The occurrence of the clusters in time was circumstantially associated with significant reductions in A. sylvaticus trapping rate that were contrary to a prevailing trend of demographic increase during the period of study. An additional, circumstantial, corollary was that an abrupt turnover between 1 + and 0 + A. sylvaticus cohorts occurred in the weeks following cluster 1, consistent with a perturbation affecting the population at this time. Over an equivalent time interval in the preceding year (2007), clusters of individuals with extreme TNF-a protein expression did not occur ( [18] ; unpublished data) and the turnover between 1 + and 0 + cohorts proceeded much more gradually (Figure 4) .