A method of marking tsetse flies is described based on the transfer of fluorescent pigment from marked males to females during copulation. Powder applied between the coxae of the male lodges in the scutellar groove of the female during mating. Powder persists well on males before mating and on females afterwards. Individual males can mark several females, and the marks of different males can be detected on females after multiple mating. Aspects of behaviour which may possibly be investigated using this method are described.

Assessments were made of the applicability of various methods of monitoring tsetse populations for control projects in Niger and Mozambique. Biconical traps proved most effective for sampling G. tachinoides in riparian habitats of Niger, and vehicle patrols for G. morsitans morsitans in savannah woodland of Mozambique. Comparisons are drawn between the various methods in terms of species, sex and physiological composition of catch, patterns of diel activity, and detection of fly concentrations.

Glossina tachinoides and G. morsitans submorsitans in Niger and G. morsitans morsitans in Mozambique conform to the general pattern of Glossina resting behaviour in that they rest on the woody parts of the vegetation during the day and on the foliage at night. G. m. submorsitans and G. m. morsitans both prefer low horizontal branches as their main day-resting sites, while G. tachinoides rests mainly on the bases of trunks. G. m. morsitans was found to exhibit a 3-phase pattern of resting behaviour: on foliage at night; low down on trunks for a short period after dawn; and on branches higher up for the remainder of the day. No relationship between temperature or relative humidity on the choice and vertical distribution of night resting sites could be found. For G. tachinoides, the microclimatic conditions of its resting sites were more favourable than ambient conditions during the middle of the day.

Examples from recent literature are used to discuss the host location behaviour of tsetse in relation to feeding patterns. Tsetse recognise potential hosts by their visual and olfactory characteristics. These, and mechanical stimulation, will activate tsetse and initiate host-oriented responses. Approach to a stationary host is by upwind flight modulated by olfactory stimuli with visual responses only at short range, while the approach to a moving host is largely mediated by visual input. Non-random feeding patterns, even where host and tsetse occupy the same habitat, may be explained by host responses to tsetse attack. There is little evidence of long range discrimination between hosts. Some epidemiological implications of the resulting feeding patterns are discussed, and the host location behaviour of tsetse is compared with that of other insects including mosquitoes.

Mobility of tsetse flies is influenced by various factors including the search for a blood-meal. Field observations on the feeding behaviour of tsetse are difficult. The use of bait animals for experimental purposes may indicate food sources but not host selection under natural conditions. Availability and density of a particular potential host species in a tsetse habitat can influence the feeding habit of the latter. Flies are capable of adapting to new host species in the absence of the usual ones. Host selection might be due principally to availability of host(s) and not based on true preference. Tsetse flies respond to visual stimuli from moving objects. Their shape, colour and size are important factors eliciting visual response. Whether these factors are distinguished separately or in combinations is not clear. Odour emanating from live hosts causes up-wind olfactory response towards the source(s), resulting in actual landing on test targets even when the hosts are out of sight. The components of odour have not been isolated. It is therefore doubtful if host selection can be associated with odour, as this might imply that tsetse can differentiate the odours of the different potential hosts. The difficulties in ascertaining true host selection by tsetse are discussed to stimulate further investigations.

Studies of some vector-borne diseases have shown genetic factors to be involved in the vector–parasite relationships and these results have often influenced measures taken to control such diseases. Novel methods of vector control have emerged from genetical studies, such methods having the dual advantages of being species specific and of causing little environmental damage. However, genetics has made little impact on trypanosomiasis research or control when compared with other vector-borne diseases, for reasons which are related to the biology of the tsetse fly and the trypanosome. Despite these natural limitations, genetical studies may yet provide useful information for those engaged in trypanosomiasis research. For example, chromosome and isozyme studies have shown that there is a large amount of genetic variation within tsetse populations, but it is not known whether such variation is related to the distribution of the disease. The infection rates of flies are known to vary between species and are dependent on fly-trypanosome interactions but it is not known whether these interactions have a genetic component. It is suggested that studies of the genetics of natural populations of tsetse, combined with selection experiments in the laboratory, could provide answers to these questions which might be of importance to control programmes and to studies of the epidemiology of trypanosomiasis.

The factors effecting establishment of salivarian trypanosomes in Glossina (tsetse flies) are reviewed. Recent observations on host–parasite relationships are outlined in the trypanosome-Glossma system. These include, in Trypanozoon, the penetration of the peritrophic membrane; the infection of the midgut cells; the finding of cyst-like multinucleate forms in midgut cells; and the observations on haemocoelic stages in laboratory and field studies. The implications of haemocoelic infection are discussed in relation to work on other Trypanosomatidae in insects and the importance of studies on the possible immune mechanisms of tsetse are outlined.

The interaction between trypanosome parasites and the surfaces of Glossina are briefly reviewed. Recent studies on the relationship between salivarian trypanosomes and LC1 and LC2 mechanoreceptors in the labrum are presented, which indicate that trypanosomes could impair receptor activity in different ways which would result in increased probing. The epidemiological and epizootological implications of these suggestions are outlined. The relevance and importance of trypanosome infection rates in Glossina are discussed.

