Texas Equine Veterinary Association

THEILER'S DISEASE Hepatitis viruses in horses are a relatively new and fascinating discovery, possibly some of the most exciting research in the last decade. In the last couple of months, two new journal articles have been published that bring new light to this subject. I will describe what we know to date, and we will follow up on any published updates at the 2019 TEVA Symposium. In the past, we called the serum hepatitis syndrome "Theiler's disease." This syndrome was assigned to the group of horses that were administered a serum or plasma product and developed severe acute hepatic necrosis one to three months later. Usually, Theiler's disease was fatal. It was assumed that the acute hepatic necrosis was secondary to a Type III Hypersensitivity reaction. I will explore the different hepatitis viruses that have been discov- ered and what we currently know about each of them. (This syndrome is not to be confused with "Tyzzer's disease." Foals affected by Tyzzer's are usually of nine-30 days of age and have acute hepatic necrosis secondary to a bacterial Clostridium piliforme infection.) Although the characterization of these viruses is recent, the discovery of this "serum sickness" is not novel. The syndrome was first described by Sir Arnold Theiler in South Africa in 1918. 1 Back in 1938, the Proceedings of the Royal Society of Medicine included an article on "Hepatitis and Jaundice Associated with Immunization against Certain Virus Diseases." 2 In this short article, it references 89 cases of jaundice in humans after 3,100 yellow fever vaccines were administered. They noted that the cases of hu- man jaundice were very similar to the occurrence of hepatitis in horses fol- lowing the administration of a horse sickness or equine encephalomyelitis serum product. It was later concluded that there was a hepatotoxic virus in the yellow fever immune serum. In 1991, a case report was published on Theiler's disease in a horse fol- lowing a routine castration. 3 The horse had received a tetanus antitoxin vac- cine in addition to sedation, antibiotics, and a tetanus toxoid vaccine just prior to the surgery. Seven weeks later, the horse succumbed to fulminant acute hepatic necrosis. In 1995, a case series was published documenting hepatic disease in eight horses following tetanus antitoxin administration. 4 These horses were periparturient mares and their newborn foals. Of these, one horse was found dead, one was treated in the hospital, and six had subclinical hepatitis. GGT was monitored in all 58 horses on the property and the initial increased levels were found from days 51-120 post inocu- lation, contrary to previous reports. The only horses on the farm that were affected were horses that received the tetanus antitoxin vaccine. However, more follow-up with the pasture mates may have been needed to document additional subclinical cases. 4 To date, numerous different biologics have been possibly implicated as leading to serum hepatitis. These include tetanus antitoxin (TAT), botuli- num antitoxin, Streptococcus equi antiserum, pregnant mare's serum, allo- genic stem cells, and the administration of equine plasma. 1 (Although two horses in this study that received allogenic stem cells were positive for the equine parvovirus, viral testing of the donor horses was negative. There- fore, the horses may have acquired the virus from either the donor serum used to culture the stem cells or another route.) In summary, a hepatopathy has been described following the administra- tion of some biologics. Affected horses have been described as exhibiting sudden death, clinical hepatitis, or subclinical hepatitis. An etiology for this hepatitis has not been described until recently. EQUINE HEPATITIS VIRUSES P iper Norton, DVM, DACVIM is a board certified internal medicine specialist doing mobile consulting in the Austin, TX area. She graduated from LSU SVM in 2007. Since that time, she pursued her love of internal medicine by completing a residency at Texas A&M CVM and by practicing at Elgin Veterinary Hospital for 7 years. She then established her own practice, Texas Equine Veterinary Associates. Although she appreciates all aspects of internal medicine, her favorite niche is neonatal medicine. She serves on the TEVA Board of Directors and is the editor of the TEVA Remuda. She also serves on AAEP's Infectious Disease Committee. by Piper Norton, DVM, DACVIM – Texas Equine Medical Associates www.texasequineva.com • page 6

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