Objective: To determine the prevalence of congenital stationary night blindness (CSNB) in Appaloosa horses in western Canada, investigate the association with the leopard complex of white spotting patterns, and further characterize the clinical and electroretinographic aspects of CSNB in the Appaloosa.
Animals studied: Three groups of 10 Appaloosas were studied based on coat patterns suggestive of LpLp, Lplp, and lplp genotype.
Procedures: Neurophthalmic examination, slit-lamp biomicroscopy, indirect ophthalmoscopy, measurement of corneal diameter, streak retinoscopy, scotopic and photopic full-field and flicker ERGs and oscillatory potentials (OPs) were completed bilaterally.
Results: All horses in the LpLp group were affected by CSNB, while none in the Lplp or lplp groups was affected. The LpLp and Lplp groups had significantly smaller vertical and horizontal corneal diameters than the lplp group had. Median refractive error was zero for all groups. Scotopic ERGs in the LpLp (CSNB-affected) group were consistent with previous descriptions. The CSNB-affected horses had significantly longer photopic a-wave implicit times, greater a-wave amplitudes, and lower b-wave amplitudes than the Lplp and lplp (normal) groups did. No differences were present in photopic flicker amplitude or implicit times. Scotopic flickers in the CSNB-affected horses were markedly reduced in amplitude and abnormal in appearance. No differences were noted in OP implicit times; however, amplitudes of some OPs were reduced in CSNB-affected horses. There were no differences in scotopic and photopic or flicker ERGs or OPs between the normal groups.
Conclusions: CSNB was present in one-third of horses studied and there was a significant association between CSNB and the inheritance of two Lp alleles. ERG abnormalities support the hypothesis that CSNB is caused by a defect in neural transmission through the rod pathway involving the inner nuclear layer.