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The effect of trypanosome infection on vector survival was observed in a line of Glossina morsitans selected for susceptibility to trypanosome infection. The differential effects of midgut and salivary gland infections on survival were examined by exposing flies to infection with either Trypanosoma congolense which colonizes midgut and mouthparts or Trypanosoma brucei rhodesiense which colonizes midgut and salivary glands. A comparison of the survival distributions of uninfected flies with those exposed to infection showed that salivary gland infection significantly reduces tsetse survival; midgut infection had little or no effect on the survival of tsetse. The significance of these findings is discussed in relation to the vectorial capacity of wild flies.

Midgut infection rates of Trypanosoma congolense in Glossina palpalis palpalis and of Trypanosoma brucei rhodesiense in Glossina pallidipes are potentiated by the addition of D+ glucosamine to the infective feed, but not to the levels of super-infection reported for G.m.morsitans, G.p.palpalis and G.pallidipes are shown to possess two trypanocidal molecules: a glucosyl lectin which can be inhibited by D+ glucosamine and a galactosyl molecule inhibited by D+ galactose. Addition of both D+ glucosamine and D+ galactose to the teneral infective feed promotes super-infection of the midguts of G.p.palpalis. The glucosyl lectin is specific for rabbit erythrocytes and is present in guts of fed G.m.morsitans and G.p.palpalis, titres of lectin activity do not increase substantially after the second bloodmeal. The galactosyl specific molecule does not show any erythrocyte specificity, although haemolytic activity is observed only in G.p.palpalis and not in G.m.morsitans. The presence of two trypanocidal molecules in some species of tsetse may account for the innate refractoriness of these flies to trypanosome infection. As D+ glucosamine also inhibits the killing of procyclic trypanosomes taken as an infective feed, it is suggested that the midgut lectin is normally responsible for the agglutination of trypanosomes in the fly midgut by binding to the procyclic surface coat, prior to establishment in the ecto-peritrophic space.

Rickettsia-like organisms (RLO) from testse midguts and mosquito cell cultures showed high levels of endochitinase activity. A line of Glossina morsitans morsitans highly susceptible to midgut trypanosome infection and with high incidence of RLO infection showed significantly greater chitinolytic activity than G. austeni which had low RLO incidence and were correspondingly refractory to midgut infection. Midgut infection rates of Trypanosoma brucei rhodesiense in G. m. morsitans showed a dose-related increase when flies were fed N-acetyl-D-glucosamine (GlcNAc) in the infective meal and for 4 subsequent days. A model is proposed for susceptibility to trypanosome infection based on the generation of GlcNAc by RLO endochitinase activity in tsetse pupae inhibiting midgut lectin in teneral flies.

Teneral Glossina morsitans morsitans from outbred and susceptible stocks infected with Trypanosoma (Nannomonas) congolense developed, respectively, three and six times higher midgut infection rates than flies of the same stock which had previously taken a bloodmeal. Non-teneral G. m. morsitans remained relatively refractory to infection when infected at subsequent feeds. Differences in susceptibility to midgut infection between teneral flies from susceptible and outbred lines of G. m. morsitans disappeared in non-teneral flies, showing that maternally inherited susceptibility to midgut infection is a phenomenon restricted to the teneral state of the fly. Laboratory reared G. m. morsitans were found to have become significantly more susceptible to trypanosome infection than wild flies from the population from which the colony was derived. The likely role of rickettsia-like organisms (RLO) in potentiating teneral susceptibility to midgut infection is discussed. The addition of the specific midgut lectin inhibitor D-glucosamine to the infective feed of non-teneral flies increased midgut infection rates to levels comparable with those achieved in teneral flies. It is concluded that the peritrophic membrane does not act as a barrier preventing non-teneral flies becoming infected. The relative refractoriness of non-teneral flies suggests that they do not play a significant part in the epidemiology of Trypanozoon or T. congolense infections.

