Epidemiology of Poliomyelitis in the United States and Its Recognition as an Infectious Disease from the Mid-19th Century to the Early 20th Century

This study reviews the role of epidemiology in the United States in the late 19th and early 20th century, which led to recognition that poliomyelitis is an infectious disease and set the stage for subsequent developments in virology and immunology, the development of inactivated and live attenuated polio vaccines, and a dramatic worldwide decrease in poliomyelitis mortality and morbidity. Epidemiological studies in the United States were systematically reviewed from the mid-19th to early 20th centuries. Isolated cases and scattered small outbreaks of poliomyelitis in the mid-19th century led to epidemics of increasing size by the end of the century, causing public consternation, especially as the disease was considered “new” and had a predilection for young children. By the 1890s, the seasonal pattern of epidemics suggested that poliomyelitis might have an infectious etiology, but direct evidence of communicability or contagiousness was lacking, so an infectious etiology was not widely suspected until the early 20th century. Reports of bacterial isolations from spinal fluid and postmortem tissues suggested that poliomyelitis might be a bacterial disease, and simultaneous outbreaks of paralytic disease in humans and animals suggested a possible zoonotic basis. Although experimental studies showed that it was theoretically possible for flies to serve as vectors of poliovirus, and occasional cases of polio were likely caused by fly-borne transfer of poliovirus from human feces to human food, a fly abatement field trial showed convincingly that flies, whether biting or non-biting, could not explain the bulk of cases during polio epidemics. In conclusion, the early application of epidemiological evidence beginning in the late 19th century strongly suggested the infectious nature of the disease, distinct from previously identified conditions. Subsequent advances in virology and immunology from 1909 to 1954 proved that poliomyelitis was a viral disease with no natural animal host and made feasible the development of an inactivated trivalent poliovirus vaccine by Salk, and, subsequently, a live-attenuated trivalent poliovirus vaccine by Sabin.