Midgut lectin activity and sugar specificity in teneral and fed tsetse

Abstract. . 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 pro-cyclic surface coat, prior to establishment in the ecto-peritrophic space.     

Genetics of two colonies of Glossina pallidipes originating from allopatric populations in Kenya

Abstract Two large colonies, originating from allopatric populations of Glossina pallidipes Austen, in the Shimba Hills and Nguruman, Kenya, which differ biologically and with respect to vectorial competence, were compared at fourteen enzyme loci using polyacrylamide gel electrophoresis. The colonies had similar levels of genetic diversity with approximately half of the loci being polymorphic, an average of 1.6-1.7 alleles per locus, and a mean heterozygosity per locus of approximately 18.4%. However, the colonies differed significantly in allele frequencies at the loci for phosphoglucomutase, glucose-6-phosphate dehydrogenase, xanthine oxidase, octanol dehydrogenase and phosphoglucose isomerase. The results were compared with earlier studies on this species and no evidence was found for selection of specific alleles during establishment or maintenance of colonies of G.pallidipes , nor were specific chromosomes, or marker genes, associated with the biological differences between the two colonies.     

Single cell recordings from tsetse (Glossina m.morsitans) antennae reveal olfactory, mechano - and cold receptors

Abstract. Action potentials from individual cells and receptor potentials were recorded from antennae (funiculi) of living tsetse flies, Glossina m.morsitans Westwood, using a 'surface-contact' recording technique. Stimuli were pressure of the electrode, changes in temperature and the vapours of l-octen-3–ol, acetone, 3–methylphenol, dichloromethane and CO₂. Two types of mechano-receptive cells were found. One type only fired action potentials when pressure was increased, the other type continued firing when pressure was maintained. Of the 182 cells tested for their sensitivity to temperature change and the odour stimuli, 19% did not respond to any of the stimuli, suggesting that we are still unaware of cues which may be of import to the flies, and 4% responded to temperature only, increasing and decreasing their activity with a decrement and increment in temperature, respectively. Of the 141 cells which were olfactory receptors, 52% responded to 1-octen-3–ol and 40% were exclusively sensitive to this substance. For 3–methylphenol these percentages were 23 and 18, for acetone 13 and 11, for dichloromethane 12 and 7, for CO₂ 13 and 13. No clustering of cell types in certain areas of the funiculi was found, nor was a difference apparent between the sexes. The majority of the olfactory cells responded by increasing their activities on odour stimulation. Inhibition was found in three cells only, in which spontaneous activity was suppressed on stimulation with 3–methylphenol. Spike responses were phasic-tonic and varied between two extremes: Cells showing relatively rapid cessation of spike activity after the end of stimulation, and cells which continued firing for several seconds or minutes after stimulation. Possible behavioural effects of the activities of the various cell types are discussed.     

Insecticide-treated cattle for tsetse control: the power and the problems

Trypanosomiasis control increasingly involves financial input from livestock owners and their active participation. If control is carried out on smaller scales than in the past, methods such as aerial and ground spraying and sterile insect techniques will have reduced application. There will be increased reliance on trypanocidal drugs, and bait methods of tsetse control--where flies are attracted to point sources and killed. If drug resistance develops, cheap and simple bait methods offer the only means of disease control that might be applied, and paid for, by stockowners themselves. The methods have been effective in some circumstances, but not in others, and it is important to understand the reasons for the successes and the failures. Analysis is presented of the results of two Tanzanian tsetse control campaigns involving the use of insecticide-treated cattle. Between 1991 and 1996, following the introduction of widespread dipping in the Kagera Region, trypanosomiasis declined from >19000 cases to <2400 and deaths from >4000 to 29. On four ranches in the region, tsetse have been almost eliminated and trypanosomiasis prophylaxis is no longer used. Similarly aggressive use of pyrethroids on Mkwaja Ranch in Tanga Region has not had such dramatic effects. Tsetse and trypanosomiasis are still common, despite high levels of prophylaxis and the deployment of approximately 200 odour-baited targets. The difference in the results is attributed to a combination of the much smaller area covered by treated animals at Mkwaja, a greater susceptibility to re-invasion and a more suitable habitat for the flies. A better understanding of the dynamics of the use of insecticide-treated cattle is needed before we can predict confidently the outcome of particular control operations.     

