Lowbury Lecture. Listeriosis: epidemiology, virulence and the significance of contaminated foodstuffs.

Recent epidemiological evidence confirms that sporadic, as well as outbreak-associated, cases of listeriosis are primarily foodborne in origin. Implicated foodstuffs include meat products, dairy products, fruit, seafood and raw and processed vegetables. Large community-acquired outbreaks in North America and Europe have been complemented by smaller outbreaks involving hospitalized patients. Anecdotal reports and case clusters of nosocomial cases also support foodborne transmission. Cross-infection may be a major mode of transmission as demonstrated in a recent outbreak in Costa Rica. The sporadic nature of outbreaks of listeriosis are more consistent with changes in organism virulence rather than host susceptibility. The population of patients at risk for listeriosis (pregnant women and immunocompromised hosts) may not vary greatly. Establishment of infection is probably dose-dependent and gastric acidity may be protective. However, other organism-specific virulence factors, such as haemolysin production, may affect the post-intestinal phase of infection. Virulence factors other than haemolysins have not been characterized as yet. In summary, acquisition of Listeria monocytogenes infection from the environment by susceptible hosts may be widespread but invasive infection remains rare and the determinants of invasion require further elucidation.