[Equine influenza outbreaks with viral antigenic drift in Berlin 1988-1991]

In this paper three outbreaks of equine influenza in Berlin (Germany) in the years of 1988, 1989 and 1991 are discussed, reporting mainly clinical, hematological, virological and some epizootiological aspects. We have detected variations from the traditional pattern of equine influenza, whereby the main clinical symptoms like cough or fever were absent in several cases. If cough was found, it was moist. Furthermore a mucous nasal discharge was present in a number of cases for a period of 4-5 days. Extreme neutropenia, lymphocytosis and predominantly an unchanged level of monocytes were observed. Several horses became ill, in spite of having been regularly vaccinated against equine influenza. As cause, a high antigenic drift of the influenza virus isolated from 1989 and 1991 is assumed in comparison to the strains that are used for the influenza vaccines available. The origin of the viruses which had caused the influenza outbreaks described could not be elucidated.