Possible over-wintering of bluetongue virus in Culicoides populations in the Onderstepoort area, Gauteng, South Africa.

Jumari Steyn,Gert J Venter,Solomon N.B Boikanyo,Karen Ebersohn,Carina Lourens,Karien Labuschagne,Estelle H Venter,Daphney Majatladi

doi:10.4102/jsava.v87i1.1371

Abstract

Several studies have demonstrated the ability of certain viruses to overwinter in arthropod vectors. The over-wintering mechanism of bluetongue virus (BTV) is unknown. One hypothesis is over-wintering within adult Culicoides midges (Diptera; Ceratopogonidae) that survive mild winters where temperatures seldom drop below 10 °C. The reduced activity of midges and the absence of outbreaks during winter may create the impression that the virus has disappeared from an area. Light traps were used in close association with horses to collect Culicoides midges from July 2010 to September 2011 in the Onderstepoort area, in Gauteng Province, South Africa. More than 500 000 Culicoides midges were collected from 88 collections and sorted to species level, revealing 26 different Culicoides species. Culicoides midges were present throughout the 15 month study. Nine Culicoides species potentially capable of transmitting BTV were present during the winter months. Midges were screened for the presence of BTV ribonucleic acid (RNA) with the aid of a real-time quantitative reverse transcription polymerase chain reaction (RT-qPCR) assay. In total 91.2% of midge pools tested positive for BTV RNA. PCR results were compared with previous virus isolation results (VI) that demonstrated the presence of viruses in summer and autumn months. The results indicate that BTV-infected Culicoides vectors are present throughout the year in the study area. Viral RNA-positive midges were also found throughout the year with VI positive midge pools only in summer and early autumn. Midges that survive mild winter temperatures could therefore harbour BTV but with a decreased vector capacity. When the population size, biting rate and viral replication decrease, it could stop BTV transmission. Over-wintering of BTV in the Onderstepoort region could therefore result in re-emergence because of increased vector activity rather than reintroduction from outside the region.

Highlights

In South Africa, bluetongue (BT) occurs annually during late summer and autumn in areas with high rainfall (Verwoerd & Erasmus 2004)
Apparent vertical transmission of bluetongue virus (BTV) in the genus Culicoides has been demonstrated in Culicoides sonorensis, the primary North American vector, after BTV nucleic acid was detected in field-collected larvae (White et al 2005)
The present study focused on the possibility that Culicoides that survived throughout winter in an area can harbour BTV and represent a possible over-wintering mechanism for the virus

Summary

Introduction

In South Africa, bluetongue (BT) occurs annually during late summer and autumn in areas with high rainfall (Verwoerd & Erasmus 2004). Culicoides vectors, relatively short-lived, are usually infectious for the rest of their life span of up to 3 months (Purse et al 2015; Wilson, Darpel & Mellor 2008). Several arboviruses such as Ross River virus (Lindsay et al 1993), West Nile virus (Goddard et al 2003; Reisen et al 2006) and Orungo virus (Cordellier et al 1982) have demonstrated the ability to be transmitted vertically by infected arthropod females. This study, did not demonstrate whether BTV RNA-positive larvae would have emerged and been able to transmit infectious virus as adults and to date there is still lack of evidence for the natural vertical transmission of BTV in Culicoides vectors (Osborne et al 2015)

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Discussion

Conclusion