Between April and July 2004, the departmental community of Mayotte, a French territory in the Indian Ocean, experienced an unusual increase in the number of infant deaths, as observed by pediatricians at the Mamoudzou Hospital Center (CHM). These infants, aged 1 to 3 months and with no specific medical history, had presented with sudden-onset cardiorespiratory distress that rapidly progressed to a fatal outcome. The hypothesis of beriberi (vitamin B1 or thiamine deficiency), was quickly raised by the hospital pediatricians, leading the French Institute for Public Health Surveillance (InVS) to dispatch an epidemiological investigation team to the site with the objectives of accurately documenting this health phenomenon, attempting to identify risk factors, and guiding the control measures to be implemented in the short and medium term. A case definition with three levels of probability, along with a reporting system and active case-finding, were established. The data collected enabled a description of the outbreak in terms of time, location, and individuals. A case-control study was conducted among 21 mothers of cases and 40 control women. This study was carried out in the homes of the participants in collaboration with health educators from the Directorate of Health and Social Affairs (DAS) of Mayotte, using a standardized questionnaire covering sociocultural, clinical, dietary, and environmental factors. The study also investigated the presence of infectious, toxic, or other known cofactors associated with beriberi. Between April 4 and July 13, 2004, 32 cases of beriberi in infants aged 1 to 4 months were reported, primarily from the municipality of Mamoudzou (53.1%). Of these cases, 20 died (case fatality rate 62.5%). The urgent implementation of vitamin B1 supplementation campaigns in early June helped bring the epidemic under control. The majority of the mothers of the affected children were not French nationals (85% of them were Comorian). The median length of stay for the mothers on the island of Mayotte was 6 years. Clinical signs suggestive of beriberi during pregnancy or the postpartum period were found in 44% of the mothers of the affected children. Dietary analysis showed that the mothers of the cases had a less diverse diet than the control mothers during their pregnancy and the postpartum period. The traditional consumption of oubou (rice porridge) during breastfeeding was more frequent among the mothers of the cases. The use of formula was reported less frequently by the mothers of the cases. None of the potential cofactors investigated could be identified despite a broad scope of the study. The results of this investigation confirm the occurrence of an infantile beriberi epidemic in Mayotte, a rare, serious, and unprecedented phenomenon in French territory. Severe dietary deficiencies due to economic and cultural factors appear to be the primary cause of this epidemic. Emergency control measures successfully contained the outbreak, but food and nutrition issues clearly constitute a vast and concerning public health problem in Mayotte. Further documentation of this problem and the implementation of longer-term control measures are a public health priority. Conducting these surveys and implementing the resulting measures constitute a public health emergency for Mayotte, for while it is true that the beriberi epidemic the island faced was successfully contained quickly, it remains true that the measures implemented addressed the consequences rather than the causes of the nutritional problems. It cannot be ruled out that, sooner or later, the food problem in Mayotte will lead to other public health emergencies, whether beriberi or other nutrition-related diseases. All of these considerations lead us to make the following recommendations: - continue thiamine supplementation for pregnant women and infants until medium- and long-term measures are developed - continue specific surveillance for beriberi - monitor the progression of the outbreak and the emergence of any new cases - evaluate the effectiveness of supplementation campaigns - Immediately strengthen preventive measures and promote a diverse diet among pregnant and breastfeeding women in Mayotte - study the feasibility of supplying Mayotte with rice of higher nutritional quality than that currently distributed - document the prevalence of lalavi and any other potential vitamin and nutritional deficiencies among pregnant women on the island - prepare a population-based study on nutritional behaviors and practices in Mayotte, in order to assess the nutritional status of the population of Mayotte, identify all potential deficiencies, and adapt prevention messages and strategies - develop, based on the results of this study and those of other research, a specific prevention strategy for beriberi in Mayotte, as well as a much broader strategy for addressing the serious nutritional problems facing Mayotte. (R.A.)