BACKGROUND
schmallenberg virus , a newly discovered orthobunyavirus , was first identified in november  <dig> from dairy cattle with unspecific and transitory symptoms, including fever, decreased milk production and diarrhoea, in north rhine-westphalia, germany  <cit> .
schmallenberg virus shows high homology to viruses of the simbu serogroup, the members of which are typically transmitted through culicoides biting midges  <cit>  and can cause congenital defects in domestic animals  <cit> .
the first clinical evidences of sbv appeared in august  <dig>  not only in germany, but also in belgium and the netherlands.
because examination of archived samples did not indicate prior sbv circulation, the most accredited hypothesis is that the virus was introduced in europe between spring and summer  <dig>  and that the excess of congenital malformations, abortions, and stillborns that have been reported in domestic ruminants since december  <dig> are the result of sbv infections in pregnant animals between summer and autumn  <dig>  <cit> .
by may  <dig>  a total of  <dig> bovine and ovicaprine herds in  <dig> european countries, including italy, were reported as being sbv-positive based on laboratory confirmation of suspected cases  <cit> .
in italy, the first laboratory-confirmed sbv case was reported at the beginning of february  <dig> from a small multi-species farm in the province of treviso, veneto region , where a dystocic goat died because of the retention of a malformed foetus  <cit> .
epidemiological investigation around this farm excluded any prior introduction of animals from other european countries, suggesting that the virus had circulated locally.
moreover, six pools of culicoides midges collected as part of the national surveillance programme for bluetongue virus  between september and november  <dig> within a 50 km radius from the first sbv case tested positive for sbv  <cit> .
this provided evidence that sbv had circulated in the local vector population almost five months before detecting the first case, but that any circulation before september  <dig> was unlikely  <cit> .fig.
<dig> map of north-east italy, including the study area , with indication of positivities for schmallenberg virus in culicoides biting midges and susceptible animals  from  <dig> to  <dig>  the first sbv-positive animal was detected in treviso province in a dystocic goat on the first semester of  <dig> .
midges were found positive in treviso province on the second semester of  <dig>  and in pordenone province on the first semester of  <dig>
evidence for the impact of sbv on animal productive performance is scarce.
a few studies have assessed the impact of sbv infection as: 1) direct impact on adult animals, including their clinical manifestations and herd- or area-level variation in key performance parameters ; 2) direct impact on newborns, including congenital malformations, abortions, and stillbirths  <cit> ; and 3) indirect impacts, including trade restrictions and treatment costs  <cit> .
although sbv may cause clinical signs of pyrexia and reduced milk production, cases are typically identified only when animals appear with congenital malformations among offspring whose mothers acquired infection during a particular period of pregnancy  <cit> .
the lack of a clear clinically overt presentation is one of the main reasons as to why there is a large degree of under-ascertainment for sbv  <cit> , especially in cattle, for which it is known that sbv infection is relatively mild, with a low incidence of birth malformations .
accordingly, the impact of sbv on adverse pregnancy outcomes  of dairy cattle is believed to be limited and lower than that on milk production or return to service  <cit> .
a recent dutch study reported that calf mortality differed very little between herds with low and high sbv seroprevalence during the second half of  <dig>  a finding that mirrors the relatively mild expression of sbv infection in adult cows  <cit> .
conversely, sbv seemed to have had a relatively more pronounced impact on milk production.
in a predefined 4-week period during which most sbv infections were expected to have occurred in the netherlands, the overall drop in milk production per cow was quantified as ~30 kg  and ~50 kg   <cit> .
however, potential confounders such as weather conditions, particularly temperature and relative humidity, which are known to affect milk production levels  <cit> , were not accounted for.
this study aimed at assessing variations imputable to sbv emergence in  <dig> productive and reproductive performance indicators of dairy cattle herds in the province of treviso, i.e.
italy’s area where sbv was at first detected  <cit>  and post-epidemic modelling studies have predicted it as one of the areas with the highest risk of sbv spread in europe  <cit> .
the analysis accounted for weather conditions and other herd-level factors that could also influence these indicators.
yet, it could only assess the presumptive/circumstantial effect of sbv on dairy cattle herd performances since no exact sbv data were available for the study area.
