Schmallenberg virus in Germany 2011-2014: searching for the vectors

Exposure of small ruminants to the Schmallenberg arbovirus in Germany from 2017 to 2018 - animal-specific and flock-management-related risk factors

The Schmallenberg virus (SBV), an emerging Orthobunyavirus of mainly ruminant hosts, caused a substantial epidemic in European ruminant populations between 2011 and 2013. The pathogen is transmitted by arthropod vectors (Culicoides spp.) and can cause reproductive disorders and severe malformations of the offspring or stillbirth. The present study aimed to assess SBV seroprevalence among German sheep and goats a few years after the first virus detection in the country (November 2011). In addition, an extensive risk factor analysis including host-specific and husbandry-related factors was implemented. Seroprevalence was determined by examining serum samples from 2759 sheep and 446 goats out of a total of 70 flocks across five German federal states. The samples were withdrawn in the period between 2017 and 2018. Using a commercial competitive ELISA, antibodies against SBV were detected in all 70 investigated flocks. A percentage of 60.1 % (1657/2759) of the sheep and 40.4 % (180/446) of the goat sera contained SBV antibodies. Generalized linear mixed modeling revealed significant effects of host species (sheep > goats), age (old > young) and sex (female > male) on SBV seroprevalence. For both species, also the farming purpose, and for goats, ectoparasite treatment and the presence of cattle on the farm played a role in terms of risk for SBV exposure. The observations from this study still emphasize a wide distribution of the pathogen in Germany. Nevertheless, the observed seroprevalence might not be sufficient to achieve effective herd immunity. Pinpointing risk factors identified susceptible populations for targeted vaccination programs to reduce potential animal losses caused by SBV.

Schmallenberg virus: a systematic international literature review (2011-2019) from an Irish perspective

In Autumn 2011, nonspecific clinical signs of pyrexia, diarrhoea, and drop in milk yield were observed in dairy cattle near the German town of Schmallenberg at the Dutch/German border. Targeted veterinary diagnostic investigations for classical endemic and emerging viruses could not identify a causal agent. Blood samples were collected from animals with clinical signs and subjected to metagenomic analysis; a novel orthobunyavirus was identified and named Schmallenberg virus (SBV). In late 2011/early 2012, an epidemic of abortions and congenital malformations in calves, lambs and goat kids, characterised by arthrogryposis and hydranencephaly were reported in continental Europe. Subsequently, SBV RNA was confirmed in both aborted and congenitally malformed foetuses and also in Culicoides species biting midges. It soon became evident that SBV was an arthropod-borne teratogenic virus affecting domestic ruminants. SBV rapidly achieved a pan-European distribution with most countries confirming SBV infection within a year or two of the initial emergence. The first Irish case of SBV was confirmed in the south of the country in late 2012 in a bovine foetus.

Since SBV was first identified in 2011, a considerable body of scientific research has been conducted internationally describing this novel emerging virus. The aim of this systematic review is to provide a comprehensive synopsis of the most up-to-date scientific literature regarding the origin of SBV and the spread of the Schmallenberg epidemic, in addition to describing the species affected, clinical signs, pathogenesis, transmission, risk factors, impact, diagnostics, surveillance methods and control measures. This review also highlights current knowledge gaps in the scientific literature regarding SBV, most notably the requirement for further research to determine if, and to what extent, SBV circulation occurred in Europe and internationally during 2017 and 2018. Moreover, recommendations are also made regarding future arbovirus surveillance in Europe, specifically the establishment of a European-wide sentinel herd surveillance program, which incorporates bovine serology and Culicoides entomology and virology studies, at national and international level to monitor for the emergence and re-emergence of arboviruses such as SBV, bluetongue virus and other novel Culicoides-borne arboviruses.

