Salmonella enterica serovar Pullorum persists in splenic macrophages and in the reproductive tract during persistent, disease-free carriage in chickens

Salmonella enterica serovar Pullorum is worldwide a poultry pathogen of con siderable economic importance, particularly in those countries with a devel oping poultry industry. In addition to the characteristic high mortality ra tes among young chicks, one of the features of Salmonella serovar Pullorum infection is that it persists for long periods in convalescent chicks in th e absence of clinical disease. This can lead to colonization of the reprodu ctive tract of chickens and at sexual maturity can result in infected proge ny through transovarian transmission to eggs. The sites of Salmonella serov ar Pullorum persistence in convalescent birds are not known, and the mechan isms of persistence are not understood. Here we show that Salmonella serova r Pullorum can persist in both the spleen and the reproductive tract for ov er 40 weeks following experimental infection in chickens. During the period of sexual maturity, Salmonella serovar Pullorum colonized both the ovary a nd the oviduct of hens and led to 6% of laid eggs being infected by Salmone lla serovar Pullorum. The colonization of several different sites of the re productive tract suggests that Salmonella serovar Pullorum may employ more than one mechanism of egg infection. Persistence occurred despite a strong humoral response, suggesting an intracellular site of infection. By use of a Salmonella serovar Pullorum strain containing a plasmid stably expressing green fluorescent protein, we demonstrated that the main site of carriage in the spleen is within macrophages. This raises interesting questions abou t the biology of Salmonella serovar Pullorum, including why there is an inc rease in bacterial numbers when birds become sexually mature and in particu lar how Salmonella serovar Pullorum avoids clearance by macrophages and whe ther it modulates the immune system In other ways.