Veterinarians should not underestimate the potential role of stress in fueling outbreaks of equine herpesvirus, according to researchers who investigated a series of cases at a stud farm in northern Germany.
The outbreak of equine herpesvirus type 4 (EHV-4) occurred at a Standardbred breeding operation in 2017.
Nine herpesviruses capable of infecting horses have been identified so far. EHV-1 and EHV-4 are considered the most important, and are closely related.
EHV-1 can cause respiratory infection, abortion, deaths in newborn foals, and neurological problems. EHV-4 causes only moderate respiratory infection in foals and horses under two years of age.
EHV-4 infections often involve no clinical signs, and infections in foals and horses may go unnoticed. EHV-4 is endemic in equine populations, and serological surveys indicate a seroprevalence of more than 80% among horses, donkeys, and mules in different geographical locations.
The outbreak of respiratory illness in Germany, described in the journal Pathogens, occurred in a group of foals and mares.
Out of 84 horses at the stud, 76 were tested and 41 horses were affected, including 20 foals, 10 stallions, and 11 mares.
Investigations by Selvaraj Pavulraj and his fellow researchers revealed the involvement of EHV-4 in all cases of respiratory illness, as confirmed by molecular-based testing techniques.
Among infected mares, 73% (8 out of 11) and their corresponding foals shed the virus at the same time.
The testing suggested the involvement of more than one animal as a source of infection due to either true infection or reactivation from a latent state. Several horses most probably harbored different EHV-4 infections that resulted in the outbreak, they said.
The study team noted that, before the outbreak, three foreign breeding mares had been introduced to the farm.
During the first week of the outbreak, 12 foals in a group of 25 showed mild to moderate respiratory illness. Clinical signs in foals were largely restricted to the upper respiratory tract, characterized by a fever, cough, nasal discharge, and poor appetite. The disease course in infected foals lasted for 7 to 14 days.
Immediately after diagnosis, biosecurity measures were applied, and management practices were implemented, including movement restrictions, assigning different personnel for handling sick animals, and the use of disinfectant foot baths and hand sanitisers.
Healthy animals that had been in contact with the sick animals were monitored daily for clinical signs.
However, EHV-4 infected horses could not be separated from healthy horses because of insufficient space. Infected and healthy horses shared common housing and feeding facilities on the farm throughout the outbreak.
Despite biosecurity measures, the disease spread to a nearby barn during the second and third weeks. Several foals and mares showed mild to moderate respiratory illness and were confirmed EHV-4-positive.
The outbreak peaked between the second and eighth weeks after detection of the first positive case. In all, the outbreak lasted 17 weeks.
Some apparently healthy mares also tested positive for EHV-4, the researchers said, and shed the virus through their nostrils continuously for up to eight weeks.
“Clinical signs were mild to moderate in foals and almost inapparent in mares and stallions, with a few exceptions,” they said.
Discussing their findings, the study team believe three main factors may have initiated the outbreak: Introducing three breeding mares with an unknown history of EHV-1 or EHV-4 to the farm, seasonal weather changes, and weaning.
Most of the EHV-4 positive foals in the current study were either unweaned or weaned recently. Foals from which the virus was isolated were unweaned during the outbreak. In addition, most of the foals together with their mothers tested positive for EHV-4.
“It is possible,” they said, “that EHV-4 was reactivated from a latent state in mares due to the above-mentioned stressful factors and resulted in infection of their corresponding foals and other foals from negative mares or stallions in contact.”
The authors noted that, during weaning, most of the foals may not have maternally derived antibodies, and weaning stress makes the foals susceptible to infection.
“In our case, weaning coincided with seasonal changes (severe rain in summer). Meanwhile, yearlings in stud were already broken in and were housed closely with broodmares and foals. A limited number of feeding spaces for horses in the paddock is also a factor attributed to stress.
“In stressed mares and stallions, EHV-4 virus might have been reactivated and spread to foals by direct and indirect means.
“This shows the importance of stress in disease outbreak and necessitates exercising stress-alleviating measures.
“Previous reports suggest that EHV-4 infection may happen throughout the year; however, the current study indicates the impact of the season in inducing stress and subsequent disease outbreak.”
They concluded: “This study shows the importance of implementing stress-alleviating measures and management practices in breeding farms in order to avoid immunosuppression and occurrence of disease.”
The study team comprised Pavulraj, Kathrin Eschke, Nikolaus Osterrieder, Sandro Andreotti and Walid Azab, all with the Free University of Berlin; and Jana Theisen, Stephanie Westhoff and Gitta Reimers, with Die Mobile Pferdepraxis.
Pavulraj, S.; Eschke, K.; Theisen, J.; Westhoff, S.; Reimers, G.; Andreotti, S.; Osterrieder, N.; Azab, W. Equine Herpesvirus Type 4 (EHV-4) Outbreak in Germany: Virological, Serological, and Molecular Investigations. Pathogens 2021, 10, 810. https://doi.org/10.3390/pathogens10070810