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Comment on SBV epidemiology

From ProMed Mail Sun 4 Mar 2012

Subject: Epidemiology of Schmallenberg virus (SBV) infection

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I am concerned that some reports in the media give an imprecise explanation of SBV epidemiology and tend to suggest that SBV is transmitting between lambs in affected flocks and spreading between flocks by this means; I would suggest that more needs to be done to ensure that the public is better informed on the epidemiology of this disease. I would also like to make some comments about Europe's vulnerability to virus invasions.

As is common knowledge, the epidemiology of SBV infection seems to be similar to that of Akabane virus and other orthobunyaviruses of the Simbu Group which affect ruminants. Useful insights can be gained from a global view of Akabane disease over the last 40 years or so since its 1st description. Akabane virus infection is endemic in many tropical and subtropical areas. In these, susceptible ruminant species become infected at an early age when fed on by ubiquitous midge and mosquito vectors and develop a long-lasting protective immunity by the time of breeding; thus, congenital abnormalities are seldom seen in endemic areas even though cases can occur when naive, susceptible animals are introduced into such areas. Under favourable environmental conditions, the vectors (and hence the virus) may spread beyond their usual range, and outbreaks of congenital disease then occur in areas where the disease process has rarely, or never been experienced before.

Essentially it is at the interface between free and endemic areas that severe outbreaks are to be expected. Acute infection following postnatal exposure, accompanied by unremarkable clinical signs, is usually not recognised to have occurred until malformed lambs and calves are born. By this time the virus is no longer present in either dam or offspring; antibodies disclose the presence of earlier infection. Akabane viruses vary in their virulence but only rarely has pathology been associated with post-natal infection (ref 1).

Outbreaks at the interface tend to occur infrequently but are recognised to have occurred in Japan, Korea, Taiwan, Australia, Israel, and Turkey (ref 1) and southern Africa to the north and south of the endemic zone. Repeated outbreaks in Turkey and Israel over the last 40 years indicate clearly that, at least from 1970 until 2010 or 2011, the eastern Mediterranean was situated at the interface between endemic and free areas.

Now that a Simbu Group virus, like bluetongue viruses in 2006 to 2010, has made the leap northwards in Europe, and without understanding the determinants that favoured the change, it is impossible to predict what will happen next. Will the virus "burn out" and the earlier status be re-established (whatever that was); are we to see a permanent shift in the interface between endemic and free zones; or, alternatively, will the epidemic wave move on as vector range extends or previously free populations of vectors become infected for the 1st time? It has been reported that SBV is most closely related to Shamonda virus, one of a number of Simbu Group viruses detected in Nigeria in the 1960s. Having not been seen since, emergence of the virus in Japan in 2002 (ref 3) illustrated that we know little of the epidemiology of such potential pathogens. One thing is clear -- our vision of the threat posed by the presence of these viruses at the edge of Europe was too complacent by far.

Several other viruses are in a similar situation to that of SBV and pose an immediate threat to the livestock of northern Europe. Not least of these are the orbiviruses causing epizootic haemorrhagic disease [EHD] of ruminants, African horse sickness [AHS], and equine encephalosis [EE]. Enhanced surveillance and applied research are surely essential at this time to prepare for the introduction of new vectorborne diseases which now seems inevitable.

References
1. Kono R, Hirata M, et al. Bovine epizootic encephalomyelitis caused by Akabane virus in southern Japan. BMC Veterinary Research 2008; 4: 20.
2. Taylor WP, Mellor PS. The distribution of Akabane virus in the Middle East. Epidemiol Infect. 1994; 113: 175-85.
3. Yanase T, Maeda K, et al. The resurgence of Shamonda virus, an African Simbu group virus of the genus _Orthobunyavirus_, in Japan. Archives of Virology 2005; 150: 361-9.

Peter Roeder OBE, Hon FRCVS, PhD, MSc, BVetMed, MRCVS
Taurus Animal Health
Hampshire