The behaviour of Sarcoptes scabiei var. hominis in human skin: an ultrastructural study

The biology of Sarcoptes scabiei var. hominis is poorly understood because of the lack of an in vivo or in vitro propagation system. To obtain more information on the mite behaviour in its natural habitat we conducted an ultrastructural study of burrows in a number of patients with common scabies. Scanning electron microscopy furnished attractive images of the tunnel, parasite body and eggs architecture and demonstrated the presence of holes in the tunnel roof probably representing aeration structures. Transmission electron microscopy showed a marked keratinocyte damage around burrowing mites, well evident ahead of the mite capitulum also. Faecal pellets containing keratinocyte micro-organelles (melanosomes and mitochondria) were documented in posterior midgut. For the first time we disclosed the adhesion mechanism of eggs to the burrow floor. We showed that the typical finger-like projections of the outer layer of the egg shell gradually disappear where the eggs are in contact with the tunnel floor. This allows the inner layer of the egg shell to fuse and stick with the damaged keratinocytes lining the tunnel floor. Our observation substantiates that Sarcoptes scabiei produces a proteolytic substance (salivary secretions?) that has a key role in its life cycle allowing burrowing, feeding and eggs-burrow adhesion.