We had a filly with an umbilical hernia and the vets advice was to have it operated on asap so we did with no problems, is there a reason why it hasn't been operated on? The fact that it is bigger is strange, i can't imagine that this could be from the flies. Let us know how you get on with the...
Equimins is just as good, you get the same amount of biotin but get 3kgs instead of 1kg for about £14! 3 Horses on our yard are on it with great results.