Never underestimate the need for salt.
I was doing a routine check on the mares yesterday and Patsy came up to greet me.
I was rather alarmed to see her like that. She had been standing in the shade of the mares shelter with the other horses so I sure wasn't expecting anything amiss.
It wasn't overly hot and humid, pretty much the same as it has been for several days.
I consulted with Shayla and with the vet. There were no signs of distress other than the rapid breathing; she was eating and drinking normally, so we ruled out colic but gave her a dose of banamine anyway.
Vet said I could bring her in in the morning if needed, but by the cool of the evening she was pretty much back to normal. I figure she was like that for at least 5 hours. I was pretty worried about her.
What I think happened is that the mares were not letting her at the salt tub, which was in the middle back of the shelter, and without enough salt she couldn't sweat enough to cool off. This morning she spent 5 minutes licking salt, and later I brought out a bucket of loose salt and added it it to the tub.
And I moved it to a better spot where nobody will get trapped and picked on.
Patsy was normal today, so I think the salt was the issue, leading to anhidrosis.