Eighty white Swiss mice were kept in laboratory of Faculty of Veterinary medicine, Duhok University, North of Iraq, for experimental using as routine animals models. In March, 2014, accidentally all mice were found a highly infested with irritant tropical rat mite Ornithonyssus bacoti. The final diagnosis of this mite was based on the morphological features. The infested mites were suffered from signs of irritation and suppurative pruritic dermatitis. By histological examination revealed different histological changes including; Hyperkeratosis, thickness in deep dermal layers and heavy leukocytes infiltration. The mites were completely removed from infested mice and skin lesion disappeared after a single subcutaneous ingestion with ivermectin.