Huge Rise in Tick-Borne Diseases
©2004 Simon Hadlington, The Independent (UK) 4 February 2004
If you go down to the woods today you could be in for a nasty surprise: ticks are on the increase. New research by scientists from the University of Oxford suggests that these small, eight-legged, blood-sucking parasites - part of the mite family - are becoming more abundant, infecting cattle, sheep and humans with a range of diseases.
While the situation in the UK would seem to be worsening, in parts of central and eastern Europe the explosion of tick-borne disease over the past 10 years has been nothing less than dramatic, and it appears to have something to do with the collapse of Communism.
Ticks are "vectors" of disease, picking up pathogens from one host animal and passing them on to another. In many tropical parts of the world, insects such as mosquitoes are the main vectors of disease, but in temperate Europe, it is ticks who are the most significant carriers.
Across the world there are about 850 species of tick. Some 20 of these are found in the UK, of which the sheep tick, Ixodus ricinus, is the most troublesome. This transmits five disease-causing agents, including those that cause red water fever in cattle, and in sheep a disease called Louping Ill and a condition known as tick fever.
Humans are also at risk from sheep ticks, which transmit the bacteria Borrelia burghdorferi that can lead to Lyme disease. In serious cases, this can result in the paralysis of certain parts of the body. In some countries of mainland Europe, the most serious illness caused by a virus transmitted by ticks is tick-borne encephalitis (TBE), which kills one in every hundred people infected.
Professor Sarah Randolph is the head of the Oxford Tick Research Group, a team of scientists attempting to understand the complex web of factors that influence how populations of ticks fluctuate, and how this relates to patterns of infection. "What we want to do in our research is to understand all the relevant factors in the transmission of disease by ticks," she says, "from the nitty-gritty of the biology of the organisms through to the bigger epidemiological picture of patterns of disease, and all the steps in between."
In this country, many reports suggest that ticks are becoming more abundant. Precise figures are difficult to obtain, but the reported incidences of Lyme disease are increasing. About 300 cases were recorded in England and Wales in 2000, compared with about 50 in 1990. While a proportion of the rise is due to more rigorous diagnosis and reporting, it is also likely, Professor Randolph says, that there has been a real and steep increase in the number of cases here, as in the rest of Europe and the US. "In fact, if we look at what we know about the numbers of infected ticks around, there should be many more cases of Lyme disease, which suggests that cases are being under-recorded."
There is anecdotal evidence, too, that farmers are suffering more than in the past, with sheep farmers reporting that ticks are more of a problem than they used to be.
In parts of Europe, the rise in incidences of TBE has been astronomical. "We have seen a massive increase in TBE in eastern Europe since the fall of Communism," says Professor Randolph. "Curiously, it appears that the more authoritarian the previous regime the greater the increase. This seems to suggest that there are some sociological factors at work."
The Oxford researchers have just started a new project investigating why these changes have occurred. One possibility is that when the regimes crumbled and employment collapsed, poorer people began to frequent tick-infested woodland, using it as a source for food. On the other hand, wealthier people, enjoying new-found freedoms, might have used the woodlands for leisure pursuits. Either scenario would have brought greater numbers of people into contact with ticks.
In order to build a complete picture of the spread of tick-borne disease, the team has constructed a detailed account of the organisms' ecology, based on hundreds of hours of work in the laboratory and the field.
"The sheep tick needs moist, shady conditions and is susceptible to drying out," says Professor Randolph. "It loves woodland and bracken, for example." There are four stages in its life cycle: egg, larva, nymph and adult. The egg hatches out in the early spring into a larva just visible to the naked eye. The larva attaches itself to a passing bird or mammal, feeds on its blood for three or four days and then drops off into the litter. Some months later, possibly having over-wintered in a state of suspended animation known as diapause, it emerges at the nymphal stage. This nymph, the size of a pinhead, will repeat the process of attaching itself to a host, feeding and dropping off to mature over several months into the next stage - the adult.
"The adult is two or three millimetres across and flat when it is unfed, but during feeding it swells to the size of a baked bean," says Professor Randolph. "It is this that people often find attached to their dogs after a walk in the woods."
Disease is spread by an infected stage of the tick feeding on its host and passing the pathogen, be it virus, bacterium or protozoan, into the host's body. Another tick then picks up the pathogen when it feeds. It then passes it on when it feeds again once it has matured into its next stage.
One of the most important factors about the tick's life cycle is that each stage feeds only once. "In this respect, there are fundamental differences between insects and ticks in terms of the transmission of disease," says Professor Randolph. "Whereas an insect such as a mosquito might feed every three or four days, making it extremely efficient at transmitting a disease, this is not the case with ticks. A tick could take several months to transmit an infection. People have tried to use models derived from insects to predict tick-based transmission systems, but this does not work."
In an effort to understand the dynamics of the tick population, the Oxford researchers are building mathematical models based on detailed information about local climate conditions in a given tick-infested area.
"We know that the microclimate - the conditions on the ground in the immediate environment of the organisms - is extremely important in the tick's life cycle," says Professor Randolph. "By using meteorological data from satellites we can obtain consistent updated information about these conditions from one month to the next and we can correlate this information with what is happening to the population."
The researchers have discovered, for example, that there is a consistent correlation between how rapidly the temperature cools in the autumn and the feeding patterns of the larval and nymphal stages of the tick the following year - something that is important in the transmission of TBE.
Interestingly, the predicted patterns of global climate change could restrict the areas where ticks are active. Warmer, drier summer conditions could make it impossible for populations to survive in some of the southern extremities of their current habitat.
"Ultimately, we would like to be able to build a model that can predict the population dynamics of the tick to show where they are likely to transmit pathogens - something that is called predictive risk mapping," says Professor Randolph. By feeding in information such as seasonal temperatures and moisture levels, it might be possible to predict the potential "infectivity" of a population of ticks in a given location.
"I feel strongly that the role of ticks in the spread of disease is an important area that has not received the attention it merits," Professor Randolph says. "Vector-borne diseases are bad enough in the northern hemisphere, but in Africa they have scarred the entire continent. Diseases carried by ticks and the tsetse fly have made it impossible for Africans to keep livestock successfully, and this has had a devastating impact on the history of the continent. If it impossible to keep oxen, the farmers cannot till the soil efficiently to plant crops nor maintain soil fertility with animal dung. It is something that the West seems largely to have turned its back on. I would like to think that the work we are doing might be able to help solve these problems."
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