Prevalence and distribution of exposure to Schmallenberg virus in Irish cattle during October 2012 to November 2013

Abstract

Background: Schmallenberg virus (SBV) was first identified in November 2011. It is a novel Orthobunyavirus (family Bunyaviridae) whose main ill effect is congenital malformation of the musculoskeletal and central nervous systems. It is borne by Culicoides spp., and has spread extensively in western Europe. The first case of SBV in Ireland was diagnosed in October 2012. It was anticipated that once the virus emerged in Ireland that there would be wide scale or nationwide spread over the course of the 2013 vector season. The objectives of this study were to determine the seroprevalence and distribution of exposure to Schmallenberg virus in Irish cattle from November 2012 to November 2013.

Methods: Samples of brain for the pathology based surveillance were collected from malformed bovine and ovine foetuses submitted for post mortem examination. These samples were tested for SBV using RT-qPCR. Three serological surveys were carried out on sera submitted for the national brucellosis eradicartion programme. A spatial analysis of both sets of data was carried out.

Results: Between October 2012 and 10th May 2013, SBV was confirmed by RT-qPCR in brain tissues from malformed foetuses obtained from 49 cattle herds and 30 sheep flocks in Ireland. In national serosurveys conducted between November 2012 until November 2013 the herd-level and animal-level SBV seroprevalences in cattle were 53 and 36 % respectively for the first survey, 51 and 35 % for the second survey and 53 and 33 % for the third survey. The herd level seroprevalence in counties ranged from 0 to 100 %, with the counties in the south and southeast having the highest seroprevalence (>50 %), the midlands a moderate herd level seroprevalence (10-50 %) while northern and north western counties had a low herd level seroprevalence (0-10 %). There was close spatial agreement between the results of the two different targeted surveillance strategies.

Conclusions: At the end of the 2012 vector season, there was widespread exposure to SBV among herds in southern and south eastern Ireland. During 2013, there was little or no evidence of further outward spread, unlike the situation in several other European countries. Given the lack of evidence for circulation of the virus since 2012, it is likely that the younger age cohort in herds previously exposed to SBV and substantial proportions of animals of all ages on the margins of affected areas are immunologically naïve to SBV, and would be susceptible to infection if the virus were to re-emerge.

Fig. 1

Chronology of confirmed cases of SBV

Fig. 2

Spatial distribution of samples collected through pathology based and serological based SBV surveillance in Ireland during 2012 and 2013. The location of RT-qPCR-positive animals, which were submitted to veterinary laboratories between 30 October 2012 and 9 May 2013, is presented as yellow triangles. The green and red dots highlight the location of farms enrolled in the 3 serological surveys during November 2012 to November 2013, with colour and size relating to the number of animals per farm that were positive to SBV

Fig. 3

County-level herd SBV prevalence, and the spatial distribution of herds positive to SBV, based on active surveillance. The colour and size of each dot refers to the number of animals per farm that were positive to SBV. a During November 2012–January 2013. b During June–August 2013. c During October–November 2013

Fig. 4

Pathological herd incidence and serological herd prevalence per 50 × 50 km square grid