It sounds as if It could be Strangles here is more info for you on it.
Even after a horse seems to have recovered completely from strangles, he may continue to shed bacteria in normal-looking nasal secretions for several subsequent weeks. In fact, researchers believe that some horses remain carriers for life, capable of spreading infections to susceptible contacts. Strangles can infect horses of any age, sex and breed, but once exposed, horses usually recover completely and develop lasting immunity. Thus the most at-risk population includes young, immunologically "naive" horses.
The infection is spread through direct horse-to-horse contact or by intermediaries, including flies, human handlers and, most commonly, communal watering troughs. Nasal discharge is variable among infected horses, but victims always run very high fevers and always develop painful abscesses in the lymphoid tissues behind or between the jaw bones, where they may impede swallowing, hence the name "strangles." Once these abscesses burst and drain, the horse usually recovers without incident.
www.nfucountryside.org.uk/newsfarming-1105.htm
Also although you may have vaccinated West Nile
it is not a guarantee that your horse will not get infected.
Hope I could help!
