US study finds changing seasonal Salmonella patterns

Researchers at the US Department of Agriculture Food Safety and Inspection Service have found that seasonal Salmonella patterns differ substantially between raw meat and poultry products. Their analysis of samples collected between 2000 and 2024 identifies a strong recurring increase in positive tests for ground beef, while several poultry and pork categories now show weak or statistically insignificant annual cycles. The findings appeared online in the Journal of Food Protection on 23 October 2026.

Michael S Williams, Iva Bilanovic, Neal J Golden and Anant X Venu modelled the proportion of samples testing positive for Salmonella, rather than the number of human infections. The researchers used a common approach to compare the timing and strength of variation across sampled products and historical periods. A positive sample records detection of the organism in the particular product tested; it does not by itself quantify exposure in consumers, the concentration of the bacteria or the probability of illness.

Ground beef is the only product group with a pronounced seasonal pattern in the most recent data, and the fitted peak has moved somewhat earlier in the year. Previous studies often found higher Salmonella positivity during summer, but the new analysis shows that the familiar seasonal profile has not persisted equally across commodities. The researchers did not demonstrate why the ground beef peak shifted, so changes in weather, sourcing, slaughter operations or processing controls should not be assigned as causes without further evidence.

The 2020–2024 results show only weak seasonality for chicken carcasses and no statistically significant cycle for comminuted turkey. The turkey series had shown stronger autumn variation in earlier years, but that pattern diminished after 2005. Positive samples may still occur throughout the year when an annual cycle is weak or absent. The measured change concerns the timing of detection and must be considered alongside the overall proportion of positive results.

Pork results also vary by product and sampling history. Earlier pork carcass data showed a summer peak, but routine carcass sampling ended in 2011, preventing direct comparisons with the 2020–2024 period. In that later period, pork cuts showed no significant seasonal pattern and comminuted pork displayed only weak variation. The latter had a small March peak and September low point in the reported analysis, with a magnitude limited relative to its overall proportion of positive samples.

To distinguish persistent annual cycles from longer-term changes, the study used a common statistical modelling approach across the sample series; Fourier terms were among the methods identified by the authors. Such a model can estimate when a recurring rise occurs and how large that seasonal component is relative to a product’s broader results. Identifying the operating or environmental changes responsible for diminished seasonality would require separate causal evidence. The weaker cycles developed over periods of roughly five to ten years.

Sampling results must also be interpreted within the structure of each surveillance programme. Differences in the products tested, sampling frequency, detection procedures and the population represented may affect the observed proportion of positive results. Average contamination levels and the risk associated with each product require separate evaluation, including sampling design and microbiological results across the year. The height of a seasonal peak measures a different property from the absolute rate of positive tests.

The findings may inform how sampling resources are distributed in meat-processing operations, but basic controls remain necessary throughout the year. Separation of raw material, hygienic equipment design, cleaning verification, temperature management and product-specific testing still follow the hazards and validation requirements of the plant. The surveillance analysis offers a way of reviewing sampling priorities, while treatment technologies and plant-specific control performance require their own validation evidence.

Where a strong increase recurs predictably, additional sampling or control verification during higher-risk months may provide a focused use of resources. The authors suggest that seasonal intervention can be cost-effective for commodities with a persistent cycle, although they do not report a universal cost-benefit figure for any specific processing technology. Ground beef presents the clearest recent seasonal signal in the data. Where no meaningful cycle remains, concentrating checks into a presumed summer peak could miss contamination occurring at other times.

Different cuts and manufacturing processes therefore require separate surveillance decisions. Ground beef, chicken carcasses, comminuted turkey, pork cuts and comminuted pork are not interchangeable sample populations; their handling steps and opportunities for contamination differ. A processor adjusting testing schedules would need to consider its own results, product characteristics, regulatory obligations and validated controls before changing frequency. Equivalent surveillance data would be needed to establish whether plants in Britain or other markets follow similar seasonal profiles.

Published on 23 October under DOI 10.1016/j.jfp.2026.100953, the study documents changes in the seasonal distribution of Salmonella-positive samples while leaving their causes unresolved. It supports closer product-by-product evaluation of historical sampling assumptions while leaving the relationship between seasonal shifts, processing interventions and human illness for further investigation. Continuous pathogen control remains necessary irrespective of whether a product retains a measurable seasonal peak.