Although previous research has documented increases in Salmonella contamination during summer months, the researchers found that these seasonal patterns have largely diminished or disappeared in poultry and pork products since approximately 2010. Additionally, the researchers observed that seasonal patterns can change over relatively short periods of five to ten years.

Ground Beef Maintains Seasonal Salmonella Peak

Of the commodities analyzed, ground beef was the only product to maintain a pronounced seasonal pattern of Salmonella contamination throughout the 25-year study period.

Based on data from 2020–2024, the researchers observed that Salmonella contamination in ground beef remained relatively low and stable from December through May, followed by a substantial increase during warmer months, peaking in August. Contamination levels in August and September were similar.

However, the timing of the seasonal pattern has shifted earlier in the calendar year. Specifically, the annual peak moved from September to August beginning in 2011, while the seasonal minimum shifted from April to March in 2006, and subsequently to February in 2011.

The researchers noted that the magnitude of seasonal fluctuations in ground beef was approximately equivalent to the current overall proportion of Salmonella-positive samples, making it the strongest seasonal pattern among the commodities analyzed.

The findings suggested that interventions targeting the approximately four-month period of elevated contamination could be an effective approach to reducing Salmonella exposure from ground beef.

Seasonal Salmonella Patterns Have Diminished in Poultry and Pork

In contrast to ground beef, seasonal patterns of Salmonella contamination in broiler chicken carcasses, comminuted turkey, and pork products have weakened substantially or disappeared.

For broiler chicken carcasses, the researchers observed that the pronounced seasonal pattern present in the early 2000s gradually diminished and changed in timing. During 2020–2024, only modest seasonal variation was observed, characterized by lower contamination levels in December and February, rather than the traditional winter minimum and summer peak.

Similarly, comminuted turkey initially exhibited a seasonal pattern characterized by elevated contamination during fall months, peaking in September. However, this pattern diminished after 2005, and no statistically significant seasonal pattern was identified for 2020–2024.

For pork, the researchers found that a strong seasonal pattern in carcass samples, characterized by an August peak, had disappeared by 2011, when routine pork carcass sampling concluded.

Analysis of pork cuts and comminuted pork samples collected during 2020–2024 identified no significant seasonal pattern in pork cuts and only a weak pattern in comminuted pork. The latter exhibited a March peak and September minimum, although the seasonal variation was small relative to the overall proportion of positive samples and was considered to have little practical significance.

Implications for Salmonella Control and Surveillance

The researchers hypothesized that changes in animal production and management practices, including indoor housing, antimicrobial interventions, and other industry controls, may have contributed to the disappearance of seasonal contamination patterns in poultry and pork. However, the study did not directly evaluate the effects of these factors.

The findings may have implications for USDA-FSIS sampling programs and Salmonella risk assessments. In particular, the researchers suggested that baseline studies, which have historically been conducted over 12 months to account for seasonal variation, could potentially be shortened for commodities that no longer exhibit meaningful seasonal fluctuations, provided that monitoring continues.

Additionally, because seasonal patterns were found to change within five to ten years, the researchers emphasized that historical findings may not accurately represent current contamination patterns, underscoring the importance of ongoing surveillance.

The researchers recommended further investigation into the factors responsible for diminishing seasonal patterns in poultry and pork, including whether successful industry interventions could be applied to reduce seasonal Salmonella contamination in ground beef. They also suggested comparing seasonal contamination patterns in food products with human salmonellosis cases to help improve foodborne illness source attribution.