Serosurveillance of Schmallenberg virus in Switzerland using bulk tank milk samples

Preparing for Emerging Zoonotic Viruses

The risk of emergence and spread of novel human pathogens originating from an animal reservoir has increased in the past decades. However, the unpredictable nature of disease emergence makes surveillance and preparedness challenging. Knowledge of general risk factors for emergence and spread, combined with local level data is needed to develop a risk-based methodology for early detection. This involves the implementation of the One Health approach, integrating human, animal and environmental health sectors, as well as social sciences, bioinformatics and more. Recent technical advances, such as metagenomic sequencing, will aid the rapid detection of novel pathogens on the human-animal interface.

Schmallenberg Virus: To Vaccinate, or Not to Vaccinate?

Schmallenberg virus (SBV), a teratogenic orthobunyavirus that infects predominantly ruminants, emerged in 2011 in Central Europe, spread rapidly throughout the continent, and subsequently established an endemic status with re-circulations to a larger extent every 2 to 3 years. Hence, it represents a constant threat to the continent’s ruminant population when no effective countermeasures are implemented. Here, we discuss potential preventive measures to protect from Schmallenberg disease. Previous experiences with other arboviruses like bluetongue virus have already demonstrated that vaccination of livestock against a vector-transmitted disease can play a major role in reducing or even stopping virus circulation. For SBV, specific inactivated whole-virus vaccines have been developed and marketing authorizations were granted for such preparations. In addition, candidate marker vaccines either as live attenuated, DNA-mediated, subunit or live-vectored preparations have been developed, but none of these DIVA-capable candidate vaccines are currently commercially available. At the moment, the licensed inactivated vaccines are used only to a very limited extent. The high seroprevalence rates induced in years of virus re-occurrence to a larger extent, the wave-like and sometimes hard to predict circulation pattern of SBV, and the expenditures of time and costs for the vaccinations presumably impact on the willingness to vaccinate. However, one should bear in mind that the consequence of seronegative young animals and regular renewed virus circulation might be again more cases of fetal malformation caused by an infection of naïve dams during one of their first gestations. Therefore, an appropriate and cost-effective strategy might be to vaccinate naïve female animals of all affected species before the reproductive age.

Schmallenberg Disease-A Newly Emerged Culicoides-borne Viral Disease of Ruminants

First appearing in 2011 in Northern Europe, Schmallenberg virus (SBV), an Orthobunyavirus of the Simbu serogroup, is associated with clinical disease mainly in ruminants such as cattle, sheep and goats. The clinical signs are characterized by abortion and congenital deformities in newborns. The virus is transmitted by Culicoides midges of the Obsoletus complex. SBV infection induces a solid protective immunity that persists for at least 4 or 6 years in sheep and cattle, respectively. SBV infection can be diagnosed directly by real-time RT-qPCR and virus isolation or indirectly by serological assays. Three vaccines are commercially available in Europe. This article provides a comprehensive literature review on this emerging disease regarding pathogenesis, transmission, diagnosis, control and prevention. This review also highlights that although much has been learned since SBV’s first emergence, there are still areas that require further study to devise better mitigation strategies.

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.

Development of within-herd immunity and long-term persistence of antibodies against Schmallenberg virus in naturally infected cattle

Background

In 2011, the teratogenic, insect-transmitted Schmallenberg virus (SBV) emerged at the German/Dutch border region and subsequently spread rapidly throughout the European continent. In cattle, one of the major target species of SBV, first antibodies are detectable between one and three weeks after infection, but the duration of humoral immunity is unknown. To assess the course of immunity in individual animals and the development of the within-herd seroprevalence, cattle kept in a German farm with a herd size of about 300 lactating animals were annually blood sampled between December 2011 and December 2017 and tested for the presence of SBV-specific antibodies.

