The Antiviral Drug Valacyclovir Successfully Suppresses Salivary Gland Hypertrophy Virus (SGHV) in Laboratory Colonies of Glossina pallidipes

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

Effect of valacyclovir on the incidence of salivary gland hypertrophy in <i>Glossina pallidipes,</i> analysis of variance and pairwise comparison of treatment means.

<p>F = 8.66.</p><p>d.f. = 2, 80.</p><p>P = 0.00039.</p><p>T values for comparison of means.</p

Effect of antiviral drug treatments in combination with clean feeding on the SGHV load in tsetse flies over three generations.

<p>At each generation flies were fed on blood supplemented with 300 µg/ml antiviral drug for 60 days, then virus load was estimated by qPCR as previously reported <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0038417#pone.0038417-AbdAlla4" target="_blank">[22]</a>. Flies fed with drug-free blood were used as control.</p

Effect of long term treatment with valacyclovir on <i>G. pallidipes</i> biology and virus prevalence.

<p>(<b>A</b>) Productivity (PPIF) and mortality of flies fed on blood supplemented with 300 µg/ml antiviral drug for 60 days<b>.</b> (<b>B</b>) SGHV virus load and SGH prevalence by qPCR and dissection, at 60 days after feeding. (<b>---</b>): threshold virus load correlated with SGH symptoms. Negative control: flies fed exclusively on clean blood; positive control: Flies fed on virus contaminated blood; valacyclovir: flies fed on virus contaminated blood with 300 µg/ml valacyclovir.</p

Dose effect of Acyclovir and Valacyclovir on tsetse flies <i>G. pallidipes</i> productivity and mortality.

<p>Flies were fed on blood supplemented with different concentrations (10 to 1000 µg/ml) of the two antiviral drugs for 60 days and the produced pupae (A) and mortality (B) were recorded. Flies fed with drug-free blood were used as control.</p

Effect of the combination of clean feeding, screening flies with non-destructive PCR for virus infection and valacyclovir treatment on the expression of SGH syndrome.

<p>Screened: teneral flies were tested with non-destructive PCR and only negative flies were used. Non-screened: teneral flies with natural variable virus infection load were used. Flies were fed on blood supplemented or not with 300 µg/ml antiviral drug for 60 days, then virus load was estimated by qPCR.</p