Study highlights how easily EHV-1 can spread among gathered horses

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Findings highlight the importance of cleaning and disinfecting stalls at events once they have been vacated, before other horses use them.
Image by TheOtherKev

The risk of subclinical shedders of equine herpesvirus-1 as sources of infection for susceptible horses have been highlighted in a just-published study.

Equine herpevirus-1 (EHV-1) is a common virus normally associated with respiratory problems in horses, but occasionally they can develop equine herpesvirus myeloencephalopathy (EHM) – the name for the neurologic form of the disease.

“While the main goal in the management of an EHM outbreak focuses on identifying early clinical disease in order to physically separate infected horses, little effort is placed towards monitoring healthy horses,” Nicola Pusterla and his fellow researchers noted in the journal Pathogens.

“The assumption that EHV-1 shedding parallels clinical disease is erroneous, as subclinical shedders have been shown to be actively involved in viral spread,” they said.

In recent years, large equestrian show venues have experienced EHV-1 outbreaks resulting in EHM cases. Various factors have been incriminated in the development of these EHM outbreaks, including a large number of competing horses and the increased movement of such horses during the shows.

Specific factors, including the number of classes competed in at the event, biosecurity-related activities at events, and recent vaccination against EHV-1, have been associated with an increased risk for EHM. “Unfortunately, by the time horses are diagnosed with EHM, EHV-1 has spread often subclinically far beyond the index case,” the study team said.

“The highly contagious nature of EHV-1 infected cases has been the demise of many show barns, racing venues, and veterinary hospitals.”

Preventive strategies aiming to reduce the risk of an EHM outbreak have mainly focused on biosecurity protocols since, at this time, no vaccine is available with the claim to prevent the neurological form of the disease.

“Unfortunately, biosecurity protocols often lack compliance, especially during non-critical times, which may predispose transmission of EHV-1 amongst horses.”

The rate of silent transmission of EHV-1 – that is, transmission without clinical disease – during EHM outbreaks can reach as high as 27%. This, they noted, is in sharp contrast to the rate of shedding of EHV-1 amongst healthy equids outside an EHM outbreak, which ranges from 0–4%, depending on age and population tested.

In a bid to document the frequency of EHV-1 shedders and their impact on environmental contamination, researchers from the University of California, Davis, collected nasal swabs from 231 healthy horses and 203 stall samples to check for EHV-1 by molecular-based quantitative polymerase chain reaction (qPCR) testing.

They were collected 28 days after the beginning of an EHM outbreak at a large hunter/jumper show venue in southern California. The horses were stabled in 21 different barns.

Six of the 231 horses (2.6%) tested qPCR-positive for EHV-1 at the genomic DNA level. Five of the positive horses originated from four barns with no previously diagnosed EHV-1 cases, while one horse was stabled in a barn with previously diagnosed EHV-1 cases.

In all, a total of 28 stall samples (13.8%) tested positive. The positive stalls accurately reflected past EHV-1 shedding of subclinically and clinically infected horses in the outbreak.

Interestingly, the study team found no association in the EHV-1 qPCR-positive status between nasal and stall swabs.

They said while testing nasal secretions of healthy at-risk horses can detect active shedding at a specific time point, the testing of stall swabs allows veterinarians to assess the EHV-1 shedding status of a horse over time.

“The study results highlight the risk of subclinical EHV-1 shedders and stalls occupied by these horses as sources of infection for susceptible horses,” they said.

“The testing of individual stalls for the presence of EHV-1 may be a more practical approach than the collection of individual nasal swabs for the monitoring and early detection of the circulating virus.”

The results, they said, also highlight the need to improve the cleanliness and disinfection of stalls used by performance horses during show events.

Discussing their findings, the authors said one of the greatest challenges in reducing the spread during outbreaks is to determine the shedding status of every single horse with direct or indirect contact with the index case.

The frequency of EHV-1 detection in the 231 healthy horses in the study was 2.6%, which is in the range of 0–4% previously reported for healthy horses not associated with an EHV-1 outbreak.

The frequency of EHV-1 detection in the study population was remarkably lower than the ones investigated in healthy horses during active EHM outbreaks. This likely relates to the time of sample collection, as the nasal swabs were collected 28 days following the diagnosis of the first index case. “The isolation protocols and strict biosecurity protocols at the show grounds likely contributed to the reduced detection frequency of EHV-1.”

They continued: “The results showed that, overall, there were more EHV-1 qPCR-positive stalls than horses. Further, the number of EHV-1 qPCR-positive stalls was greater in barns with previously diagnosed clinical cases.”

The findings, they said, show the impact of clinical disease on subclinical infection and secondary environmental contamination. EHV-1 has been shown to remain stable and infectious under various environmental conditions such as water for up to three weeks, they noted. The presence of environmental EHV-1 should trigger measures aimed at cleaning and disinfecting stalls once they have been vacated and before new show horses use the stalls.

“Because of the cumulative effect of EHV-1 shedding in infected horses, the swabbing of stalls for qPCR testing may be a more practical way to monitor horses compared to testing individual nasal swabs. This situation would mostly apply to multi-day horse events with large numbers of competing horses and would by no means replace routine biosecurity protocols.”

They said future studies are needed to refine the swabbing protocol to focus mostly on areas with the highest contamination load. “Further, to reduce costs of testing, the pooling of environmental samples for the testing of EHV-1 should also be evaluated.”

The results showed that a relatively small number of healthy horses were subclinically infected with EHV-1. While the level of EHV-1 shedding was variable, some of the subclinically infected horses shed EHV-1 at levels similar to clinically infected horses.

The study team comprised Pusterla, Samantha Barnum, Amy Young and Carrie Finno, all with the University of California, Davis; Eric Mendonsa and Steve Lee, with Fluxergy in Irvine, California; and Steve Hankin and Skyler Brittner, with Desert International Horse Park, in Thermal, California.

Pusterla, N.; Barnum, S.; Young, A.; Mendonsa, E.; Lee, S.; Hankin, S.; Brittner, S.; Finno, C.J. Molecular Monitoring of EHV-1 in Silently Infected Performance Horses through Nasal and Environmental Sample Testing. Pathogens 2022, 11, 720. https://doi.org/10.3390/pathogens11070720

The study, published under a Creative Commons License, can be read here

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