Surveillance of listeriosis in Los Angeles County, 1985-1986. A first year's report.

After a large food-borne outbreak of listeriosis in Los Angeles County, California, in 1985, the California State Department of Health Services instituted mandatory reporting of Listeria monocytogenes by clinical laboratories. From September 1, 1985, through August 31, 1986, 94 cases of listeriosis were reported in Los Angeles County for an annual crude incidence rate of 12 cases per million persons. Of the 94 cases, 37 (39%) were in neonates and/or their mothers and 57 (61%) were nonperinatal. The overall case fatality rate was 31%, with a perinatal case fatality of 16% (6 fetal and 23 nonperinatal); this compares with an epidemic perinatal case fatality rate of 32%. No significant differences were observed in age-adjusted, race-specific incidence rates among nonperinatal cases or race-specific incidence rates among perinatal cases. All but 2 of the nonperinatal patients had a known predisposing risk factor for the development of listeriosis, the most common of which was a prior history of steroid therapy. A clustering of cases was not identified. No common food sources were apparent. Patients presenting as perinatal cases were more likely to have ingested Mexican-style cheese, ice cream, and yogurt than those presenting as nonperinatal cases. Improved case ascertainment through mandatory reporting and laboratory-based surveillance will establish meaningful baseline levels of listeriosis.