Horse transportation may lead to increased shedding, transmission and reactivation of equine herpesvirus-2 (EHV-2), evidence suggests.
Transporting horses is known to increase the risk of respiratory disease, with the sustained higher head position believed responsible for opportunistic bacterial proliferation in the lower respiratory tract.
However, immunosuppression and stress are also suspected to play a role in transport-related disease and are associated with viral reactivation.
While EHV-1 has been extensively studied due to the economic impact of outbreaks in the equine industry, much remains unknown about the role of EHV-2 and EHV-5 in disease. Both are very common and have been isolated from healthy horses, from those with mild respiratory symptoms, and from horses with more severe disease (both respiratory and non respiratory).
University of Sydney researcher Katharine Muscat and her colleagues set out to learn more about the behaviour of equid herpesviruses in horses, exploring reactivation and changes in infection and shedding associated with transport, as well as any potential contributions to transport-related respiratory disease.
The 12 horses used in the study were divided into two groups, each of which made an eight-hour road trip by truck around New South Wales.
Serum samples were collected before, immediately after, and two weeks after the truck journey.
Respiratory tract endoscopy and tracheal washes were collected before and five days after transportation. Nasal swabs were collected before, immediately after, and one and five days following transport.
DNA-based screening of nasal samples was undertaken for EHV-1,-2,-4,-5 using the quantitative polymerase chain reaction (qPCR) method.
All horses were negative for EHV-1 and EHV-4 throughout.
In contrast, most were positive for EHV-2 and EHV-5.
Although no horses were actively shedding EHV-4 in nasal secretions, all but one horse (11/12) had a detectable antibody/immune response to the virus, indicating previous exposure.
No significant difference in EHV-5 viral load was seen in association with transport overall, but prevalence of EHV-2 increased over time, with three horses becoming positive after transport.
Six of the horses were found to have persistent neutrophilic airway infiltrates after the journey, indicating subclinical respiratory disease, the study team reported in the journal Frontiers in Veterinary Science.
No horses were qPCR positive for either of the alphaherpesviruses – that is, EHV-1 and EHV-4 – nor did any seroconvert (display a significant increase in antibody levels) to either virus.
However, four out of the nine horses positive for either EHV-2 or EHV-5 based on the qPCR testing before transport developed neutrophilic airway inflammation.
Five horses showed increasingly positive readings on qPCR for EHV-2 after transportation and seven out of eleven horses that were positive for EHV-2 after transport shared strains of high sequence similarity with other horses in the study.
“Similarity of EHV-2 sequences across several horses suggested high levels of transmission.”
One EHV-2 virus detected in one horse after transport was genetically different to all other EHV-2 viruses isolated, and may represent a reactivated virus, they reported.
“The clinical significance of EHV-2 and EHV-5 remains in question,” the researchers said.
“However, these results indicate that transportation may lead to increased shedding, transmission and reactivation of EHV-2 and EHV-5, but not EHV-1 and -4.”
Unlike previous work focusing on the role of alphaherpesviruses (EHV-1 and -4), the research suggests that investigation of the gammaherpesviruses (EHV-2 and -5) in transport-related disease should not be dismissed, they said, particularly given that these viruses can encode suppressive immunomodulators that may affect host health.
No association was found between gammaherpesvirus detection and the occurrence of subclinical transport associated respiratory disease.
“In addition, no evidence for reactivation of either EHV-1 or EHV-4 could be detected in association with the transport event.”
They continued: “The detection of shedding of gammaherpesviruses in horses both before and after transport reflects the ubiquity of these viruses in horse populations.”
Differences in the shedding patterns over time between EHV-2 and EHV-5 could be consistent with a previous suggestion that reactivation of these viruses may be triggered by different biological stimuli.
Results from a related study confirmed that all horses involved in the study experienced stress in relation to the transport event as shown by an increase in frequency of stress-inducing behaviors during transport and elevated cortisol levels in serum after transport.
“For horses positive for EHV-2 prior to transport, shedding significantly increased after the transport event, however, there was no association between a particular time point after transport and change in viral load.”
Six out of 11 horses positive for EHV-2 had highest viral load 5 days after transport. However, three of those six horses were negative for EHV-2 prior to transport.
“This study has shown that detection of EHV-2 and EHV-5 viral infection in horses is likely to increase with transportation due to stress-associated reactivation of latent infections or increased shedding in actively infected hosts.
“This shedding may also increase the opportunity for horizontal transmission in the closed airspace of the transport vehicle or subsequently after transport.”
This potential increase in spread of infectious agents, such as EHV, may have important implications for controlling and preventing transmission of pathogens from the transported horses, such as the time interval before such horses are integrated with others in active training or for breeding purposes.
“The ubiquitous nature of these viruses and their ability to concurrently infect horses makes interpretation of shedding patterns difficult,” they continued.
“No direct association between sub-clinical respiratory disease and gammaherpesvirus infection was detected. Nevertheless to reduce the risk of infection and transmission of respiratory pathogens, stringent biosecurity measures should continue to be in place whenever horses are transported.”
The full study team comprised Muscat, Barbara Padalino, Carol Hartley, Nino Ficorilli, Pietro Celi, Peter Knight, Sharanne Raidal, James Gilkerson and Gary Muscatello, variously affiliated with the University of Sydney, the University of Melbourne, Charles Sturt University, the University of Bari in Italy and the University of Hong Kong.
Muscat KE, Padalino B, Hartley CA, Ficorilli N, Celi P, Knight P, Raidal S, Gilkerson JR and Muscatello G (2018) Equine Transport and Changes in Equid Herpesvirus’ Status. Front. Vet. Sci. 5:224. doi: 10.3389/fvets.2018.00224