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Title: Assessing the potential impact of Salmonella vaccines in an endemically infected dairy herd

Author
item LU, ZHAO - Cornell University
item GROHN, YRJO - Cornell University
item SMITH, REBECCA - Cornell University
item WOLFGANG, DAVID - Pennsylvania State University
item Van Kessel, Jo Ann
item SCHUKKEN, YNTE - Cornell University

Submitted to: Journal of Theoretical Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/30/2009
Publication Date: 8/21/2009
Citation: Lu, Z., Grohn, Y.T., Smith, R.L., Wolfgang, D.R., Van Kessel, J.S., Schukken, Y.H. 2009. Assessing the potential impact of Salmonella vaccines in an endemically infected dairy herd. Journal of Theoretical Biology. 259(4):770-784.

Interpretive Summary: Salmonella are a major group of food-borne pathogens known to affect both man and animals. Dairy cattle are a known reservoir of these bacteria and reduction of Salmonella prevalence in herds is important to prevent human Salmonella infections. Typical on-farm control measures include removing infectious animals from the herd, vaccination, and improved hygiene management. Vaccines have been developed for controlling Salmonella transmission in dairy herds; however, these vaccines are imperfect and a variety of vaccine effects on susceptibility, infectiousness, Salmonella shedding level, and duration of infectious period have been observed. In this study we used a mathematical modeling approach to assess the potential impact of imperfect Salmonella vaccines on prevalence over time and the eradication criterion. A deterministic compartmental model was developed with both replacement (cohort) and lifetime (continuous) vaccination strategies, and applied it to a Salmonella Cerro infection in a dairy farm. To understand the uncertainty of prevalence and identify key model parameters, global parameter uncertainty and sensitivity analyses were performed. The results show that imperfect Salmonella vaccines reduce the prevalence of Salmonella Cerro but the impacts differ depending on the vaccine effect. Among three vaccine effects that were being considered in the model, decreasing the length of the infectious period is the most effective in reducing the endemic prevalence in the herd. Analysis of the model results showed that reducing Salmonella prevalence in the herd should be possible by choosing vaccines that have either a single vaccine effect at relatively high effectiveness, or two or more vaccine effects at relatively low effectiveness. The model also showed that continuous vaccination is more effective than single time vaccination of animals that are brought into the herd. The model developed in this study may be used as a tool for guiding the development of new Salmonella vaccines for cattle.

Technical Abstract: Salmonella spp. in cattle are contributing to bacterial foodborne disease for humans. Reduction of Salmonella prevalence in herds is important to prevent human Salmonella infections. Typical control measures are culling of infectious animals, vaccination, and improved hygiene management. Vaccines have been developed for controlling Salmonella transmission in dairy herds; however, these vaccines are imperfect and a variety of vaccine effects on susceptibility, infectiousness, Salmonella shedding level, and duration of infectious period were reported. To assess the potential impact of imperfect Salmonella vaccines on prevalence over time and the eradication criterion, we developed a deterministic compartmental model with both replacement (cohort) and lifetime (continuous) vaccination strategies, and applied it to a Salmonella Cerro infection in a dairy farm. To understand the uncertainty of prevalence and identify key model parameters, global parameter uncertainty and sensitivity analyses were performed. The results show that imperfect Salmonella vaccines reduce the prevalence of Salmonella Cerro. Among three vaccine effects that were being considered, decreasing the length of the infectious period is most effective in reducing the endemic prevalence. Analyses of contour lines of prevalence or the critical reproduction ratio illustrate that reducing prevalence to a certain level or zero can be achieved by choosing vaccines that have either a single vaccine effect at relatively high effectiveness, or two or more vaccine effects at relatively low effectiveness. Parameter sensitivity analysis suggests that effective control measures through applying Salmonella vaccines should be adjusted at different stages of infection. In addition, lifetime (continuous) vaccination is more effective than replacement (cohort) vaccination. The presented study may be used as a tool for guiding the development of Salmonella vaccines.