The herpes virus is a large family of viruses. Most mammalian species are susceptible to at least one type of herpes virus. There are five known subtypes in horses, EHV-1 to EHV-5, but Equine Herpes Virus-1 (EHV-1) and EHV-4 are the two most severe forms of the virus. EHV-3 is another type of major herpes virus, although it is normally associated with coital exanthema (PETMD, 2018); also known as genital horse pox, a sexually transmitted disease in horses.
(EHV-1) causes acute respiratory disease in foals and yearlings, and is the biggest viral cause of equine abortion. EHV-1 can also cause paralysis and this affects horses of all ages and types. It often first involves weakness of the hind limbs but then progresses to involve all four limbs and results in either death or a requirement for euthanasia. (Carson, 2010) The reason for this is that some EHV-1 strains cause encephalomyelopathy, affecting both the brain and spinal cord, which may precede or accompany abortion epizootics. (Piero, 2000).
The impacts of EHV-1 can be devastating due to mass loss of horses; according to Maanen et al. (2000) abortion due to EHV-1 infection is a major source of loss to the horse breeding industry. There is huge welfare concern to herds where horses are infected and with preventing infection in the first place, because of this there is a need for the management of horses, and the prevention strategies put in place to avoid EHV infection, to advance and for new procedures to emerge.
2.0 Preventative Management Strategies
There are several key recommended management procedures that contribute to the prevention of EHV infection and spreading.
Allen (2002) suggests 3 key preventative strategies for EHV-1 epidemics: vaccination, division and subsequent isolation and stress reduction.
For the vaccination aspect, there are many vaccines available that claim to protect the horse against different strains of EHV-1 virus.
Firstly, there are inactivated vaccines. There are some available to protect against the respiratory disease form of EHV-1 and two available to protect against both respiratory disease and abortion. Next, there is a modified live vaccine, a single manufacturer provides a licensed modified live EHV-1 vaccine. It is indicated for the vaccination of healthy horses 3 months of age or older as an aid in preventing respiratory disease caused by equine herpesvirus type 1 (EHV-1). For the neurological form of EHV-1, there are no vaccinations available, however there are vaccines that reduce nasal shedding and thus the spread of infection. (AAEP, 2018)
The division strategy suggests that horses should be separated into like groups; avoid mixing different age groups and pregnant with non-pregnant mares. Maanen (2002) also recommends that horses should be divided into age-matched groups and to segregate pregnant from non-pregnant mares.
The division aspect also advises that for maximal protection, group size should be as small as the physical facilities allow, with each group kept under conditions that limit the transmission of virus between established groups. It is also advised that groups of pregnant mares should be separated into foaling groups that have been established early in gestation and that are based on similar delivery dates and how many foals each mare has previously had. (Allen, 2002)
Both Allen (2002) and Maanen (2002) recommend that new horses should be quarantined for at least 21 days and that isolation facilities should be created and staff educated on the EHV disease.
With regard to the stress reduction aspect of Allen (2002) key preventative strategies, the latently infected horse is usually a silent carrier who, in response to external stressors, sheds infectious EHV-1 into the environment. The aim is to minimise stress experienced by the horse – stress caused by crowding, poor nutritional state, heavy parasite infestation, lengthy transport, disruption of established social groups, cold or wet weather, group weaning and other disease states (Bryans, 1981), thus minimising risk of shedding of EHV-1 into the environment.
The 3 described preventative strategies for EHV-1 epidemics (vaccination, division with subsequent isolation and stress reduction) are not isolated actions. Each is an integral part of the overall process of preventing and controlling the size of epidemic EHV-1 disease. The exclusion of any one of the interrelated procedures may compromise the odds for success. (Allen, 2002)
Figure 1.1 A diagram of the initiation and progression of a typical epidemic of EHV-1 infection (vertical arrows) and locations along the progression cycle at which specific control measures can be applied for interrupting transmission of the virus (horizontal arrows). L = latent virus; R = reactivated virus; S = shed virus; I = infecting virus. (Allen, 2002)
2.1 Issues with preventative management strategies. Whilst the above preventative management strategies may have some effect on the spread and initial infection of EHV-1, there are some issues with carrying out these strategies and their effectiveness on prevention of EHV-1.
Goehring et al. (2010) reported that during an outbreak of EHV-1 in a veterinary teaching hospital, despite institution of rigorous biosecurity precautions at the time of admission of the index case, EHV-1 infections spread to 6 other horses that were hospitalized at the James L. Voss Veterinary Teaching Hospital, including 2 that served as sources of infection for horses on their home premises after discharge. The biosecurity precautions taken included division of the original EHV-1 infected case (see Fig 1.2), separate staff for the infected horse, disinfectant foot baths and foot mats, the adjacent 4 stalls to the infected horse used as a barrier to separate the areas of infection to those not infected and no further inpatients were taken by the hospital.
Figure 1.2 Equine barn floor plan of the James L. Voss Veterinary Teaching Hospital. (Goehring et al. 2010)
These results suggest that the preventative management strategies put in place that are also suggested above by Allen (2002) and Maanen (2002) had little effect on preventing the spread of EHV-1.
Walter et al. (2013) documented an EHV-1 outbreak and noted that from day 1, quarantine was imposed for broodmare barn 7 and from day 5, different grooms attended separate barns. Facilities were entered through hygienic barriers in which personnel had to change clothes. Horse movements were prohibited and all horses remained indoors. However, new cases of fever were detected in nearly all barns during the outbreak. Separation of horses, which were suspected to shed virus based on clinical symptoms such as fever, EHM and/or abortion in the affected barns, was deemed impossible. Isolation facilities were not available and barn construction could not be changed fast enough, so horses were able to maintain nose-to-nose contact. Their conclusion was that strict containment procedures employed from day 5 of the outbreak did not prevent spread of virus on the premise.
These results show that the preventative management strategies suggested by Allen (2002) and Maanen (2002) are not always possible if the equine facilities are limited or small and in particular, the division and isolation aspect and so therefore the efficacy of the strategies is not high in the Walter et al. (2013) report.