Detection of EHV-4 and physiological stress patterns in young Thoroughbreds consigned to a South African auction sales

Equine respiratory infection is an important cause of disease and economic loss worldwide.  EHV-1 and -4 are important agents associated with infectious upper respiratory tract disease (IURD).  Risks associated with IURD include host, environmental, management and pathogen-specific factors.  In addition, higher detection rates of EHV-1 and -4 occur in juvenile horses and during the colder winter months.  Physiological stress is arguably one of the more important risk factors, with associations reported between EHV-1 and -4 shedding and externally derived stressors.  Stress may further increase the susceptibility of naïve animals to new infections.  Transport and the subsequent confinement, handling and management at sales events may contribute to physiological stress. 

Little is known about the prevalence of equine herpesvirus-1 and -4 (EHV-1 and -4) in South African Thoroughbreds at auction sales.  The bringing together of young Thoroughbreds from various populations, together with the stress associated with transporting them to sales could very well cause shedding and transmission of EHV-1 and -4.  This study sampled 90 young Thoroughbreds consigned from 8 farms from 3 different South African provinces representative of the South African breeding demographic.

Horses were tested by qPCR via :

• nasal swabs for EHV-1 and -4 DNA;
• blood samples for EHV-1 and -4 antibodies on arrival and departure;
• nasal swabs obtained serially from those horses with high temperatures and/or nasal discharge.

In addition daily faecal samples were used to determine faecal glucocorticoid metabolite (FGM) concentrations as a measurement of physiological stress.

Results

EHV -1 and -4 detection: No EHV-1 was detected, but EHV-4 DNA was detected in 13/90 (14.4%) of the horses from 7 out of 8 of the farms.  Repeated incidents of EHV-4 DNA detection occurred in 30.8% of the positive horses.  The longest period of continuous detection was four days.  Similarly, the longest interval between consecutive detections was four days.  Nasal swabs from 1.1% of the horses were positive for EHV-4 DNA on the day of arrival, while 7.8% were positive on the day of departure.

EHV-4 detection and clinical signs: All 13 EHV-4 DNA positive  horses showed either nasal discharge alone, or both high temperatures and nasal discharge.  However 84.4% of the remainder of the horses that were EHV-4 DNA negative, also showed one or both of these clinical signs.  The high temperature lasted for less than 24 hours in 7/8 horses with concurrent detection of EHV-4 DNA.

EHV-1 and -4 serology: Upon arrival at the sales complex 1.1% of the study population showed serological evidence of prior exposure to EHV-1, and 93.3% had previously been exposed to EHV-4.  No instances of seroconversion were recorded between arrival and departure.  Only one horse of the 13 EHV-4 DNA positive horses was EHV-4 seronegative on arrival and remained so on departure seven days later.

FGM concentration : The average FGM concentrations for the eight farms increased (with increased variability) after arrival, before decreasing in concentration and variability for most of the remainder of the study period.  During the adaptation phase, FGM concentrations were 64% higher on the day of arrival, and 93% higher on one day after arrival, when compared with three days after arrival.  No discernible increase in FGM concentrations was associated with the auction phase.

Conclusions

EHV-4 DNA was detected in some young Thoroughbreds consigned to a South African auction sale.  Most of these horses had been previously exposed to EHV-4 and very few to EHV-1 prior to their arrival at the sale.  The combination of stressors associated with their transport and arrival was associated with most horses showing a physiological stress response.  These, other stressors and commingling inherent to the current worldwide consignment process increase the risk association with IURD in young horses.  The transport and arrival phases are key areas for future investigation into management practices to reduce the impact of physiological stress on the health and welfare of young Thoroughbreds during sales consignment.

Publication : BMC Veterinary Research (2015) 11:126; DOI 10.1186/s12914-015-0443-4

Research Team :

Department of Companion Animal Clinical Studies, Faculty of Veterinary Science, University of Pretoria : Marcha Badenhorst, Patrick Page;

Department of Anatomy and Physiology, Faculty of Veterinary Science, University of Pretoria : Andre Ganswindt, Peter Laver;

Equine Research Centre, Faculty of Veterinary Science, University of Pretoria : Alan Guthrie;

Section of Reproduction, Faculty of Veterinary Science, University of Pretoria :  Martin Schulman