When veterinarians evaluate horses for lameness, they usually provide the owners with written descriptions of what they observe. For example, “Lame at trot, left fore, +3 ... ,” is what you might find scrawled in a vet’s notes. While “lame at the trot” is pretty clear-cut, what does the “+3” mean?
The number is part of a “lameness grading system” adopted by most veterinarians. The system ranges from 0 to 5. Zero is often referred to as “negative” (-), meaning the horse shows no lameness—1 through 5 grades are always referred to as “positive” (+), meaning a degree of lameness is visible.
Here are the definitions the American Association of Equine Practitioners assigns to each of the numbers within the lameness grading system:
0 - Lameness not perceptible under any circumstances.
1 - Lameness is difficult to observe and is not consistently apparent, regardless of circumstances (e.g., weight carrying, circling, inclines, hard surface, et cetera).
2 - Lameness is difficult to observe at a walk or when trotting in a straight line but consistently apparent under certain circumstances (e.g., weight carrying, circling, inclines, hard surface, et cetera).
3 - Lameness is consistently observable at a trot under all circumstances.
4 - Lameness is obvious at a walk.
5 - Lameness produces minimal weightbearing in motion and/or at rest or a complete inability to move.