Glucose-6-phosphate dehydrogenase (G6PD) deficiency is a well-characterized 
X-linked inherited disorder in humans but has not been reported in horses. We 
describe a persistent hemolytic anemia and hyperbilirubinemia due to a severe 
G6PD deficiency in an American Saddlebred colt. Other abnormalities in the 
colt's erythrocytes as compared with those of healthy horses (n = 22-35) 
included increased activities of hexokinase and pyruvate kinase, decreased 
concentrations of reduced glutathione and reduced nicotinamide adenine 
dinucleotide phosphate (NADP), and increased concentration of oxidized NADP. 
Morphologic abnormalities included eccentrocytosis, pyknocytosis, anisocytosis, 
macrocytosis, and increased number of Howell-Jolly bodies. Scanning and 
transmission electron microscopic examinations revealed that eccentrocytes had 
contracted to spherical regions and thin collapsed regions. Eccentrocytes were 
more electron dense than were normal erythrocytes when examined by transmission 
electron microscopy. When exposed to acetylphenylhydrazine, erythrocytes from 
the G6PD-deficient colt produced more and smaller Heinz bodies than did 
erythrocytes from normal horses. Abnormalities in the colt's dam included 
presence of eccentrocytes and pyknocytes; her average erythrocyte G6PD activity 
was slightly below the range of reference values.