Epidemiology and Risk Factors of Equine Parvovirus-Hepatitis, Hepacivirus, Pegivirus caballi, and Pegivirus equi in U.S. Horses

Abstract

Emerging equine viruses such as Equine parvovirus-hepatitis (EqPV-H), Equine hepacivirus (EqHV), Pegivirus caballi (P. caballi), and Pegivirus equi (P. equi) have recently gained recognition as significant contributors to equine infectious disease ecology and hepatic pathology. EqPV-H, in particular, has been implicated in Theiler’s disease (TD), a potentially fatal form of acute hepatitis in adult horses. Despite growing global interest, limited data are available describing their molecular epidemiology in the United States. This study was designed to address that gap through a comprehensive molecular survey aimed at evaluating prevalence and potential host-associated risk factors for these viruses across multiple states. A total of 1,195 equine serum samples were collected from diagnostic submissions and surveillance programs across Alabama, Georgia, and Texas. Using optimized quantitative PCR and RT-qPCR assays targeting conserved regions of each viral genome, the study quantified viral prevalence and examined associations with host demographics, including age, sex, breed, and geographic origin. Statistical analyses, performed in RStudio, employed both univariable and multivariable logistic regression models to identify significant predictors of viral positivity, supported by pairwise Fisher’s exact and Wilcoxon rank-sum tests for prevalence and viral burden comparisons, respectively. EqPV-H showed the highest prevalence (19.3%), followed by EqHV (5.6%) and P. caballi (1.7%), with only two cases of P. equi detected. EqPV-H infection was significantly associated with breed, age, and sex, whereas EqHV was primarily associated with breed, particularly Thoroughbreds and Quarter Horses. Demographic analysis revealed that EqPV-H-positive horses were significantly older, and male horses had 1.62 times the odds of infection compared to females. Breed-specific associations were also identified: Tennessee Walking Horses had higher odds of EqPV-H positivity (OR = 2.46), while Quarter Horses (OR = 4.16) and Thoroughbreds (OR = 9.64) showed increased odds of testing positive for EqHV. EqPV-H-positive horses also exhibited significantly higher viral loads compared to animals positive for EqHV or pegiviruses. No host-related risk factors were identified for pegiviruses, though P. caballi-positive cases were only identified from horses in Georgia. Viruse sequences identified in this study were similar to the reported ones in the United States and other regions. This largest molecular survey highlights the widespread distribution of EqPV-H and EqHV in U.S. horses and underscore the importance of continued surveillance, particularly in high-risk breeds and settings. The data provides a foundation for developing preventive strategies and improving understanding of the epidemiology and potential clinical impact of these emerging equine viruses. These results expand the epidemiological landscape of equine hepatotropic viruses in in horses in the United States, offering valuable insights into their ecological niches and transmission patterns. EqPV-H’s dominance in both prevalence and viral load reinforces its central role as a hepatotropic pathogen of concern. While EqHV remains largely subclinical, its occurrence in specific high-performance breeds raises questions regarding chronic infection. In conclusion, this molecular survey represents one of the most extensive state-level assessments of EqPV-H, EqHV, P. caballi, and P. equi in the United States to date. The findings substantiate the endemic nature of EqPV-H and EqHV and delineate host and regional factors influencing their distribution. Collectively, these results underscore the urgency of integrated monitoring systems and molecular diagnostics to track the spread of these emerging equine viruses and mitigate their impact on equine health and industry sustainability.