Investigation into the seasonal salmonellosis in lactating dairy cattle

(Accepted 11 April 2007; first published online 17 May 2007)SUMMARYSporadic salmonellosis has been reported in mature lactating dairy cattle in the southwesternUnited States and is an intriguing problem in that Salmonella can be cultured from faecal samplesof these cattle throughout the year. However, it is pathogenic only during late summer/earlyautumn and in certain years. We sampled apparently healthy (n=10) and diarrhoeic (n=10)cattle during an outbreak on a 2000 head dairy in 2003. The following year, monthly faecal(from the same 30 head), total mixed ration, water, and pen soil samples were collected forSalmonella culture. No serogroup, serotype, genetic, or antimicrobial susceptibility differenceswere observed in comparison of isolates from healthy and sick cattle. During year 2 of the study,Salmonella was routinely cultured (although highly variable from month to month) from thecattle and the environment, although no outbreak of salmonellosis was observed.INTRODUCTIONDairy cattle serve as an important reservoir forSalmonella and have been implicated in cases of hu-man salmonellosis [1, 2]. The United States NationalAnimal Health Monitoring System’s Dairy ’96 studyreported 5.4% of milk cows shed Salmonella and27.5% of dairy operations had at least one cowshedding Salmonella [3]. Salmonella has been isolatedfrom all ages of dairy cattle and throughout the pro-duction process. Mature dairy cattle typically appearasymptomatic while shedding this pathogen in theirfaeces [4–7] and while young calves are more suscep-tible to salmonellosis, cases in adult cattle have beenreported [8–10].Previous research conducted by our laboratorydemonstrated significant variation in the prevalenceof faecal Salmonella in healthy, lactating dairy cattle,not only among farms across the United States [11],butalsoinfarmswithinasmallgeographicareaandinindividualfarmsfromseasontoseason[7].Additionalresearch examined production parameters (heifers vs.mature cows, lactation status, stage of lactation andheat stress) on Salmonella prevalence [6, 12]. Whileminor differences were noted in Salmonella shedding,results were generally inconsistent with no significanttrends noted. Although heat stress did not result inany Salmonella shedding differences as measured byfaecal incidence in the morning compared to theafternoon, in one experiment Salmonella prevalenceaveraged nearly 100% [6].