RESULTS
the productive and reproductive performance indicators of dairy cattle herds in the study area are summarized by semester in table  <dig>  data were collected monthly from january  <dig> to june  <dig> at the herd level .
the average number of herd-month observations by semester per each indicator varied from  <dig>  to  <dig> , depending on the number of missing values.
some indicators  showed a clear seasonal pattern, with a tendency towards decreased milk production in the second semester  of each year  and increased number of newborns in the first semester  of each year .
accordingly, the average number of inseminations per pregnancy leading to a healthy newborn increased in the second semester of each year.
the indicator average lactation length showed a drop in jul-dec  <dig>  which was preceded in first half of  <dig> by another drop as compared to the previous semesters.
the same was observed for the average calving-to-conception interval.table  <dig> mean values  of the productive and reproductive performance indicators of dairy cattle herds of treviso province, north-east italy.
data were collected monthly at the herd level , from january  <dig> to june 2012
acalculated as the ratio between the average number of inseminations per pregnancy  and the total number of registered newborns  of the following 9th month
bincludes only healthy calves that survived for at least 21 days after birth and the farmers were then obliged to register to the local health authority
the mixed-effects linear models showed that only two  performance indicators of interest varied significantly with respect to the reference semester .
these indicators were the average lactation length and the average calving-to-conception interval .
except for the second semester of  <dig>  all other semesters showed a significantly longer lactation length – also known as days in milk  – than that of the reference semester .
looking specifically at the same semesters over the years, herds’ lactation length was, on average,  <dig>  days longer in the second semesters of  <dig> compared to the reference semester .
concerning the average calving-to-conception interval, a significant difference of approximately 4 days was observed between the second semester of  <dig> only  and the reference semester  .table  <dig> adjusted estimates of the effects of semester on the average lactation length and average calving-to-conception interval of  <dig> dairy cattle herds of treviso province, italy, obtained from the final mixed-effects linear models with first order autoregressive covariance structure fitted to monthly measurements
aestimates are adjusted for variables  <dig> and 12– <dig> and for mean temperature , all included as fixed effects.
random effects were set at the farm level and used sinusoidal terms for seasonal control
bestimates are adjusted for variables 12– <dig>  and for mean temperature , all included as fixed effects.
random effects were set at the farm level and used sinusoidal terms for seasonal control
†also referred to as marginal means, are the means after controlling for the other covariates included in the models
the effects of the control variables included in the final models for dim and calving-to-conception interval are reported in table  <dig>  except for the herd size and cows’ average age, all the other covariates in the final model for dim were significantly associated with the outcome.
an increase in the average monthly temperature, calving-to-conception interval, proportion of pregnant cows, and proportion of artificial  inseminations performed in the farm was significantly associated with an increase in dim.
conversely, an increase in the length of the dry period and proportion of culled cows was associated with a decrease in dim.
except for the herd size, cows’ average age, average monthly temperature and proportion of pregnant cows, all the other covariates in the final model for the calving-to-conception interval were significantly associated with the outcome and had associations similar to those found for dim.table  <dig> associations of the control variables included in the ‘semester-based’ mixed-effects linear models for the average lactation length and average calving-to-conception interval of  <dig> dairy cattle herds of treviso province, italy, with first order autoregressive variance structure fitted to monthly measurements
aestimates are adjusted for semester , included as fixed effect.
random effects were set at the farm level and used sinusoidal terms for seasonal control
the trimester-based analysis  revealed that dim and calving-to-conception interval were again the only indicators with significant variations as compared with the reference trimester .
the lactation length was, on average,  <dig>  days significantly longer  in oct-dec  <dig>  than in oct-dec  <dig> , but this was not the case  in oct-dec  <dig> .
regarding the calving-to-conception interval, a difference of  <dig>  days was observed between the last trimester of  <dig>  and the reference trimester .
the associations of the control covariates  did not differ from those of the semester-based analysis.table  <dig> adjusted estimates of the effects of trimester on the average lactation length and average calving-to-conception interval of  <dig> dairy cattle herds of treviso province, italy, obtained from the final mixed-effects linear models with first order autoregressive covariance structure fitted to monthly measurements .
aestimates are adjusted for variables  <dig> and 12– <dig> and for mean temperature , all included as fixed effects.
random effects were set at the farm level and used sinusoidal terms for seasonal control
bestimates are adjusted for variables 12– <dig>  and for mean temperature , all included as fixed effects.
random effects were set at the farm level and used sinusoidal terms for seasonal control
calso referred to as marginal means, are the means after controlling for the other covariates included in the models
aestimates are adjusted for trimester , included as fixed effect.
random effects were set at the farm level and used sinusoidal terms for seasonal control
discussion
this study assessed the potential impact of sbv emergence on a number of productive and reproductive performance indicators of dairy cattle herds in a sbv-affected province of north-east italy, while accounting for a range of potential confounders, including weather conditions.
results indicated a statistically significant drop in the average lactation length and in the average calving-to-conception interval during the second semester of  <dig>  but none of the other indicators showed significant variations.
these results were reproduced using a narrower temporal window based on trimesters.
