Equine serum hepatitis, formerly known as Theiler’s disease, is caused by infection with equine parvovirus-hepatitis virus (EqPV-H). Most infected horses are asymptomatic, and only 2% develop clinical signs of liver disease.

Horses can become infected sporadically in late summer to early fall (the mode of transmission is thought to be insect related) or through receiving certain biologic products. Products known to have transmitted the virus to horses in the past include:

• Tetanus antitoxin (TAT)
• Botulism antitoxin
• Pregnant mare's serum
• Streptococcus equi antiserum
• Allogenic stem cell preparations
• Equine plasma products

When infected, those who show clinical signs usually do so 4-13 weeks after receiving these products.

The 2% of affected horses who develop hepatitis may show one or more clinical signs and die typically within 72 hours after onset of the signs.