Memorandum 39
Supplementary submission from the Institute
for Animal Health
EFFECT OF
CLIMATE-CHANGE
ON VECTOR-BORNE
DISEASES
Introduction:
The Intergovernmental Panel on climate-change
(IPCC) has predicted that in the early part of the 21st Century
temperatures will increase, by up to 6.2oC and that maximum warming
will occur at higher latitudes and during the winter, and that
night time temperatures will increase more than day time. It is
also predicted that precipitation (rainfall) patterns will change
and that there will be an increase in extreme weather events.
Current climatic changes seem to be in line with these predictions.
Some vector-borne (ie insect-borne) diseases
are likely to be among those most strongly affected by such changes
because insect vectors:
— Are poikilothermic (cold-blooded)
and therefore respond directly and quickly to changes in temperature.
— Are small and easily desiccated,
therefore dying rapidly if humidity decreases, but survive for
longer periods if the humidity increases.
— Breed in aquatic environments so
will have more breeding sites if rainfall increases and less if
it decreases.
— Are highly mobile and so can move
quickly on the wind, over large distances. If climate-change brings
more extreme weather events eg increased windiness, this is likely
to increase the dispersion of such vectors.
— Are most active at night rather than
during the day
Consequently, climate-change is likely to have
profound effects upon many insect-borne diseases. Temperature
increases are probably the most important change and may have
the following effects:—
For the insect vector an increase in temperature
may cause:
— An increase in metabolic rate leading
to more frequent blood-feeding and hence an increase in the opportunity
to transmit. Also, as blood-feeding fuels egg production, this
will also cause increases in vector population size.
— An extension in the geographical
range of some insect vectors, as most prefer warm climates, leading
to an extension in the range of the pathogen.
— An extension in the period of vector
activity over a greater proportion of the year, as winters decrease
in duration and severity, thus facilitating extended pathogen
transmission and easier over-wintering of the pathogen.
For the pathogen an increase in temperature may
cause:
— An increase in the replication rate
of the pathogen in the vector, leading to earlier transmission.
— More efficient transmission by an
increased proportion of a vector population leading to an overall
increase in transmission rate.
— Additional insect species to become
vectors, leading to transmission by a wider spectrum of vectors.
Virtually all of these changes impacting on
both the vector and the pathogen have already been documented
for bluetongue virus (BTV) and have led to 12 BTV incursions into
Europe since 1998 when there were only 2 in the previous 40 years.
Furthermore, these recent and on-going incursions have included
many areas across northern Europe when previously this virus only,
sporadically affected, SW Iberia and a few Greek islands. Other
diseases are likely to follow where BTV has led.
May 2008
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