Schmallenberg virus update

Tarlinton, R.

Abstract

Since emerging in Europe in 2011 Schmallenberg Virus (SBV) has affected over 6000 holdings over most of the continent. The virus causes mild disease in adult ruminant animals (fever, milk drop and diarrhoea) but readily crosses the placenta of sheep, cattle and goats to replicate in fetal neurological tissue causing distinctive deformities including hydranencephaly and arthrogryposis dependent on the stage of fetal development at infection (Tarlinton and others 2012). The virus is genetically part of the Simbu serogroup of Orthobunyaviridae which includes viruses such as Akabane virus which have been responsible for sporadic large scale outbreaks of very similar foetal deformities in Australia and Japan. Much of the predictions about SBVs spread and impact have been extrapolated from knowledge of Akabane. Both viruses are transmitted by Culicoides sp. biting midges; however, Culicoides brevitarsis, the main vector of Akabane in Australia, is a tropical species, the virus is endemic in Northern areas of Australia and most animals are infected and become immune before their first pregnancy. Large outbreaks occur during unusual weather conditions where the vectors host range is altered. C. obsoletus, one of the species identified as transmitting SBV, is a temperate species with a range that extends over the whole of Europe (Tarlinton and others 2012). Since emerging SBV has overwintered and gradually spread out over the range of the midges. It is expected to continue to do so and to become endemic in Europe (European Food Safety Authority 2012). Individual holdings have reported loss rates of lambs, kids and calves in excess of 30% (Jeckel and others 2012), though loss rates vary greatly from holding to holding with official estimates (based on cases reported through government laboratories) are that less than 8% of herds in any one region have been affected (European Food Safety Authority 2012). Maternal losses due to birthing trauma and noticeable drops in milk production in dairy animals have also added to farm financial losses due to SBV (Tarlinton, unpublished data). Akabane outbreaks in Japan are managed by strategic deployment of vaccines. Quite crude killed adjuvanted vaccines for Akabane usually produce an effective immune response against viral infection (Tarlinton and others 2012). At least one killed adjuvanted SBV vaccine has been submitted to the Veterinary Medicines Directorate and when this becomes available will give real management options for minimising the impact of the virus. There remain several unanswered problems with the virus, particularly in the areas where it differs from Akabane, that make assessment of the long term impact of the virus difficult. SBV is expected to become endemic, though loss rates from year to year will depend on what level of sero-conversion occurs at the herd level annually (this may be affected by weather conditions that affect midge abundance). Initial results indicate that some UK herds (particularly high production indoor herds) have sero-conversion rates as low as 25%, indicating that they will be vulnerable to ongoing problems with SBV unless vaccinated (Tarlinton and Daly 2013). The effects of SBV on early fertility is also controversial as Akabane is not thought to affect pregnancy in the first 28 days (for sheep) or first trimester (for cattle). Anecdotal reports of poor fertility in sheep and cattle in summer and autumn 2012 have been attributed to SBV, though are difficult to untangle from effects due to nutrition and other infectious diseases. This is certainly an area of research that needs further work. Another area of potentially greater long term impact is that infectious SBV can be detected in bull semen for some time (up to 60 days at least) after initial infection (Pro-med mail archive number 20130123.1511878) with implications for trade that have resulted in many countries outside the EU imposing trade bans on semen, embryos and live animals. This presentation will give an update on the current clinical and epidemiological situation with SBV in Europe as well as an update on current research on the virus.

Citation

Tarlinton, R. (2014). Schmallenberg virus update. Cattle Practice, 22, 65

• Journal Article Type: Article
• Publication Date: Jan 1, 2014
• Deposit Date: Feb 28, 2025
• Journal: Cattle Practice
• Electronic ISSN: 0969-1251
• Peer Reviewed: Not Peer Reviewed
• Volume: 22
• Pages: 65
• Public URL: https://nottingham-repository.worktribe.com/output/3116398
• Related Public URLs: https://www.researchgate.net/publication/287076050_Schmallenberg_Virus_Update