What is Thrush?
It's a bacterial, fungal and/or yeast infection most commonly found in collateral grooves or central sulcus of the frog. These microorganisms are anaerobic (can't live in oxygen). Usually there is not just one type, but a soup of different types of microorganisms in the infected area.
There is definite causal link between wet conditions and the infection but that's not the whole story. Its true horses will often get these infections in the wet season and it will clear up in summer when it is hot and dry; BUT, there are wild horses that constantly live in wet swampy conditions and they have healthy feet. How is that possible? It's possible because there are other important factors to a healthy foot besides dry conditions. Proper diet and lifestyle are very important. Lots of turnout and exercise is good, standing in a stall 24/7 in caustic urine is bad. A varied diet of low carb plants that can only be found with turnout in a large natural area is good; a diet high in simple sugars (grain, fruit, sweet feeds with molasses) is bad.
Number one (if he can stand it) is to get your horse off anything that is high in sugar and carbs like pellets with high carbs, apples, fruit, carrots and grain (fungus loves sugar). If he is working hard and needs the grain supplement this may not be realistic.
The next thing is the trim; bring the walls and heels down enough so the frog is engaged with every step. This will improve circulation in the frog and cause the frog to grow. As Pete Ramey says, you may be able to out run the infection if the frog grows faster than the infection can eat through the frog.
Since the microorganisms are anaerobic, it helps to keep the central sulcus and collateral grooves open and to cut away all tatters and flaps on the frog that can trap dirt and manure. Exposure to air will kill the little critters. Less is more; leave as much tough outer frog as possible. Also, pick out his feet as often as possible since your horse's hooves quickly get packed with dirt and manure thus producing more anaerobic conditions.
External treatments can help also. Starting off the treatment with a ½ hour soaks every other day (or weekly if that's all the time you have) will be more effective than a topical spray or liquid. One of my instructors Cindy Sullivan; calls this reducing the bio load meaning; it will kill a lot more bugs than a topical treatment will. I have used Pete Ramey's cure of 2 ounces of Lysol concentrate to 1 gallon of water and ½ hour soaks. Other products to soak with include White Lightning and Clean Trax which can be obtained off the web or a soak of 50/50 apple cider vinegar and water. I have used white lightning and it was effective but expensive.
In between your soaks you could also use various topical treatments on a daily basis. When you apply your favorite topical treatment, put on hoof boots (with foam pads!) and go for a walk; it will keep the product on the affected area longer and keep mud and manure out. Here are some topical treatment suggestions:
If you see heel cleavage like this, you have thrush.