What can we do to prevent listeriosis in 2006?

Investigations of outbreaks of listeriosis during the 1980s demonstrated that lis-teriosis is a foodborne disease. With the exception of 4 large outbreaks occurring in the United States, the United Kingdom, and France, each of which involved 1100 cases, most listeriosis outbreaks have involved few cases (usually !20 cases). Throughout Europe and the United States, the majority of cases of listeriosis reported to public health officials are apparently not linked to a common source and are, therefore , considered to be sporadic. The role of foodborne transmission in sporadic listeriosis was demonstrated in the 1990s in the United States by 2 case-control studies that associated the consumption of undercooked poultry, non-reheated hot dogs, soft cheeses, and delicatessen counter foods with sporadic listeriosis. The ability of Listeria mono-cytogenes to grow at refrigeration temperatures , coupled with the ubiquitous presence of the bacterium (e.g., in unpas-teurized milk, in dairy products, and in raw and processed meats, as well as in poultry, vegetables, and seafood), makes this bacterium a serious threat to susceptible consumers. In outbreaks, the vehicle of transmission is usually a highly contaminated, ready-to-eat food. However, in sporadic cases, different foods could be implicated, and factors such as the duration and conditions of storage, food handling, and modes of food preparation can influence the frequency and level of contamination. In this issue of Clinical Infectious Diseases , Varma et al. [1] studied the food preferences associated with 169 cases of sporadic listeriosis. Their study shows that the disease is associated with the consumption of hummus prepared in a commercial establishment and with eating melon at a commercial establishment. However, contamination by these foods would explain !20% of the cases. Patients may have been contaminated by many different products that this study did not identify because of the lack of power or the biases inherent in the design of the study. Case-control studies for listeriosis are a challenge, because several conditions inherent to this illness can bias the study. Listeriosis occurs mostly in elderly patients who have an underlying disease that can interfere with their food habits, so matching case patients and control subjects with the same underlying disease (e.g., cancer of the same organ and the same type of diabetes) is crucial but is not always possible. The combination of listeriosis, underlying disease, and, often, older age makes the interview difficult or impossible. The investigators will, then, have to rely on surrogates …