AThe initial high fever could suggest viral infection, but is sounds like it was not followed by localizing signs typical of respiratory or gastrointestinal viral attack.

The main localizing sign here is edema of the throat and limbs. Edema with warmth suggests vasculitis–inflammation of the blood vessel walls that makes them leaky. In many cases of vasculitis, damage to the blood vessels also produces hemorrhage. However, these can be difficult to see unless they occur just under the mucous membranes of the lips and nose where there is no pigmented skin or hair to hide their presence. Vasculitis can happen as a result of trauma, infection, purpura hemorrhagica, equine infectious anemia, and possibly equine viral arteritis.

Purpura hemorrhagica (purpura) is a common cause of edema affecting several parts of the body. It can be a sequel to strangles, which is common in younger horses. In chronic strangles, the horse’s immune response produces antibodies that bind with antigen (parts of the strangles bacteria); these complexes can sometimes become deposited in the walls of the blood vessels and set off inflammation. So-called bastard (atypical) strangles can be difficult to diagnose as the infection is buried in deep lymph nodes of the head and neck or has spread to less commonly affected parts of the body such as the abdomen. An abscess in the belly might explain the colic experienced by this colt. The swelling in the throatlatch area and jaw of this colt could be inflammation triggered by strangles in deep lymph nodes. In cases of purpura where the cause is not obvious, I like to endoscopically examine (scope) the upper respiratory tract and the guttural pouches. I have found this very helpful in cases where I was unsure if the horse had bastard strangles; sometimes the affected lymph nodes can be seen pressing on, or draining into, the guttural pouches. It can also reveal hemorrhages from purpura affecting the mucosal lining of the respiratory tract