Susceptibility of calves to challenge with Salmonella typhimurium 4/74 andderivatives harbouring mutations in htrA or purE

Salmonella typhimurium 4/74 is highly virulent for cattle after oral challe nge, causing severe diarrhoea, which is; sometimes associated with systemic spread of the micro-organism. Although susceptible to oral challenge, grou ps of cattle were found to be relatively resistant to subcutaneous challeng e with this strain. The virulence of S. typhimurium 4/74 harbouring mutatio ns in htrA and purE was also assessed in cattle. Although S. typhimurium 4/ 74 htrA and purE are attenuated following oral challenge in mice, cattle we re highly susceptible to oral challenge with these mutants. As with the par ent S. typhimurium 4/74 strain, cattle exhibited greater susceptibility to oral compared to challenge with S. typhimurium htrA and purE mutants. Follo wing challenge with sublethal levels of S. typhimurium 4/74 calves produced significant levels of antibodies to S. typhimurium soluble extract. No cor relation was detected between interferon gamma levels in sera and susceptib ility to infection by any route. The concentrations of the acute-phase-asso ciated protein haptoglobin were increased in the sera of five of six cattle inoculated subcutaneously, although increases in concentration were smalle r in cattle inoculated orally.