fly. Epidemic peaks can result from minor alterations in the environment, such as changes in temperature, humidity and vegetation, that alter the ecological balance.

The geographical distributions of east and west African trypanosomiasis are shown in Figure 14a.5. The at-risk population is about 60 million, and it is estimated that about 300000 new cases occur annually. Regrettably, less than 10% of this number reach medical attention.

West African Trypanosomiasis

Although the tsetse fly is adapted to feeding on a wide variety of mammals, the slow rate of progression of this disease makes the human reservoir of prime importance. The main vectors are G. palpalis palpalis, G. palpalis gambiensis, G. fuscipes fuscipes and G. tachinoides. These

East African Trypanosomiasis

The epidemiology of endemic disease is zoonotic in nature, human infection being acquired from species of tsetse fly that inhabit the savanna and usually feed on a wide variety of domestic and wild animals. The bushbuck is probably the most important animal reservoir, it can live close to human habitation; important domestic reservoirs include cattle, dogs, sheep and goats. Human infection usually follows entry into woodland areas infested by the tsetse fly, the principal species involved being G. morsitans morsitans, G. morsitans centralis, G. swynnertoni, G.pallidipes and G. fuscipes fuscipes. Hence, the infection tends to have a patchy distribution, affecting predominantly adult men.

Epidemic disease is associated with changes in the distribution of G. morsitans populations, resulting in increased feeding on humans by tsetse flies, possibly caused by an alteration in the distribution of wild animals. It has a different transmission cycle, the human and domestic animal reservoirs predominating. In consequence, men, women and children are equally affected.

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