Nakamura R M, Easterday B C, Pawlisch R, Walker G L
Bull World Health Organ. 1972;47(4):481-7.
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.
对美国中西部地区自然发生的呼吸道疾病的研究表明，猪流感仍然流行，且存在温和型以及经典型猪流感。还发现了临床上与猪流感相似但病因不明的呼吸道疾病暴发。在一些农场，猪流感首先出现在产仔栏。在前些年发生过该病的一些农场并未出现。这种消失可能是由于种畜的淘汰或超免疫，或者是由于改为剖腹产获得的猪的饲养方式。对自然感染或实验感染猪的血清学研究表明，抗体滴度在几个月内逐渐上升。在屠宰场采集的血清的血清学研究中证实了这一观察结果，该研究表明，年长的种猪的抗体滴度始终高于年轻的市场猪。这些结果无法用R.E.肖普提出的关于猪流感病毒生存和传播的肺线虫假说解释。有人提出，种猪在动物间流行期间充当恢复期携带者和猪流感病毒的储存宿主。
