- 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.
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- 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.
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- 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.
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- 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.
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- 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.
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- 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."
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- 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."
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- 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.
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- 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."
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- 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.
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- 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.
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- "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.
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- "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."
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- 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.
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- 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."
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- 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.
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- "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."
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- 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.
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- 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.
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- "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.
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- "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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- © 2004 Independent Digital (UK) Ltd
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- http://news.independent.co.uk/world/science_medical/story.jsp?story=494696
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