a reduced number of dim is strictly related to a decrease in the number of days between calving and conception, also known as days open, as cows that are re-impregnated early in lactation will enter earlier the dry period.
shortening the lactation period by means of early start of re-insemination  is a desirable target in modern dairy farm management .
extended lactation periods are in fact an indicator of reproductive problems in the herd .
the advantage of short lactation cycles is that more peaks of lactation will occur in a given time span, resulting in more milk yield and more calves delivered.
moreover, cows that are re-inseminated early in lactation perform better in terms of pregnancy rates  <cit> .
conversely, increased days open affect farm profitability via increased breeding costs coupled with reduced milk production due to the post-peak flattening of the lactation curve.
besides, increased days open in a previous parity have been associated with a higher risk of death and live culling around the subsequent calving, resulting in increased replacement costs  <cit> .
it follows, therefore, that the significant decrease in dim and days open observed here actually represents a positive outcome, which is not as such imputable to sbv emergence.
if sbv would have had  effects on these two indicators via, for instance, the onset of clinical disease, non-return and adverse pregnancy outcomes, an increase, rather than a decrease, in dim and days open had to be expected.
as it is highly unlikely that sbv improved the performance of dairy cattle herds, other factors than sbv are likely to be involved, e.g.
the voluntary waiting period, artificial insemination technique, calving season , farm management policy, herd size, production level and parity  <cit> , but also mere yearly variation.
as some of these factors, such as the herd size, season, production level and parity were actually controlled for in our models, other  changes in farm management practices would probably explain the observed differences.
besides this, co-morbidities in the farms might have had an effect on the performance indicators.
however, only three farms notified problems of mastitis caused by staphylococcus aureus and streptococcus uberis, but no other diseases were reported during the second semester of  <dig>  making concurrent health problems unlikely to have contributed significantly to the observed decrease in dim and days open.
the main limitation of this study is that it was not possible to distinguish between “sbv-infected” and “sbv-free” herds, nor between different gradients of within-herd sbv prevalence.
rather, we assessed the overall impact of sbv based on a pre- and post-emergence design, assuming a homogeneous spread of infection across the study area.
this assumption has some plausibility given the limited extension of the study area and the evidenced ability of sbv to spread rapidly and extensively  <cit> .
moreover, in the study area the prevalence of sbv in biting midges of the obsoletus complex has been reported to be much higher  than that of btv  <cit> .
similar findings have been reported from other european countries  <cit> , suggesting a higher vector competence for sbv, which might counterbalance its shorter viraemia compared to btv  <cit> , a feature that draws further attention to the sbv potential for spreading.
accordingly, a model for between-farm sbv spread has shown that non-midge-borne transmission routes are unnecessary to explain the rapid spread of sbv  <cit> .
given that all herds in the study area were assumed to be equally exposed to sbv and effects at the herd level might be detectable at a finer scale depending on the within-farm prevalence, our results might be biased towards no significant effect because of insufficient insights into the spatial heterogeneity of sbv spread within the study area.
similarly, the use of relatively large temporal windows , because of the difficulties in pinpointing the exact 1– <dig> week period in which animals were clinically ill, could have masked an actual effect.
however, it can also be argued that any significant effect of sbv, including repercussions on farm profitability, would be better appreciable in a six to three month period interval, as finer temporal subdivisions would have complicated the interpretation of the results given the occurrence of small-scale punctual fluctuations in the productive indicators.
after all, a study in the netherlands  <cit>  was able to detect significance variations in herds’ performances attributable to sbv using a 3-month reference period.
as mentioned in the introduction, veldhuis et al.
<cit>  were able to demonstrate a significant, yet limited, reduction in milk production and calf mortality in the netherlands during the hypothesized sbv transmission period by distinguishing between herds with high and low seroprevalence and between farms with and without reported clinical cases.
the same approach allowed to highlight an increased rate of abortions, malformed lambs, dystocia and lamb mortality in sbv-positive versus sbv-negative sheep flocks in belgium  <cit> .
the lack of prevalence or clinical data for the herds under study was the main reason as to why we could not apply such an analytic approach, which was certainly likely to provide more insights in the impact of sbv.
however, veldhuis et al.
<cit>  did not distinguish between herds based on their sbv infection levels, but performed separate analyses for the dutch national population of dairy cattle herds and for the subgroup of herds reporting malformations in newborn calves, and found small but significant variations in several productive parameters.
therefore, the present study can only make the point that, irrespective of the sbv infection levels and rates of clinical illness, overall at the province level the productive performance of dairy cattle herds in the study area did not seem to have been significantly affected by the emergence of sbv, at least not in a way that there were detrimental effects on farm profitability.
this is in line with the results of wernike et al.