Maintaining the puparial stage of successive generations of a population of tsetse 3 degrees C lower than normal reduced the numbers of rickettsia-like organisms (RLO) carried by emerging flies. The susceptibility of these flies to midgut infection with Trypanosoma congolense was also significantly reduced compared with control flies held at normal temperature. These results support the view that the relationship between RLO and susceptibility is quantitative-teneral flies with heavier RLO infections being more susceptible to trypanosome infection.

A midgut trypanolysin and an agglutinin from Glossina palpalis subspecies were isolated and partially characterized using anion-exchange chromatography and polyacrylamide gel electrophoresis. FPLC fractions of midgut extracts of Glossina palpalis palpalis caused agglutination and lysis of two trypanosome species (Trypanosoma congolense and Trypanosoma brucei brucei), although Glossina palpalis gambiensis caused only agglutination. The trypanolysin and agglutinin were active only in the posterior midguts, were heat labile above 50 degrees C, had a periodic cycle of 'activity' in response to bloodmeal intake and were not affected by protease inhibitors or trypsin but were inactivated by pronase. The lytic substance contained two proteins with approximate molecular weights (Mr) of 12,000 and 10,000 Da respectively. The agglutinin had an approximate Mr of 67,000 Da. Gamma-irradiation of the two subspecies caused a temporary inhibition of trypanolytic and agglutinin activities in midgut extracts.

Male tsetse, when infected in the laboratory with trypanosomes of the subgenus Trypanozoon, usually produce greater salivary gland infection rates than females of the same species. We show that a single sex-linked gene model can be fitted to most recently published data for salivary gland infection rates in tsetse. The maturation of Trypanosoma congolense infections is shown to be independent of fly sex. The possible effects of genetic control of maturation of Trypanozoon infections in tsetse populations on the transmission of sleeping sickness are considered.

The tsetse immune system has recently been shown to be involved in trypanosome maturation; lectin secreted in the midgut, normally responsible for preventing the establishment of midgut infections, induces established midgut trypanosomes to mature. We now show that a second lectin, present in tsetse haemolymph, is essential to complete the maturation process. Interactions between tsetse lectins and parasite surface coats probably determine trypanosome transmissibility and may be partly responsible for the distribution of trypanosomiasis in Africa.

Differences in susceptibility to infection with Trypanosoma congolense between F1 families of Glossina morsitans morsitans indicated that susceptibility is maternally inherited in this species of tsetse fly. Twelve F1 families, six selected for susceptibility and six selected for refractoriness to infection, have been bred for up to 13 generations. The reciprocal differences demonstrated in the F1 generation persisted in these selected families over many generations, indicating that susceptibility/refractoriness to T. congolense infection is extrachromosomally inherited in G. m. morsitans. Repeated 'backcrossing' to males of the opposite strain showed that infection rates within families were independent of the contribution of the male parent. Susceptible families had a mean midgut infection rate of 76.9% and a mature (hypopharyngeal) infection rate of 47.9%. In the refractory families 88.9% of the flies failed to develop an infection, 11.1% had midgut infections and only 6.3% developed mature infections. Levels of midgut infection remained remarkably constant within families over generations, whether refractory or susceptible, while maturation rates varied between generations and between sexes. Males matured a significantly greater proportion of midgut infections than females in the susceptible families. It is suggested that the inheritance of susceptibility/refractoriness relates primarily to the establishment of midgut infections in G. m. morsitans, and that maturation of midgut infections is dependent upon environmental factors such as diet, and differences between sexes probably reflecting differences in rates of bloodmeal digestion.(ABSTRACT TRUNCATED AT 250 WORDS)

Trypanosomes infecting the midgut of wild-caught tsetse from Zimbabwe were found on electron microscopic examination to be penetrating the peritrophic membranes and so entering the ecto-peritrophic space in the mycetome region of the midgut. Other trypanosomes entered midgut cells, behaviour similar to that previously reported in laboratory-infected tsetse colonies.

