Climate keeps tick-borne diseases in check

The blood-sucking ticks that spread microbes, causing disease in livestock and people, are very sensitive to the weather. So different sorts of microbes cycle between ticks and their hosts in the UK and in other parts of Europe where the summers are warmer and drier. This has obvious implications for the possible effects of global warming on the spread of vector-borne diseases, such as malaria, Oxford University scientists said today, Monday 29 March 2004, at the Society for General Microbiology’s meeting in Bath.

“For example, there are five distinct strains of viruses carried by the commonest tick species in Europe. Even though the animals they live on – woodmice, birds, deer, sheep, goats – are widespread, each strain of virus is restricted in its range because of climate”, says Professor Sarah Randolph of the Department of Zoology at the University of Oxford. “The viruses have evolved to cope with different seasonal patterns of warmth and dryness, because these affect how the carrier ticks interact with their animal hosts”.

Transfer of the virus between hosts occurs at long intervals, because the ticks feed only once during each stage of their life cycle. This means that these viruses evolve slowly, but changes in environmental conditions can occur much more rapidly, allowing them to shift their ranges, appearing in new places and disappearing in others.

The researcher’s findings explain why one of these viruses, which causes the brain inflammation called Western tick-borne encephalitis in humans, has failed to reach Britain through France, even though it is spread via common rodents. Instead, a related strain of the virus that infects sheep, louping ill, entered the UK via Ireland several hundred years ago, probably having spread originally from the highlands of northern Spain.

Viruses transmitted by ticks that infest sheep could have been widely spread by farmers’ livestock movements, but each new virus type has remained isolated, apparently trapped by specific weather requirements. Louping ill and tick-borne encephalitis viruses now appear to be getting closer to each other via Scandinavia.

“Each genetic strain of microbe has evolved to survive in particular hosts. These viruses evolve slowly as ticks only feed once during each stage in their life cycle, which imposes a strong brake on the speed they can spread or change”, says Prof Randolph. “Changes in environmental conditions occur much more rapidly and could allow these viruses to shift their ranges, appearing in new places and disappearing in others”.

“Only if we understand the current situation for such complex biological systems can we hope to control the diseases already threatening people in Western Europe, and predict how climate change might allow other diseases to spread in the future. Lessons learnt from tick-borne diseases may be applied to insect-borne diseases such as malaria and dengue fever”, says Prof Randolph.

“Contrary to the common knee-jerk reaction, recent marked increases in tick-borne encephalitis and Lyme disease are almost certainly not due solely to climate change. We need to focus on the right causes to achieve effective control”, says Prof Randolph.