Ocular:

Rats and mice may develop red staining around the eyes and nostrils when they are distressed, e.g., by disease, trauma, etc. This staining is due to the accumulation of porphyrins produced by the Harderian gland, a lacrimal gland. Though a normal constituent of tears in rodents, lacrimal porphyrin is produced in limited amounts and rodents keep themselves clean of debris through frequent grooming. Porphyrin staining in distressed animals occurs because stress stimulates porphyrin production in tears and distressed animals groom themselves less often.