The cyclic transmission of African trypanosomes is subject to the successful passage through successive ‘biological barriers’ associated with the ecology and physiology of the host(s) and vector(s), and the ability of the parasite to adjust to the various substrates. This paper is an analytical compilation of factors which play a relevant role in the complex pathway characteristic of trypanosome transmission. These are discussed under the following headings: (a) physiology of the parasite; (b) physiology of the vector; (c) ecology of the vector; (d) feeding habits of the vector; (e) physiology of the vertebrate host; and (f) behaviour of the vertebrate host.

In intertropical Africa, various haematophagous Diptera are considered responsible for the maintenance and spread of cases of cattle trypanosomiasis in regions where tsetse flies are absent. In comparing early observations with the results of two recent tsetse control campaigns, this paper raises the question of whether the role of machanical vectors is as important as that normally attributed to them, and stresses the insufficiency and impressive nature of available information on this important epidemiological problem, which requires much more intensive research.

The epidemiology of T. b. gambiense is reviewed in the light of recent findings, and potential animal reservoir hosts are listed. The discovery of distinct zymodemes in pigs with isozyme characteristics identical with stocks previously isolated only from man, and the human plasma resistant character (BIIT positive) of these stocks, is indicative of a zoonotic source of infection of man in some areas of West Africa. The likely epidemiological differences between savanna and forest foci are emphasised.

The focality of West African sleeping sickness is discussed in relation to the various hypotheses which have been propounded to explain this phenomenon. Recent observations which suggest Glossina can move longer distances on wind fields than hitherto supposed, and its possible implications as an explanation of some residual foci, is discussed. The possible impact of onchocerciasis on the distribution of foci is mentioned.

There is increasing evidence that, under laboratory conditions, trypanosome populations which have the same genetic constitution by nature of the fact that they were derived from a cloned population but differ phenotypically in their expression of different surface variable antigens also differ in their infectivity and growth characteristics in certain environmental conditions. We have reviewed the evidence available for the existence of such a phenomenon in T. vivax and T. brucei variant populations and conclude that, in view of the fact that no physiological function has yet been attributed to the surface variable glycoprotein antigen, such differences most likely indicate concomitant expression of other genes controlling physiological processes which determine the parasite's ability to multiply in a given environment. The question of whether such a linkage between expression of a given surface variable antigen and expression of other physiological control genes is stable over extended periods of time or exists in all members of a trypanosome population displaying a given variable antigen is also raised but the experimental data at present available is too limited to allow any conclusions to be drawn.

The epidemiological importance of differences in transmissibility, infectivity, antigenicity, pathogenicity and drug sensitivity among species and strains of the African pathogenic trypanosomes is well known. These aspects of trypanosomiasis have been studied particularly intensively in the case of Trypanosoma brucei, but less attention has been given to T. congolense and T. vivax, the two most serious trypanosomal pathogens of domestic livestock. Our recent studies have included experiments involving the cyclical transmission by Glossina morsitans of stocks of T. congolense from locations in East and West Africa to rabbits, calves and sheep, and examinations of formation of local skin reactions and the antigenicity of the trypanosomes developing in these hosts. Differences have been observed in the occurrence of local skin reactions in different host species, and antigenic differences have been found between trypanosome stocks from different areas by serological tests and immunisation and challenge experiments. Differences in the serological responses of rabbits to different trypanosome stocks have also been observed during estimations of antibody levels using enzyme-linked immunosorbent assays.

The control of Trypanosoma rhodesiense sleeping sickness by clinical methods is often complicated by the existence of animal reservoir hosts which greatly contribute to the persistence of the disease in many parts of East Africa. The control of sleeping sickness should, therefore, include control of the vector and elimination of the reservoir hosts. Another factor which is often ignored, when planning sleeping sickness control measures, is the influence of cultural practices of the resident human population on the epidemiology of the disease in any given location in East Africa. At Lower Kitete, northern Tanzania, trypanosomiasis due to T. rhodesiense is occupational in nature so that many of the individuals are infected in Lossitete Forest while engaged in various activities. The factors favouring infection within the area, and the role played by the cultural practices of the Masai around the area in bringing about the outbreak of an epidemic, are discussed. The importance of understanding the cultural practices and occupational tendencies of the Masai in Lower Kitete, where trypanosomiasis outbreaks have occurred in the last 10 years, is stressed. With such an understanding the management of trypanosomiasis at Lower Kitete may be successful.

