Polybius states that this occurred when Hannibal was passing through a very rough path in order to leave the lands of the Celts and reach Etruria. The primary difficulty was that the path was in a large bog. As is said in the third book,
"Many of the horses lost their hooves by the continuous march through the mud. Hannibal himself on the sole remaining elephant got across with much difficulty and suffering, being in great pain from a severe attack of ophthalmia, which finally led to the loss of one eye as he had no time to stop and apply any treatment to it, the circumstances rendering that impossible."