Cavalry miasma - what's all that mist in the air? At first gander it appears that an "attack" of it could possibly have caught up to him. Two of the other fellows seem jovial and the boyo on the right seems to be in a daze.
If you want to know the basis of my diagnosis…the key is the old oak tree flanked by a small grove of pine. It is I would suppose you could call this an imbalance between green (life) and lichen and diseased gray (death) An environmental metaphor if you will. A bad spot.
Any kind of night or morning air that hangs up like that is not good for cavalry. I do not know where this miasma is coming or even if it is smoke it is still bad for human lungs. Probably laughing about the stink or the funk or the bad odor of the field.
If I could receive additional information I might also propose that it could be related to the gentleman having a bad reaction to breakfast.