Swine influenza: epizootiological and serological studies.

Studies of naturally occurring respiratory diseases in the midwestern parts of the USA showed that swine influenza is still prevalent and that mild forms as well as the classical forms of swine influenza occur. Outbreaks of respiratory disease of unknown etiology that are clinically similar to swine influenza were also found. On some farms, swine influenza occurred first in farrowing pens. It did not occur on some farms where the disease had occurred in previous years. This disappearance may have resulted from the elimination or hyperimmunization of breeder animals or from a change to the raising of swine obtained by caesarean section. Serological studies of swine with natural or experimental infections showed that antibody titres rose gradually for several months. This observation was corroborated in serological studies of sera obtained at the abattoir, which showed that older breeder swine had consistently higher titres than the younger market swine. These results cannot be explained by the lungworm hypothesis proposed by R. E. Shope for the survival and transmission of swine influenzavirus. It is suggested that breeder swine act as convalescent carriers and as the reservoirs of swine influenzavirus between epizootics.