Description of an unusually large outbreak of nervous system disorders caused by equine herpesvirus 1 (EHV1) in 2009 in Belgium Beschrijving van een ongewoon grote uitbraak van zenuwstoornissen veroorzaakt door het equiene herpesvirus 1 (EHV1) in 2009 in België

Neurological diseases caused by neuropathogenic strains of EHV1 are being reported with increasing frequency. Consequently, concern is being voiced within the US horse industry that the neurologic form of EHV1 may be intensifying in prevalence and/or morbidity and mortality. In Belgium, outbreaks of EHV1induced abortions are an annually recurrent phenomenon, but outbreaks of equine herpes myelitis (EHM) are rare. This report describes an unusually large outbreak of EHV1-induced EHM that involved at least 13 different premises. Seven of them were characterized in more detail and were included in this study. A morbidity of 26% was seen, with an EHM incidence of 43% in the affected horses. The outbreak was characterized by rapid occurrence of ataxia and paralysis immediately after disappearance of the fever. EHV1 was diagnosed by means of virus isolation and/or seroconversion. The isolated virus was classified either as neuropathogenic or as belonging to group 4 after sequencing in the ORF30 and ORF68 regions, respectively. The extent of this outbreak and the high percentage of neurological disease, along with the fact that EHM is only sporadically seen in Belgium, might indicate that the neurological form of EHV1 is possibly also emerging in Belgium.