Climate change not always to blame for disease outbreaks - Veterinary Practice
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InFocus

Climate change not always to blame for disease outbreaks

DON’T blame climate change
every time there is a new outbreak
of a parasitic disease in Europe,
the real reason for the changing
patterns of infection in humans
and animals may be much more
subtle, Sarah Randolph of the
University of Oxford told the
VAWM meeting.

Professor Randolph, from the
department of zoology, is an expert on
the factors determining the spread of
tick-borne diseases and led a European
Union-funded project looking at the
increasing risks of human tick-borne
encephalitis infections.

The disease is fatal in around 1%
of cases and “there has been an order
of magnitude rise in the number of
cases across many countries in central
and eastern Europe over the past two
decades”, she said.

Yet there has been considerable
variation between countries in the pattern of human cases which do not
appear to be related solely to local
environmental conditions and the
project attempted to explain the
reasons why.

Its findings show that economic
and social changes have just as
important an influence on the
epidemiology of TBE as changes in
the population of disease vector, Ixodes
ricinus
, she explained. So these factors
must be understood if attempts to
reduce the numbers of human cases
are to succeed.

Changing temperature can certainly
have an impact on the abundance of
ticks, she acknowledged.

Unlike blood-sucking insect
species, ticks only feed once during
each life-cycle stage and between feeds
they develop in the undergrowth
where they may experience high
mortality in adverse conditions;
however, the risk of humans or domestic animals becoming infected
with the viral or bacterial diseases
carried by ticks is a combination of
biological and behavioural factors.

But even when those biological
factors would appear to increase the
threat of disease transmission to
humans, the relationships may also be
more complex than they might first
appear.

Part of the study looked at Ixodes
population in the UK which is not
currently a focus of TBE but has
experienced increasing numbers of
another condition transmitted by ticks,
Lyme disease.

In Wales the population of ticks
found in the undergrowth increased
with the growth in population of the
main wildlife host, deer.

Yet in Dorset the study showed
that there were fewer ticks where deer
were most abundant. It appears that
beyond a certain population density
for the deer, “questing” ticks find a host so quickly that few remain in the
undergrowth to pose a risk to human
health.

Light rather than heat

Even where environmental factors
were shown to be affecting the tick
population, the study’s findings were
surprising. Long-term analysis of tick
numbers suggested that the sharp
increase in the population from
around 1990 onwards was a result of
increased light levels rather than
temperature.

Tick numbers appear to have
increased in line with reduced
industrial pollution following the
collapse of the Soviet system. At the
same time, eastern European
agriculture also suffered a decline and
much agricultural land reverted to
scrub, providing a more suitable
environment for the ticks.

Economic factors had an even
greater influence on the risks of human exposure to these ticks. There
were both winners and losers in the
changes which followed the adoption
of a market economy by former
Warsaw bloc countries and disease
cases occurred most often at both
ends of the social spectrum.

The newly wealthy members of
society enjoyed more opportunities for
the sort of rural leisure activities that
would bring them into contact with
the ticks. At the same time the poor
became dependent on foraging for
food in forest areas to supplement
their diet.

Examination of medical records in
Poland showed that there was a
disproportionately high number of
cases of TBE in the unemployed.

So the political response to the
increasing numbers of TBE cases
needed to focus on providing low-cost
vaccination services to protect these
vulnerable groups against this readily
preventable disease, she said.

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