Psittacosis was first described as a human disease in Europe in 1879, but was rarely reported in the United States before a pandemic that occurred in 1929-1930. This U.S outbreak was traced to parrots imported for the 1929 Christmas trade and resulted in the first ban on the importation and interstate shipment of psittacine birds in the United States. This surveillance summary is based on individual case reports submitted by state and territorial health departments. As with other diseases, the true incidence of psittacosis is higher than the reported incidence. Many cases are so mild that they go undiagnosed, since the signs do not point specifically to psittacosis and the disease may be self-limited.
Psittacosis is a disease of low incidence that tends to occur sporadically.
For this reason, it may not be considered in the differential diagnosis of a patient's illness, particularly when the patient's exposure history is not known. Underreporting of confirmed cases also occurs, and psittacosis is not a reportable disease in all states. The clinical, laboratory, and epidemiologic information on the case reports varies widely in its completeness. Thus, statements about reported frequencies should not be interpreted literally. No distinct seasonality was observed for the 112 cases with known month of onset.