Sentence:  There are several reasons why a manual on this disease should be a part of the veterinary literature of the day, the chief one being that <WSD>fistula</WSD> of the withers is a very prevalent disease of horses and thus exacts a big toll from the horse industry. 
Meaning of the word marked with <WSD> tags:  An abnormal connection or passageway between organs or vessels that normally do not connect.