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LIVESTOCK MATTERS
16
SUMMER 2014 ISSUE
EXTERNAL PARAS I TES
In the UK, the sheep tick Ixodes ricinus is by
far the most important vector of tick-borne
disease. Survival of ticks in the environment
is dependent upon the availability of large
mammals, including sheep, cattle, hare, and
deer, on which adult ticks can feed and the
presence of a moist microclimate when not
feeding. Thus ticks and their associated
diseases remain largely confined to certain
areas of upland grazing, while they remain
absent from well-drained, more intensively
farmed lowland areas.
The tick I.ricinus has a three year life-cycle.
During this time only around 17 days are
spent feeding, with the remaining time being
spent on the ground.
Ticks are generally inactive during winter,
but become active in spring in response to
rising environmental temperature. During the
following 4-8 weeks the tick must feed before
final exhaustion and drying-up.
In some regions of the UK, particularly in the
west of the country (e.g. parts of the Lake
District and Wales), a second period of tick
activity occurs in the autumn. This may
extend into the winter months if conditions
are very mild.
Tick-borne fever
(TBF)
Tick-borne fever is a severe bacterial infection
of sheep caused by the rickettsia Anaplasma
phatocytophilum.
The incubation period, following natural
exposure of sheep to infected ticks, is 4-8
days. The primary clinical sign is high fever
which may last for seven days. During this
time appetite is depressed and coughing may
be observed.
Infection causes profound immune system
depression and thus a greatly increased
susceptibility to secondary infections.
These may include pasterellosis, listeriosis
and other bacterial, viral and fungal
infections. Definitive diagnosis of TBF
requires the examination of blood smears
under a microscope.
However, infection of pregnant ewes will
often lead to abortion and high mortality
rates. In lambs, tick pyaemia is the most
common complication associated with
A. phagocytophilum infection. This is caused
by the spread to the joints and sometimes
spinal cord of the normally commensal skin
bacteria, Staphylococcus, following
introduction via tick-bites.
Infection of naive breeding rams during
tupping and the preceding six to eight
weeks will have a profound negative impact
on fertility.
Louping-ill
Louping-ill is an acute viral disease affecting
the brain and spinal cord of infected sheep,
though infection of other vertebrate species
can also occur.
All ages of sheep are equally susceptible
to infection. However, in regions where the
disease is endemic, older sheep often have
effective immunity and thus disease is often
seen in lambs or, more commonly still, in
yearlings returning from winter tack and
reintroduced to tick infected pastures.
Clinical signs may vary from mild transient
inco-ordination to sudden death. Progression
from inco-ordination to paralysis, convulsions
and death within 24-48hrs is however, the
usual outcome. Co-infection with Anaplasma
phatocytophilum (see above) often gives rise
to mortality rates approaching 100 per cent.
Louping-ill should be considered when sheep
are exhibiting signs of neurological disease
or sudden death. Disease occurs in areas
where ticks are active or transportation from
such areas has recently taken place.
Definitive diagnosis of louping-ill requires
sampling of the brainstem at postmortem or
blood sampling to assess antibody response.
Ticks
Control of tick-borne
disease
l
Avoiding the grazing of tick-infested
pastures during periods of peak
tick activity.
l
Regular plunge-dipping or application
of synthetic pyrethroid pour-on products
to minimise sheep contact with ticks.
l
Controlled introduction of non-pregnant
ewes to tick-infested pasture during
low risk periods to allow immunity
to develop.
l
Oxytetracylines are effective in the
treatment of clinical cases of
TBF
and in some situations may be used
prophylactically.
l
Vaccination of breeding stock against
louping-ill should be considered.
l
Avoidance of introduction of naive
pregnant ewes to tick-infested pasture.
Transmission of tick-borne disease
predominantly occurs during periods
of peak tick activity. Control is most
effective when attention is given to
controlling the level of exposure to
infected ticks as well as maximising
sheep immunity.
Measures which can be taken to
reduce losses due to tick-borne
disease include:
Reference
1
For further information on
the Sustainable Control Of
Parasites in Sheep please
visit
www.scops.org.uk
Ticks spend most of their life on the ground - only
feeding for 17 days of their three year life