Bilateral Convergent Strabismus with Exophthalmos in cattle - what do we know?

The aim of this paper is to describe the most important aspects of the so called "Bilateral Convergent Strabismus with Exophthalmos (BCSE)", the most common aspect of strabismus in cattle. The disease has been known since the end of the XIX century, but no definitive conclusion has been found in respect to the pathogenesis and the etiology. BCSE may affect all major breeds and is known worldwide. It is characterised by a progressive bilateral symmetric anterior- medial rotation of the bulbi associated with a slight to severe protrusion of the eyeball (exophthalmos). The onset of the defect is around the first year of age. It usually ends up with severe visual deficit that might modify the behavior of the affected animal. Although there is few literature, defects in the lateral rectus muscle and the retractor bulbi muscle of the eye and in their appendant nerves (CN III and CN VI, respectively) were suspected to be the cause of BCSE. The presence of "red ragged fibers" in the lateral and median rectus muscles were also found in affected animals. It is common to note changes of character, probably related to insecurity resulting from visual difficulties: the affected animals become scary, move with difficulty, struggling to enter the milking stations, over-react to the routine operations that are carried out in the barn. As to the authors' knowledge, there are no useful therapies to cure or even improve the disease. The majority of the authors consider the BCSE as an inherited defect. However, the exact mode of inheritance has not been found yet. Recent studies carried out on German Brown cattle affected by BCSE showed a significant SNPs association with the disease within two candidate genes located on BTA5 (PLXNC1) and BTA18 (RDH13). Although the role of these genes could not be definitively confirmed, it is most likely that these candidate genes or nearby located structural mutations may be responsible for BCSE in German Brown cows.