Listeriosis: a primer

Although L. monocytogenes was recognized as an animal pathogen over 80 years ago,3 the first outbreak confirming an indirect transmission from animals to humans was reported only in 1983, in Canada's Maritime provinces.4 In that outbreak, cabbages, stored in the cold over the winter, were contaminated with Listeria through exposure to infected sheep manure. A subsequent outbreak in California in 1985 confirmed the role of food in disseminating listeriosis. Since then Listeria has been implicated in many outbreaks of food-borne illness, most commonly from exposure to contaminated dairy products and prepared meat products, including turkey and deli meats, pâté, hot dogs and seafood and fish.5 Newborns may present clinically with early- (< 7 days) or late-onset forms of infection (= 7 days). Those with the earlyonset form are often diagnosed in the first 24 hours of life with sepsis. Early-onset listeriosis is most often acquired from the mother through transplacental transmission. Late-onset neonatal listeriosis is less common than the early-onset form. Clinical symptoms may be subtle and include irritability, fever and poor feeding.13 The mode of acquisition of lateonset listeriosis is poorly understood, but acquisition of the organism after birth is implicated, since there is usually no maternal infection in such cases. The organism is not felt to be spread through breast-feeding. Given the characteristics of Listeria and the risk of infection, what can be done to minimize and prevent infection? The Canadian Food Inspection Agency and the US Centers for Disease Control and Prevention have made recommendations to protect against listeriosis in the home. People with a poor immune system and pregnant women are advised to take even more rigorous precautions by avoiding foods that may become contaminated with Listeria (Box 2).5,7