Failure to nurse or a depressed demeanor is an indication that a foal is ill. Diseases that affect newborn foals are almost always life-threatening and can progress very rapidly.

When a foal does not nurse, the mare's udder becomes engorged with milk, and ultimately it streams or steadily drips. A sick foal often approaches and nuzzles the udder but does not latch and nurse. The foal's face may become glazed with dried milk. A young foal that is not regularly nursing can become severely dehydrated in just a few hours.

Any time a newborn foal stops nursing or seems depressed, something must be done immediately. The younger the foal, the more urgent the situation.