Update re Equine Herpesvirus Neurological Disease Outbreak

Equine herpesviruses are endemic in horse populations worldwide and equine herpesvirus 1 (EHV1) respiratory disease and abortion are diagnosed each year in Ireland. EHV1 neurological disease is less common but when it occurs, often results in fatalities. Vaccination is a useful aid in the control of abortion and respiratory disease but no vaccine has an efficacy claim against the neurological form of the disease.

On the 20th of February the FEI were notified of a suspected outbreak of EHV1 neurological disease in showjumpers at the CES Valencia Spring Tour in Spain and reacted promptly to cancel the rest of the event. To-date there have been 18 deaths and the dispersal of horses from the venue has resulted in the spread of the virus to 10 countries with confirmed cases in Belgium, Denmark, Spain, France, Germany, Italy, Qatar, Slovakia, Sweden and Switzerland. Multiple abortions have been confirmed on one affected premises in Germany where two adult horses and one neonatal foal have died.

The FEI response included cancelling international events in 12 countries on the European mainland from 1 March to 11 April 2021 and blocking almost 4,000 horses on their database. These horses are not permitted to attend any FEI event until they have fulfilled the necessary testing requirements.

In Ireland, Horse Sport Ireland in consultation with the Department of Agriculture, Food and the Marine and the Irish Equine Centre developed a protocol for horses arriving from the continent. This includes two weeks of isolation and during this period, a testing protocol which requires two blood samples and nasopharyngeal swabs (NPS) collected at least 10 days apart. The NPS are tested by PCR. The first swab is for the timely detection of any potential problem after arrival and the second is for clearance from quarantine. As any PCR test is only a snapshot in time the antibody titre of the two blood samples is compared to determine if the horse seroconverted during the quarantine period indicating recent exposure to virus.

There has been much speculation concerning the cause of the outbreak, in particular whether this is a new aggressive strain of the virus. However there is no indication to date from genetic characterisation, that this a novel strain. The Valencia virus belongs to Clade 10 which includes viruses detected previously in France, Ireland and the United Kingdom as a result of ongoing virus surveillance programmes. There is no evidence that these Clade 10 viruses are particularly neuropathogenic but investigation of the virus isolated in Valencia is ongoing. At present it appears that environmental conditions at the venue where hundreds of horses were stabled in a tented facility may have contributed to the build- up of a large virus challenge. High stocking density in any shared air space particularly if ventilation is poor, facilitates the spread of respiratory viruses.

Fortunately to-date it appears that the virus has been contained on approximately 30 affected premises and the FEI have announced a return to international sport in mainland Europe on 12 April following a six-week shutdown. The Return To Competition protocols include a mandatory negative PCR test for EHV-1 for shows with more than 400 horses of any category and overnight stabling. Where competition takes place over consecutive weeks with horses stabled in the same airspace, the horses must be PCR tested for EHV-1 every Monday in order to remain on venue, regardless of whether they compete or not. For overnight stabling of not more than 200 horses sharing the same airspace, boxes must be grouped in units of up to 16 boxes with a minimum six-metre distance in all directions between the units. Vaccination has been shown to reduce virus shedding and the FEI recommend that at events vaccinated horses be stabled separately from unvaccinated.

It is anticipated that the measures adopted by the FEI will allow for a safe resumption of international sport and provide reassurance of the high health of equestrian sport horses to veterinary authorities internationally. In Ireland we need the co-operation of all horse owners to protect our existing health status. Coolmore and Fethard Equine Hospital with support from Department of Agriculture have imported 10,000 vaccine doses from the USA and made them available to different sectors of the industry. EHV1 neurological disease is not common here but both single cases and outbreaks with multiple fatalities occur sporadically. The disease primarily affects adult horses and mares appear to be more vulnerable than geldings or stallions. Horses usually present with hind limb ataxia and urinary incontinence several days after respiratory disease, pyrexia or lethargy which may go unnoticed. By the time the horse has neurological signs the temperature and appetite are usually normal and the antibody titre is often high. Sometimes the disease is very severe in onset and the horse presents in lateral recumbency, unable to rise. To confirm the diagnosis the sample of choice is an NPS for PCR.

The disease is not Notifiable but is effectively contained by adherence to the Code of Practice. Click here for more information.

Ann Cullinane MVB PhD
6th April 2021