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known effective antiviral therapy for the treatment of EAV infected horses. However, most naturally infected animals recover without any complications. Severely affected horses can be treated symptomatically with nonsteroidal anti-inflammatory drugs/antipyretics, diuretics, and supportive care to control the fever and oedema, with rest from training and breeding. There is no effective treatment of neonatal foals with interstitial pneumonia or the pneumoenteric syndrome other than antibiotic treatment to prevent secondary bacterial infections (Balasuriya 2014). Although several therapeutic approaches to clear the


carrier state or eliminate semen infectivity have been explored (gonadotropin releasing hormone [GnRH] antagonists, anti-GnRH vaccination, antiviral compounds, and single-layer centrifugation) (Dowsett et al. 1991; Fortier et al. 2002; Clement et al. 2005; Turkstra et al. 2005; Burger et al. 2006; Morrell and Geraghty 2006; Janett et al. 2009; Wenzinger et al. 2010; Morrell et al. 2013), none of them can guarantee clearance of persistent infection in stallions or complete elimination of semen infectivity. A pilot study by Burger et al. (2006) demonstrated vaccination of EAV carrier stallions twice 4 weeks apart with either Improvac1 or Equity1


stopped the shedding of EAV in semen 4–6 months after the first immunisation and from all subsequent semen collections. However, all vaccinated stallions had decreased libido, scrotal size, total sperm number, percentage of normal sperm, and motility. Other than these side effects, this study demonstrated that vaccination against GnRH might represent a means of successfully treating EAV-shedding breeding stallions. However, further studies are needed to confirm these findings. Because of the side effects of GnRH vaccination, there are no nonsurgical means currently available that are without potential adverse side effects for eliminating the carrier state other than surgical castration; this results in loss of the commercial value of the stallion as a breeding animal.


Vaccination programme


Routine vaccination of horses against EAV There is an attenuated modified live virus (MLV) vaccine (ARVAC)1 licensed for use in the USA and Canada. This vaccine is administered intramuscularly and has been widely used in the USA for the control and prevention of EAV infection since 1985. The MLV vaccine has been demonstrated to be safe and efficacious, inducing protective immunity in vaccinated horses (McCollum 1969a, b, 1986; Harry and McCollum 1981). This vaccine elicits neutralising antibodies within 5–8 days post-vaccination, with a peak in antibody titres observed between 7 and 14 days post-vaccination (Doll et al. 1968; McCollum 1969a,b, 1986; Harry and McCollum 1981; McCollum et al. 1988; Summers- Lawyer et al. 2011; Zhang et al. 2012). Neutralising antibodies may last for at least 2 years, and revaccination frequently boosts the serologic response, providing protective immunity for an extended period of time. It has been determined that neutralising antibody titres of at least 1:64 are necessary to ensure protection (Fukunaga et al.


revaccinations with the MLV vaccine are recommended as established in EVA control and prevention guidelines (USDA- APHIS 2004; OIE 2015). A small proportion of vaccinated horses may develop febrile reactions with transient lymphopenia, and the attenuated virus can be sporadically


isolated from nasal secretions and buffy coat cells up to 7 days post-vaccination but rarely up to 32 days post- vaccination (Harry and McCollum 1981; McCollum 1981; McKinnon et al. 1986; McCollum et al. 1988; Timoney et al. 1988, 2007; Summers-Lawyer et al. 2011). It has been shown that primary vaccination with the MLV vaccine provides efficient protection against clinical disease but does not consistently prevent reinfection of vaccinated horses (Zhang et al. 2012). Thus, EAV infection in vaccinated horses can still induce limited viraemia and viral shedding in nasal secretions (McCollum 1986; McCollum et al. 1988). Vaccination of pregnant mares with the MLV vaccine is


not recommended, particularly during the last 2 months of gestation due to the high risk of abortion (Broaddus et al. 2011b). Similarly, it is not recommended to vaccinate foals aged <6 weeks or dams with foals aged <6 weeks. Foals should be vaccinated at age 6 months or later when maternal antibodies decline, hence avoiding neutralisation of the vaccine virus by passively transferred antibodies. However, younger foals can be vaccinated in high-risk situations. Vaccination of foals prior to the onset of puberty is strongly recommended, as it provides protective immunity and, resistance to the development of the carrier state in colts.


Even though early studies have claimed that there is a


single serotype of the virus broadly neutralised by polyclonal antisera against the VBS of EAV, more recent in vitro studies demonstrated that neutralising antibodies induced post-MLV vaccination do not neutralise all EAV strains with the same efficacy (Balasuriya et al. 1997, 2004b, 2013, 2014; Zhang et al. 2010a; Balasuriya 2014). Nevertheless, the MLV vaccine has so far been proven effective in preventing outbreaks of EVA and establishment of carrier state in stallions. The MLV vaccine is not currently licensed in European


countries or Japan. Instead, an inactivated EAV vaccine (Artervac)1 is licensed for use in several European countries including the UK, Ireland, France, Hungary and Denmark. Due to its reduced immunogenicity compared to the MLV vaccine, a booster immunisation is required after 3–4 weeks of primary vaccination with semiannual revaccinations. Although this vaccine induces neutralising antibodies, its efficacy in preventing EVA and establishment of persistent infection in stallions is substantially less characterised than that of the MLV vaccine.


1990). Annual


Vaccination of breeding stallions In the USA, it is recommended that seronegative stallions be vaccinated with the MLV vaccine (ARVAC) at least 28 days before the breeding season, followed by an isolation period of 28 days after first-time vaccination during which the animal is not bred (naturally or by means of artificial insemination) or in direct contact with other horses due to the potential risk of transient shedding of vaccine virus in the semen or in nasal secretions (USDA-APHIS 2004). Even though vaccinated stallions usually do not shed vaccine virus in either semen or urine, very low levels of virus (<1 PFU/mL) were reported in the semen of one stallion at 4 and 6 days post-vaccination in one experimental study (Summers-Lawyer et al. 2011). Annual revaccination before the start of the breeding season is important and highly recommended. In addition, as current serological tests cannot distinguish vaccinated from naturally infected animals, an EVA vaccination certificate confirming the stallions’ seronegative status prevaccination should be


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