A phyto-sociological analysis of the distribution of riverine tsetse flies in Burkina Faso

In Burkina Faso, Glossina palpalis gambiensis Vanderplank and G. tachinoides Westwood (Diptera: Glossinidae) are the main cyclic vectors of trypanosomiasis. The vegetation type along river banks is an important factor determining the distribution and abundance of these tsetse. The following work investigated the relation between the plant species present (including the disturbance level) and tsetse distribution and abundance, using three ecotypes, described by P.C. Morel in 1978. These were the Guinean, Sudano-Guinean and Sudanese gallery forests. In the Mouhoun River basin, these three ecotypes are found successively from upstream to downstream. Berlinia grandiflora, Syzygium guineense and Cola laurifolia and finally Acacia seyal and Mitragyna inermis were the best indicators for the Guinean, Sudano-Guinean and Sudanese gallery forest ecotypes, respectively, as suggested by Morel. However, other species such as Pterocarpus santalinoides and Mimosa pigra were not ecotype specific. Trap catches confirmed that G. palpalis and G. tachinoides are predominant in Guinean and Sudanese gallery forests, respectively, and that both species are well represented in the Sudano-Guinean ecotype. Tsetse densities dropped significantly in disturbed Sudano-Guinean and Sudanese gallery forest sites. However, this was not the case for both species in Guinean or for G. tachinoides in half-disturbed Sudanese gallery forest sites, confirming their high resilience to human-made changes. The importance of a detailed consideration of riverine ecotypes when predicting tsetse densities is discussed.     

Sex pheromone of the tsetse species, Glossina austeni: isolation and identification of natural hydrocarbons, and bioassay of synthesized compounds

Copulatory responses of male Glossina austeni (Newstead) (Diptera: Glossinidae), that were elicited after contact with frozen female tsetse, were not observed after solvent washing of cuticular lipids. Chromatographic analysis of extracts from laboratory-reared and field-collected G. austeni females yielded natural hydrocarbons that were highly stimulatory to males. Most of this activity was produced by compounds in the alkene fraction. Gas chromatograms (GC) contained five natural alkenes; these were separated by preparative GC for bioassays conducted in Tanzania. The two major alkenes were identified using gas chromatography-mass spectrometry (GC-MS) to be 13,17-dimethyltritriacont-1-ene and 13,17-dimethylpentatriacont-1-ene, after the samples had undergone derivatization using dimethyl disulphide and saturation with deuterium. These alkenes and natural alkanes were quantified from G. austeni of both sexes from laboratory and field samples to confirm that their presence was consistent in this species. Trials of synthetic samples resulted in the order of biological activity for the stereoisomers of 13,17-dimethyltritriacont-1-ene as follows: S,R-33:1 > R,S- 33:1 > S,S-33:1 > R,R-33:1. Dose-response data showed an ED(50) at 5 microg per treated, solvent-washed male decoy. Of the four stereoisomers of 13,17-dimethylpentatriacont-1-ene, R,R-35:1 showed the most activity. This is the first report of alkene-induced sexual activity in males of the genus Glossina.     

User-friendly models of the costs and efficacy of tsetse control: application to sterilizing and insecticidal techniques

