A new parvovirus has been identified which is capable of causing lift-threatening liver disease in horses.
Researchers focused on a horse that contracted equine serum hepatitis, also known as Theiler’s disease, after receiving a dose of equine-derived tetanus antitoxin. The horse in Nebraska died 65 days after treatment with the antitoxin.
Dr Thomas Divers and his colleagues identified the previously unknown parvovirus in the serum and liver of the dead horse, as well as in the administered antitoxin.
The equine parvovirus they discovered was genetically analysed and found to be a new species in the genus Copiparvovirus. Other members of the genus Copiparvovirus include parvoviruses that infect pigs, cows, and sea lions, as well as a recently identified virus found in horse cerebrospinal fluid.
The new virus, tentatively known as equine parvovirus-hepatitis (EqPV-H), is more closely related to pig and cow Copiparvovirus than to the only other known horse Copiparvovirus, indicating different evolutionary origins.
The study team, writing in the journal Emerging Infectious Diseases, said their work confirmed that a tetanus antitoxin contaminated with the virus was able to cause infection in two horses.
They also determined that EqPV-H is an endemic infection in horses.
The researchers tested 100 clinically normal adult horses, finding evidence of the virus in the blood of 13 of them. In all, 15 of the horses were seropositive for the virus. This suggests that most horses that become infected with EqPV-H do not develop signs of disease.
The evidence linked the virus with equine serum hepatitis and showed it could be transmitted through contaminated biological products, they said.
Equine serum hepatitis, sometimes called idiopathic acute hepatitis, is a serious and often life-threatening disease of horses. It was first described in 1919 in South Africa by Sir Arnold Theiler.
Theiler observed hundreds of cases of a highly fatal form of hepatitis after experimental vaccination studies to prevent African horse sickness.
The disease has since been described in horses in many areas of the world after treatment with a variety of equine serum products, including tetanus antitoxin, botulinum antitoxin, antiserum against Streptococcus equi, pregnant mare’s serum, and equine plasma.
The clinical disease has a high rate of death, but some horses survive, and survivors have not been reported to have evidence of persistent liver disease.
Dr Divers and his fellow researchers say parvoviruses are common and can have many effects on their hosts, not always causing disease. Parvovirus hepatitis in other species is rare, although it can be associated with acute hepatitis in humans after transfusion of contaminated blood products.
“Although the overall incidence of clinically recognized serum hepatitis in adult horses receiving tetanus antitoxin is low, tetanus antitoxin has been the most commonly reported blood product associated with the disease in the United States for the past 50 years.”
Commercial tetanus antitoxin is heat-treated to inactivate the virus, and phenol and thimeresol are added as preservatives.
“However, the two successful transmissions of EqPV-H to experimentally inoculated horses confirmed that EqPV-H can be transmitted from heat-treated commercially available tetanus antitoxin.”
Animal parvoviruses are very resistant to heat inactivation and solvent detergent treatments, the study team said.
The findings of the serological survey of 100 clinically sound horses, with evidence of the virus found in 13% of the animals without biochemical evidence of liver disease, suggests that most horses that become infected with the virus do not develop clinical disease.
“This finding would be compatible with epidemiologic data on Theiler’s disease outbreaks in which clinical hepatitis develops in only 1.4% to 2.2% of horses receiving equine blood products.”
The researchers said follow-up studies that include horses living in different geographic areas are necessary to define the true prevalence and genetic diversity of the virus.
“Why some horses develop severe and often fatal disease after EqPV-H infection and others do not also remains unknown and requires further investigation.”
Several findings in the study suggest that EqPV-H infections in horses can often persist, they said.
The study team comprised Thomas Divers, Bud Tennant, Arvind Kumar, Sean McDonough, John Cullen, Nishit Bhuva, Komal Jain, Lokendra Singh Chauhan, Troels Kasper Høyer Scheel, Ian Lipkin, Melissa Laverack, Sheetal Trivedi, Satyapramod Srinivasa, Laurie Beard, Charles Rice, Peter Burbelo, Randall Renshaw, Edward Dubovi, and Amit Kapoor.
They are variously affiliated with Cornell University in New York State, Nationwide Children’s Hospital in Ohio, North Carolina State University, Columbia University in New York, the University of Copenhagen in Denmark, Hvidovre Hospital in Copenhagen, Rockefeller University in New York, Kansas State University, and the National Institute of Dental and Craniofacial Research, part of the National Institutes of Health in Bethesda, Maryland.
Divers, T. J., Tennant, B. C., Kumar, A., McDonough, S., Cullen, J., Bhuva, N….Kapoor, A. (2018). New Parvovirus Associated with Serum Hepatitis in Horses after Inoculation of Common Biological Product. Emerging Infectious Diseases, 24(2), 303-310. https://dx.doi.org/10.3201/eid2402.171031.