Latest research in tick-borne disease epidemiology and future questions
Friday seminar by Sarah Randolph (NOTE THE TIME AND VENUE)
Many different vector-borne diseases come and go around the world. New ones appear, old ones creep into new territories or erupt in incidence and may then persist at high levels or decline in significance. Emergence in the evolutionary sense involves genetic change in the microbe or the vector, opening up new biological opportunities for microbial circulation. My interest lies in emergence in the geographical sense, involving the sequential steps of arrival, establishment and spread of the biological partners. While permissive biological interactions between microbes, vectors and hosts are the sine qua non, we now recognise that such events are commonly triggered and facilitated inadvertently by human activities. Travel and trade around the world has a long history of transporting vectors and infectious agents. The exploitation of Siberia by the construction of roads from west to east in the 17th and 18th centuries allowed the reverse flow of Siberian strains of tick-borne encephalitis virus (TBEv), followed in the 20th century by the transport of Far-Eastern strains of TBEv in the 100,000s of wild animals that were moved from east to west to act as sustainable sources of food during the Soviet era. Commerce or politics, they both had unforeseen consequences.
European strains of TBEv cause thousands of cases of severe CNS pathology each year, appearing in new foci and at higher levels of incidence over the past two decades. This is one of the few vector-borne diseases whose epidemiology on a continental scale has been explained; discovery of the cellular and molecular basis of viral transmission between ticks feeding together on rodents lead to a definition of the environmental conditions necessary for persistent enzootic cycles. We can therefore be confident in concluding that variable rates of the biological processes that generate variable numbers of infected ticks in space and time are not sufficient to explain the observed epidemiological patterns in space and time. The variable contact rate between infected ticks and humans must also be taken into account: i.e. risk = hazard x exposure.
Best analysed to date are the abrupt and generally persistent increases in TBE incidence in central and eastern Europe that can be understood as an unforeseen consequence of post-Soviet socio-economics, as both poverty and wealth increased with the transition to the market economy; both increase exposure to ticks through greater reliance on natural foods and more recreational activity, respectively, in the tick’s forest habitat. Is the emergence and increasing incidence of TBE in the Nordic countries, on the latitudinal boundary of this disease’s range, due more to the impact of biological factors on hazard than the impact of human factors on exposure?
Superimposed on the general trends are occasional annual spikes in TBE incidence, sometimes caused by unusual weather (2006) and sometimes by economic crisis (2009), acting through abrupt changes in human activities rather than the slower responses of natural enzootic cycles. Identifying the high-risk sectors of the population should allow better targeting of intervention: TBE is almost entirely preventable by vaccination.
Sarah E. Randolph
Department of Zoology, University of Oxford, UK