I purchased a horse two weeks ago (my first horse in over 35 years) who had a pre-purchase exam and passed. The farrier came out this week and when he pulled off the first shoe he noticed white line disease. In speaking with the vet he said he would not have noticed the white line disease unless he pulled off the shoe. I'm just wondering how long does it take for white line to develop. Is it something she picked up in her new environment?
No White line is a bacterial infection that develops over time - typically they would have a crack or abscess hole that isn't cleaned properly and the infection spreads through the white line. Unless her new environment and really muddy and she had a crack or something I don't think it happened with her new home.
We have a TB who recently developed this - it's been no problem for him honestly. The farrier stripped the hoof wall where the infection was, we kept it clean and sprayed with 'save a hoof' and kept his stall super clean, it healed right up and grew out again. He was totally sound even right after they peeled his hoof wall away. This was right when they finished - looks pretty scary but he didn't even really notice.
It takes more than a couple weeks for white line disease to develop... It's an 'opportunisitic' type of fungus which usually sets in after the wall of the hoof becomes weakend or damaged (poor conformation, not well balanced, irregular trimming, especially flat shoed horses, etc) all contribute to an improperly balanced hoof and the contraction of white line especially after the tearing of the hoof wall. Once the hoof wall seperates theres nothing to really support the boney column you then become more likely to contract laminitis. That being said it's likely your horse is prone to white line and probably this didn't happen under your care.
The veterinarian could not have seen a small to moderate whiteline infection with the shoe in place unless it was so extensive that it was causing deformity in the hoof wall, or was a large enough cavity to show up on a radiograph. Or it was so big it was causing instability between the bone and hoof wall ,making the horse lame. How fast it grows varies a lot. I assume you did not get pre purchase radiographs?