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state in stallions, cancellation of equestrian events, and banning of horse movement (Balasuriya et al. 2013; Balasuriya 2014). The occurrence and spread of EVA has been facilitated by the increasing volume of horse movement around the world for competition or breeding purposes, and by the trade of fresh or frozen semen and embryos (Balasuriya et al. 1998; Timoney 2000a,b; Olguin Perglione et al. 2010; Zhang et al. 2010a; Broaddus et al. 2011a; Miszczak et al. 2012). The development of the carrier state in stallions directly


results in reduction in the commercial value of these animals, higher costs required for breeding, and barriers in the national and international movement of persistently infected stallions and commercialisation of their semen and embryos. Control and preventive measures aimed at mitigating the repercussions of EVA are focused on prophylactic vaccination and the control of stallions used for natural or artificial breeding by serological testing and subsequent virological assessment of their semen before initiation of each breeding season as well as during pre-export and import quarantines (USDA-APHIS 2004; OIE 2013, 2015). The implementation of strict EVA surveillance programmes capable of identifying carrier stallions could potentially lead to EAV eradication. However, considering the large value of international trade of horses, semen, and embryos between countries, the enactment of EVA eradication programmes will require joint international efforts towards the implementation of a universal code of practice.


Concluding remarks, current challenges, and future directions


Equine viral arteritis continues to be an important infectious disease of horses; its occurrence can be associated with significant economic loss to the equine industry due to abortions, neonatal mortality and establishment of the carrier state. Even though outbreaks of the disease have not been reported in the USA since 2006–2007, EVA is endemic in most European countries and is of concern for the international movement of horses. There is sufficient evidence that the increase in the incidence of EVA in past years is associated with the movement of horses for breeding purposes and the commercialisation of semen. EVA prevention and control strategies are primarily focused on the identification of carrier stallions and, thus, significant efforts are being undertaken in this regard. Current OIE-approved diagnostic assays, including the VI


and the VNT for detection of EAV and its antibodies, require trained and highly experienced personnel, extended turnaround times, high costs, and can present difficulties in standardisation between laboratories. In addition, handling and processing of a large number of samples can be technically challenging. More contemporary assays of equal sensitivity should be accepted as alternative tests to current prescribed tests. Among these, RT-qPCR and a novel RT- iiPCR assay have been demonstrated to be highly sensitive and accurate for the detection of EAV in semen, other clinical samples, and tissues from abortions. Furthermore, modern serological assays that offer ease of standardisation among laboratories should be implemented for detection of EAV antibodies, such as competitive enzyme-linked immunosorbent assay or microsphere immunoassays (Luminex).


Equine viral arteritis control and prevention measures are


primarily based on the identification of carrier stallions and the vaccination of seronegative horses, including stallions. Commercially available vaccines (ARVAC and Artervac) have been shown to be protective and can prevent establishment of the carrier state, although ARVAC is better characterised than the inactivated product. It is not possible, however, to differentiate vaccinated from infected horses with current serological assays and available vaccines. Thus, development of second generation vaccines that would allow identification of infected and vaccinated animals could be highly beneficial for the equine industry, facilitating preimportation testing. Even though the mechanism and host factors that are


determinant in the establishment and maintenance of EAV carrier state in stallions are not fully understood, current ongoing studies are focused on better understanding of the pathogenesis of the carrier state and further identifying host factors (e.g. genetics) associated with persistent infection. The results of this research could lead to the development of genetic or other tests to identify stallions that are more prone to establish EAV long-term infection and novel therapeutics to induce clearance of the carrier state other than surgical castration. In conclusion, EVA remains an important disease of


equids and a major impediment to the international movement of horses. EVA can be a significant cause of abortion and, thus, horse owners, farm managers, and veterinarians should consider it in a differential diagnosis with other abortigenic agents such as equine herpesviruses. Future research focused on improvement of diagnostic assays, vaccines, and additional understanding of the EAV carrier state is of priority.


Authors’ declaration of interests No conflicts of interest have been declared.


Ethical animal research Ethical review not applicable for this review article.


Source of funding


This work was supported by the Kentucky Agricultural Experiment Station, College of Agriculture, University of Kentucky and the Agriculture and Food Research Initiative competitive grant number 2013-68004-20360 from the United States Department of Agriculture National Institute of Food and Agriculture (USDA-NIFA).


Acknowledgements


The authors would like to gratefully acknowledge the intellectual and creative input of Dr N. James MacLachlan (Department of Veterinary Pathology, Microbiology and Immunology, University of California, Davis, California, USA) and the late Dr William H. McCollum (Maxwell H. Gluck Equine Research Center, Department of Veterinary Science, University of Kentucky, Lexington, Kentucky, USA). The authors would also like to acknowledge Ms Kathleen M. Shuck for critical reading of the manuscript and Ms Diane Furry for assistance in figure preparation.


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