If you haven't done so yet, I recommend doing a forum search (the tab is at the top along the blue menu). There are lots of threads on this disease.
The first thing to consider is whether or not you live in an endemic area. If you do, and your horse gets much pasture or trail time, there's a high probability he's been exposed throughout his life. If not, it's more likely he hasn't been exposed.
The next step would be to have the Vet test his blood. If you live in an endemic area, he will probably have a titer, but the newer tests are supposed to give more information as to the stage of infection.
Next, you decide if to treat and with what.
Finally, you need to figure out how to prevent the horse from being infected again.
I think when you have more information, like blood test results, specifics on the symptoms you're observing, we can give you more help/information to consider.
You also may find it useful to go to Cornell's website and search Lyme Disease.
I have had two diagnosed with both chronic and acute LD. Symptoms most definitely are NOT universal and can vary from horse to horse and symptoms can also mimic those of other illnesses. One of mine had no symptoms other than a late lead change while the other had full blown ataxia with little control over her hind end. Too much for me to cover before coffee but a search will yield good results....Your vet should run a Western Blot and an Alissa. In the field snap test is not accurate.
There are multiple threads even started by me! The lyme Clique.....if you want more answers pm me and we can chat...the symptoms are far from universal very individual...I had one horse that got aggressive and the other just was lame up front....go figure....
Mai Tai aka Tyler RIP March 1994-December 2011
Grief is the price we pay for love- Gretchen Jackson
"And here she comes. Is it a bird? Is it a plane? No, it's ZENYATTA!"