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Before attributing cause and consequence to climate change, the precise patterns of change must be known. Ground records across much of Europe show a 1-2 °C rise in temperatures in 1989 with no significant rise since then. The timing and spatial uniformity of this pattern, relative to changes in the distribution and incidence of many vector-borne diseases, are sufficient to falsify most simple claims that climate change is the principal cause of disease emergence. Furthermore, age-specific increases in incidence indicate causes other than, or in addition to, climate change. Unfortunately, many public health professionals repeat the received wisdom that climate change is worsening the burden of indirectly transmitted infections; this 'expert opinion' soon becomes consensus dogma divorced from quantitative evidence. The pressing need is to gather appropriate data to test the simple concept that the composition and relative importance of disparate multifactorial factors, commonly integrated within a causal nexus, will inevitably vary with the geographical, cultural, socio-economical, wildlife, etc. context. The greatest impact of warming occurs at the geographical limits of current distributions, where low temperatures limit the hazard of infected vectors. Within core endemic regions, changing exposure of humans to this hazard, through changing socio-economic factors is evidently more important amongst both the poor and the wealthy.

During the 1990s, Milan Labuda's experimental results established a new paradigm for the study of tick-borne viruses that has since been strengthened by its demonstrated effectiveness in explaining the epidemiology of tick-borne encephalitis (TBE). This brief review summarizes the essential features of the transmission of tick-borne pathogens such as TBE virus. Leukocytes migrate between tick feeding sites, bearing infective virions and providing a transport route for the virus between co-feeding ticks independent of a systemic viraemia. Such tick-borne pathogens are thus transmitted from tick to tick via vertebrates; the ticks are the reservoirs as well as the vectors, while the vertebrate is the transient bridge. The aim is to bring the related but non-synonymous terms (co-feeding and non-systemic) to the attention of workers who use simple PCR screening to identify additional vertebrate reservoir hosts of vector-borne pathogens that are not in fact maintained in nature through systemic transmission., (Copyright © 2011 Elsevier GmbH. All rights reserved.)

Although the presence of tick-borne encephalitis (TBE) virus circulating in tick populations depends on large-scale patterns of climate, and the local density of infected ticks depends on the abundance of mammalian hosts, the risk of human infection depends on the access and use by human populations of tick-infested habitats, particularly forests, at the landscape level. We investigated the incidence of reported TBE cases in rural parishes (i.e., municipalities) in Latvia. The following major characteristics of parishes were considered: whether their environment is suitable for tick and tick-host populations (depending on land cover); whether the local human population is likely to enter the forest on a regular base (depending on land use); and whether the spatial distributions of these two aspects are likely to intersect, through access rules (as a function of land ownership). The results indicated that all three aspects are important in explaining and predicting the spatial distribution of TBE cases in the rural areas of Latvia. The concept of landscape is here given new depth by consideration of its physical structure, its use by human populations, and its accessibility as modulated by ownership.

There is no doubt that all vector-borne diseases are very sensitive to climatic conditions. Many such diseases have shown marked increases in both distribution and incidence during the past few decades, just as human-induced climate change is thought to have exceeded random fluctuations. This coincidence has led to the general perception that climate change has driven disease emergence, but climate change is the inevitable backdrop for all recent events, without implying causality. Coincidence and causality can be disentangled using tick-borne encephalitis (TBE) as a test case, based on the excellent long-term data for this medically significant European disease system. Detailed analysis of climate records since 1970 has revealed abrupt temperature increases just prior to the dramatic upsurge in TBE incidence in many parts of central and eastern Europe. Furthermore, the seasonal patterns of this temperature change are such as might have favoured the transmission of TBE virus between co-feeding ticks. Nevertheless, the pattern of climate change is too uniform to explain the marked heterogeneity in the timing and degree of TBE upsurge, for example in different counties within each of the Baltic countries. Recent decreases as well as increases in TBE incidence must also be taken into account. Instead of a single cause, a network of interacting factors, acting synergistically but with differential force in space and time, would generate this epidemiological heterogeneity. From analysis of past and present events, it appears that human behavioural factors have played a more significant role than purely biological enzootic factors, although there is an explicit causal linkage from one to the other. This includes a range of abiotic and biotic environmental factors, together with human behaviour determined by socio-economic conditions. Many of the abrupt changes followed from the shift from planned to market economies with the fall of Soviet rule. Comparisons between eight countries have indeed revealed a remarkable correlation between poverty indicators and the relative degree of upsurge in TBE from 1993. Against this background of longer-term shifts in TBE incidence, sudden spikes in incidence appear to be due to exceptional weather conditions affecting people's behaviour, which have a differential impact depending on socio-economic factors. This new perspective may also help explain the epidemiology of Crimean-Congo haemorrhagic fever around the eastern Mediterranean region, including the current exceptional epidemic in Turkey.

The abilities of the most common European genospecies of Borrelia burgdorferi sensu lato to survive blood meals taken by ticks feeding on birds were analyzed. A pattern of differential survival of the spirochetes in feeding ticks was observed. The result is consistent with the concept of selective transmission of Lyme borreliosis spirochetes.

Vector-borne pathogen systems are limited by climate, principally through its impact on the distribution, abundance and population dynamics of the vectors and on the interaction of vectors with their hosts. This leads to the prediction that climate has been a significant factor in the evolution of these pathogens and, by implication, will be important in future events on both evolutionary and ecological time-scales. New phylogenetic trees of organisms, that describe their evolutionary history, are routinely available, but explanations for the driving force and constraints that have shaped the evolutionary origin of new pathogen strains are rare. Under the force of environmental pressures that change in space and time, which pathogens are likely to emerge in new places and where will this be? To answer such questions, data from molecular biology can be linked with data from satellites that allow us to characterize environmental conditions on a global scale. Broad-scale patterns in the satellite imagery are consistent with the pattern of new tick-borne flaviviruses having evolved sequentially in a cline across the world. So far, the distribution of three tick-borne flaviviruses from western Europe have been captured simultaneously by the same 10 satellite-derived seasonal climatic variables. Once this procedure is extended to the remaining viruses in this clade, we shall test for significant matches between their phylogenetic tree and an independent tree derived from quantitative descriptions of the viruses' 'eco-space'. This will indicate the extent of key ecological drivers for specific evolutionary events, whose biological basis can then be explored in detail.