Equine Herpesvirus Type 1 (EHV-1) is endemic in horse populations in Australia and worldwide. This virus can cause abortions, respiratory disease and less commonly, neurological disease. As a result, disease caused by EHV-1 has significant economic implications in the equine industry. EHV-1 shares a high degree of genetic similarity to EHV-4 but is distinct, as EHV-4 is the major cause of rhinopneumonitis in foals but is rarely involved with abortions (Ma et al., 2013; Heldens et al., 2001). The greatest economic impact of EHV-1 in the equine industry occurs through sporadic or epidemic abortion storms in breeding herds. These abortions usually occur in late gestation between eight to eleven months but can occur as early as four months. EHV-1 is endemic in Australia and is a notifiable disease in all states except WA and Tasmania. Control of EHV-1 is multifactorial with both vaccine and farm management being integral to control.
In June and July 2020, four thoroughbred broodmares in a herd of 10 at a large thoroughbred horse stud in the Hunter region aborted over a two-month period. These mares had been vaccinated for EHV-1 using an inactivated bivalent herpesvirus vaccine 'Duvaxyn EHV1,4'.
Mare 1 was an 18-year-old bay thoroughbred mare that aborted on the 7th June 2020, during the 8th month of gestation. Her vaccination history comprised of two vaccinations in her 5th and 7th month of gestation in 2020 and 10 vaccinations prior to that between 2015 and 2019.
Mare 2 was a 15-year-old bay thoroughbred mare that aborted on the 23rd of June 2020, during the 9th month of gestation. Her vaccination history comprised of three vaccinations in her 5th, 7th and 9th month of gestation in 2020 and 13 vaccinations prior to that between 2015 and 2019.
Mare 3 was a 12-year-old bay thoroughbred mare that aborted on the 1st July 2020, during the 9th month of gestation. Her vaccination history comprised of three vaccinations in her 5th, 7th and 9th month of gestation in 2020 and 17 vaccinations prior to that between 2013 and 2019.
Mare 4 was an 8-year-old bay thoroughbred mare that aborted on the 14th July 2020, during the 10th month of gestation. Her vaccination history comprised of three vaccinations in three months in 2020 and nine vaccinations prior to that between 2016 and 2019.
These mares were all in the same paddock prior to the first mare aborting, remained in this paddock and were not moved to a different paddock or into smaller groups until after the second abortion occurred.
All mares were clinically well prior to aborting with no overt clinical signs supportive of abortigenic disease. No vaginal discharge or inappetence was observed prior to the abortions occurring.
Mare 1 returned a positive result for EHV-1 via PCR on pooled foetal and placental tissues.
Mares 2, 3 and 4 returned positive results for EHV-1 via PCR using two swabs of pooled foetus and placenta in PBGS viral transport media.
EHV-1 Virus Neutralisation Tests were conducted in June 2020 following the abortions of the first two mares using paired samples 14 days apart. Titres were also conducted on the eight other mares in the same herd in an attempt to determine whether they were at risk of aborting. Two of the eight mares later aborted (Mare 3 and Mare 4) but their titres were not significant in predicting these abortions as the titre of Mare 3 remained the same (218), and the titre of Mare 4 halved (from 512 to 256).
Aetiology and Pathogenesis
Equine Herpes Virus 1 (EHV-1) is classified as a member of the herpes virus subfamily Alphaherpesvirinae and the genus Varicellovirus (Ata et al., 2018). The horse is the natural host to herpes viruses 1, 2, 3, 4 and 5, of which EHV-1 is considered the most important due to diverse clinical presentations and potential to cause large-scale economic losses (Lunn et al., 2009). EHV-1 is associated with three different clinical presentations. In young immunologically naïve horses, the virus may present as a mild upper respiratory tract infection with nasal discharge, coughing and fever that may be overlooked. In pregnant mares, this virus causes spontaneous abortions usually within 30 days following initial exposure, often with no overt clinical signs and with the placenta covering the foetus (Australian Veterinary Association, 2002). If the mare is exposed in late gestation, foals may be born alive with signs of weakness, dyspnoea and jaundice and usually die within three days (Australian Veterinary Association, 2002). EHV-1 may also present with the development of neurological signs during an active infection. This is called equine herpes myeloencephalopathy (EHM) (Wilson, 1997).
An integral component of EHV-1 pathogenesis is the ability to establish a cell-associated viraemia after primary infection or during its reactivation from a latent infection (Kydd et al., 2012).
The consequence that is central to each of these clinical syndromes is necrotising vasculitis and thrombosis resulting from a lytic infection of the endothelial cells lining blood capillaries (Patel & Heldens et al., 2005).
