Scabies mite inactivated serine protease paralogues are present both internally in the mite gut and externally in feces

The scabies mite, Sarcoptes scabiei, is the causative agent of scabies, a disease that is common among disadvantaged populations and facilitates streptococcal infections with serious sequelae. Previously, we encountered large families of genes encoding paralogues of house dust mite protease allergens with their catalytic sites inactivated by mutation (scabies mite inactivated protease paralogues [SMIPPs]). We postulated that SMIPPs have evolved as an adaptation to the parasitic lifestyle of the scabies mite, functioning as competitive inhibitors of proteases involved in the host–parasite interaction. To propose testable hypotheses for their functions, it is essential to know their locations in the mite. Here we show by immunohistochemistry that SMIPPs exist in two compartments: 1) internal to the mite in the gut and 2) external to the mite after excretion from the gut in scybala (fecal pellets). SMIPPs may well function in both of these compartments to evade host proteases.