By John Brown, DVM, senior technical services veterinarian, Zoetis

The number of cage-free egg layers has soared in recent years, accounting for approximately 39% of the national table-egg laying flock in January 2024.¹

With the increased population of cage-free layers, questions have been raised about the impact on the prevalence of foodborne Salmonella.

The predominant Salmonella serotype in layers that’s of concern is Salmonella Enteritidis; Salmonella Typhimurium can also occur, although to a lesser extent.²

Studies comparing the prevalence of Salmonella among different types of production systems for layers have yielded different results.³ Personally, I haven’t seen much difference in Salmonella prevalence between caged and cage-free hens. The risk for Salmonella seems to be less related to the type of system and highly related to management, and the best control of Salmonella I’ve witnessed is achieved when good management is coupled with vaccination.

Potential for horizontal transmission

It’s probably true that among hens in cage-free systems there is more potential for horizontal transmission of Salmonella simply because birds have more access to each other.

Although there are pens within cage-free systems that separate different groups of birds, the birds within each section have access to each other’s manure and to dust created by birds. Feces and dust are reservoirs for Salmonella. Cage-free production also raises the risk of birds coming into contact with rodents, which are Salmonella vectors.

Asking for trouble when management chores are neglected

Issues with Salmonella are certain to rise when sanitation, rodent control and biosecurity are lacking. When any of these important management chores are neglected, Salmonella will be a problem in any type of production system.

It’s important to remember that simple, everyday biosecurity measures, such as changing shoes between buildings, help prevent people tracking in Salmonella as well as a host of other pathogens into poultry houses.

Vaccination plays key role

The industry has done a phenomenal job regarding vaccination of layer flocks against Salmonella. When Salmonella Enteritidis first came to prominence in the layer industry, it was more often transmitted vertically from breeders to offspring. Once breeder producers upped testing, eliminated positive flocks and implemented Salmonella vaccination programs, vertical transmission was brought under control.

Vaccination programs are obviously going to differ, but most involve use of live and inactivated Salmonella vaccines because each kind has its own attributes.

One example of a Salmonella vaccine protocol for layers would start with administration of a live Salmonella Typhimurium vaccine (such as Poulvac^® ST) given at 1 day of age and another at 2 to 3 weeks of age; some producers give a third dose at 6 to 10 weeks of age. Live Salmonella vaccines are thought to elicit a broad host-immune response.³

Protection from live vaccines, however, may not last as long as laying hens are in production, so an inactivated vaccine is also often given around 12 to 14 weeks of age. Inactivated vaccines generally require injection and may help protect against multiple strains of Salmonella,³ although one of the live Salmonella vaccines (Poulvac ST) has been shown to provide cross-protection.⁴

It takes time for vaccine-initiated immunity to develop, so hens need to be protected from Salmonella in the meantime — which, here again, underscores the importance of good sanitation and biosecurity.

References

¹ U.S. Department of Agriculture, Economic Research Service. Livestock, dairy, and poultry outlook: February 2024 (Report No. LDP-M-356). Accessed March 13, 2024, at https://www.ers.usda.gov/webdocs/outlooks/108574/ldp-m-356.pdf?v=5226.1.

² UB Consulting. Analyzing the growth of the cage-free egg market. Feb. 25, 2022. https://www.urnerbarry.com/News/1219575/UB-Consulting-Analyzing-the-Growth-of-the-Cage-Free-Egg-Market. Accessed May 13, 2022.

³ Jia S, et al. Challenges in vaccinating layer hens against Salmonella Typhimurium. Vaccines. 2020;8(4):696.

⁴ Muniz E, et al. Evaluation of the effectiveness and safety of a genetically modified live vaccine in broilers challenged with Salmonella Heidelberg. Avian Pathol. 2017;46(6):676-682.

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