Four herpesviruses can infect the respiratory tract of horses, including Equine Herpesvirus-1 (EHV-1), EHV-4, EHV-2, and EHV-5. EHV-1 is the most pathogenic and can induce respiratory disorders, abortions and neurological disorders. With the recent EHV-1 outbreak in November 2025 linked to the Women’s Professional Rodeo Association World Finals and Elite Barrel Race event held in Waco, Texas, interest in the virus has piqued.
Dr. Come Thieulent, Dr. Udeni Balasuriya and their team at the Louisiana State University School of Veterinary Medicine are investigating one of the most difficult challenges in EHV control: identifying where the viruses persist inside the horse during periods of latency. Viral latency allows a pathogenic virus to exist in a dormant state within a cell until activated, potentially by a stress event, leading to active infection and viral shedding.
Understanding where this virus remains dormant is critical. This silent persistence is a major driver of unexpected reactivation and transmission during stress, transfer or commingling events. Thus, identification of cells latently infected with EHVs is critical for better understanding the mechanisms of latency and viral reactivation. That being said, few studies have described the identification of latent infection using novel molecular techniques.
To study latency at low viral levels, the LSU team is using digital PCR (dPCR) as a detection assay. Compared with conventional PCR or real-time PCR, dPCR offers greater precision and absolute quantification of latent viruses. Samples are prepared similarly to those for real-time PCR, but are then separated into thousands of partitions, each ideally containing one or zero templates. It allows detection of viral DNA at very low levels, which is particularly important for latent infections.
One of the key components of the research is the examination of peripheral blood mononuclear cells (i.e. white blood cells) as a site of viral persistence. While neuronal tissues have historically been associated with herpesvirus latency, blood mononuclear cells have not yet been investigated as a reservoir for latent EHVs. They could represent a more accessible and practical sample type in live horses. Identifying latent virus in blood could eventually improve understanding of which horses may be at greater risk of reactivation and active shedding.
Latency patterns vary across EHV types. EHV-1 and EHV-4 are most commonly associated with latency in neural tissues, specifically in the trigeminal ganglion. However, EHV-1 has been shown to establish latency more broadly, with viral DNA detected in the lymphoid tissues. Other EHVs, such as EHV-2 and EHV-5, persist primarily in lymphoid cells. These differences in tissue tropism likely influence patterns of reactivation, shedding and detection.
The LSU research is ongoing and addresses a foundational gap in understanding the biology of EHVs. As recent multi-state outbreaks have demonstrated, the inability to identify silent carriers remains one of the greatest obstacles to controlling EHV transmission. By clarifying where latent viruses reside and refining tools to detect them, this work may ultimately support more informed surveillance, risk management and outbreak response strategies.
