Ulcerative Dermatitis in Mice

Ulcerative dermatitis is a relatively common cause of morbidity in laboratory mice, characterised by reddened skin and hair loss, usually on the dorsum, neck, ears, or legs of the mouse. Ulcerative dermatitis is characterised by intense pruritis, leading to self-trauma as the mouse scratches the area, worsening the lesion, leading to an exudative lesion, pain and, in severe cases, bleeding and a full-thickness skin defect. In the latter cases, euthanasia may be indicated on humane grounds, leading to loss of a valuable mouse from the breeding colony, or from study. This skin condition may be particularly relevant in studies involving C57BL/6 strain of mice. Better management of this condition is relevant to Reduction of animal numbers in breeding colonies and ageing studies, as mice are not lost from the cohort due to incidental conditions.

The aetiology of ulcerative dermatitis is poorly understood and therapies such as NSAIDs, topical steroids or antibiotics are generally unrewarding. A useful refinement to management of these mice involves the use of topical Green Clay (Bentonite). Bentonite is listed in the US pharmacopoeia (USP) and has been used for centuries for its anti-inflammatory, adsorptive and antiseptic properties.

The treatment is in two parts:

Firstly to trim the sharp points of the claws (nails) on the mouse's hind feet in order to reduce self-trauma due to scratching. Note that only the tips of the claws are removed, avoiding the "quick" or the blood vessel within the claw. This may need to be repeated weekly until the skin has healed.

Secondly, Green Clay paste is applied to the affected skin once or twice daily until the lesion has healed (usually 5-14 days). The clay is first sterilised by oven-heating to 180 deg C for 3 hours. To prepare the green clay, 4g of clay is mixed with 4mL of Reverse Osmosis (RO) water, mixed to a paste and allowed to stand for one hour. It is then applied in a thin layer to the skin lesion(s) using a cotton bud or similar applicator. Once mixed, clay paste can be kept for up to 3 days in a sealed container.

As with all skin conditions, identification of the issue at an early stage and prompt treatment promote successful outcome. Mice which have full skin-thickness lesions, or skin lesions greater than 20mm diameter (approximate area the size of a 10 Euro cent coin), or which show intense scratching despite treatment should be humanely killed.

Adams et al. (2016) describe the use of toenail trimming alone to treat ulcerative dermatitis. A video of the procedure can be viewed here.