[header] How to clean the sheath of a horse [title] Understand the pros. [step] In general, arguments in favor of washing a horse's sheath focus on hygiene and reducing the chances of infection. Bacteria tend to colonize around the smegma. 
The exact nature of the secretion and the likelihood of infection vary from horse to horse. No matter what, an excessive build-up can lead to irritation and inflammation in any horse.