An Overview of Tsetse Fly Repellents: Identification and Applications

Tsetse flies are vectors of the parasite trypanosoma that cause the neglected tropical diseases human and animal African trypanosomosis. Semiochemicals play important roles in the biology and ecology of tsetse flies. Previous reviews have focused on olfactory-based attractants of tsetse flies. Here, we present an overview of the identification of repellents and their development into control tools for tsetse flies. Both natural and synthetic repellents have been successfully tested in laboratory and field assays against specific tsetse fly species. Thus, these repellents presented as innovative mobile tools offer opportunities for their use in integrated disease management strategies.

Identification of Semiochemical Candidates Involved in Glossina Palpalis Gambiensis Larviposition Site Selection and Behavioural Responses of Adult Gravid Females

Tsetse flies (Diptera: Glossinidae) are the cyclical vectors of human and animal trypanosomes. This viviparous insect develops and produces a single larva at 10-day intervals deposited in specific sites. In some species aggregation of larvae has been shown and seems to be mediated by both physical factors and volatile semiochemicals of larval origin. In this context, this study aims to identify chemicals emitted during the pupariation process in Glossina palpalis gambiensis. Volatile Organic Compounds (VOCs) emitted by larvae were identified using static headspace solid-phase microextraction and gas-chromatography mass-spectrometry (GC-MS) analysis. Electrophysiology and behavioural assays were performed on gravid females to confirm VOCs behavioural activity and attractiveness. GC-MS results revealed ten chemicals emitted during the pupariation process of G. p. gambiensis larvae. Among these chemicals, gravid females were shown to detect nine of them during coupled gas chromatography - electroantennographic detection tests. Behavioural assays highlighted two compounds were as attractive as pupae and one compound and a blend of four compounds were more attractive than pupae. Although the larval origin of some of them needs to be confirmed as they may also likely produced by micro-organisms, these compounds induced significant behavioural responses in the laboratory. Further experiments have to explore the biological activity and competitiveness of these compounds in the field. This work opens interesting opportunities for behavioural manipulation and control of tsetse flies.

Therapeutic efficacy of β-sitosterol treatment on Trypanosoma congolense infection, anemia development, and trans-sialidase (TconTS1) gene expression

Background

African animal trypanosomiasis hinders sustainable livestock productivity in sub-Saharan Africa. About 17 million infected cattle are treated with trypanocides annually but most of the drugs are associated with drawbacks, necessitating the search for a promising chemotherapeutic agent.

Objectives

In this study, the effects of β-sitosterol on Trypanosoma congolense infection were investigated along with its effect on the trans-sialidase gene expressions.

Results

Oral treatment with β-sitosterol at 15 and 30 mg/kg body weight (BW) for 14 days significantly (p < 0.05) reduced parasitemia and ameliorated the parasite-induced anemia. Also, the parasite-induced increase in serum urea level and renal histopathological damage scores in addition to renal hypertrophy was significantly (p < 0.05) reverted following treatment with 30 mg/kg BW β-sitosterol. The compound also significantly (p < 0.05) down-regulated the expression of TconTS1 but not TconTS2, TconTS3, and TconTS4. Correlation analysis between free serum sialic acid with the TconTS1 and TconTS2 gene variants revealed negative correlations in the β-sitosterol-treated groups although they were non-significant (p > 0.05) in the group treated with 15 mg/kg BW β-sitosterol. Similarly, a non-significant negative (p > 0.05) correlation between the biomolecule and the TconTS3 and TconTS4 gene variants was observed in the β-sitosterol-treated groups while positive correlations were observed in the infected untreated control group.

Conclusion

The observed effect of β-sitosterol on T. congolense infection could make the compound a possible template for the design of novel trypanocides.

Quality Control and Mating Performance of Irradiated Glossina palpalis gambiensis Males

Simple Summary

In vector control programs based on the sterile insect technique (SIT), the biological quality of sterile males must be as high as possible to compete with their wild counterparts. This study evaluates the biological quality and mating performance of sterile male Glossina palpalis gambiensis produced in a mass-rearing facility in Burkina Faso. It shows that irradiation had no effect on the adult emergence rate but reduced the percentage of operational flies and male fly survival. Irradiation had no effect on mating performances, as all females were inseminated, and the sterile males competed well with unirradiated males for virgin females in walk-in field cages. However, the sterility rate induced in females was lower than expected (89.67%). This study indicates that, under experimental field cage conditions, the G. p. gambiensis males produced at the CIRDES are well-suited for use in area-wide integrated pest management (AW-IPM) programs that have an SIT component. The reduction in the sterility rate highlights the importance of regularly monitoring the biological parameters of sterile males and the radiation source.

Abstract

The biological quality of sterile male insects produced in a mass-rearing facility is a prerequisite for the success of the SIT, which is a component of area-wide integrated pest management (AW-IPM). Indeed, sterile male insects released in the field must have a good mating performance in order to compete with wild males, but they must also present the required level of sterility. In the present study, the biological quality of sterile male Glossina palpalis gambiensis produced in a mass-rearing insectary was assessed through quality control testing. The mating performance of irradiated males was assessed in walk-in field cages. Irradiation had no effect on adult emergence but significantly reduced the percentage of operational flies (from 89.58% to 79.87%) and male survival (from 5 to 4 days, on average). However, irradiation did not impact the sterile male insemination potential, with all females inseminated and more than 80% of the spermathecae completely filled. The rate of induced sterility in females was 89.67% due to a dose rate decrease of the radiation source. Moreover, sterile males were able to compete successfully with untreated fertile males for untreated females in walk-in field cages. This study confirmed that the flies were still competitive and stressed the importance of regularly checking the radiation source parameters.

Evaluation of different blood-feeding frequencies on Glossina palpalis gambiensis performance in a mass-rearing insectary

Background

The main challenge to the successful mass-rearing of the tsetse fly in insectaries, especially in Africa, is a sustainable supply of high-quality blood meals. As such, the collection of high-quality blood in large quantities can be an important constraint to production. One possible strategy to lessen the impact of this constraint is to modify the blood-feeding frequency. In the study reported here, we evaluated the effect of three blood-feeding frequencies on the colony performance of Glossina palpalis gambiensis, a riverine tsetse fly species.

Methods

The effect of three, four and six blood-feedings per week on female survival and productivity were evaluated over a 30-day period. Progeny emergence rate and flight ability were also evaluated.

Results

Female survival was significantly higher in flies fed four times per week (87%) than in those fed three (72%) and six times per week (78%; P < 0.05). Productivity was similar between flies fed four and six times per week (457 and 454 larvae) but significantly reduced in flies fed three times per week (280 larvae produced; P < 0.05). Both emergence rate and flight ability rate were also similar between flies fed four times per week (97 and 94%, respectively) and six times per week (96 and 97%, respectively), but they were significantly reduced when flies were fed three times per week (89 and 84%, respectively; P < 0.05).

Conclusions

Blood-feeding frequency could be reduced from six times per week to four times per week without affecting mass-rearing production and progeny quality. The implications of these results on tsetse mass-rearing production are discussed.