Often a dam's full udder is noticed by the owner, indicating that the foal is not nursing. Bloody feces have not been associated with rotavirus infection, and fever is not a consistent clinical sign. Foals less than 14 days of age often are more severely affected and potentially can die from the dehydration and electrolyte imbalances within six hours. Clinical signs in foals two or more months of age can be quite different. Sometimes the only indication of the disease is cow-pile consistency feces with no other signs of sickness.
Older foals have more energy reserves and can better cope with the disease.
Foals from two days to five months of age can be affected, and most have diarrhea for three to four days, but the condition can last as long as a week. Even in the midst of the worst farm outbreaks, mares and other adult horses are not affected by rotavirus. In two separate research studies, a small percentage of mares were shown by serology to be infected with rotavirus at the time that their foals were ill, but none showed any signs of sickness. Although each species has its own variety of rotavirus, one of the surface proteins remains the same. Human test kits are commonly used to diagnose animal rotavirus in as little as five to 10 minutes.