Thrush prospers when horses are exposed to dirty or unsanitary conditions. Bacteria are actually responsible for tearing down the soft tissue of the hoof that is known as thrush. This type of bacteria requires very little or no oxygen to survive. The decomposing tissue along with manure, urine, and dirt forms that black, somewhat pasty discharge and disgusting odor.
The bacteria do require a moist environment, but the rain is not the problem. Manure, urine, mud, and even general debris in the stable area are the issue. Once that stuff gets packed in the hoof then the bacteria will go to work.
If you do not observe any evidence of thrush until there is moisture then you may need to proactively manage the living conditions using the tips below.
Remove manure and urine from the stall and paddock daily
Ensure that the stalls and paddocks have adequate drainage so that there is no standing liquid (such as manure and urine soup) for bacteria to feed off
Clear the 3 grooves (sulci) of the hoof with a hoof-pick daily. Use a hard bristle brush, if necessary. Dish washing soap in water has an antibacterial agent and is good to include.
Over-grown hooves, pads, and shoes not routinely changed/reset along with inadequate exercise can contribute to the problem. Blood circulation to and from the lower extremities of the legs is dependent on the horse moving, so exercise is important for healthy feet.
Hydrogen peroxide and copper sulfate are common applications for light to moderate thrush conditions. Severe conditions may require an evaluation and treatment from a veterinarian.