Ticks and politicsThe rise of tick-borne diseases in Eastern EuropeRecent rises in tick-borne diseases across the world are not just a matter for medicine. |
To borrow a phrase from history, the epidemiology of tick-borne diseases is 'a riddle wrapped in a mystery inside an enigma'. The biology of the tick, its hosts and the pathogens it transmits are complex enough. Add to that mix some human behaviour and high politics, and you have a major scientific challenge. One that ecologist Professor Sarah Randolph of the Oxford Tick Research Group has enthusiastically taken on.
Professor Randolph, previously a Wellcome Senior Research Fellow, has been awarded a Wellcome project grant to explore the dramatic rise in tick-borne diseases (specifically tick-borne encephalitis, TBE, and Lyme disease) in the Baltic States - Latvia, Lithuania and Estonia. In the early 1990s, these countries saw increases in TBE incidence of up to 22-fold in just one year, far higher than elsewhere in the ticks' natural range. Professor Randolph aims to test the hypothesis that the increase is at least partly linked to the collapse of communism and the social upheaval that followed. Proving this requires disentangling the real from the apparent, the biological from the non-biological.
The march of the ticks
Tick-borne diseases are on the increase across the northern hemisphere, but the pattern of emergence is anything but uniform. So attributing the rise to the currently fashionable 'cause' of climate change is, Professor Randolph argues, far too simplistic. "There is no question at all that vector-borne diseases, tick-borne diseases in particular, are hugely affected by climate. But it is absolutely questionable that all the 'bad' scenarios we see are due to climate change. It needs to be tested. There may be quite different explanations."
For some years now, Professor Randolph and her group have been investigating the biological processes that drive tick populations and thus the dynamics of the diseases they carry. They are, she says, "inches away from having a model that works", that can help them predict how tick populations will respond to key environmental variables.
Professor Randolph uses highly innovative techniques, developed by her husband Professor David Rogers, which exploit meteorological satellite imagery to provide vital temperature and moisture data over space and time. These can be matched against disease incidence reports to look for explanations for the patterns of emergence and change. "Along the southern edge of the tick's distribution, for example, increasingly dry summers will play a important role in determining limits to the disease. This is because moisture conditions are critical to tick survival."
"But," Professor Randolph stresses, satellite imagery can only take you so far. The real answers lie on the ground, in the study of the ticks themselves and their hosts, including humans. Even under similar climatic conditions, the same vector and the same host range can give you highly variable epidemiological patterns."
To explain these patterns, Professor Randolph aims to disentangle the multitude of contributory factors. "You have to understand and quantify the processes if you want to get a proper explanation of what's going on. All biological systems are complex, built up of similar processes but operating at different rates and in different combinations, so the outcome is hugely variable. TBE across Europe is a great example of this. In Sweden, for example, a doubling in incidence in 1984-86 and again in 2000 contrasts with extraordinarily sudden increases in 1993 in Latvia and Lithuania but decreases since 1997 in Hungary - changing climate and host availability may be important, but variable sociological factors may shape the outcome."
Human behaviour
The ecology of the tick is complex enough, but another factor may be having an even greater impact on disease: what people are doing. Looking at charts plotting the incidence in reported TBE, the dramatic rise within a year or two of the fall of communism is obvious, and Professor Randolph was not the first to see it. But the differences between countries are equally striking - in the Czech Republic, for example, which was never so disadvantaged under communism, TBE roughly doubled. But in Poland and the Baltic States, the increase was much more dramatic at the point of communist collapse. But correlation is not causation. "As it happens, there was also a temperature leap a few years before communism collapsed, and it is not unreasonable that, after a time lag, there may have been a major response to a minor change in temperature. The increase in TBE in Sweden, however, occurred before the rise in temperature."
How do you go about quantifying the effects of political change? Professor Randolph happily admits this is new territory for her, and is clearly relishing the challenge. She has had the good fortune to find a Latvian assistant, Dana Sumilo, and so gains unprecedented access to the excellent public health records in the Baltic States.
