You don't have to have wet ground for the horse to develop thrush. You can be in the middle of a drought and still have a horse develop thrush
There are lots of single reasons and combinations of those single reasons as to why Horse "A" develops thrush and Horse "B" does not --- or they both do.
1. Ground conditions. The Ph balance is way off on our land; it is very acidic. Acidic equals a breeding ground for bacterial/fungal "things".
When the ground is hot, the heat carries up into the hoof that is standing on it. Since frogs do have blood pumping thru them, they hold a bit of moisture by nature.
So you have the heat from the ground the frog is standing on, permeating up into the frog that will retain a bit of moisture anyway, and along comes the possibility of thrush.
2. Diet. Diet can make a huge difference in some horses. Too much starch in grain-based products.
3. Cleanliness of stalls. If the horse comes in for any part of the day or night, the urine in the shavings can further aggravate the problem. I have one horse with chronic thrush issues.
He has the run-in stall with grid mats on top of limestone crush. Even though his stall is picked clean of manure every day, I have to change his shavings every three days to keep his shavings dry.
He has 22 acres of high and dry pasture to run on, so it isn't like he is confined to a small dirt area that is overcrowded with horses.
4. Immune System is certainly far from the least of issues. Horses with weakened immune systems (no matter how good you feed them) are more likely to be prone to thrush.
The horse I mentioned in #3 three has environmental and food allergies, so it's no surprise I battle thrush on him. He also has a less-than-grade 1 club hoof which, they are prone to thrush.
Depending on the severity of the thrush there are products on the market that are non-invasive to the healthy tissue.
1. Absorbine Hooflex "Thrush Remedy" is as non-invasive yet helpful as a product can get. It is oil-based so does not dry the hoof out. After picking and brushing the hooves clean, I apply it, then let the horse stand while I brush him and get him ready for turnout; giving the product ample time to soak in.
I use it on the hooves as a preventative, a couple times a week, during the hot/humid months. I don't have near as many issues during the wet/cold months as I do during the warmer months. Absorbine Hooflex Thrush Remedy Absorbine (Farrier Hoof Care - Thrush Treatment)
Tractor Supply also carries this product.
If the horse has he's-going-lame issues, it's time for something else.
White Lightening is a good soaker for serious thrush issues. I've never used it but I have never heard anything bad about it.
ToMorrow (for cow mastitis) does wonders. I have used it with huge success on my thrush-prone horse. I've seen it in Tractor Supply, I have bought it at my local Co-Op but we are very rural and they cater to the cattle farmers.
Soaking the hooves in Betadyne/water/clorox helps. Use enough Betadyne to make the water almost black but only a capful of clorox; all that in about a gallon of water.
I have a three inch deep rubber pan that I use. You only need enough water to barely cover the heel bulbs; it's the bottom and the sulci (crack between the heel bulbs) that you need to soak.
All my horses will stand still to soak their hooves but when I was teaching them, a wheel-barrow full of hay in front of their noses did wonders to get them to stand still
I hope this helps and hasn't been too overwhelming