'Vector-borne' infections are transmitted between humans and animals by an in-between 'vector' or agent. These vectors are often ticks and mosquitoes, but mammals (usually rodents) can also be responsible. Weather affects the population dynamics of the hosts and the transmission of the disease, with temperature and humidity having key influence.
The researchers investigated the rise of NE in Belgium. NE is present throughout most of Europe and common symptoms include high fever, headache and nausea, but it can have more serious consequences. It has become the most important cause of infectious acute kidney failure in Belgium and is fatal in extreme cases.
The NE virus is carried by rodents, particularly the bank vole. It is passed on to humans via airborne traces of infectious excrement. The bank vole's staple food is 'mast' or seeds of deciduous broad-leaf trees, such as acorns and beechnuts. Recent high levels of mast, possibly the result of climate change, could provide an explanation for the increases in NE, via an increased autumn food supply for the bank vole.
The study examined fluctuations in NE, changes in 'mast' abundance and climatic data from 1985 to 2007. The results suggest recent changes in climate have encouraged the spread of NE by bank voles in Belgium. The combined effect of increasingly hotter summers and autumns is seen to match a growth in incidences of NE in recent years.
A total of 2,048 NE cases were registered during this 23 year period, with numbers increasing throughout. The record year in 2005 saw 372 cases, which was considered a near-epidemic for Belgium. This situation has also been confirmed in neighbouring countries such as Germany, where a record 1,687 NE cases were officially registered in 2007.
From 1993 onwards, all NE peaks were preceded by an abundance of mast in the previous autumn. The average temperature in the period 1996-2007 of 11.4°C was significantly higher than the previous decade (10.7°C), although rainfall was similar in the two periods. This suggests that a high availability of mast, and therefore a higher survival of bank voles in autumn-winter, provides a link between climate change and fluctuations in NE.
By investigating data from each season, the study demonstrated a number of significant relationships between cases of NE and climate. For example, outbreaks of NE appear to be influenced by the following:
• A cold and moist summer three years previously causing growth of trees and abundant mast two years later
• A hot summer two years previously causing stimulation of bud formation and abundant mast one year later
• A warm autumn one year previously promoting the survival of bank voles
This demonstrates that simple climate variables can help predict increases in NE and inform prevention policies by health authorities. The authors also suggest that climate change could affect other emerging infections in Western Europe, such as Lyme disease, carried by both ticks and rodents, and which can affect the joints, nervous system and heart.