Results

During the monitored period, the within-herd seroprevalence declined from 74.92% in 2011 to 39.93% in 2015 and, thereafter, slightly increased to 49.53% in 2016 and 48.44% in 2017. From the animals that were tested in 2014 and 2015 for the first time (between 24 and 35 months of age) only 14.77% and 7.45%, respectively, scored positive. Thereafter, the seropositivity rate of this age group rose markedly to 58.04% in 2016 and 48.10% in 2017 indicating a circulation of SBV. Twenty-three individual animals were consistently sampled once per year between 2011 and 2017 after the respective insect vector season, 17 of them tested positive at the first sampling. Fourteen animals were still seropositive in December 2017, while three cattle (17.65%) became seronegative.

Conclusions

The regular re-emergence of SBV in Central Europe is a result of decreasing herd immunity caused by the replacement of animals by seronegative youngstock rather than of a drop of antibody levels in previously infected individual animals. The consequences of the overall decline in herd seroprevalence may be increasing virus circulation and more cases of fetal malformation caused by infection of naïve dams during gestation.

Limited added value of fungal ITS amplicon sequencing in the study of bovine abortion

Bovine mycotic abortion is sporadic and caused by different ubiquitous and opportunistic fungi. Recently, a broad spectrum of bacterial opportunists involved in bovine abortion was revealed by 16S rRNA gene amplicon sequencing. We hypothesized that fungal organisms potentially involved in bovine abortion also might remain undetected by conventional culture. In this retrospective study, we therefore applied fungal internal transcribed spacer 2 (ITS2) region amplicon sequencing to 74 cases of bovine abortion submitted to our diagnostic service. The investigation was complemented by fungal culture and, retrospectively, by data from bacteriological, virological and parasitological analyses and histopathological examination of placentas. Fungal DNA was found in both the placentas and abomasal contents, with 92 fungal genera identified. In 18 cases, >75% of the reads belonged to one specific fungal genus: Candida (n = 7), Malassezia (n = 4), Cryptococcus (n = 3), unidentified Capnodiales (n = 3), Actinomucor (n = 1), Cystofilobasidium (n = 1), Penicillium (n = 1), Verticillum (n = 1) and Zymoseptoria (n = 1) with one case harboring two different genera. By culture, in contrast, fungal agents were detected in only 6 cases. Inflammatory and/or necrotizing lesions were found in 27/40 histologically assessed placentas. However, no lesion-associated fungal structures were detected in HE- and PAS-stained specimens. Complementary data revealed the presence of one or more non-fungal possible abortifacient: Chlamydiales, Coxiella burnetii, Leptospira spp., Campylobacter fetus subsp. fetus, Streptococcus uberis, Escherichia coli, Streptococcus pluranimalium, Bacillus licheniformis, Campylobacter fetus subsp. fetus, Serratia marcescens, Trueperella pyogenes, Schmallenbergvirus, Neospora caninum. The mycobiota revealed by sequencing did not differ between cases with or without a possible infectious etiology. Our study suggests that amplicon sequencing of the ITS2 region from DNA isolated from bovine abortion does not provide additional information or new insight into mycotic abortion and without complementary analyses may easily lead to a false interpretation of the role of fungal organisms in bovine abortion.

Schmallenberg Virus: A Novel Virus of Veterinary Importance

In late 2011, unspecific clinical symptoms such as fever, diarrhea, and decreased milk production were observed in dairy cattle in the Dutch/German border region. After exclusion of classical endemic and emerging viruses by targeted diagnostic systems, blood samples from acutely diseased cows were subjected to metagenomics analysis. An insect-transmitted orthobunyavirus of the Simbu serogroup was identified as the causative agent and named Schmallenberg virus (SBV). It was one of the first detections of the introduction of a novel virus of veterinary importance to Europe using the new technology of next-generation sequencing. The virus was subsequently isolated from identical samples as used for metagenomics analysis in insect and mammalian cell lines and disease symptoms were reproduced in calves experimentally infected with both, this culture-grown virus and blood samples of diseased cattle. Since its emergence, SBV spread very rapidly throughout the European ruminant population causing mild unspecific disease in adult animals, but also premature birth or stillbirth and severe fetal malformation when naive dams were infected during a critical phase of gestation. In the following years, SBV recirculated regularly to a larger extend; in the 2014 and 2016 vector seasons the virus was again repeatedly detected in the blood of adult ruminants, and in the following winter and spring months, a number of malformed calves and lambs was born. The genome of viruses present in viremic adult animals showed a very high sequence stability; in sequences generated between 2012 and 2016, only a few amino acid substitutions in comparison to the initial SBV isolate could be detected. In contrast, a high sequence variability was identified in the aminoterminal part of the glycoprotein Gc-encoding region of viruses present in the brain of malformed newborns. This mutation hotspot is independent of the region or host species from which the samples originated and is potentially involved in immune evasion mechanisms.

