Lu Zhao, Gröhn Yrjö T, Smith Rebecca L, Karns Jeffrey S, Hovingh Ernest, Schukken Ynte H
Department of Population Medicine and Diagnostic Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY, 14853, USA,
Bull Math Biol. 2014 Mar;76(3):541-65. doi: 10.1007/s11538-013-9931-5. Epub 2014 Feb 28.
Salmonella is a major cause of bacterial foodborne disease. Human salmonellosis results in significant public health concerns and a considerable economic burden. Dairy cattle are recognized as a key source of several Salmonella serovars that are a threat to human health. To lower the risk of Salmonella infection, reduction of Salmonella prevalence in dairy cattle is important. Vaccination as a control measure has been applied for reduction of preharvest Salmonella prevalence on dairy farms. Salmonella vaccines are usually imperfect (i.e., vaccines may provide a partial protection for susceptible animals, reduce the infectiousness and shedding level, shorten the infectious period of infected animals, and/or curb the number of clinical cases), and evaluation of the potential impacts of imperfect Salmonella vaccines at the farm level is valuable to design effective intervention strategies. The objective of this study was to investigate the impact of imperfect Salmonella vaccines on the stochastic transmission dynamics in an adult dairy herd. To this end, we developed a semi-stochastic and individual-based continuous time Markov chain (CTMC) vaccination model with both direct and indirect transmission, and applied the CTMC vaccination model to Salmonella Cerro transmission in an adult dairy herd. Our results show that vaccines shortening the infectious period are most effective in reducing prevalence, and vaccines decreasing host susceptibility are most effective in reducing the outbreak size. Vaccines with multiple moderate efficacies may have the same effectiveness as vaccines with a single high efficacy in reducing prevalence, time to extinction, and outbreak size. Although the environment component has negligible contributions to the prevalence, time to extinction, and outbreak size for Salmonella Cerro in the herd, the relative importance of environment component was not assessed. This study indicates that an effective vaccination program against Salmonella Cerro spread in the herd can be designed with (1) vaccines with a single high efficacy in reducing either the infectious period or susceptibility of the host, or (2) if such single high efficacy vaccines are not available, vaccines with multiple moderate efficacies may be considered instead. These findings are also of general value for designing vaccination program for Salmonella serotypes in livestock.
沙门氏菌是细菌性食源性疾病的主要病因。人类沙门氏菌病引发了重大的公共卫生问题和相当大的经济负担。奶牛被认为是几种对人类健康构成威胁的沙门氏菌血清型的关键来源。为降低沙门氏菌感染风险，降低奶牛群中沙门氏菌的流行率很重要。疫苗接种作为一种控制措施已被用于降低奶牛场收获前沙门氏菌的流行率。沙门氏菌疫苗通常并不完美（即，疫苗可能为易感动物提供部分保护，降低传染性和排菌水平，缩短感染动物的感染期，和/或抑制临床病例数量），在农场层面评估不完美的沙门氏菌疫苗的潜在影响对于设计有效的干预策略很有价值。本研究的目的是调查不完美的沙门氏菌疫苗对成年奶牛群中随机传播动力学的影响。为此，我们开发了一个具有直接和间接传播的半随机且基于个体的连续时间马尔可夫链（CTMC）疫苗接种模型，并将CTMC疫苗接种模型应用于成年奶牛群中沙门氏菌塞罗型的传播。我们的结果表明，缩短感染期的疫苗在降低流行率方面最有效，降低宿主易感性的疫苗在减小暴发规模方面最有效。具有多种中等效力的疫苗在降低流行率、灭绝时间和暴发规模方面可能与具有单一高效力的疫苗具有相同的效果。尽管环境因素对牛群中沙门氏菌塞罗型的流行率、灭绝时间和暴发规模的贡献可忽略不计，但未评估环境因素的相对重要性。本研究表明，针对牛群中沙门氏菌塞罗型传播的有效疫苗接种计划可以通过以下方式设计：（1）使用在降低感染期或宿主易感性方面具有单一高效力的疫苗，或者（2）如果没有这种单一高效力的疫苗，则可以考虑使用具有多种中等效力的疫苗。这些发现对于设计家畜沙门氏菌血清型的疫苗接种计划也具有普遍价值。
