EQUINE INFLUENZA IN WEST-BERLIN, 1988 TO 1989

Equine influenza virus, subtype A/equi 2 strains, which differed in their haemagglutinin and neuraminidase properties from known strains, were isolated during an epidemic in Berlin. Detailed serological examinations demonstrated that similar strains were also responsible for an epidemic in March 1988, although no virus was isolated. Serological tests indicated that the majority of affected horses possessed inadequate or no antibodies to surface antigens of the new virus strains. Zero or low titre antibodies to haemagglutinins and neuraminidases of the prototype viruses incorporated in the current vaccines indicate that animals were only partially immunized, not immunized, or the immunity induced was short-lived. The rapid succession of influenza epidemics in horses is therefore most probably due to extensive antigenic drift of the subtype A/equi 2 and the reduced ability of currently available vaccines to induce cross protecting immunity together with inadequate attention to the recommendations for the use of vaccines.