AN EQUINE INFLUENZA OUTBREAK IN THE CZECH -REPUBLIC IN THE FALL OF 1995 - AN EPIDEMIOLOGIC-STUDY

A mild outbreak of acute respiratory infection was reported in racing horses in the fall of 1995. Four studs were investigated for the sourc es and routes of infection. In five horses from two herds, virus isola tes were obtained which, in preliminary typing experiments, were ident ified as the influenza A/equi 2 virus. The presence of this illness in all the examined herds was confirmed by a rise in specific antibody t itres. The affected animals included both older vaccinated horses and young horses not yet vaccinated. Epidemiological studies suggested tha t the spread of infection occurred in situations where infected and no n-infected horses were together, most frequently at races. Newly infec ted horses brought the infection back to their studs where further ani mals became infected. Some of them, still being in the incubation peri od without any clinical signs, may have taken part in another race and passed the infection onto healthy animals from other studs. Since the races usually took place in 7-day intervals, the epidemiological chai n remained continuous. The causes of the outbreak of infection in vacc inated horses are analysed. The length of post-vaccination immunity, t he antigenic composition of vaccines and their completion with new str ains of the influenza virus are discussed.