D+Glucosamine, which has been shown in vitro to specifically inhibit tsetse midgut lectin activity, when fed to Glossina morsitans morsitans with the infective feed significantly increased midgut infection rates of Trypanosoma congolense and T. brucei rhodesiense. All flies infected with T.b. rhodesiense and maintained on a diet of blood with D+Glucosamine throughout their lives developed midgut infections. Midgut extracts from flies bred for refractoriness to infection with trypanosomes showed significantly greater erythrocyte agglutinating activity and were more trypanocidal in vitro than flies bred for susceptibility to trypanosome infection. It is concluded that susceptibility to trypanosome infection in tsetse is mediated through midgut lectins.

Three experimental groups of male Glossina morsitans morsitans were infected at their first feed with procyclic forms of different stocks of Trypanosoma congolense and subsequently maintained on a diet containing rabbit antiserum to one of these stocks. Control flies were similarly infected but were then maintained on normal rabbit serum. Dissection of the flies 19-21 days post infection showed a reduction in both immature and mature infection rates in all groups fed on antiserum by comparison with control flies. These results suggest that vaccination with a single procyclic T. congolense strain could reduce transmission of both homologous and heterologous T. congolense strains which might, in certain epidemiological circumstances, provide an alternative or additional control method for African trypanosomiases.

Removal of certain components from pig or cow blood greatly reduces the maturation rate of Trypanosoma congolense infections in Glossina morsitans fed on such diets. In particular, delipidation of serum has the same effect as complete removal of dietary serum with few midgut infections reaching maturity. The addition of gamma-globulin to red cells partially restores the ability of immature infections to transform suggesting that serum lipids or other serum factors are not acting directly on midgut trypanosome metabolism but indirectly through interactions between dietary constituents and the flies' metabolic processes. As susceptibility to T. congolense infection is known to be maternally inherited in G. morsitans, it is suggested that serum factors act through the flies' symbionts to induce maturation in midgut infections.

The process of maturation of Trypanosoma congolense Broden in tsetse has been shown to be initiated by lectin secreted in the fly midgut. In the present study the duration of lectin signal required to induce maturation was determined by the sequential addition or removal of a specific lectin inhibitor (D+glucosamine) to the diet of infected male Glossina morsitans Westwood. An established midgut infection of T.congolense was found to require, at most, 72 h exposure to midgut lectin to begin the process of maturation. Longer exposure to midgut lectin increased the frequency of maturation, suggesting clonal variation in response to lectin stimulation occurs within trypanosome stocks. It is suggested that this variation corresponds to differences in lectin binding sites on the trypanosome surface. Midgut trypanosomes retained their ability to mature throughout their life in the fly; when lectin activity in the midgut was inhibited, the trypanosomes remained as procyclic forms but when this inhibition was removed maturation was able to proceed. This indicates that the process of maturation is dependent upon a signal from the fly and is not predetermined by the trypanosomes undergoing a fixed number of division cycles. The possible role of lectins in the maturation of trypanosomes in vitro is discussed.

Feeding D + Glucosamine to Glossina morsitans throughout their life significantly reduced the proportions of midgut infections which developed into mature infections with three different stocks of Trypanosoma congolense. In one stock of T. congolense, maturation was completely blocked by this carbohydrate, which is known to specifically inhibit tsetse midgut lectin activity. Similar experiments with T. brucei showed that D + Glucosamine also inhibited maturation and, when combined with results from previous experiments with the same carbohydrate, indicated that lectin inhibition also had significant effects on T. brucei infections. It is concluded that midgut lectin secreted by the fly is responsible for triggering maturation of procyclic T. congolense and T. brucei. The maturation response of midgut trypanosomes to lectin stimulation varies between species and between stocks of the same trypanosome species, probably reflecting differences in numbers of lectin binding sites which are determined by trypanosome genotypes.