Serological testing of Schmallenberg virus in Swedish wild cervids from 2012 to 2016.

BACKGROUND: Schmallenberg virus (SBV) first emerged in Europe in 2011, and in Sweden in late 2012. The virus was still circulating in parts of Europe in 2015. In recent testing, the virus has not been detected in Swedish domestic animals, indicating that it is no longer circulating in Sweden. It is not known if the virus has circulated and is still circulating in Swedish wild cervid populations and whether wildlife can act as virus reservoirs. The aim of this study was to investigate whether SBV has circulated, and is still circulating among wild cervids in Sweden. RESULTS: Ninety-two sera from moose (Alces alces, n = 22), red deer (Cervus elaphus, n = 15), fallow deer (Dama dama, n = 44), and roe deer (Capreolus capreolus, n = 11) were collected and analyzed for antibodies against SBV. The sampling occurred in the southern and middle part of Sweden during three time periods: 1) before the vector season in 2012, 2) after the vector season in 2012, and 3) after the vector season in 2015. Animals from periods 1 and 2 were of varying ages, whereas animals collected in period 3 were born after the vector season 2013. Animals from period 1 (n = 15) and 3 (n = 47) were seronegative, but, 53% (16 of 30) of animals from period 2 were seropositive, determined by SBV competitive ELISA. Samples from period 2 were additionally analyzed for SBV-neutralizing antibodies. Such antibodies were detected in 16/16 SBV-N-antibody-positive, 3/12 negative and 2/2 doubtful sera. The two tests were in accordance at SBV-neutralizing antibody titers of 1:32 or higher. CONCLUSION: Our results show that SBV circulated among wild cervids during the vector season of 2012. Three years later, no SBV-antibodies were detected in animals born after the vector season 2013. The likely absence of SBV circulation in Sweden, in contrast to other parts of Europe, might be explained by the annual occurrence of a vector-free season due to climate conditions. Interpretations are limited by the small sample-size, but the results suggest that the SBV competitive ELISA has high specificity but might have slightly lower sensitivity compared to a seroneutralization assay, when using samples from wild cervids.

Tick-borne encephalitis.

Tick-borne encephalitis (TBE), a zoonotic arbovirosis caused by tick-borne encephalitis virus (TBEV), is an increasing public health concern. Infections result in neurological symptoms in humans and the virus has rapidly expanded to new geographical areas. Three subtypes are currently present in different parts of Europe and Asia. The virus is transmitted by ticks, mainly Ixodes spp., between small mammals such as rodents, which serve as virus amplifying hosts. Humans are infected sporadically, either by a tick bite or by ingestion of infected milk or milk products. Other mammals (e.g. ruminants) can also be infected, but most of the time do not show clinical signs. In contrast to rodents, other wild and domestic mammals probably play only a very small direct role in maintaining TBEV in an area, but they might play an important role as hosts in sustaining a large tick population. Therefore, the virus prevalence and the occurrence of TBE can be influenced by several environmental, genetic and behavioural factors associated with the virus, the vectors or the hosts, and understanding these factors is essential for implementation of effective control measures. This article reviews virus characteristics and the epidemiological and clinical aspects of TBEV infections and examines pathogenesis, diagnostic approaches and control measures.

A six-year study on respiratory viral infections in a bull testing facility.

Viral infection dynamics and bovine respiratory disease (BRD) treatment rates were studied over six years at a Swedish bull testing station with an 'all in, all out' management system. In August of each of the years 1998-2003, between 149 and 185 4-8-month-old calves arrived at the station from 99 to 124 different beef-breeding herds, and remained until March the following year. Only calves that tested free from bovine viral diarrhoea virus (BVDV) were allowed to enter the station and original animal groups were kept isolated from new cattle in their original herds for three weeks before admission. Although neither prophylactic antibiotics, nor BRD vaccines were used, less than 0.7-13.2% (mean 5%) of the calves (n=970) required treatment for BRD during the first five weeks following entry. This was probably due, at least in part, to the season (the summer months) when the animals were commingled. In the six-month period August-February, 38% of the animals were treated one or more times for BRD and mortality was 0.7%. Hereford and Aberdeen Angus calves had significantly higher treatment rates than Charolais, Simmental and Blonde d'Aquitaine. Serological testing on samples obtained in August, November and January indicated that bovine parainfluenza virus 3 (PIV-3) infections occurred each year before November after entry. Bovine coronavirus (BCoV) infections also occurred every year, but in 3/6 years this was not until after November. Bovine respiratory syncytial virus (BRSV) infections occurred only every second year and were associated with a treatment peak and one death on one occasion (December). The herd remained BVDV free during the entire study period. The infection patterns for PIV-3 and BCoV indicated a high level of infectivity amongst bovine calves, whereas the incidence for BRSV was observed at a lower level. Although the rearing of the animals differed from conventional beef production, the study has shown that commingling animals from many sources is not necessarily associated with high morbidity within the first few weeks after arrival. By preventing BRD soon after commingling the prerequisites for protective vaccination at entry might be improved. Applied management routines are discussed.

Dynamics of virus infections involved in the bovine respiratory disease complex in Swedish dairy herds.

The dynamics of bovine respiratory syncytial virus (BRSV), bovine parainfluenza virus 3 (PIV-3), bovine corona virus (BCoV) and bovine viral diarrhoea virus (BVDV) infections were studied in 118 dairy herds in south western Sweden. By using serology on paired samples from three approximately 7 vs. approximately 15-month-old calves per herd, the propagation of infections was investigated over about a 1-year period. The results implied that at least 74% of calves had experienced one or more of the monitored infections at the age of approximately 7 months (Sample 1, Spring); 30%, 48%, 34% and 8% were seropositive to BRSV, PIV-3, BCoV and BVDV, respectively. Seroconversions to BRSV, PIV-3, BCoV and BVDV occurred in 26%, 38%, 50% and 3% of seronegative animals and 63% had antibodies against two or more infections at approximately 15 months (Sample 2). In total, 90-97% of animals that were seropositive in Sample 1 remained positive in Sample 2. A significant association was found between BVDV and BCoV (P = 0.01). Moreover, a significantly higher proportion of herds in which no calves had a recorded history of respiratory disease (n = 15) were classified as negative to all four infections monitored when compared to herds in which disease was observed (P = 0.0002). This study showed a high infection burden in young animals and effective spread of BRSV, PIV-3 and BCoV in one area of Sweden. BVDV infections were restricted to a few herds, reflecting the effect of a voluntary control program against BVDV in Sweden.