The possible infection of an unborn foal through the mare's blood supply may be an exception to the hypothesis that PHF is an infectious and not a contagious disease. Among horses, E. risticii primarily causes disease in adult horses. But all ages, breeds and both sexes are susceptible to the infection. There is some field evidence suggesting that recovered animals are susceptible to reinfection. PHF ranges from inapparent to fatal infections.
Clinical cases occur in two stages.
This stage often goes unnoticed and the animal returns to normal. The second stage occurs several days to a week following the initial fever. In 1 to 3 days, the horse may develop a watery, profuse, often projectile diarrhea that will persist for 1 to 10 days. If the diarrhea is not treated properly, dehydration leading to shock and death can occur. Other signs of the disease that may develop are laminitis, colic and subcutaneous edema.