Pooling of laying hen environmental swabs and efficacy of Salmonella spp. detection

Authors: Jones, Deana; Gast, Richard; REGMI, PRAFULLA - Purdue University; WARD, GARRETT - University Of Georgia; ANDERSON, KENNETH - North Carolina State University; KARCHER, DARRIN - Purdue University

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/16/2020
Publication Date: 6/1/2020
Citation: Jones, D.R., Gast, R.K., Regmi, P., Ward, G.E., Anderson, K.E., Karcher, D.M. 2020. Pooling of laying hen environmental swabs and efficacy of Salmonella spp. detection. Journal of Food Protection. 83(6):943-950.

Interpretive Summary: Laying hen environmental swabs are required in the U.S. for farms with > 3,000 hens producing eggs for the shell egg market. Some U.S. states also have requirements for environmental monitoring during egg production. The swabs are assessed for the presence of pathogens, generally Salmonella Enteritidis (SE). The cost burden for the swab collection and laboratory assessments are assumed by the egg producer. A single swab testing positive for SE will result in the entire flock being positive. For these reasons, there has been interest in swabbing of environmental swabs to reduce the cost burden. The current study was undertaken to determine the ability of low (10 cfu) and high (100 cfu) dose inoculum to be detected in single, 2, or 4 swab pools of manure scraper or drag swabs. Isolates of Salmonella Enteritidis, Kentucky, Heidelberg, and Typhimurium were each utilized. A flock of egg laying hens were monitored at all required FDA environmental sampling times. Low dose inoculations were difficult to detect in single and pooled manure scraper swabs due to the amount of competing flora present. High dose inoculations in manure drag swabs could be detected similarly in single and two drag swab pools for most isolates. To ensure low levels of Salmonella can be detected in all allowable types of environmental swabs, single swab analysis produces the most consistent results.

Technical Abstract: Environmental testing for Salmonella Enteritidis is required for U.S. shell egg producers with > 3,000 hens on a farm. The egg producer assumes all costs for the mandatory testing. According to the FDA Egg Rule, either manure scraper or drag swabs can be collected according to published guidelines and requirements. The current study was undertaken to determine the efficacy of Salmonella detection in single, two, and four swab pools of either manure scraper or drag swabs. Resistant isolates of Salmonella Enteritidis (1000 ppm streptomycin; SE), Heidelberg (200 ppm nalidixic acid (NA); SH), Typhimurium (200 ppm NA; ST), and Kentucky (200 ppm NA; SK) were utilized. Low (approximately 8.4 CFU) and high (approximately 84 CFU) dose inocula were introduced onto a single swab within a pool. A single flock of each conventional cage (manure scraper swabs) and cage-free barn (drag swabs) were monitored throughout the study at the ages required under the FDA Egg Rule. The greatest and most consistent recovery of inoculum was found in single swab samples. For low dose inocula, it was difficult to recover isolates from single manure scraper swabs (57.9 – 29.2 %) and decreased as more swabs were added to the pool. Recovery of isolates in manure scraper swabs was greater for high dose inoculum, though SH exhibited difficulty competing with naturally occurring flora. One and two swab pools of drag swabs had similar rates of recovery at both low and high dose SE, SH, and ST. When SE and SK were combined in an inoculum, SE was recovered at a much higher rate than SK for all types of swabs and doses of inocula. Pooling of two drag swabs allowed for similar detection of low and high dose Salmonella tested in the current study, but the pooling of manure scraper swabs decreased detection of low dose Salmonella.