There are two possible sources of EHV-1 infection on a farm; through the introduction of the EHV-1 virus from outside the herd or from a resident horse that reactivates a previously latent infection. EHV-1 is highly infectious and transmission occurs through inhalation of infective aerosols or through direct contact with infective ocular or nasal discharge (Ata et al., 2018). The virus attaches to and infiltrates the mucosa of the upper respiratory tract and begins to replicate (Brinsko et al., 2011). Aborted foetuses, foetal membranes and fluids are also a significant source of the virus (Gardiner et al., 2012). EHV-1 can survive in the environment up to three weeks in the absence of disinfection and in cool, wet conditions. Aborting mares can shed the virus for up to two weeks despite demonstrating no clinical signs and can then be re-bred in the next cycle. Foals can be infected early in life by the dam regardless of whether she is vaccinated. The virus establishes latent infections and can then continue to silently spread to other foals before and after weaning (Gilkerson et al., 1999). Abortion can occur from two weeks to several months following exposure to the virus without the mare showing any clinical signs (American Association of Equine Practitioners, 2021).
A sero-epidemiological investigation at a large thoroughbred stud farm in the Hunter Valley in 1999 demonstrated that the prevalence of EHV-1 antibody positive mares was 26.2% and 11.4% in foals (Gilkerson et al., 1999). Serological studies from different countries have continued to show that EHV-1 has a significantly lower prevalence than EHV-4, which could partially be explained by the nature of EHV-4 infection occurring throughout the year and EHV-1 infection usually occurring during the pregnancy season (Patel & Heldens, 2005). Serological evidence suggests that 30% of the Australian adult equine population has had previous exposure to EHV-1 (Australian Veterinary Association, 2002).
As with other herpesviruses, latency occurs with EHV-1 leading to subclinical shedding during sporadic reactivation of the virus for unknown reasons throughout the entire life of the host. If host immunity has decreased over time due to lack of vaccination frequency, reinfection can occur risking subsequent abortions in the future (Brinsko et al., 2005).
The outcome of EHV-1 infection is influenced by various risk factors including the age, immune status, physical condition, the nature of infection (primary, reinfection or reactivation), the pathogenic potential of the virus strain and the management of mares after the abortions (Ata et al., 2018). Mares and foals have been found in previous studies to be a reservoir of EHV-1, where the virus may be transmitted prior to and after weaning and from foals as young as 30 days old (Foote et al., 2004). A study in 2004 found evidence of viral DNA excreted in nasal swab samples of young, unvaccinated foals and their dams using PCR (Foote et al., 2006). A more recent sero-epidemiological investigation found that despite implementing vaccination programs in equine herds across Australia, EHV-1 and EHV-4 circulate in vaccinated populations of mares and unweaned foals confirming the continuation of the silent cycle of spread of this virus (Foote et al., 2006).
The samples to be collected from suspected EHV-1 cases are nasal, foetal or placental swabs for quantitative PCR testing (dry swabs into PBSG), acute and convalescent serum collection for serology and whole blood for detection of cell-associated viraemia. Age, breed, sex and vaccination history should be noted (Donald, 1998). Quantitative PCR testing was conducted in this case and is considered the diagnostic test of choice due to high sensitivity and specificity (Ata et al., 2018). Both placental and foetal tissues should also be submitted to the laboratory, but biosecurity measures are indicated when handling these tissues (American Association of Equine Practitioners, 2021).
Virus neutralisation testing is recommended to confirm a recent infection and previous studies have shown that neutralising antibody titres to the virus rapidly increase after natural infection and by one week following experimental infection (American Association of Equine Practitioners, 2021; Animal Health Diagnostic Centre, 2021). Increases in antibody titres of at least four-fold are determined to be significant and confirm a positive diagnosis of EHV-1 (Hartley et al., 2005). The interpretation of virus neutralisation tests is complicated by the fact that EHV-4 strongly cross reacts with EHV-1 and there are no maternal or foetal serological tests that identify an antibody response specifically to EHV-1 or EHV-4 (Patel & Heldens, 2005). To complicate this limitation further, the available vaccinations cover both EHV-1 and EHV-4. Titre values that exceed 250 indicate a possible EHV-1 infection but titres less than 250 can be found in vaccinated horses, early acute infections or horses previously infected with EHV-1 or EHV-4 (Khusro et al., 2020). The results of serological tests from a single collection are therefore unable to be interpreted with any degree of confidence so it is recommended to use paired samples collected 14 to 21 days apart between acute and convalescent samples (Hartley et al., 2005; AAEP, 2021).
