Methemoglobin reductase deficiency in a cat.

During ovariohysterectomy of a 6-month-old domestic shorthair cat, extravasated blood appeared brown. Methemoglobinemia was considered likely on the basis of discoloration of oxygenated blood. The absence of clinical signs of toxicosis, inability to identify a source of oxidant that could cause methemoglobinemia, and persistence of cyanotic-appearing tongue and mucous membranes in this cat made a drug-induced cause unlikely, and was suggestive of a congenital disorder. Subsequent analysis revealed 49.6% methemoglobin (normal, 0 to 1.1%), resulting from erythrocyte methemoglobin reductase deficiency (21% of normal activity).