white line disease??

["Carol Harrison" <capape@worldnet.att.net>]

Hi,
Could someone shed some light on white line disease.  A friend of mine has
two horses with it now.  One is an old mare that founder many years ago and
has had white line disease for a number of years.  Just this last week
another horse came up very lame and the vet said white line disease also. 
The horses are stalled with runs during the day and are out on a couple of
acres at night.  She is located south of Houston so the soil is sandy, the
humidity is high, and usually conditions are wet with this year being
exceptionally dry.  What causes white line disease?  Could there be a
problem with the farrier (ie. long toe)?  What are preventative measures? 
Would a supplement help?  

I'm going to visit her tomorrow and will take along any responses.

Thanks,
Carol and Electric Shahck (Mom, remember to pack my mosquito net)