As long as the two populations remain in balance and the Ph in the hind gut is stable at a slightly acidic level, the horse is healthy. When this balance is upset, the horse may eat, but be unable to extract the nutrients he needs from his food. Many things upset the microbial balance in the horses gut. Antibiotics can kill beneficial gut bacteria. Heavy loads of parasites or even the dewormers used to kill them off can upset the balance.
Viruses affect the gut population.
Elderly horses benefit greatly with the use of porbiotics due to the fact they have a reduced digestion of fiber, protein and phosphorus relative to young horses fed the same diet. Unfortunately, the micro flora balance can be upset in all horses quicker that it can be restored. An abrupt change in feed can cause changes in the hind gut and fecal output within hours. Many of us have seen it when transporting horse or putting them into new situations. Probiotics can curb the symptoms of severe diarrhea within 24 hours, but it takes about three days to see real changes in the microbial balance, and three weeks to reestablish good gut performance completely.