This report by the European Food Safety Authority (EFSA) provides an overall assessment of the impact of the infection on animal health, animal production and animal welfare of the provisionally named “Schmallenberg” virus (SBV) together with a characterization of the pathogen that was first detected in Germany in 2011. In Europe mid May 2012, 3745 holdings have been reported with SBV cases confirmed by laboratory testing across eight Member States.
The Authority reviewed the epidemiological reports noting that SBV has been detected in cattle, sheep, goats and a bison. SBV antibodies have been detected in deer and no other species are known to be affected. EFSA also confirms that new studies support the initial assessment undertaken by the European Center for Disease Control and Prevention, that it is very unlikely that SBV poses a risk to humans.
In terms of transmission routes, recent entomological investigations have identified SBV in field samples of biting midges of the Culicoides obsoletus group. Data from EU BT-NET for 2007-2011 indicates that the Culicoides obsoletus group is widespread in Europe. However this dataset is not representative for all countries in Europe, the sampling methods are not harmonized and there is some evidence of misidentification of the Culicoides species. Currently there is no evidence of any other route of transmission other than transplacental or vector borne routes.
EFSA coordinated the collation of SBV epidemiological data during 2011-2012 in order to obtain comparable data for Europe. The maximum proportion of reported sheep holdings with SBV confirmed was 4% per country and 7.6% per region while for cattle less than 1.3 % of holdings were reported as SBV confirmed at both country and regional level. The data collected indicates that the impact of SBV is greater in sheep holdings than cattle. This assessment of impact should be interpreted with caution however, since the case ascertainment are dependent on the disease being notifiable or not, the level of awareness of different stakeholders and the diagnostic capacity available in the Member State. No data is currently available on within herd impact.
The impact on animal welfare and animal production was not assessed due to lack of data. It is necessary to investigate the impact of SBV infection on return to service, milk yield and rates of dystocia.
In order to assess the impact of SBV (spatial and temporal spread, proportion of affected holding and potential projection of arthrogryposis hydranencephaly syndrome cases) three models were used. The model considers a wide vulnerable period for both susceptible known species given that no information is yet available specifically for SBV. The geographical spread model was broadly able to capture the observed dynamics of SBV in Europe during 2011 in terms of duration of the transmission period and the number of infected holdings within a region. However, estimates for the within-region force of infection critically depend upon the level of under-ascertainment (due to lack of identification of affected holdings, vulnerable period used in the geographical spread model) of infected holdings, which sero-prevalence data suggest could be substantial. The probability of SBV surviving over the winter and subsequently spreading in 2012 is difficult to assess because of a lack of data. However, previous experience with BTV8 indicates that vector borne viruses can overwinter, if SBV overwinters the geographical spread model predicts that SBV is most likely to re-emerge between mid-April and the end of May 2012 and is likely to be of a similar size to the one occurred in 2011, though in regions previously unaffected (assuming post-infection immunity). From the prediction of the geographical spread model, the most likely affected areas for next season are expected to be at the south and east regions of the previously-affected areas.
The model of geographical and seasonal within holding transmission using bluetongue virus (BTV) parameters suggests virus transmission and spread becomes possible at temperatures around 15°C with a temperature optimum between 18°C and 19°C. The analysis of the preceding 29 years daily temperatures suggests that most of Europe has a suitable climate for within holding vector borne transmission.
The projection model used to evaluate impact was based on the geographical spread model and reported affected holdings up to the month of March. The analysis of impact is limited to regions where calving and lambing data was available to EFSA. The model shows that further cases of arthrogryposis hydranencephaly syndrome (AHS) are likely to be very rare in lambs for the year 2012 after the month April, clearly predicting the observed pattern, since negligible number of affected sheep herds were reported in the month of April and May. However the projection model predicts that further cases in calves could be observed until July, consistent also with the observed pattern for this species, in which the newly reported affected holdings are mainly cattle holdings. The model predicts that the regions that have the highest infection and impact figures in cattle (and AHS cases) and those regions with the highest infection and impact figures in sheep (and AHS cases) are in general regions with large number of holdings (high livestock density).
In regions with SBV confirmed holdings, assuming a high prevalence of infection and post infection immunity, impact in the 2012-2013 calving and lambing season should be low. However, assuming SBV survived the winter of 2011, the models suggest that in unaffected regions or regions with low prevalence with suitable temperatures for within herd transmission by vectors and high density of susceptible species (cattle and sheep) SBV infection is likely to spread.
EFSA puts forward a number of recommendations to fill the knowledge gaps, these include but are not limited to: continuing serological investigations in affected regions and regions neighbouring affected areas, within herd and animal level impact investigation, monitoring putative vector population, setting SBV host vector transmission parameters, investigating other routes of transmission, host susceptibility, virulence and vulnerable period during gestation. Furthermore, the possible origins of the virus should be investigated as more information becomes available on the virus characteristics and infection epidemiology.