<cit>  who monitored a farm near the city of schmallenberg, germany, between may  <dig> and january  <dig>  all tested animals were sbv-positive after week  <dig> of  <dig>  yet, no decrease in milk yield nor diarrhoea was observed, only transient fever.
moreover, no abortions, stillbirths or malformed calves were observed despite that at the end of september  <dig> most cows were between  <dig> and 175 days of pregnancy, i.e.
the critical gestation period.
similarly, the relatively low number of sbv-associated congenital malformations compared to the high level of sbv infection as indicated by seroprevalence studies suggest that these occur only rarely in calves  <cit> .
furthermore, calves infected in utero can clear sbv infection  <cit> , and farms adopting calving patterns with the critical period falling mostly into periods of low vector activity  can experience lower impact  <cit> .
therefore, although it cannot be excluded that in some instances the impact of sbv may have been considerable at the individual farm level, it was not unexpected to find here that overall at the province level this impact did not appear to be significant.
our results agree, to a major extent, with those of a recent study on the impact of sbv on milk production, reproductive performance, and calf mortality in dairy cattle in kleve district, germany  <cit> .
these authors could only find a small increase in the number of inseminations in the assumed sbv period, but no significant changes in calf mortality or milk production were found, indicating that sbv had a very limited impact, but also that there is considerable variation in the extent of such impact between countries.
underreporting and under-ascertainment of sbv cases is known to be considerable  <cit> .
this was especially true at the beginning of the epidemic, when the mild and transitory nature of the clinical disease, combined with the  lack of diagnostic insights, led many cases to pass unnoticed.
even after the discovery of the virus and the launch of the international alert, many farmers might have not reported suspected cases because of the fear of bearing financial consequences.
thus, even if the reported cases could merely depict the enforcement of enhanced monitoring/control activities by local authorities, development of diagnostic tools and recognition of the lack of sbv zoonotic potential, a possible explanation for the absence of an evidenced negative effect of sbv in the study area may be due, to some extent, to the absence of reported clinical cases in  <dig>  <cit> .
the major impact of sbv originated from restrictions on international trade of susceptible animals, including semen and embryos  <cit> .
of course the rapid spread of the virus in a completely naïve host population resulted in some direct impacts.
however, whether this scenario will happen again is a matter of discussion, especially since sbv is likely to remain endemically in europe, allowing for herd immunity to persist  <cit> .
new sbv outbreaks will therefore occur if the level of herd immunity declines or introductions of novel sbv strains occur.
nonetheless, based on our study, it seems that the future impact of sbv on dairy cattle will depend more on the trade restrictions applied than on the worsening of herds’ performance.
it should be reminded, however, that our results are not based on empirical data on the exact sbv status of the herds, so we are likely to have underestimated the effect of sbv because the proportion of sbv-infected animals within a herd is variable and probably never  <dig> %.
moreover, the detrimental effects of sbv infection are only present during a limited period of gestation, and a large part of the animals in the herds may have been infected outside this critical period, having no detectable effect on performance.
CONCLUSIONS
we assessed the potential impact of sbv emergence on several productive and reproductive performance indicators of dairy cattle herds in a sbv-affected province of north-east italy, accounting for weather conditions and other herd-level factors that could also influence these indicators.
significant decreases in the average lactation length and calving-to-conception interval were observed during the likely sbv infection period.
however, such decreases actually represent a positive outcome which is not imputable to sbv emergence per se, but rather mirrors other beneficial changes in farm management.
while it is highly unlikely that sbv would have improved the performance of dairy cattle herds, the lack of significant variations in dairy herds’ productive performance imputable to sbv is in line with the relatively mild expression of acute sbv infection in adult cattle.
however, our results also contradict previous reports on reproductive problems, and may have underestimated the effect of sbv emergence since no data on the sbv status of the herds and animals therein were available to inform the analysis on the infection levels of the cattle population under study.
although the emergence of sbv might have significantly challenged some individual farms provided that most sbv infections had occurred in the critical gestation period, overall at the province level their productive performances did not seem to have been significantly affected, at least not in a way that there were detrimental effects on farm profitability.