Vaccinations are considered an important strategy in preventing EHV-1 infections (Allen et al., 2004). The vaccination helps to prevent the stimulation of respiratory infection, systemic dissemination of the virus through cell-associated viraemia and the reactivation of the virus following a period of latency (Minke et al., 2004). It is recommended that foals are vaccinated at 3-5 months of age with a second dose 4-6 weeks later and a booster recommended every 3-6 months. The booster vaccination requirements are frequent as herpesviruses do not stimulate long-lasting host immunity. It is also recommended to vaccinate pregnant mares in the fifth, seventh and ninth month of gestation to help prevent EHV-1-induced abortions (Gonzalez‐Medina & Newton, 2015). It is recommended that non-pregnant mares in contact with pregnant mares are also vaccinated (Australian Veterinary Association, 2002). The vaccine has been reported to reduce the incidence of abortion following experimental challenge and also the clinical signs of respiratory disease in vaccinated animals (Heldens et al., 2001).
Prevention should not only rely on vaccination as it provides limited protection against the virus, viral shedding and is not 100% effective in preventing abortions. Heldens et al. 2001, found that in a study using the same vaccine as administered on this stud that despite vaccinated animals not being fully protected against EHV-1, vaccination clearly reduces clinical signs and duration of virus shedding and amount of virus shed. It was also concluded in this study that the vaccination of foals and pregnant mares significantly reduces the risk of abortions and outbreaks of respiratory disease caused by circulating field viruses.
There are several EHV-1 vaccines manufactured around the world, some approved for abortion control and some not, with Australia having only a single choice of vaccine with 'Duvaxyn EHV1,4'.
Management Aspects to reduce the incidence of abortions and minimising the risk of spread to other studs
One of the major challenges in controlling the spread of EHV-1 is recognising the horses that are likely to shed the virus and represent a contagious disease risk. There are various management strategies that can be implemented to ensure farm biosecurity and safety in the equine industry. It is recommended that pregnant mares are separated from non-pregnant mares and kept in small groups according to the gestational stage (Brinsko et al., 2011). Other horses should also be kept in age-matched groups. A closed herd is considered a fundamental management strategy but if maintaining a closed herd is not possible, resident horses should be kept separate to visiting horses introduced to the property. If horses are introduced, they should be quarantined for a minimum of thirty days with background, health status and vaccination status investigated. This quarantine period may be reduced if the history and health status is known. Records should be kept of horse movements between paddocks on the property, as this information is useful in the event of an abortion storm (Allen et al., 2004). Transportation equipment and other objects in contact with horses should be disinfected regularly. The number of visitors to the property should be limited and a logbook kept with visitor details. Potential stressors including poor nutrition, concurrent disease, travel, adverse environmental conditions and overcrowding, should be minimised to reduce reactivation of latent herpesvirus infection (Allen et al., 2004).
It is also imperative for staff to be educated on the disease, and preventative measures with emergency abortion plans implemented. In the event of an abortion, mares that have aborted should be immediately segregated from the rest of the herd and diagnostic tests should be performed to rule out EHV-1. Mares that have aborted should remain isolated for 14-28 days to maintain biosecurity, because if they are positive for EHV-1 they can continue to shed the virus through the respiratory route for at least 10 days (Brinsko et al., 2011; Australian Veterinary Association, 2002).
To manage an EHV-1 outbreak, measures for limiting the spread have been described using the acronym "DISH" (Allen et al., 2004). This involves disinfection of areas contaminated by the virus from aborted material, isolation of affected horses, submission of clinical samples to the laboratory and the implementation of hygienic measures to ensure biosecurity. Abortion kits should be kept in readily-accessible areas and contain occlusive barrier clothing (gloves, disposable mask, disposable coveralls and boot covers), halter and lead rope, iodine scrub and body bags with ties. It is recommended that aborted foetuses plus the placenta are placed in two body bags and tied securely. The rest of the herd that have been exposed should be removed from the paddock and isolated individually or in small groups of two or three mares until they foal or abort. Following abortion, lime can be spread in the areas where the foetus and placenta were found, and these areas can be fenced off to limit virus spread (Australian Veterinary Association, 2004).
Eradication is not an option due to the presence of latently infected horses in herds. It remains endemic in Australia and vaccination does not provide complete protection from the virus. An optimal control strategy is a combination of a vaccination program and implementation of a preventative herd management program (American Association of Equine Practitioners, 2021).
EHV-1 continues to cause significant economic losses in the equine industry worldwide. The incidence of disease associated with this virus continue to increase due to the cycle of silent infection that spreads throughout equine herds allowing infection to remain endemic. Until research provides a vaccine that offers complete protection in equine herds, it is imperative to combine frequent vaccinations in foals and pregnant mares with effective management practices in prevention and control. These should be implemented in equine enterprises to reduce the incidence of EHV-1 and the various complications that are caused by infection.