Immune Support and the Horse

Does your horse have a healthy and strong immune system?  How do you know?  The most common problems we associate with a poorly functioning immune system are recurrent infections and even allergies, but what about other conditions?  Even in the case of infections, we most often attribute a poor immune response to more severe infections that may even result in hospitalization, but are there other clues or more subtle situations that may indicate that we have a problem?  The immune system is complex and involved in more bodily processes than one might just realize.