Apparent acaridal dermatitis in a C57BL/6 Nya mouse colony

Dermatitis in a breeding colony of black (C57BL/6 Nya) mice was characterized by intense pruritus leading to self-mutilation and death. The cause appeared to be infestation with the mite, Myobia musculi. Ulceration of the skin resulted in exudation of serum proteins and exposure of the animals' immune defenses to microorganisms and irritants, with subsequent hyperplasia of regional lymph nodes and spleen. Affected mice had an increase in sessile plasma cells, serum immunoglobulin concentration, and percent of circulating granulocytes. The mites were eliminated by treatment with a dichlorvos-ronnel combination in liquid form, which was dispensed onto the bedding with an automatic syringe. The incidence of dermatitis was reduced to zero. Our experience with M musculi in two other strains of mice, both white, indicates that pathogenicity of this mite for mice varies according to the strain, sex, age, and individual differences in sensitivity of the mice as well as the mating ratios employed.