Invisible but dangerous, the bacteria carrier state is starting to reveal its secrets

Abstract

The analysis of bacterial carrier state in domestic animals is relatively recent, despite the important role of carriers in the epidemiology and evolution of infectious diseases. The most documented examples are zoonoses, i.e. animal diseases transmitted to humans, primarily through food. The carrier animal is infected by bacteria without manifesting symptoms of this infection ; these bacteria are then transmitted to another susceptible human or animal host. In some cases, carrier state prevalence in animals is very high and its degree of importance is related to the frequency and/or the seriousness of the diseases it induces. The phenomenon is misunderstood and a better understanding of the mechanisms involved could improve its control, thus reducing disease risk in humans. The use of analytical tools in relevant experimental models has enabled the initial analysis of bacterial and host factors involved in the carrier state. The most documented example is the intestinal carrier state, which was compared in several models: Salmonella enterica serovar Typhimurium or Salmonella enterica serovar Enteritidis in chickens, Campylobacter jejuni in the same host species and Escherichia coli O157:H7 in ruminants. This work shows that the factors involved are numerous in both the bacterium and the host. Each model has revealed the involvement of factors of which a few are in common. Regulating or modulating mechanisms that help to explain this balance between the bacterium and its host are also highlighted.