Gastroenteritis is the inflammation of the stomach, small and large intestines. The severity of the infection seen in the extent of mucosal irritation, invasion or even ulceration, causes very acute symptoms like severe and sudden vomiting and diarrhea.
The definition of inbred is someone who is born from people who are closely related, or is something that exists in a person or animal from birth. When two cousins get married and have a child, this is an example of a time when the child isinbred.
It simply means that at one point in its life, the horse was raced or went into race training and it's a thoroughbred. There's also an OTSTB, same thing but a standardbred, and OTQH, same thing but a quarter horse. It applies to horses at all levels of training no matter how long it's been since the horse was raced.