When horses do not eat (anorexia), even for just half a day, it is very common for them to appear jaundiced.
The jaundiced/yellow appearance is due to a build up of bilirubin in the blood stream.
What is bilirubin:
A by-product of red blood cell breakdown. White cells that gobble up old or damaged red blood cells release it, and it is transported by carrier proteins within the blood to the liver.
Here the liver joins it to a sugar group, and it is then released in bile.
The bile travels down little ducts to the intestines.
Bilirubin is excreted in the faeces or resorbed to be excreted in the urine.
This process can be disrupted, and bilirubin levels can increase in the blood, causing the membranes in the mouth and eyes to look yellow.
Why does hyperbilirubinemia happen? (High bilirubin levels)
Rapid destruction of red blood cells e.g toxicity or auto immune disease
Reduced function of the liver
Blocked ducts carrying the bile out into the intestines
Fasting: This is very common in horses. If the cause for not eating is treated, then the yellow membranes go away.
Allow your horses access to lots of hay or grass, and ensure they can get some form of shelter in the cold weather.