Butchers' was are caused by:

Occupational handlers of meat, poultry or fish have a high incidence of hand was where the skin is in prolonged contact with moist animal flesh. HPV-2 is frequently found in butchers' was, but HPV-7 is present in a third to a half of lesions. Ref: Rook's textbook of dermatology, 8th edition, Pg 33.47