 schmallenberg virus, a newly discovered orthobunyavirus, was first identified in november from dairy cattle with unspecific and transitory symptoms, including fever, decreased milk production and diarrhoea, in north rhine-westphalia, germany. schmallenberg virus shows high homology to viruses of the simbu serogroup, the members of which are typically transmitted through culicoides biting midges and can cause congenital defects in domestic animals. the first clinical evidences of sbv appeared in august not only in germany, but also in belgium and the netherlands. because examination of archived samples did not indicate prior sbv circulation, the most accredited hypothesis is that the virus was introduced in europe between spring and summer and that the excess of congenital malformations, abortions, and stillborns that have been reported in domestic ruminants since december are the result of sbv infections in pregnant animals between summer and autumn. by may a total of bovine and ovicaprine herds in european countries, including italy, were reported as being sbv-positive based on laboratory confirmation of suspected cases. in italy, the first laboratory-confirmed sbv case was reported at the beginning of february from a small multi-species farm in the province of treviso, veneto region, where a dystocic goat died because of the retention of a malformed foetus. epidemiological investigation around this farm excluded any prior introduction of animals from other european countries, suggesting that the virus had circulated locally. moreover, six pools of culicoides midges collected as part of the national surveillance programme for bluetongue virus between september and november within a 50 km radius from the first sbv case tested positive for sbv. this provided evidence that sbv had circulated in the local vector population almost five months before detecting the first case, but that any circulation before september was unlikely.fig. map of north-east italy, including the study area, with indication of positivities for schmallenberg virus in culicoides biting midges and susceptible animals from to the first sbv-positive animal was detected in treviso province in a dystocic goat on the first semester of. midges were found positive in treviso province on the second semester of and in pordenone province on the first semester of evidence for the impact of sbv on animal productive performance is scarce. a few studies have assessed the impact of sbv infection as: 1) direct impact on adult animals, including their clinical manifestations and herd- or area-level variation in key performance parameters; 2) direct impact on newborns, including congenital malformations, abortions, and stillbirths; and 3) indirect impacts, including trade restrictions and treatment costs. although sbv may cause clinical signs of pyrexia and reduced milk production, cases are typically identified only when animals appear with congenital malformations among offspring whose mothers acquired infection during a particular period of pregnancy. the lack of a clear clinically overt presentation is one of the main reasons as to why there is a large degree of under-ascertainment for sbv, especially in cattle, for which it is known that sbv infection is relatively mild, with a low incidence of birth malformations. accordingly, the impact of sbv on adverse pregnancy outcomes of dairy cattle is believed to be limited and lower than that on milk production or return to service. a recent dutch study reported that calf mortality differed very little between herds with low and high sbv seroprevalence during the second half of a finding that mirrors the relatively mild expression of sbv infection in adult cows. conversely, sbv seemed to have had a relatively more pronounced impact on milk production. in a predefined 4-week period during which most sbv infections were expected to have occurred in the netherlands, the overall drop in milk production per cow was quantified as ~30 kg and ~50 kg. however, potential confounders such as weather conditions, particularly temperature and relative humidity, which are known to affect milk production levels, were not accounted for. this study aimed at assessing variations imputable to sbv emergence in productive and reproductive performance indicators of dairy cattle herds in the province of treviso, i.e. italy’s area where sbv was at first detected and post-epidemic modelling studies have predicted it as one of the areas with the highest risk of sbv spread in europe. the analysis accounted for weather conditions and other herd-level factors that could also influence these indicators. yet, it could only assess the presumptive/circumstantial effect of sbv on dairy cattle herd performances since no exact sbv data were available for the study area.  the productive and reproductive performance indicators of dairy cattle herds in the study area are summarized by semester in table  data were collected monthly from january to june at the herd level. the average number of herd-month observations by semester per each indicator varied from to, depending on the number of missing values. some indicators showed a clear seasonal pattern, with a tendency towards decreased milk production in the second semester of each year and increased number of newborns in the first semester of each year. accordingly, the average number of inseminations per pregnancy leading to a healthy newborn increased in the second semester of each year. the indicator average lactation length showed a drop in jul-dec which was preceded in first half of by another drop as compared to the previous semesters. the same was observed for the average calving-to-conception interval.table mean values of the productive and reproductive performance indicators of dairy cattle herds of treviso province, north-east italy. data were collected monthly at the herd level, from january to june 2012 acalculated as the ratio between the average number of inseminations per pregnancy and the total number of registered newborns of the following 9th month bincludes only healthy calves that survived for at least 21 days after birth and the farmers were then obliged to register to the local health authority the mixed-effects linear models showed that only two performance indicators of interest varied significantly with respect to the reference semester. these indicators were the average lactation length and the average calving-to-conception interval. except for the second semester of all other semesters showed a significantly longer lactation