Although, theoretically can be spread in utero, through colostrum or milk, most likely spread is through infected blood natural vectors - tabnid flies, when interrupted in a blood meal on a viremic horse flies carry virus to next meal, same or another horse close by. Animals that survive acute phase become chronically infected immune system temporarily controls virus, decrease in viremia.
Antigenic change leads to fever and virus spike.
Large amount of non-neutralizing antibody.