HgCl₂ challenge in Brown Norway rats lead to dermatitis instead of arthritis

A wide range of methods are described to produce adjuvant arthritis in rats by antigen exposure. Studies using these methods are rarely controlled histologically though the result can be paw dermatitis instead of arthritis. Three male Brown Norway rats were injected s.c. with HgCl₂ (1 mg Hg/kg body weight) on five alternating days following closely a well described scheme for induction of adjuvant arthritis. Extent of paw oedema was assessed sonographically. Location and extent of inflammatory responses were inspected histologically. Swollen reddish and painful paw oedema started to develop on day 13 increasing until day 16. Oedema increased skin-to-bone and skin-to-skin distance across the inflamed paws significantly. Histological examination on day 16 revealed marked dermatitis with dense cellular infiltrates, single cell necrosis and fibrin exudation. In contrast, no inflammatory responses were observed in the joints. Use of a well described scheme for induction of adjuvant arthritis produced dermatitis of the paw with identical time course, clinical and sonographic appearance as expected for arthritis. This observation strongly suggests the need to check the histology on location and the kind of inflammatory response when a model for adjuvant arthritis is altered or used for the first time.