Post-Epidemic Distribution of Schmallenberg Virus in Culicoides Arbovirus Vectors in Poland

Pooled samples of female and male Culicoides midges (5146 and 332 pools, respectively) that corresponded to a total number of 124,957 specimens were collected between 2013-2017 in the vicinity of cattle barns that were distributed throughout Poland were analyzed for the presence of Schmallenberg virus (SBV) RNA. Sixty-six pools tested positive (1.2%) with mean C_t value of 34.95. The maximum likelihood estimated infection rate (MLE) was calculated at 0.53 per 1000 individuals; however, it peaked in 2016 with MLE of 3.7. Viral RNA was detected in C. obsoletus/scoticus complex, C. punctatus, and C. pulicaris pools. Moreover, viral material was present in nulliparous (virgin) Culicoides females (MLE 0.27) and for the first time reported in males (MLE 0.34), which suggests the possibility of transovarial route of SBV or virus RNA transmission, as both do not fed on host blood. The accuracy of targeted versus random SBV surveillance in Culicoides vectors was compared. The relationship between infection rate (expressed as minimum infection rate; MIR), in addition to MLE, was compared with the density of virus infected midges (DIM). In conclusion, the SBV infection rate in the vector was significantly higher in 2016 as compared to other surveillance years; this is consistent with the simultaneous increase in SBV seroprevalence (seroconversion) in ruminants during the same year.

Longitudinal monitoring of Culicoides in Belgium between 2007 and 2011: local variation in population dynamics parameters warrant cautious use of monitoring data

BACKGROUND

Several European countries suffered important economic losses during the past decade due to the emergence of bluetongue and Schmallenberg viruses. Both are viruses of veterinary importance and are spread by Culicoides spp. This triggered many European countries to start Culicoides population monitoring. Recently a one year monitoring study at 16 sites in Belgium revealed that important variation existed in Culicoides abundance and species diversity between collection sites. In order to analyze whether this variation is consistent over years, a detailed analysis of monitoring data collected at seven locations in Belgium between 2007 and 2011 was performed in this study. At all locations, biting midges were collected with OVI black light traps set-up in close proximity to livestock.

RESULTS

In total, 42 different Culicoides species were morphologically identified. Species of the subgenus Avaritia represented 83% of all collected midges. Nevertheless, important differences in species composition were found between sites. Furthermore, statistical differences between sites were found for the total and maximum annual abundance, showing that a consistent higher or lower number of Culicoides could be collected depending on the selected collection site. Yearly, up to 16 and 30-fold differences in total and maximum annual abundances between sites, respectively, were found. Also the month in which most Culicoides were collected varied greatly between years, both at local (from May to October) and country level [May (2008), June (2010), July (2009), August (2011), October (2007)]. Finally, the average vector-free period over all sites and years was 173 days and could roughly be defined between November and the end of April. Interestingly, important yearly variations of up to two months in the duration of the vector-free period were found between the studied collection sites. In contrast to the abundance parameters, no specific sites could however be identified where monitoring consistently showed shorter or longer vector-free periods.

CONCLUSIONS

In conclusion, our results show that the selection of collection sites for Culicoides monitoring, even in a small country such as Belgium, strongly influences abundance parameters and that yearly variation in seasonality occurs. This emphasizes that care should be taken when using such parameters in risk assessments for transmission of Culicoides-borne diseases and that more clear and strict guidelines for Culicoides monitoring should be considered when monitoring data are used for legislative purposes.

Reproducibility and relevance in insect-arbovirus infection studies

Experimental infections of insects with arboviruses are performed to achieve a variety of objectives but principally to draw inferences about the potential role of field populations in transmission or to explore the molecular basis of vector-pathogen interactions. The design of such studies determines both their reproducibility and the extent to which their results can be extrapolated to natural environments, and is constrained by the resources available. We discuss recent findings regarding the effects of nutrition, the microbiome, co-infecting agents and feeding methods on the outcome of such experiments, and identify resource-efficient ways to increase their relevance and reproducibility, including the development of community standards for reporting such studies and better standards for cell line and colony authentication.

Evidence of extensive renewed Schmallenberg virus circulation in Belgium during summer of 2016 - increase in arthrogryposis-hydranencephaly cases expected

A seroprevalence study carried out between June and September 2016 in the Belgian sheep population showed a significant increase in overall (from 25% to 62%) and between-herd (from 60% to 96%) seroprevalence against Schmallenberg virus (SBV) during this period, indicating the most extensive recirculation of SBV since its original emergence in 2011. SBV recirculation was confirmed by the detection of SBV RNA-positive Culicoides obsoletus complex midges collected in the region of Antwerp in August 2016, reaching a minimum infection rate of 3%. The recirculation of SBV in the largely unprotected ruminant population during summer 2016 will likely cause an increase in the number of arthrogryposis-hydranencephaly cases in newborn ruminants during the coming months.