length – also known as days in milk – than that of the reference semester. looking specifically at the same semesters over the years, herds’ lactation length was, on average,  days longer in the second semesters of compared to the reference semester. concerning the average calving-to-conception interval, a significant difference of approximately 4 days was observed between the second semester of only and the reference semester.table adjusted estimates of the effects of semester on the average lactation length and average calving-to-conception interval of dairy cattle herds of treviso province, italy, obtained from the final mixed-effects linear models with first order autoregressive covariance structure fitted to monthly measurements aestimates are adjusted for variables and 12– and for mean temperature, all included as fixed effects. random effects were set at the farm level and used sinusoidal terms for seasonal control bestimates are adjusted for variables 12– and for mean temperature, all included as fixed effects. random effects were set at the farm level and used sinusoidal terms for seasonal control †also referred to as marginal means, are the means after controlling for the other covariates included in the models the effects of the control variables included in the final models for dim and calving-to-conception interval are reported in table  except for the herd size and cows’ average age, all the other covariates in the final model for dim were significantly associated with the outcome. an increase in the average monthly temperature, calving-to-conception interval, proportion of pregnant cows, and proportion of artificial inseminations performed in the farm was significantly associated with an increase in dim. conversely, an increase in the length of the dry period and proportion of culled cows was associated with a decrease in dim. except for the herd size, cows’ average age, average monthly temperature and proportion of pregnant cows, all the other covariates in the final model for the calving-to-conception interval were significantly associated with the outcome and had associations similar to those found for dim.table associations of the control variables included in the ‘semester-based’ mixed-effects linear models for the average lactation length and average calving-to-conception interval of dairy cattle herds of treviso province, italy, with first order autoregressive variance structure fitted to monthly measurements aestimates are adjusted for semester, included as fixed effect. random effects were set at the farm level and used sinusoidal terms for seasonal control the trimester-based analysis revealed that dim and calving-to-conception interval were again the only indicators with significant variations as compared with the reference trimester. the lactation length was, on average,  days significantly longer in oct-dec than in oct-dec, but this was not the case in oct-dec. regarding the calving-to-conception interval, a difference of  days was observed between the last trimester of and the reference trimester. the associations of the control covariates did not differ from those of the semester-based analysis.table adjusted estimates of the effects of trimester on the average lactation length and average calving-to-conception interval of dairy cattle herds of treviso province, italy, obtained from the final mixed-effects linear models with first order autoregressive covariance structure fitted to monthly measurements. aestimates are adjusted for variables and 12– and for mean temperature, all included as fixed effects. random effects were set at the farm level and used sinusoidal terms for seasonal control bestimates are adjusted for variables 12– and for mean temperature, all included as fixed effects. random effects were set at the farm level and used sinusoidal terms for seasonal control calso referred to as marginal means, are the means after controlling for the other covariates included in the models aestimates are adjusted for trimester, included as fixed effect. random effects were set at the farm level and used sinusoidal terms for seasonal control discussion this study assessed the potential impact of sbv emergence on a number of productive and reproductive performance indicators of dairy cattle herds in a sbv-affected province of north-east italy, while accounting for a range of potential confounders, including weather conditions. results indicated a statistically significant drop in the average lactation length and in the average calving-to-conception interval during the second semester of but none of the other indicators showed significant variations. these results were reproduced using a narrower temporal window based on trimesters. a reduced number of dim is strictly related to a decrease in the number of days between calving and conception, also known as days open, as cows that are re-impregnated early in lactation will enter earlier the dry period. shortening the lactation period by means of early start of re-insemination is a desirable target in modern dairy farm management. extended lactation periods are in fact an indicator of reproductive problems in the herd. the advantage of short lactation cycles is that more peaks of lactation will occur in a given time span, resulting in more milk yield and more calves delivered. moreover, cows that are re-inseminated early in lactation perform better in terms of pregnancy rates. conversely, increased days open affect farm profitability via increased breeding costs coupled with reduced milk production due to the post-peak flattening of the lactation curve. besides, increased days open in a previous parity have been associated with a higher risk of death and live culling around the subsequent calving, resulting in increased replacement costs. it follows, therefore, that the significant decrease in dim and days open observed here actually represents a positive outcome, which is not as such imputable to sbv emergence. if sbv would have had effects on these two indicators via, for instance, the onset of clinical disease, non-return and adverse pregnancy outcomes, an increase, rather than a decrease, in dim and days open had to be expected. as it is highly unlikely that sbv improved the performance of dairy cattle herds, other factors than sbv are likely to be involved, e.g. the voluntary waiting period, artificial insemination technique, calving season, farm management policy, herd size, production level and parity, but also mere yearly variation. as some of these factors, such as the herd