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To investigate the microbiology and epidemiology of the 1,700 sporadic cases of listeriosis that occur annually in the United States, we developed a multilocus enzyme electrophoresis (MEE) typing system for Listeria monocytogenes. We studied 390 isolates by MEE. Eighty-two electrophoretic types (ETs) were defined. Two distinct clusters of ETs, ET group A (ETGA) and ET group B (ETGB), separated at a genetic distance of 0.440, were identified. Strains of ETGB were associated with perinatal listeriosis (P = 0.03). All strains of H antigen type a were in ETGA, while all strains of H antigen type b were in ETGB. Among 328 clinical isolates from cases of listeriosis, 55 ETs of L. monocytogenes were defined. Thirty-four ETs were identified among 62 isolates from food products. The mean number of strains per ET (5.2) was significantly higher among clinical isolates than among food-borne isolates. Examination of isolates from outbreaks further documented the link between cases and contaminated food products. In one investigation, we found 11 different ETs, ruling out a single common source as a cause of that outbreak. By examining a large number of isolates collected over a specified time in diverse geographic locations in the United States, we have begun to establish a baseline for the study of the epidemiology of listeriosis by MEE.

After a decade-long resurgence, including a large cutaneous diphtheria outbreak in Washington State, the diphtheria incidence rate in the United States reached its lowest recorded level ever in 1980-two patients (0.01 case per million). Mortality paralleled the decline in incidence rate. Only 143 of the 3,141 US counties reported noncutaneous diphtheria patients during 1971-81; most were located in the West. The highest attack rates were experienced by children less than 15 years old (0.8 case per million) and by American Indians (22.6 cases per million). Persons immunized with three or more doses of diphtheria toxoid had a lower death-to-case