Polyplax serrata: Histopathology of the skin of louse-infested mice

The sequence of changes in the inflammatory reaction in the skins of mice infested with the louse, Polyplax serrata (Burmeister) (Anoplura), was examined in detail throughout 12 weeks of infestation. The reaction was manifest in 2 phases. The first phase, lasting for 4 weeks after initial infestation, was characterized by an increase (followed by a decrease) in epidermal thickness, invasive neutrophils, eosinophils, and lymphocytes. After an initial increase in the first week (hyperemia), the numbers of vessels containing blood decreased steadily until the fifth week and remained low until the twelfth week. Arteriolar vasoconstriction was seen regularly after the second week. The second phase, lasting for the next 8 weeks, was characterized by another increase (followed by a decrease) in epidermal thickness, tissue lymphocytes, and monocytes. A sustained increase in mast cell numbers occurred, accompanied by degranulation of these cells. Fibroblast proliferation was also increased.
The data indicate that the histopathologic reaction of mice to sucking lice is basically similar to that shown in sheep infested with the sheep ked shown earlier. There is a suggestion that the skin changes represent the basic protective mechanism necessary for acquired host resistance both for the ked-sheep and for systems involving the Anoplura.