size, season, production level and parity were actually controlled for in our models, other changes in farm management practices would probably explain the observed differences. besides this, co-morbidities in the farms might have had an effect on the performance indicators. however, only three farms notified problems of mastitis caused by staphylococcus aureus and streptococcus uberis, but no other diseases were reported during the second semester of making concurrent health problems unlikely to have contributed significantly to the observed decrease in dim and days open. the main limitation of this study is that it was not possible to distinguish between “sbv-infected” and “sbv-free” herds, nor between different gradients of within-herd sbv prevalence. rather, we assessed the overall impact of sbv based on a pre- and post-emergence design, assuming a homogeneous spread of infection across the study area. this assumption has some plausibility given the limited extension of the study area and the evidenced ability of sbv to spread rapidly and extensively. moreover, in the study area the prevalence of sbv in biting midges of the obsoletus complex has been reported to be much higher than that of btv. similar findings have been reported from other european countries, suggesting a higher vector competence for sbv, which might counterbalance its shorter viraemia compared to btv, a feature that draws further attention to the sbv potential for spreading. accordingly, a model for between-farm sbv spread has shown that non-midge-borne transmission routes are unnecessary to explain the rapid spread of sbv. given that all herds in the study area were assumed to be equally exposed to sbv and effects at the herd level might be detectable at a finer scale depending on the within-farm prevalence, our results might be biased towards no significant effect because of insufficient insights into the spatial heterogeneity of sbv spread within the study area. similarly, the use of relatively large temporal windows, because of the difficulties in pinpointing the exact 1– week period in which animals were clinically ill, could have masked an actual effect. however, it can also be argued that any significant effect of sbv, including repercussions on farm profitability, would be better appreciable in a six to three month period interval, as finer temporal subdivisions would have complicated the interpretation of the results given the occurrence of small-scale punctual fluctuations in the productive indicators. after all, a study in the netherlands was able to detect significance variations in herds’ performances attributable to sbv using a 3-month reference period. as mentioned in the introduction, veldhuis et al. were able to demonstrate a significant, yet limited, reduction in milk production and calf mortality in the netherlands during the hypothesized sbv transmission period by distinguishing between herds with high and low seroprevalence and between farms with and without reported clinical cases. the same approach allowed to highlight an increased rate of abortions, malformed lambs, dystocia and lamb mortality in sbv-positive versus sbv-negative sheep flocks in belgium. the lack of prevalence or clinical data for the herds under study was the main reason as to why we could not apply such an analytic approach, which was certainly likely to provide more insights in the impact of sbv. however, veldhuis et al. did not distinguish between herds based on their sbv infection levels, but performed separate analyses for the dutch national population of dairy cattle herds and for the subgroup of herds reporting malformations in newborn calves, and found small but significant variations in several productive parameters. therefore, the present study can only make the point that, irrespective of the sbv infection levels and rates of clinical illness, overall at the province level the productive performance of dairy cattle herds in the study area did not seem to have been significantly affected by the emergence of sbv, at least not in a way that there were detrimental effects on farm profitability. this is in line with the results of wernike et al. who monitored a farm near the city of schmallenberg, germany, between may and january all tested animals were sbv-positive after week of yet, no decrease in milk yield nor diarrhoea was observed, only transient fever. moreover, no abortions, stillbirths or malformed calves were observed despite that at the end of september most cows were between and 175 days of pregnancy, i.e. the critical gestation period. similarly, the relatively low number of sbv-associated congenital malformations compared to the high level of sbv infection as indicated by seroprevalence studies suggest that these occur only rarely in calves. furthermore, calves infected in utero can clear sbv infection, and farms adopting calving patterns with the critical period falling mostly into periods of low vector activity can experience lower impact. therefore, although it cannot be excluded that in some instances the impact of sbv may have been considerable at the individual farm level, it was not unexpected to find here that overall at the province level this impact did not appear to be significant. our results agree, to a major extent, with those of a recent study on the impact of sbv on milk production, reproductive performance, and calf mortality in dairy cattle in kleve district, germany. these authors could only find a small increase in the number of inseminations in the assumed sbv period, but no significant changes in calf mortality or milk production were found, indicating that sbv had a very limited impact, but also that there is considerable variation in the extent of such impact between countries. underreporting and under-ascertainment of sbv cases is known to be considerable. this was especially true at the beginning of the epidemic, when the mild and transitory nature of the clinical disease, combined with the lack of diagnostic insights, led many cases to pass unnoticed. even after the discovery of the virus and the launch of the international alert, many farmers might have not reported suspected cases because of the fear of bearing financial consequences. thus, even if the reported cases could merely depict the enforcement of enhanced monitoring/control activities by local authorities, development of diagnostic tools and recognition of the lack of sbv zoonotic potential, a possible explanation for the absence of an evidenced negative effect of sbv in the study area may be due, to