Emerging animal diseases: from science to policy - Favv
50 2.2 Methodology Surveillance of WNV circulation can be performed at different levels of the WNV transmission cycle: birds, horses, mosquitoes, humans. It relies on passive surveillance systems (based on clinical diagnosis of WNV infection in horses or in humans, or on analysis of enhanced bird mortality rates) or by active surveillance (development of a specific epidemiological network for prospective WN virus and/or WN virus-antibody detection in birds, horses,...). Each surveillance system has to fit to epidemiological, climatic and geographical conditions in the exposed area. Most of the european countries have organised a passive surveillance, including a survey on human and horse neurological cases occurring during late springautumn, which is the preferential period of WNV infection in relation to favourable climatic conditions and high vector density in our temperate regions. In Europe, bird mortality cannot be considered currently as a good indicator of WNV activity since bird mortality related to WNV infection is rare, whereas it has been largely reported in Israel from 1998 and in North America from 1999 (Deubel et al., 2001). Seroconversion surveys on naïve bird (captive sentinel birds, domestic poultry, and identified wild birds) and horse populations have also been used to demonstrate WNV circulation (Murgue et al., 2001). This active surveillance seems particularly indicated in those areas where outbreaks have occurred and/or in areas considered especially at risk because of their climatic and/or land use characteristics (river deltas, frequently flooded areas,..). Mosquito collections in areas where newly infected animals have been detected are important for isolating and characterizing circulating WNV strain. However, mosquito collections are not a practical method for routine surveillance of transmission, because this costly method lacks some sensitivity. 2.3 Results of West Nile virus surveillance in Europe We’ll summarize studies that have been undertaken to provide incidence data on WNV in Europe. Infection outbreaks Since the 1990’s, two major human outbreaks have been reported, in Bucharest, Romania in 1996 and in Volgograd, Russia in 1999. Reinforced surveillance the following years identified some more cases each year (see the following table), suggesting that in those areas, the virus could be endemic. Human WNV infection foci have also been identified in Czech Republic, France, Hungary, Portugal and Spain. Small outbreaks involving only horses have been described in Italy in 1998 and in France along the Mediterranean cost in 2000, 2004 and 2006. In Europe, WN disease outbreaks are erratic and spatially and temporary limited phenomena, occurring quite unpredictably. Evidence of WNV circulation WNV presence has been evidenced in Eastern and Southern Europe (Koopmans et al., 2007). The low rate of clinically apparent infections imposes serological surveillance of the population to evaluate true prevalence of WNV infection. Significant percentage of serologically positive persons have been found in different surveys in regions with reported WNV activity (Romania, Spain, Russia) (Lozano & Filipe, 1998, Bofill et al., 2006, Cernescu et al., 2000, Platonov et al., 2001).
A broad range of seroprevalence against WNV (1-15%) amongst horses was described in France, Spain, Croatia, Romania and Russia (Jimenez-Clavero et al., 2007, Ozkul et al., 2006, Vasil’ev et al., 2005, Madic et al., 2007, Murgue et al., 2001). Surveillance studies of neurological cases in humans and horses in the UK and The Netherlands did not find any evidence for WNV circulation (Rockx et al., 2006, Morgan, 2006). No WNV activity could be demonstrated in recent reports from Austria, Germany or the United Kingdom (Weissenbock et al., 2003, Phipps et al., 2008, Linke et al.? 2007). Moreover, WNV has also been detected in mosquito pools in France, Portugal, Moldavia, Romania, Slovenia, Czech Republic, Ukraine and Belarus (Koopmans et al., 2007). 51