ISSN 2398-2993      

Schmallenberg disease

obovis

Introduction

  • Cause: Orthobunyavirus of the Simbu serogroup.
  • Signs: mostly unspecific; fever, drop in milk yield, diarrhea, abortions, birth of malformed fetuses.
  • Diagnosis: clinical diagnosis can be difficult due to the unspecific nature of the signs and the lag period between infection and birth of fetal malformations; laboratory diagnosis consists of virology, pathology and serology.
  • Treatment: supportive treatment only; preventative treatment via inactivated vaccines.
  • Prognosis: variable; mostly good for adult animals, poor for fetuses due to abortion and/or malformations.
Schmallenberg is a disease of sheep and goats, in addition to cattle. In small ruminants the infection appears to be mostly subclinical, although there have been reports of decreased milk yield in flocks of dairy sheep and goats.
 

Pathogenesis

Etiology

  • SBV is still a relatively novel Orthobunyavirus, unknown in Europe prior to 2011 Schmallenberg virus.
  • Two main clades have been identified, one for each of the outbreaks reported (2011-13 and 2016).
  • Multiple species of Culicoides spp midges were found positive to SBV genomic markers, confirming the suspicion of their role as vectors. Although the mechanisms used by the virus to persist over winter are still unclear, a study found evidence of transovarial transmission within the female vector. It has also been suggested that high viral burdens within infected fetuses and amniotic fluid could be one of the overwintering mechanisms.
  • Once passed from vector to final host, vertical transmission occurs in the first and early second trimester of pregnancy.
  • Direct horizontal transmission between cattle and/or sheep has not been confirmed, despite the virus being present in most body fluids. SBV could be found in semen of infected bulls.

Predisposing factors

General

  • Both domestic and wild ruminants have been shown to be at risk, however, only indirect evidence of infection was found in wild ruminants via serological tests.

Specific

  • Pregnant animals in the early stages of pregnancy (first and early second trimester).

Pathophysiology

  • Teratogenic virus with a tropism for the Central Nervous System (CNS) and musculoskeletal tissues in the developing embryo.
  • Uninfected female Culicoides spp acquire the virus with a blood meal from a viremic animal. Viral replication within the vector occurs during the extrinsic incubation period (EIP). This can vary between 9-41 days (as it is dependent on local environmental conditions). Once viral replication has reached sufficient levels within the vector, the next blood meals will transmit infection in the saliva.
  • If viremia in the bovine host occurs during the critical stages of gestation, the virus can cross the placental barrier and cause Schmallenberg disease in the fetus.

Timecourse

  • 1-3 days incubation from the infected midge bite.
  • 1-6 days duration for the acute disease. An outbreak may last up to 4 weeks.
  • Months later for the teratogenic consequences to be evident.

Epidemiology

  • The disease is able to spread quickly in a naïve herd as long as the favorable environmental conditions of the vector are maintained.
  • As herd immunity is established (either via natural infection or vaccination), virus re-emergence is not expected to happen for several years (3-6 according to studies on similar viruses of the Simbu serogroup), until the majority of herd immunity has waned at the same time as a return of favorable conditions for the vectors.
  • For example, low level virus circulation was detected in 2016-17 in the UK national flock and surveys carried out on bulk milk from dairy herds across the Country showed a variable level of positivity to SBV antibodies.

Diagnosis

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Treatment

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Prevention

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Outcomes

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Further Reading

Publications

Refereed Papers

  • Recent references from PubMed and VetMedResource.
  • Endalew, Abaineh D et al (2019) Schmallenberg disease - a newly emerged culicoides-borne viral disease of ruminants. Viruses 11 (11), 1065 PubMed.
  • Collins et al (2019) Schmallenberg virus: a systematic international literature review (2011-2019) from an Irish perspective. Ir Vet J 72, 9 PubMed.
  • Kęsik-Maliszewska J, Larska M, Collins Á B & Rola J (2019) Post-epidemic distribution of Schmallenberg virus in Culicoides arbovirus vectors in Poland. Viruses 11 (5), 447 PubMed.
  • Stokes J E, Tarlinton R E, Lovatt F, Baylis M, Carson A & Duncan J S (2018) Survey to determine the farm-level impact of Schmallenberg virus during the 2016–2017 United Kingdom lambing season. Vet Rec 183 (22), 690 PubMed.
  • Haider N, Cuellar A C, Kjær L J, Sørensen J H & Bødker R (2018) Microclimatic temperatures at Danish cattle farms, 2000-2016: quantifying the temporal and spatial variation in the transmission potential of Schmallenberg virus. Parasit Vectors 11 (1), 128 PubMed.
  • De Regge N (2017) Akabane, Aino and Schmallenberg virus-where do we stand and what do we know about the role of domestic ruminant hosts and Culicoides vectors in virus transmission and overwintering? Curr Opin Virol 27, 15–30 PubMed.
  • Claine F, Coupeau D, Wiggers L, Muylkens B & Kirschvink N (2015) Schmallenberg virus infection of ruminants: challenges and opportunities for veterinarians. Vet Med 6, 261–272 PubMed.
  • Wernike K, Holsteg M, Schirrmeier H, Hoffmann B & Beer M (2014) Natural infection of pregnant cows with Schmallenberg virus - a follow-up study. PLoS ONE 9 (5), e98223 PubMed.
  • Schulz C, Wernike K, Beer M & Hoffmann B (2014) Infectious Schmallenberg virus from bovine semen, Germany. Emerg Infect Dis 20 (2), 338–339 PubMed.
  • Elbers A, Stockhofe-Zurwieden N & van der Poel W (2014) Schmallenberg virus antibody persistence in adult cattle after natural infection and decay of maternal antibodies in calves. BMC Vet Res 10 (1), 103 PubMed.
  • Seehusen F, Hahn K, Herder V et al (2014) Skeletal muscle hypoplasia represents the only significant lesion in peripheral organs of ruminants infected with Schmallenberg virus during gestation. J Comp Pathol 151 (2-3), 148–152 PubMed.
  • Wernike K, Eschbaumer M, Schirrmeier H, Blohm U, Breithaupt A, Hoffmann B et al (2013) Oral exposure, reinfection and cellular immunity to Schmallenberg virus in cattle. Vet Microbiol 165 (1), 155–159 PubMed.
  • Wernike K, Eschbaumer M, Schirrmeier H et al (2013) Oral exposure, reinfection and cellular immunity to Schmallenberg virus in cattle. Vet Microbiol 165 (1-2), 155–159 PubMed.
  • Rasmussen L D, Kristensen B, Kirkeby C et al (2012) Culicoids as vectors of Schmallenberg virus. Emerg Infect Dis 18 (7), 1204–1206 PubMed.
  • Reusken C, van den Wijngaard C, van Beek P et al (2012) Lack of evidence for zoonotic transmission of Schmallenberg virus. Emerg Infect Dis 18 (11), 1746 PubMed.
  • Hoffmann B, Scheuch M, Hoper D, Jungblut R, Holsteg M, Schirrmeier H et al (2012) Novel orthobunyavirus in Cattle, Europe, 2011. Emerg Infect Dis 18 (3), 469–72 PubMed.

Other sources of information

  • Waine K, Busin V & Strugnell B (2019) Getting the Most out of On-Farm Post-Mortems: A Guide for Veterinary Surgeons. AHDB, UK. Website: https://ahdb.org.uk.
  • European Centre for Disease Prevention and Control (ECDC) Facts About Schmallenberg Virus. Website: www.ecdc.europa.eu.

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