some extent, to the absence of reported clinical cases in. the major impact of sbv originated from restrictions on international trade of susceptible animals, including semen and embryos. of course the rapid spread of the virus in a completely naïve host population resulted in some direct impacts. however, whether this scenario will happen again is a matter of discussion, especially since sbv is likely to remain endemically in europe, allowing for herd immunity to persist. new sbv outbreaks will therefore occur if the level of herd immunity declines or introductions of novel sbv strains occur. nonetheless, based on our study, it seems that the future impact of sbv on dairy cattle will depend more on the trade restrictions applied than on the worsening of herds’ performance. it should be reminded, however, that our results are not based on empirical data on the exact sbv status of the herds, so we are likely to have underestimated the effect of sbv because the proportion of sbv-infected animals within a herd is variable and probably never %. moreover, the detrimental effects of sbv infection are only present during a limited period of gestation, and a large part of the animals in the herds may have been infected outside this critical period, having no detectable effect on performance.  we assessed the potential impact of sbv emergence on several productive and reproductive performance indicators of dairy cattle herds in a sbv-affected province of north-east italy, accounting for weather conditions and other herd-level factors that could also influence these indicators. significant decreases in the average lactation length and calving-to-conception interval were observed during the likely sbv infection period. however, such decreases actually represent a positive outcome which is not imputable to sbv emergence per se, but rather mirrors other beneficial changes in farm management. while it is highly unlikely that sbv would have improved the performance of dairy cattle herds, the lack of significant variations in dairy herds’ productive performance imputable to sbv is in line with the relatively mild expression of acute sbv infection in adult cattle. however, our results also contradict previous reports on reproductive problems, and may have underestimated the effect of sbv emergence since no data on the sbv status of the herds and animals therein were available to inform the analysis on the infection levels of the cattle population under study. although the emergence of sbv might have significantly challenged some individual farms provided that most sbv infections had occurred in the critical gestation period, overall at the province level their productive performances did not seem to have been significantly affected, at least not in a way that there were detrimental effects on farm profitability. 
similarly, during the last trimester of which is most likely to cover the sbv infection period in the study area, there was an average decrease of −4 days and −7 days compared to the same trimesters of the years and respectively. using a comprehensive data set from a sbv-affected province in north-east italy, this study aimed at assessing the potential impact of sbv emergence on productive and reproductive performance indicators of dairy cattle herds, accounting for weather conditions and other herd-level factors that could also influence these indicators. none of the other indicators showed significant variations, confirming the relatively mild expression of sbv infection in cattle. although the emergence of sbv might have significantly affected the productive performance of some individual farms, we concluded that overall at the province level there were no significant variations attributable to sbv, at least not in a way that would lead to negative effects on farm profitability. however, the observed decreases actually represent a positive outcome that is not as such imputable to sbv emergence, but rather reflects other beneficial changes in farm management. taking the second semester of as reference, significant decreases in the average lactation length and calving-to-conception interval were observed relative to the same semesters of the years and respectively. schmallenberg virus has spread across europe since mid- causing unspecific and transitory symptoms in ruminants and congenital malformations in their offspring. evidence for the impact of sbv on cattle productive performance is limited.
 schmallenberg virus has spread across europe since mid- causing unspecific and transitory symptoms in ruminants and congenital malformations in their offspring. evidence for the impact of sbv on cattle productive performance is limited. using a comprehensive data set from a sbv-affected province in north-east italy, this study aimed at assessing the potential impact of sbv emergence on productive and reproductive performance indicators of dairy cattle herds, accounting for weather conditions and other herd-level factors that could also influence these indicators.  a total of farms with an average of cows per farm were monitored monthly from january to june mixed-effects linear models for longitudinal data were used to assess the average variation in herds’ performance indicators over semesters and trimesters therein. taking the second semester of as reference, significant decreases in the average lactation length and calving-to-conception interval were observed relative to the same semesters of the years and respectively. similarly, during the last trimester of which is most likely to cover the sbv infection period in the study area, there was an average decrease of −4 days and −7 days compared to the same trimesters of the years and respectively. however, the observed decreases actually represent a positive outcome that is not as such imputable to sbv emergence, but rather reflects other beneficial changes in farm management. none of the other indicators showed significant variations, confirming the relatively mild expression of sbv infection in cattle.  although the emergence of sbv might have significantly affected the productive performance of some individual farms, we concluded that overall at the province level there were no significant variations attributable to sbv, at least not in a way that would lead to negative effects on farm profitability. electronic supplementary material the online version of this article contains supplementary material, which is available to authorized users. keywords sbvorthobunyavirusdairy cattleitalyissue-copyright-statement© the author 2015 