Herd-level infectious disease surveillance of livestock populations using aggregate samples

All sectors of livestock production are in the process of shifting from small populations on many farms to large populations on fewer farms. A concurrent shift has occurred in the number of livestock moved across political boundaries. The unintended consequence of these changes has been the appearance of multifactorial diseases that are resistant to traditional methods of prevention and control. The need to understand complex animal health conditions mandates a shift toward the collection of longitudinal animal health data. Historically, collection of such data has frustrated and challenged animal health specialists. A promising trend in the evolution toward more efficient and effective livestock disease surveillance is the increased use of aggregate samples, e.g. bulk tank milk and oral fluid specimens. These sample types provide the means to monitor disease, estimate herd prevalence, and evaluate spatiotemporal trends in disease distribution. Thus, this article provides an overview of the use of bulk tank milk and pen-based oral fluids in the surveillance of livestock populations for infectious diseases.

Association of clinical signs after acute Schmallenberg virus infection with milk production and fertility in Swiss dairy cows

Since its first occurrence in August 2011 in Germany and the Netherlands, the Schmallenberg virus (SBV) spread rapidly across Europe, where it caused production losses and abortions in ruminants as well as congenital malformations in the offspring of affected animals. Several studies have investigated the impact of SBV on fertility and production parameters in dairy cows at herd level. However, the impact of clinical disease at the animal level remained undetermined. This study aimed at estimating the impact of clinical disease during and after an infection with SBV on production and fertility parameters in individual Swiss dairy cows. Sixty-seven case and twenty-four control herds were selected according to whether cows had been showing clinical signs indicative of SBV during the epidemic from July to December 2012 in Switzerland. Of these 91 farms, production and fertility data from 388 cows with clinical signs from case herds were collected over a time period of four years, and compared to data from 932 cows without clinical signs originating from case or control herds. Milk yield, somatic cell count, number of inseminations and non-return at day 56 were analysed by means of hierarchical multivariable regression analysis. A significant drop in milk yield was observed in all groups during the SBV epidemic compared to the time before the infection, which amounted to 1.9kg per test day for clinical animals, 1.1kg for non-clinical animals from case herds and 0.6kg for non-clinical animals from control herds. A prolonged effect on milk yield was observed in clinical cows for about one year, suggesting that animals with clinical disease might not return to their previous milk production level in the current lactation after an acute infection with SBV. Clinical animals showed a significantly higher somatic cell count during the epidemic compared to the time before the infection. The number of inseminations per cow and production cycle was higher for clinical animals during the epidemic compared to the time periods before and after, but not significantly higher than for non-clinical animals from case and control herds. No difference regarding non-return at day 56 was found. Although the overall impact of the SBV epidemic in Switzerland was limited, the consequences could be substantial in farms with a high prevalence of clinical disease.

Moving past serology: Diagnostic options without serum

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Traditional serology has moved beyond blood as a test medium.

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A number of different samples and tissues are now frequently used in veterinary diagnosis.

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Testing can be robust and accurate and opens up the field to a variety of new opportunities.

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Molecular testing allows direct testing for the agent on a variety of tissues and samples, and pools.

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Pooling of samples can allow for more efficient and cost-effective testing.

Detecting antibodies formed in serum in response to infection is the traditional function of serology. Diagnostic modalities have included complement fixation tests, agar gel immune-diffusion, radioimmunoassay, ELISA and immunofluorescence. More recent technology now allows for the direct detection of pathogens by PCR. This review details the options for diagnostic testing using specimen types other than serum, identifying the advantages and disadvantages of these options and providing evidence for more widespread use of these techniques and specimen types.

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

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.