Periodic Opthalmia

["RACHAEL WARD" <rward@admin.hilconet.com>]

Five years ago, I bought my first Arab and discovered (after purchase) that
the horse was "moon blind" and was told by my vet that it was incurable and
that no one knew what caused the eye to flare up periodically.  I sold Rob
to some people in the Hill Country (cedar trees, hills) and they kept him
for 3 years.  I bought him back last week after being told that he hadn't
had any problems with his eye in the time they had had him.  Well, to make
a long story short, I got back to my place and for the first 24 hours he
was fine.  I started riding him and he seemed to be more or less the same
horse he was 5 years ago.  But, he has started rubbing his eyes and acting
irritated and I think that he probably has an allergy to some plant or bush
that isn't present in the Hill Country.  (I'm in South Texas, brush,
mesquite trees, etc.)  Now I'm debating whether I should return the horse.
I feel like if I keep him he will probably be permanently and completely
blind within 2 or 3 years.  I did Limited Distance rides on him before and
we were Top Ten in all but one ride and would love to compete on him again.
 I love Rob with all of my heart and I think he feels the same about me,
when I picked him up he hadn't seen me in 3 years and came straight to me
and licked my cheek.  But, if keeping him is going to hurt him I don't want
to do it.  Also, I don't know whether he and I could do distance riding if
he was totally blind.  Which is what I believe will happen eventually.  Has
anyone out there ever had to deal with this particular disease?  Rob had
tunnel vision in his right eye and perfect vision in his left when I sold
him.  I  haven't taken him to a vet this week but it appears to me that he
is completely blind now in the right eye and partially blind in the left. 
(He walked right into a tree at dusk when I turned him out Thursday)  I
need some advice!!  Rachael and Rob in South Texas