Equine gastric ulcer syndrome (EGUS) is extremely common in horses, with a prevalence of 80-100% in Thoroughbreds in active race training. It can affect both the squamous and glandular regions of the stomach, and is associated with signs of recurrent colic, weight loss, diarrhea, and poor performance. The risk factors and pathophysiology of EGUS, especially in the glandular region, remain poorly understood. Consequently, prevention and treatment are highly challenging and largely empirical. Our group was the first to demonstrate the presence of the equine gammaherpesviruses EHV-2 and EHV-5 in squamous and glandular regions of the equine stomach. Moreover, we found these viruses more frequently in gastric ulcer biopsy tissue samples compared to normal gastric tissue samples. Interestingly, these viruses are commonly associated with general malaise and poor performance, two common signs of EGUS. For this current proposal, our goal is to use equine gastric organoids to study the relationship between equine gammaherpesviruses and cells from the glandular regions of the stomach, which will greatly increase our understanding of the role these viruses play in EGUS. To this end, we will establish and characterize equine stem cell-derived gastric organoids, guided by wellestablished protocols for human and mouse gastric organoids (Aim 1). We will study the infection kinetics of (Subaim 2.1), and the efficacy of antiviral drugs against (Subaim 2.2), equine gammaherpesviruses in equine gastric organoids, using standard molecular virology and cellular methodologies. Studying the role of equine gammaherpesviruses in EGUS directly addresses Zweig’s research priority for the 2023/24 funding period related to the digestive health of horses. Equine gastric organoids will provide a critical model system to (i) study the interplay between gammaherpesviruses and the equine stomach and (ii) evaluate the efficacy of antiviral drugs for the treatment of EGUS, all without the immediate need for in vivo experimentation in horses. Our studies are important for New York State’s horse racing because the high prevalence of EGUS negatively impacts the health and athletic performance of racehorses. In addition, our research might lead to novel efficacious treatment options for EGUS, such as the use of herpes antiviral drugs.