As communism collapsed, both poverty and wealth increased. People turned to small-scale private farming, taking them and their livestock into tick-infested forests and fields. They also went to gather berries and mushrooms to supplement their diet. Among the more wealthy, outdoor leisure activities increased. Sumilo will interrogate public health archives and also look for other measures of social and behavioural change that can be quantified. "You have to ask why people might go into tick-infested places, and then think of the sort of indices that might be relevant, that may be recorded in the archives."
Professor Randolph is convinced that such first-hand evidence is essential. "You need someone on the ground. It's no good sitting in Oxford and having some airy-fairy notion of life in Eastern Europe. You're in danger of making simplistic assumptions and interpretations of the data." As an example, she points to a graph showing a sudden fall in TBE in Hungary. "I've just learned that, at that time, people there had to start paying for TBE diagnosis - so is this a real fall in incidence, or just a fall in reporting?"
It's this attention to details that marks Professor Randolph out. She is as far away from 'airy fairy' as it is possible for someone to be. She sets herself and others very high standards, and has no time for simplistic solutions to complex problems. Simple, yes - but simplification, she knows, comes only from proper, deep understanding. If the dynamics of tick-borne disease is the enigma, she is well on track to crack the code.
More correctly called Lyme borreliosis, Lyme disease is the most commonly reported tick-borne infection in Europe and North America. The term ‘Lyme disease’ was first used following a cluster of rheumatoid arthritis cases in young people in the town of Old Lyme, Connecticut, USA, in the mid-1970s. The disease, though not its biology, has been known in Europe since the 1880s.
Borrelia burgdorferi, a type of spirochaete (thin spiral-shaped bacteria), transmitted through tick bites. In Europe the main vector is Ixodes ricinus, the sheep or castor bean tick.
It often begins with flu-like feelings of headache, stiff neck, fever, muscle aches and tiredness. About 60 per cent of people notice an expanding rash days or weeks later – a red blotch or widening ‘bullseye’ ring round the bite point. The disease should be treated at this stage as further symptoms may develop up to a year later – from severe headaches, arthritis and swollen joints to heart problems, memory loss and difficulty in concentrating.
If caught early, the disease is treatable with antibiotics; however, later stage disease does not respond as well.
Also known as spring–summer encephalitis, TBE is a viral infection transmitted by Ixodes ricinus ticks. The virus can also be passed on in untreated milk and milk products from infected cows, goats, or sheep. It occurs mainly in Scandinavia, central and eastern Europe, and countries of the former Soviet Union. A closely related disease occurs in China, Korea, Japan, and eastern areas of Russia.
After 1–8 days, symptoms such as fever, headache and muscle pain. A month later a quarter of patients develop a second stage involving central nervous system disorders such as meningitis or encephalitis. In about 1 per cent of cases, this can be fatal. A few patients may experience longer-term balance and coordination problems, headaches and hearing defects.
With immunoglobulin injections, soon after bite.
See also
- Professor Sarah Randolph of the Oxford Tick Research Group at the University of Oxford: Research interests
Further reading
Randolph S E (1998) Ticks are not insects: consequences of contrasting vector biology for transmission potential. Parasitol. Today 14: 186-92
Hay S I, Randolph S E, Rogers D J (eds) (2000) Remote Sensing and Geographical Information Systems in Epidemiology. Academic Press, London. 357 pp
Randolph S E (2001) The shifting landscape of tick-borne zoonoses: tick-borne encephalitis and Lyme borreliosis in Europe. Phil. Trans. Roy. Soc. B 356: 1045-56
Rogers D J, Randolph S E, Snow R W, Hay S I (2002) Satellite imagery in the study and forecast of malaria. Nature 415: 710-15
Rogers D J, Randolph S E (2003) Studying the global distribution of infectious diseases using GIS and RS. Nature Reviews Microbiology 1: (in press)