The clinical manifestations of infection were analysed during an outbreak of 34 cases of Yersinia pseudotuberculosis serotype Ia infection. The diagnosis was based on the results of enzyme immunoassay and verified by stool culture in five cases. The first four patients were pupils from the same school, and information on any signs and symptoms of infection was obtained by questionnaire from pupils of the school. A blood sample was obtained from 101 children. Strong IgM and IgG antibody responses to Yersinia pseudotuberculosis Ia were found in 13 (25%) of the 52 pupils who reported signs and symptoms of infection and in 9 (18%) of the 49 with no manifestations of infection. The vigorous immune response also resulted in effective opsonization of the causative microorganism. The most common symptoms were fever and abdominal pain. Three children operated on because of suspected appendicitis were found to have mesenteric lymphadenitis. Only one patient developed reactive arthritis. Analysis showed that a remarkable proportion of Yersinia pseudotuberculosis infections may be subclinical, and that individual Yersinia pseudotuberculosis strains may have different capacities to cause postinfection complications.

Passive immunization with monoclonal antibodies specific for the lipopolysaccharide (LPS) O-side chain protected mice against intravenously given lethal doses of Yersinia enterocolitica O:3 bacteria. On the other hand, passive immunization with monoclonal antibody specific for the LPS core oligosaccharide did not protect mice. Neither antibody was able to protect mice against orally given lethal doses of bacteria. These results indicate that the O-side chain functions as an important antigenic structure during infection, and that immunity to it probably offer protection also in the in vivo situation.