An interactive programme, incorporating a deterministic model of tsetse (Diptera: Glossinidae) populations, was developed to predict the cost and effect of different control techniques applied singly or together. Its value was exemplified by using it to compare: (i) the sterile insect technique (SIT), involving weekly releases optimized at three sterile males for each wild male, and (ii) insecticide-treated cattle (ITC) at 3.5/km(2). The isolated pre-treatment population of adults was 2500 males and 5000 females/km(2); if the population was reduced by 90%, its growth potential was 8.4 times per year. However, the population expired naturally when it was reduced to 0.1 wild males/km(2), due to difficulties in finding mates, so that control measures then stopped. This took 187 days with ITC and 609 days with SIT. If ITC was used for 87 days to suppress the population by 99%, subsequent control by SIT alone took 406 days; the female population increased by 48% following the withdrawal of ITC and remained above the immediate post-suppression level for 155 days; the vectorial capacity initially increased seven times and remained above the immediate post-suppression level for 300 days. Combining SIT and ITC after suppression was a little faster than ITC alone, provided the population had not been suppressed by more than 99.7%. Even when SIT was applied under favourable conditions, the most optimistic cost estimate was 20-40 times greater than for ITC. Modelling non-isolated unsuppressed populations showed that tsetse invaded approximately 8 km into the ITC area compared to approximately 18 km for SIT. There was no material improvement by using a 3-km barrier of ITC to protect the SIT area. In general, tsetse control by increasing deaths is more appropriate than reducing births, and SIT is particularly inappropriate. User-friendly models can assist the understanding and planning of tsetse control. The model, freely available via http://www.tsetse.org, allows further exploration of control strategies with user-specified assumptions.     

The activation of resting tsetse flies (Glossina) in response to visual and olfactory stimuli in the field

ABSTRACT Studies were conducted in Zimbabwe of the responses of Glossina morsitans morsitans Westwood and Glossina pallidipes Austen resting in a refuge to various host stimuli. Tsetse took off in response to 100% ox odour, 0.08% carbon dioxide or a visual stimulus consisting of a 0.75 × 0.75 m black target placed c . 5 m from the refuge moving at 4^o s^-1, but the level of response was low with only 35%, 19% and 29% responding, respectively. Tsetse did not take off in response to any one of 25% ox odour, 0.8% carbon dioxide, acetone (3 μg 1^-1) or octenol (0.03 μg 1^-1). In the absence of any host stimuli, flies emerged from the refuge later on hotter days (35–37^oC) than on cooler days (32–34.5^oC). Male G.pallidipes emerging later in the afternoon contained significantly more haematin than those emerging relatively earlier. There were no significant differences between the responses of G.m. morsitans and G.pallidipes. It is suggested that the initial activation of resting flies is primarily mediated through endogenous, rather than host, stimuli.     

Analysis of the mating activity of male tsetse flies Glossina m. morsitans and G. pallidipes in the laboratory

ABSTRACT. The responses of male Glossina morsitans morsitans West-wood and Glossina pallidipes Austen to freeze-killed females were examined in the laboratory. Analyses were performed using a specially designed, automated, computer-based, recording system. G. pallidipes were more active than G. m. morsitans , interacting with the female decoys twice as often. Interactions with the decoys divided broadly into short-stay (<60 s) and long-stay, full copulatory attempts. For G. m. morsitans 90% of interactions resulted in full copulatory attempts, the mean duration of which was >1 h. For G. pallidipes only 40% of interactions resulted in full copulatory attempts, the mean duration of which was 35 min. The initiation of interactions showed a clear V-shaped activity pattern in G. m. morsitans but in G. pallidipes only a morning peak was observed. In neither species was there a tendency for full copulatory responses to be initiated in any specific period of the diurnal activity pattern. The results indicate that the two species have very different mating systems, and represent an initial step in the quantification of these differences.     

Tsetse mating behaviour: effects of age and hunger in Glossina morsitans morsitans and G.pallidipes

ABSTRACT. The effects of age and hunger on the responses of male Glossina morsitans morsitans Westwood and G.pallidipes Austen to freeze-killed female decoys, were examined in the laboratory. In both species, activity, estimated as the total number of interactions between males and decoys, increased with both age and hunger. Interactions were divided into short-stay (<60 s) and long-stay, full copulatory responses. In both species, young, unfed males were significantly less likely to attempt to copulate with a decoy after encounter than were fed males. Among fed males the proportion of interactions that proceeded to full copulatory attempts did not change with increasing age, but decreased consistently with increasing hunger. At all ages and hunger levels, G.pallidipes were more active than G.m.morsitans. However, after encountering a decoy, G.pallidipes were less likely to attempt to copulate than G.m.morsitans. In both species the duration of copulatory attempts did not change with age, but declined with increasing hunger. Copulatory attempts by G.pallidipes were significantly shorter than those of G.m.morsitans. The results are discussed in relation to the behaviour of tsetse in response to control devices such as traps and targets.