Leopard Complex - Vision Issues

Vision Issues

Congenital stationary night blindness is an opthalmologic disorder in horses which is present at birth (congenital), non-progressive (stationary) and affects the animal's vision in conditions of low lighting. Horses with CSNB may be hesitant to enter dimly-lit places - such as indoor arenas, dark stalls, or trailers - and be apprehensive when in such conditions, which may interfere with handling or riding. CSNB is usually diagnosed based on the owner's observations, but some horses have visibly abnormal eyes: poorly-aligned eyes (dorsomedial strabismus) or involuntary eye movement (nystagmus). The condition can be confirmed using electroretinography, from which a "negative ERG" indicates CSNB. While the retina is a normal shape, the nerve signal triggered when light reaches rod cells does not reach the brain. Rod cells in the retina are connected to bipolar cells, which transmit the nerve impulse to the next set of neurons. It is thought that these cells fail to undergo the basic chemical reaction for nerve impulse transmission, which involves shuttling of calcium (Ca2+).

Congenital stationary night blindness has been linked with the leopard complex since the 1970s. The presence of CSNB in non-leopard breeds and horses suggested that the two conditions might be located on close, but separate genes. However, one study used ERG findings to diagnose all the homozygous Lp subjects with CSNB, while all heterozygotes and non-Lp horses were free from the disorder. The gene to which Lp has now been localized encodes a protein that channels calcium ions, a key factor in the transmission of nerve impulses. This protein, which is found in the retina and the skin, existed in fractional percentages of the normal levels in homozygous Lp/Lp horses. A 2008 study theorizes that both CSNB and leopard complex spotting patterns are linked to the TRPM1 gene.

Equine Recurrent Uveitis (ERU) is also present in the breed. Appaloosas have an eightfold greater risk of developing Equine Recurrent Uveitis (ERU) than all other breeds combined. Up to 25% of all horses with ERU may be Appaloosas. Uveitis in horses has many causes, including eye trauma, disease, and bacterial, parasitic and viral infections, but ERU is characterized by recurring episodes of uveitis, rather than a single incident. If not treated, ERU can lead to blindness, which occurs more often in Appaloosas than in other breeds. Up to 80% of all uveitis cases are found in Appaloosas, with physical characteristics including light colored coat patterns, little pigment around the eyelids and sparse hair in the mane and tail denoting more at-risk individuals. Researchers may have identified a gene region containing an allele that makes the breed more susceptible to the disease.