21/01/2020
**When Equine Herpesvirus-1 goes "viral" on social media**
Equine herpesvirus type 1 (EHV-1) is a member of the subfamily Alphaherpesvirinae and the genus Varicellovirus.
EHV-1 is an endemic disease in the UK (e.g. commonly found in this part of the world) that was first described in the 1930s and is present in most equine populations worldwide. Infection with EHV-1 can result in the development of one of several disease syndromes: a) mild-moderate respiratory disease, b) last trimester abortions (and neonatal foal death) and 3) paralytic neurological disease known as equine herpesvirus myeloencephalopathy (EHM). The latter being the syndrome currently affecting horses at Crofton Manor.
The syndrome which develops depends on several factors including age, immune status of the host (e.g. if antibodies are present or absent), the type of infection (primary infection, re-infection and reactivation) and the pathogenicity (nastiness) of the strain. EHV-1 is an OIE (World Organization for Animal Health) listed disease and towards the end of 2019 in Europe was producing respiratory disease in the UK, Denmark, France, Germany, Ireland and the Netherlands, reproductive disease in Denmark and neurological disease in France, Netherlands and Switzerland.
EHV-1 is highly contagious and usually transmitted by direct horse-to-horse contact (e.g. via respiratory droplets and nasal discharge). EHV-1 can also be spread via contact with aborted fetal and placental tissues, or by indirect contact with contaminated feed, water, fomites, people, vehicles, clothing and equipment (or actually anything that has had contact with infected horses). EHV-1 can also travel short distances of around 15 meters as an aerosol (EHV-1 is a large virus so cannot spread far) if the horse is excreting a high dose of virus in respiratory secretions (aerosolized by snorting). Interesting, the levels of virus in nasal secretions is higher if this is the first EHV-1 infection the horse has seen , when compared to levels of virus in nasal secretions from horses which have had multiple previous infections, or if the virus has been reactivated from latent infection (see below). In these latter cases, there is very little opportunity for aerosol transmission.
Characteristic to herpesviruses, EHV-1 results in latent infections where the virus will “hide” in the body sitting quietly until reactivated via mechanisms such as stress (transportation, competitions etc.), pregnancy (considered a stressor and hence why this disease is frequently observed on breeding farms), or immunosuppression (compromised immune system). Horses are usually infected whilst young with EHV-1 then establishing latent infections. In some areas up to 90% of horses will be latently infected with EHV-1. Reactivation of EHV-1 may or may not be associated with visible (clinical) disease, however in both cases the virus will be shed from the respiratory tract and therefore reactivation of latent EHV-1 is a potent source of infectious virus for transmission to horses in the immediate vicinity. In fact, most horses will be re-infected multiple times during their lifetime and as a result.
Vaccines are available for EHV-1 and are used in “peace times” to obtain herd level protection (e.g. where a minimum number of horses are vaccinated to reduce the disease transmission to un vaccinated animals), but their use in response to an outbreak is contra-indicated. EHV-1 vaccines do not prevent a horse becoming infected but can reduce the severity of clinical signs (e.g. respiratory tract disease in young horses and abortion storms in mares) and thus viral shedding. At present vaccine manufacturers have not evaluated the performance of these vaccines on EHV-1 induced paralytic neurological disease. Vaccination must therefore only be performed where clear disease history is known since evidence suggests that EHV-1 infection may be enhanced by the presence of sub-neutralising antibody levels and in some cases may lead to the development of more severe clinical disease including the neurological form. This is because sub-neutralising levels of antibodies (generated either to the vaccine or by natural infection) can help the virus gain entry to cells allowing more virus to be produced rather than neutralize it. When thinking about vaccination it should also be remember that it takes time for the immune response to reach a level at which it becomes protective (known as onset of immunity). For EHV-1 onset of immunity is 3 weeks after completion of the primary course (around 6 weeks- so if you vaccinated today your horse would be considered protected after 9 weeks) and the duration of immunity (how long an animal is protected for) is licensed as 6 months. Vaccination is a valuable tool, but must be used carefully and not in response to an outbreak in areas where EHV-1 is actively circulating.
Instead strict on-off yard biosecurity, disinfection of premises, vehicles and equipment, destruction of contaminated bedding/feed/manure, movement restrictions and diagnostic screening should be used.
Let’s be proud of Crofton Manor and those who have followed in their footsteps by implementing control strategies, whether directly involved in the outbreak or not. However, let’s keep discussions on this topic FACTUAL and PROPORTIONAL to the risk, and above all remember exactly how this virus is transmitted. It will be interesting to learn the history of the index case from Crofton Manor, to establish as soon as possible how EHV-1 gained access in the first place. Is this reactivation of latent infection in a horse that has been at Crofton Manor for a while (Most likely and most probable given no more cases have been confirmed outside of Crofton)? Or is this a primary infection, or a reactivation of latent infection in a recently transported/imported horse new to Crofton Manor (much less likely but more worrying as the transmission pathway is then incomplete)? This is important to understand to determine the risk posed to the wider community (e.g. was Crofton Manor the index case or did the virus come from somewhere else first, if so where?)
Lastly, let’s pay our condolences to the people who have lost much loved horses in this outbreak.
**please share this post if you can, so we can put to bed some of the hysteria (and subsequent witch hunts) associated with how far this virus can transmit in the air and also the very poor recommendations made by some for vaccination**
