Human listeriosis has been a reportable disease since 1986, and although our knowledge of the epidemiology remains incomplete, recent analysis of both epidemic and sporadic disease has provided considerable insight into the epidemiology and transmission of L. monocytogenes. Four major epidemics of listeriosis have occurred in North America and one in Switzerland since 1981. The cases were linked to the ingestion of contaminated dairy products in three of the epidemics, contaminated cabbage in one, and no source could be implicated in the fifth outbreak. Studies of dairy products, raw vegetables, and sausages have documented the presence of L. monocytogenes in low numbers, suggesting that this organism may be a common contaminant of both processed and unprocessed foods of plant and animal origin. Because the organism can survive and multiply at refrigerator temperatures and in a wide range of pHs, even a small amount of contamination may be significant.
Most human cases are sporadic, and the source of the infection is usually unknown.
Listeriosis is seen almost exclusively in neonates, pregnant women, and immunocompromised individuals. The clinical presentation ranges from sepsis and meningitis in neonates and immunocompromised patients to a nonspecific febrile illness in pregnant women. Transplacental infection occurs often possibly because of deficiencies in local im-munoregulation at the placenta, and results in premature labor, septic abortion, and neonatal listeriosis. Outside of the perinatal period, the most common presentation of listeriosis is meningitis. The pathogenesis of listeriosis is related to several factors, including host immunity, inoculum size, and possibly strain-specific virulence factors.