An unappetising, greasy substance in the creases of the frog often lead the horse owner to suspect thrush. Frequently, this diagnosis will be confirmed by a vet or farrier on the basis of this symptom. On closer investigation, however, it turns out that only a small percentage of the hooves with damp frog sulci actually have thrush.
Sweat glands in the frog sulci provide for a damp climate in the tight folds. In this way, the elasticity of the frog horn is ensured, and the anaerobic bacteria which live here (as everywhere in tight folds on mammalian skin) and which suppress germs, disease and putrefaction, find a suitable environment.
In the confines of the frog sulcus the continually formed horn cannot be worn away in any other way (so that is does not become ever tighter in the sulcus) than through decomposition to a fluid mass. This decomposition is done by the anaerobic bacteria. The acid produced is well known for preventing rot and putrefaction, which is why we have, from time immemorial, preserved vegetables in acidic form (pickled cabbage, etc.) A decomposition of frog horn into an acidic or caseous mass is therefore the natural course of events in the frog. It is illogical to remove this moist, acidic environment (with thorough cleansing), because without it, disease and putrefaction can take hold in this region. If, however, this decomposition grease does not smell acidic-caseous, but rather musty-rotten, a high sensitivity in the frog region is also noticeable, even lameness. In this case we are dealing with thrush.
In most hoof textbooks (Ruthe, Koerber, et al.) poor hoof care and insufficient stable hygiene are usually cited as causes of thrush. Frequently the blame will also be given to incessant wetness or dampness. As control measures, dessicating and disinfecting treatments are advised, in the form of covering bandages, specialized shoeing, cleansing with iodine-mixtures, etc.
However, many years of studying hooves and investigating successful treatments of thrush have brought to light another picture of the cause of this disease. Moisture, at any rate, is not responsible (think of the Camargue horses, which live for weeks in flooded pastures and retain healthy hooves). Poor hoof care can only partly be identified as a cause (for example, with horses kept standing most of the time, unable to move freely), for who looks after the hooves of the wild, large breeding herds?