The objective of this study by A.F.A. Pires and colleagues at Michigan State University was to identify risk factors associated with persistence of Salmonella shedding in finishing swine.

In a paper published in Preventive Veterinary Medicine, they describe a longitudinal study they conducted in 18 cohorts of pigs from three finishing sites of one swine production company.

Among the 446 Salmonella isolates (isolated from 187 pigs), there were 18 distinct serovars. The six most common serovars were S. enterica serovar Derby (47.3 per cent), S. Agona (27.4 per cent), S. Johannesburg (10.5 per cent), S. Schwarzengrund (2.7 per cent), S. Litchfield (2.5 per cent) and S. Mbandaka (2.2 per cent).

Survival analysis techniques, Kaplan-Meier methods and Log-rank test were used to estimate the duration of Salmonella shedding in days and to evaluate differences in shedding associated with risk factors at different organisational levels: isolate (serovar), pig, cohort and site.

The risk factors at the pig-level were: sex, age and individual health status; and the risk factors at the cohort-level were: health risk, treatment and 'at-risk pigs' proportions, nursery and barn environment Salmonella status and prior exposure to the same serovar in the nursery or barn environment.

Survival analysis using acceleration failure time models, with a log-normal distribution, was applied to investigate risk factors associated with Salmonella persistence (175 pigs) and serovar-specific persistence (151 pigs) during the study period.

Pigs detected Salmonella-positive for the first time at 10 weeks of age had a longer duration of shedding, than pigs first detected at an older age.

The duration of shedding was shorter among pigs infected with S. Derby, S. Johannesburg and other serovars as compared to pigs infected with S. Agona.

A significant difference was observed among sites.

Cohorts with pig treatment proportions greater than the median were more likely to have a shorter duration of Salmonella shedding.

Pigs from cohorts with nursery positive pools greater than the overall mean had a longer duration of Salmonella shedding as compared to pigs from cohorts with nursery pools less than or equal to the mean.

Pires and colleagues conclude their results suggest that the duration of Salmonella shedding may depend on Salmonella serovar, pig age at the time of infection, farm site and cohort-level risk factors.

They added that identification of risk factors associated with the duration of shedding may allow more targeted interventions for the control Salmonella by evaluation of control measures not only for prevalence reduction but also to decrease the duration of shedding. Such measures may decrease the risk of contamination of pork and subsequent risk of foodborne illness.

Reference

Pires A.F.A., J.A. Funk and C. Bolin. 2014. Risk factors associated with persistence of Salmonella shedding in finishing pigs. Preventive Veterinary Medicine. 116 (Issues 1-2):120-128.

October 2014