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INTRODUCTION
The sheep ked Melophagus ovinus Linnaeus, 1758 is an ectoparasite of sheep that has been largely eradicated in the UK as a consequence of the widespread use of pesticides (Small, 2005). However, it persists in the feral Soay sheep population of Hirta, St Kilda in the Outer Hebrides off the north-west coast of Scotland (Craig et al. 2008). The parasite fauna of these sheep has been monitored over the past 2 decades during an annual capture of study sheep each August and several nematode species, the tapeworm Moniezia expansa and the protists Eimeria spp., Giardia duodenalis and Cryptosporidium parvum have been recorded (Craig et al. 2006, 2007, 2008).
Sheep keds are wingless dipteran flies of the family Hippoboscidae that feed on host blood via their piercing mouthparts. The relationship with the host is intimate and obligatory, with the whole reproductive cycle taking place in the fleece of the host; the female ked produces a single full-grown larva periodically, which becomes firmly attached to the wool and forms a puparium in situ (Small, 2005). Transfer from sheep to sheep is by direct contact, typically from ewe to lamb (Small, 2005). The blood-feeding habit and intimate relationship with the host make keds suitable vectors for blood-borne parasites and the sheep ked is known to transmit the protist, Trypanosoma (Megatrypanum) melophagium (Hoare, 1972).
At first there was controversy whether T. melophagium Flu, 1908 was a parasitic flagellate of the sheep ked, M. ovinus, rather than the sheep, because the trypanosomes were easy to find in the insect host whereas infection in the mammal was cryptic. The development of methods for culture of the trypanosomes direct from sheep blood, coupled with the meticulous description of the developmental stages in the ked, revealed the true life cycle of this digenetic parasite (Hoare, 1923). Transmission was demonstrated to occur when infective metacyclic trypanosomes from the hindgut were applied to the mucous membranes of the sheep's mouth; this would be achieved naturally by sheep removing infected keds from the fleece during grooming and chewing them in the mouth (Hoare, 1923). T. melophagium is regarded as non-pathogenic to the sheep host (Hoare, 1972), although heavy infestation with keds causes irritation and damage to the...