Managing hytrosavirus infections in Glossina pallidipes colonies: feeding regime affects the prevalence of salivary gland hypertrophy syndrome

Abstract

Many species of tsetse flies are infected by a virus that causes salivary gland hypertrophy (SGH) syndrome and the virus isolated from Glossina pallidipes (GpSGHV) has recently been sequenced. Flies with SGH have a reduced fecundity and fertility. Due to the deleterious impact of SGHV on G. pallidipes colonies, several approaches were investigated to develop a virus management strategy. Horizontal virus transmission is the major cause of the high prevalence of the GpSGHV in tsetse colonies. Implementation of a "clean feeding" regime (fresh blood offered to each set of flies so that there is only one feed per membrane), instead of the regular feeding regime (several successive feeds per membrane), was among the proposed approaches to reduce GpSGHV infections. However, due to the absence of disposable feeding equipment (feeding trays and silicone membranes), the implementation of a clean feeding approach remains economically difficult. We developed a new clean feeding approach applicable to large-scale tsetse production facilities using existing resources. The results indicate that implementing this approach is feasible and leads to a significant reduction in virus load from 10(9) virus copies in regular colonies to an average of 10(2.5) and eliminates the SGH syndrome from clean feeding colonies by28 months post implementation of this approach. The clean feeding approach also reduced the virus load from an average of 10(8) virus copy numbers to an average of 10(3) virus copies and SGH prevalence of 10% to 4% in flies fed after the clean fed colony. Taken together, these data indicate that the clean feeding approach is applicable in large-scale G. pallidipes production facilities and eliminates the deleterious effects of the virus and the SGH syndrome in these colonies.

Figure 1. Prevalence of SGH in different tsetse colonies of G. pallidipes.

The flies were randomly selected at different time points from the different colonies and dissected to determine status of the salivary glands. Numbers between brackets are the mean SGH prevalence percentage. The line is the smoothed regression.

Figure 2. Handling, feeding and management of tsetse colony.

(A) Regular in vitro membrane feeding system, (B) intermediate phase, and (C) final phase of the clean feeding system.

Figure 3. Effect of long-term clean feeding system on SGH prevalence in the Seibersdorf Tororo G. pallidipes colonies by fly dissection.

The flies were randomly selected at different time points from the different colonies after implementation of the clean feeding system and dissected to determine status of the salivary glands. Numbers between brackets are the mean of SGH prevalence percentage; lines are Loess smoothed regressions.

Figure 4. Effect of long-term clean feeding system on the prevalence of GpSGHV in the Seibersdorf Tororo G. pallidipes colonies.

Virus load was determined by qPCR of the main colony before initiation of the clean feeding system and from the three colonies Clean feeding 1 (CFC-1), Clean feeding 2 (CFC-2) and normal feeding system (NFC) after 28 months of implementation of the clean feeding system. (---): threshold virus load correlated with SGH symptoms; solid lines are Loess smoothed regressions.