Investigation of an unexplained mass illness

An episode of spontaneous vomiting among 6 students in a class of 26 fifth-graders was reported to the regional focal point of the ARS and investigated by the Cire. A medical examination of one of the students concluded that there was no infectious cause, as the symptoms subsided outside of this class. Several causes may be responsible for "outbreaks of digestive symptoms": food poisoning (including occasional microbiological contamination of tap water), person-to-person transmission, exposure to an environmental chemical pollutant, or psychogenic syndrome. Given the definition and current understanding of psychogenic syndromes, or "unexplained collective syndrome," several criteria identified during the investigations supported this type of event: nonspecific digestive symptoms, the existence of an index case, a generally higher incidence rate among girls (in this episode, incidence rates were 38% among girls and 7% among boys), and the spread of symptoms “by sight.” Conversely, the evidence gathered did not support the other hypotheses: exclusion of a collective food poisoning outbreak (sporadic occurrence of cases, low overall attack rate and difference between girls and boys, absence of a suspected environmental source). In conclusion, the occurrence of several cases of vomiting in close succession and within the same community meets the definition of an epidemic phenomenon. The hypothesis of a psychogenic syndrome was considered the most likely based on the investigations conducted. A persistent, characteristic odor of vomit (particularly acidic) could be the trigger for symptoms, especially in vulnerable individuals. Such mass syndromes can easily lead to a crisis as long as they remain unexplained or no consensus exists within the community regarding their cause. Unexplained mass illness can occur in a tense social context (conflicts between employees and management, uncertainty about the future, relocation, reorganization, etc.). The scientific literature reports multifactorial causes, but the multidisciplinary approach to these issues, which is relatively recent, has not yet been the subject of many scientific publications.