A scanning electron microscope study of L. cuprina larvae and the development of blowfly strike in sheep

Abstract

Scanning electron microscopy was used to determine whether first instar larvae were capable of physically damaging sheep skin and thus initiating blowfly strike without predisposing conditions. In addition, damage at the lesion site was monitored throughout the infection period. It was found that first instar larvae do not possess the mouth hooks seen in second and third instars. They do however have sets of spines within the oral cavity which, with the spines on the creeping welts, may be able to abrade the outermost skin cell layers. Damage to the epidermis was observed within the first 8 h after larval implantation on sheep. The dermis was exposed within the first 24 h and after this time the normal skin architecture was rapidly destroyed. At 72 h areas of wool were missing and the collagen matrix of the dermis was clearly visible. Predisposing conditions such as fleece rot or wounding were not necessary for strike initiation. It is suggested that a combination of physical irritation and abrasion together with enzymes released by larvae enable them to initiate formation of the strike lesion. Thus, the only mandatory predisposing condition for fly strike may be wetted wool and skin.