In the United States, foodborne illnesses remain a significant public health concern, with Salmonella being a key contributor. Every year, over 1.3 million people are affected by Salmonella infections, many of which are linked to contaminated or undercooked poultry meat and eggs. As consumers become more conscious of food safety, the growing issue of antimicrobial resistance (AMR) in Salmonella strains isolated from retail chicken meat has sparked alarm among researchers and food safety authorities alike.
A recent study conducted by researchers at the University of Illinois Urbana-Champaign sheds light on the alarming trends of antimicrobial resistance in certain Salmonella serovars isolated from chicken meat. This research not only reveals the diversity of Salmonella strains but also highlights the mounting challenge of controlling the spread of drug-resistant infections.
The Complexity of Salmonella Control
Salmonella is not a singular pathogen but rather a genus that includes thousands of strains, or serovars, each with unique characteristics. This diversity makes the pathogen exceptionally challenging to control, especially when some serovars develop multi-drug resistance. To make matters worse, Salmonella infections in poultry are often hard to detect, as birds carrying the bacteria may not exhibit any signs of illness.
“One of the key issues is that some Salmonella serovars do not cause disease in poultry, even though they can still be harmful to humans,” explains Csaba Varga, assistant professor of epidemiology at the University of Illinois and a leading expert on disease spread. “Chickens can appear healthy, but they may harbor dangerous strains of Salmonella that can infect humans when the meat is consumed.”
This stealthy nature of the pathogen complicates efforts to monitor and control its spread in poultry farms and retail chicken meat.
Tracking Antimicrobial Resistance in Chicken Meat
In an effort to monitor the presence and resistance patterns of Salmonella, the National Antimicrobial Resistance Monitoring System (NARMS) has been collecting samples from retail chicken meat since 2002. This longitudinal dataset allows researchers to track the prevalence of Salmonella over time and study how different serovars respond to antibiotics.
In the recent study, Varga’s team analyzed nearly 40,000 samples of chicken meat collected between 2013 and 2020. Approximately 7.7% of these samples (around 3,000) tested positive for Salmonella, with the most common serovars being S. Kentucky, S. Typhimurium, S. Infantis, and S. Enteritidis. Among these, S. Kentucky was the most frequently detected, accounting for 35% of the positive samples.
Interestingly, while S. Kentucky is prevalent in poultry, it poses a relatively low risk to human health because it rarely causes infections in humans. In contrast, the other three serovars—particularly S. Infantis—are more concerning due to their ability to cause illness in humans and their growing resistance to antibiotics.
The Rise of S. Infantis: A New Threat
One of the most troubling findings of the study is the dramatic rise in the prevalence of S. Infantis. Between 2015 and 2020, the proportion of positive Salmonella samples that contained S. Infantis skyrocketed from 3% to nearly 40%. This sudden increase raises alarm because S. Infantis has recently been found to carry a multidrug-resistant plasmid known as pESI.
“This plasmid harbors several genes that make S. Infantis resistant to a wide range of antibiotics,” notes Nasim Sohail, a visiting research scholar and one of the study’s authors. “Not only does this plasmid contribute to the serovar’s resistance, but it also contains virulence genes that enhance the pathogen’s ability to cause disease.”
The rise of multidrug-resistant (MDR) S. Infantis is not confined to the United States. Similar trends have been observed globally, making this serovar an emerging public health threat. As the poultry industry continues to grapple with controlling S. Infantis, researchers are concerned that existing mitigation strategies may no longer be effective.
Changes in Serovar Prevalence and Resistance
In addition to the rise of S. Infantis, the study revealed interesting trends in the prevalence of other Salmonella serovars. S. Typhimurium, which has long been a concern due to its association with human infections, has shown a marked decline in recent years. This decline is attributed to the widespread use of live attenuated vaccines targeting S. Typhimurium in poultry.
However, while vaccinating against S. Typhimurium has reduced its prevalence, this success may have inadvertently created an opportunity for other serovars, like S. Infantis, to thrive. “When we control one serovar, it can leave a gap that another serovar might fill,” explains Varga. “In this case, S. Infantis has taken advantage of that opening and become more prevalent.”
Regional Variations and Potential Causes
The study also examined the spatial distribution of Salmonella serovars across the United States. High concentrations of S. Typhimurium were found on the East Coast, while S. Kentucky was more common in the southern and western states. Environmental factors, as well as differences in chicken farming practices, may contribute to these regional variations, but more research is needed to fully understand the underlying causes.
The Need for Serovar-Specific Control Strategies
The emergence of MDR S. Infantis represents a critical challenge for the poultry industry. Existing control measures, such as vaccinations and antibiotic treatments, may not be sufficient to curb its spread. According to Varga, future control efforts must take a more serovar-specific approach, with tailored interventions designed to target the unique characteristics of each strain.
“Our findings show that while we’ve made progress in controlling S. Typhimurium, we now face a new challenge with S. Infantis,” says Varga. “We need to develop new intervention strategies to effectively manage these emerging serovars, particularly those with multidrug resistance.”
Implications for Consumers
While the poultry industry works to address the growing threat of resistant Salmonella strains, consumers can play an important role in reducing the risk of infection. Proper food handling practices, such as thoroughly cooking chicken to the correct internal temperature and avoiding cross-contamination in the kitchen, remain essential in preventing foodborne illnesses.
“We may never completely eliminate Salmonella from the food supply, but by taking precautions, consumers can significantly reduce their risk of illness,” advises Varga.
Conclusion: Addressing a Growing Threat
The rise of antimicrobial-resistant Salmonella strains in retail chicken meat is a concerning development for both public health and the poultry industry. With the rapid spread of MDR S. Infantis, there is a pressing need for more research and the implementation of effective, targeted control strategies. By staying vigilant and prioritizing food safety, we can work together to mitigate the risks associated with this persistent pathogen.
