Chickens and eggs in the built environment

The built environment includes housing for livestock, which may have consequences for the transmission of zoonotic diseases in our food supply. Different animal housing strategies provide an ideal opportunity to test the effects of crowding and degree of confinement on the microbiology of built environments. In the case of egg-laying chickens, housing strategies include conventional cages, enriched cages, cage-free, free-range, pasture-raised, and organic. Proponents of these different strategies often claim that Salmonella Enteritidis transmission is reduced by their preferred approach, while some studies have detected no differences.

Adult chicken in a cageChicken housing strategies are currently in the news because six states are suing California over new regulations on how egg-laying chickens are housed. In 2008, California voters passed Proposition 2, an initiative to prohibit the confinement of some farm animals in a manner that does not allow them to turn around freely, lie down, stand up, and fully extend their limbs, except as needed for veterinary care. Proposition 2 goes into effect in 2015 and specifically addresses three types of livestock confinement: veal crates, sow gestation crates, and battery cages for chickens. California is the only state in the country to extend such protections to chickens. In this regard, the United States lags behind the European Union who banned battery cages in 2012 and Germany, Switzerland, Sweden, and Austria who banned battery cages for egg-laying hens earlier. Reducing the degree of crowding in cages is not the same as banning cages and is a long way from the chickens that roam the pastures of Point Reyes. But this hasn’t stopped the Wall Street Journal from decrying the future cost of a California omelet and six states from suing over a 2010 bill extending the requirements to out of state producers who sell eggs in California.

Such measures are largely (and appropriately) motivated by animal welfare concerns. Still the potential effects on disease control are often cited, particularly Salmonella Enteritidis, even though it only occurs in about 1 in 20,000 eggs in the United States (Ebel and Schlosser 2000). What do we know about how chicken housing strategies affect the microbiology of these built environments and the potential for disease transmission? A search of the literature using Web of Science suggests that these systems are ripe for microbial diversity studies. Due to its low incidence in chicken flocks, Salmonella can be hard to detect. Consequently, Holt et al. (2011) suggest that environmental monitoring may be more sensitive for detecting differences in the prevalence of Salmonella among various egg production systems. Indeed a study of chicken flocks by the European Food Standards Agency found that dust samples more readily detected the presence of Salmonella than fecal samples (EFSA 2006). A review of effects on housing systems on egg safety suggested that the currently available body of research on this subject is inconclusive but indicated that factors such as climate, hen breed, disease status, presence of vectors such as insects and rodents, and facility age are important (Holt et al. 2011).

Ebel, E., and W. Schlosser. 2000. Estimating the annual fraction of eggs contaminated with Salmonella enteritidis in the United States. Int. J. Food Microbiol. 61:51—62.

EFSA. 2006. Preliminary report. Analysis of the baseline study on the prevalence of Salmonella in laying hen flocks of Gallus gallus. EFSA J. 81:1—71.

Holt, P. S., R. H. Davies, J. Dewulf, R. K. Gast, J. K. Huwe, D. R. Jones, D. Waltman, and K. R. Willian. 2011. The impact of different housing systems on egg safety and quality. Poult. Sci. 90:251—262.

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Holly Ganz

Holly Ganz is a project scientist at UC Davis working with Jonathan Eisen on the microbiomes of built environments where animals live.