Abstract
Many species of tsetse flies are infected with a virus that causes salivary gland hypertrophy (SGH) symptoms associated with a reduced fecundity and fertility. A high prevalence of SGH has been correlated with the collapse of two laboratory colonies of Glossina pallidipes and colony maintenance problems in a mass rearing facility in Ethiopia. Mass-production of G. pallidipes is crucial for programs of tsetse control including the sterile insect technique (SIT), and therefore requires a management strategy for this virus. Based on the homology of DNA polymerase between salivary gland hypertrophy virus and herpes viruses at the amino acid level, two antiviral drugs, valacyclovir and acyclovir, classically used against herpes viruses were selected and tested for their toxicity on tsetse flies and their impact on virus replication. While long term per os administration of acyclovir resulted in a significant reduction of productivity of the colonies, no negative effect was observed in colonies fed with valacyclovir-treated blood. Furthermore, treatment of a tsetse colony with valacyclovir for 83 weeks resulted in a significant reduction of viral loads and consequently suppression of SGH symptoms. The combination of initial selection of SGHV-negative flies by non-destructive PCR, a clean feeding system, and valacyclovir treatment resulted in a colony that was free of SGH syndromes in 33 weeks. This is the first report of the use of a drug to control a viral infection in an insect and of the demonstration that valacyclovir can be used to suppress SGH in colonies of G. pallidipes.
Highlights
In sub-Saharan Africa, tsetse flies (Glossina spp.) are the vectors of two debilitating diseases, human sleeping sickness and the cattle disease nagana [1] and are a major constraint to the development of sustainable livestock production systems and the reduction of hunger and poverty [2]
The highest concentration (1000 mg/ml) of both acyclovir and valacyclovir significantly reduced the fecundity expressed as the number of pupae produced per initial female (PPIF) (Fig. 1A)
The application of clean feeding alone decreased salivary gland hypertrophy (SGH) prevalence to an average of 1.9% (22/1154) (Fig. 3B) negative control). These results clearly demonstrated that valacyclovir alone suppressed GpSGHV replication (P,0.001) and maintained the virus infection far below the threshold level of 109 viral copies required to develop SGH in G. pallidipes [22]
Summary
In sub-Saharan Africa, tsetse flies (Glossina spp.) are the vectors of two debilitating diseases, human sleeping sickness and the cattle disease nagana [1] and are a major constraint to the development of sustainable livestock production systems and the reduction of hunger and poverty [2]. Due to the lack of vaccines, the difficulty of drug treatment for sleeping sickness, and the development of resistance to the available trypanocidal drugs for nagana [3], vector control remains the most efficient strategy for the sustainable management of these diseases [4]. The successful eradication of Glossina austeni from Unguja Island (Zanzibar) using an area-wide integrated pest management approach including the sterile insect technique (SIT) (1994–1997) [5] encouraged several African countries to incorporate the SIT in their national tsetse control programs. G. pallidipes flies with SGH have no or reduced progeny and could jeopardize eradication programs with an SIT component against this species
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