Ecological surveys were carried out to assess the population characteristics and epidemiological importance of Glossina palpalis palpalis (Rob-Desv.), 1830 in the human trypanosomiasis focus of Vavoua in the forest zone of Ivory Coast. G. p. palpalis was widespread in all components of the ecosystem: forest, interstitial savannahs, coffee plantations, tracks and villages. Most flies were caught at the forest edge. Sex ratio and age composition of populations varied in relation to trapping sites. Open areas were dispersal places. Plantations provided resting, feeding and breeding sites giving rise to close man–fly contact. Here, teneral flies could become infected and later on transmit trypanosomes to plantation workers and their families. The role of man and domestic pigs together with that of fly behaviour and genetic factors in the transmission of the disease are discussed. Methods of tsetse control are suggested in the light of the findings.

It has become increasingly noticeable that common features of many forest and forest/ savanna foci of West African human trypanosomiasis (WAT) are peri-domestic tsetse populations closely associated with domestic pigs. Recent evidence from various sources almost completely proves that, in at least some WAT foci, domestic pigs play an active role as reservoirs of Trypanosoma brucei gambiense. On the basis of this assumption, various aspects of Glossina palpalis and G. tachinoides local distribution and ecology, in relation to domestic pig and human populations, are discussed.

In those WAT foci where domestic pigs are kept, tsetse are widely distributed in forest islands, forest/plantations, and riverine forests, but major concentrations are in villages containing pigs. Close pig/fly relationships at the village level, involving peri-domestic feeding, breeding and resting behaviour on the part of flies, could be important factors in maintaining T.b. gambiense transmission in pig populations.

Endemic transmission of WAT to man could occur, either in villages where low pig densities dictate that a significant proportion of the fly population must also feed on man, or away from the village as a result of contact with infected flies which have strayed from a village transmission cycle.

The development of epidemic conditions could be explained, on the basis of changes in pig density and resultant changes in fly feeding behaviour. The sudden reduction of a pig population could result in an increased number of infected flies feeding on man, until such time as the fly population became stabilized on a reduced pig population (or on other hosts if pigs disappeared totally). During the period of increased feeding of flies on man, T.b. gambiense transmission could be intense.

Minor flare-up of WAT could occur if small numbers of infected pigs were introduced into a village area which had no previous history of pig husbandry. If a loose man/fly association persisted in such a village, endemic WAT could become a characteristic feature of that village.

Because of the fluidity of pig distribution and density, infected pigs could be instrumental in dispersing T.b. gambiense from village to village within a WAT focus and contribute to the persistent character of the focus. Pig movements involving much greater distances could also be important to the transfer of T.b. gambiense strains from one focus to another.

The geographical distribution and the biology of the different species of tsetse fly are closely connected with the vegetation. Any modification in the vegetation cover may therefore change, in one way or another, the dynamics of tsetse populations and, consequently, influence the transmission of trypanosamiasis. Irrigation and rural development programmes which often drastically change the natural vegetation of vast areas will therefore result in major changes in the incidence, epidemiology and epizootiology of trypanosomiasis.

Only very limited studies have been carried out on this important problem and it is very difficult, if not impossible, to make a review of this subject as the data available are very fragmentary and often relate to specific situations.

The main purpose of this article is not only to review the data and information available at present, but also to identify and stress our lack of knowledge and the need for further research.

The problems created by establishment of irrigation and rural development programmes, industrial scale plantations, irrigated cultivations, livestock rearing centres are highlighted without neglecting the sociological and medical aspects of population transfers connected with these development programmes.

The conclusions attempt to identify the research which should be carried out in the field and propose some recommendations drawing attention to both the health services and to those responsible for planning and establishing irrigation and rural development programmes.

The spread of human trypanosomiasis towards Central Africa from sources in the Congo at the beginning of the present century, has been linked to population changes and movements between the affected states.

The re-appearance of the disease in this part of Africa during the last 10 to 15 years raises again the question of how this disease is propagated. Although present day population movements are not so pronounced as those at the beginning of the century, these are significant population movements along major lines of communication (large rivers particularly). Of equal importance are disease foci which straddle national boundaries. Migrant populations (fishermen from Nigeria and displaced persons or refugees) also play important roles in the dispersal of the disease.

Even within certain foci, the mobility of rural populations helps to sustain the endemicity of the disease over large areas.

The risks of introducing or increasing trypanosomiasis must not be neglected when populating new areas and when developing agro-industrial complexes which require large labour forces.

During the 1977/78 dry season an experiment was conducted in the Volta Noire source tributaries area of Upper Volta, to assess the feasibility of controlling human trypanosomiasis by reducing the vector, Glossina palpalis gambiensis, by a combination of techniques. The objective was to suppress the target tsetse population by chemical means and then to achieve a further reduction or eradication by the release of sterile males.

Sixteen kilometres of the fringing forest of the upper River Guénako were treated with the synthetic pyrethroid decamethrin (OMS-1998) applied from a helicopter as an aerosol at a dosage of 0.2 g/ha. Two applications at 14 days interval reduced the target G.p. gambiensis population by over 95% and caused important changes in the qualitative composition of both G.p. gambiensis and G. tachinoides populations. Subsequent over-flooding of the residual G.p. gambiensis population by sterile males was easily achieved.