An outbreak of lameness amongst Merino lambs, associated with the presence of Hyalomma ticks,
was investigated on a farm in the south-western Free State, South Africa. The purpose was to follow
the progress of the condition and to determine the extent of involvement of the two Hyalomma species
which occur in the region. The flock of experimental sheep (n = 460) ranged free in natural veld under
extensive farming conditions. During September and October 1993, adult ewes and lambs in this flock
were examined at weekly intervals to determine tick identity, abundance and attachment-site preferences
on lame and unaffected animals. Lameness occurred only among lambs, of which 68 were affected
during the 8-week period. Hyalomma ticks tended to aggregate and mean numbers of ticks/aggregation
were significantly higher on lame lambs (mean = 11,3) than on either the unaffected lambs (mean = 6,9) or
the ewes (mean = 7,1). Most tick aggregations (72,4%) on the lame lambs occurred on the lower legs and
feet, 34 out of 55 of these on the fetlocks or interdigital clefts. Hyalomma truncatum dominated (> 97%)
on all animals examined. Only 15,8% of the Hyalomma marginatum rufipes recovered from the lame
animals were attached to the legs. At two other localities at which H. marginatum rufipes was more
abundant, or even dominant, few ticks attached to the lower legs or feet. Those that did were mostly H.
truncatum (> 90%). Both H. marginatum rufipes and H. truncatum may attach to the same ventral and
anogenital body regions, but H. truncatum has a tendency to attach also to the feet and lower legs.
Although attachment of one or a few ticks in the axillar region or upper legs may cause lameness in
sheep, the attachment of ticks to the interdigital clefts and fetlocks almost always causes lameness.
The latter condition is more likely to occur in regions where H. truncatum dominates.
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