During 1978-1981, B.S. Pomeroy at the University of Minnesota grew Salmonella-free turkeys primarily by selecting Salmonella-free hatchlings, feeding Salmonella-free feed and isolating the flock. “Hatching eggs from a primary breeder over this period (1978-81) resulted in salmonella-free day-old poults from which 7500 hens and 600 toms were selected for breeders each of the 4 years. Approximately 2.5 million poults were produced over the 4 years.”
“Phase 3 involved a primary breeder-hatchery that had a 10-year history of S. sandiego infection in its breeder flocks and poults. A vaccination program using an autogenous oil-adjuvant bacterin supplementing other sanitation and management efforts resulted in elimination of S. sandiego. Because the breeder went out of business, it was not possible to determine if the freedom from salmonella could be sustained over a period of years.”
In 1981, John Silliker wrote, “The Nurmi concept has been described as the first new lead for control of Salmonella in poultry in 35 years”. … “The term “competitive exclusion” has been used to describe the phenomenon. Colonization of the intestinal tract with normal gut flora apparently discourages colonization with salmonellae.”
The USDA Agricultural Research Service (ARS) developed competitive exclusion (CE) as a control for Salmonella in food animals, but was stymied by the U.S. Department of Health and Human Services Food and Drug Administration’s (FDA) insistence on defined cultures.
In their 2012 paper, Doyle and Erickson wrote, “Several CE and probiotic products are commercially available (Callaway et al., 2008); however, CE products are not approved for use in livestock and poultry in the U.S. because of concerns regarding the potential for virulence and antibiotic resistance genes being transmitted by the undefined microbes. … Importantly, U.S. regulations require that the bacteria present in such products be identified but by the very complex nature of CE cultures, the types and numbers of bacteria present in the mixture could vary from lot-to-lot. Other concerns with CE products are the potential presence of antimicrobial resistance genes and the possibility of transferable virulence genes in the unidentified/undefined bacteria of CE products.”
In contrast to FDA’s objection to only feeding animals identified bacteria, FDA permits raw milk cheeses with unknown bacteria for human consumption. Back inoculation, sour mash, back slopping, and “cave aged” are other techniques employing unknown cultures permitted for human foods.
Concerns for animal welfare over human welfare can also be illustrated by the 1933 National Poultry Improvement Plan (NPIP) that continues today. NPIP eradicated the poultry pathogens Salmonella Pullorum and Gallinarum.
However, supporting the principle of, “There ain’t no such thing as a free lunch,” Doyle and Erickson, citing Baumler, et al., and Velge, et al., wrote, “It has recently been proposed that the eradication of S. Gallinarum opened an ecological niche, which allowed the introduction of S. Enteritidis into poultry flocks.” Following outbreaks involving eggs, NPIP included vaccination for control of S. Enteritidis.
In a 2011 review, Foley, et al., wrote, “Coinciding with the decrease of S. Enteritidis, S. Heidelberg and S. Kentucky have emerged as the predominant serovars in commercial broilers. The emergence of S. Heidelberg as the most commonly detected serovar in chickens following the implementation of NPIP and the corresponding decline in S. Enteritidis infections could signify that S. Heidelberg is occupying the ecological niche left by the decline of S. Enteritidis.”
Similarly, Doyle and Erickson cautioned, “An evaluation of the effectiveness of an intervention should take into consideration the microbial ecology of the animal or plant to avoid unintended consequences such as an alternative pathogen colonizing the host in the absence of the targeted pathogen.”
In their 2012 paper, Doyle and Erickson reviewed the literature on preharvest interventions for foodborne zoonoses. Their extensive review included: on-farm management and hygienic practices, feed and water treatments, macronutrient diet formulation, antibiotics and growth-enhancing additives, prebiotics, probiotics, synbiotics, bacteriophages, bacteriocins, immunotherapy, vaccines, breeding and multiple interventions.
They concluded, “Effective food safety interventions to reduce or control foodborne pathogens are needed throughout the food continuum, from the farm to the end user. Current production and processing procedures for livestock and poultry and fresh fruits and vegetables do not have sufficiently robust food safety interventions to ensure pathogen-free fresh meat and produce products. Since there is no single widely accepted food safety intervention that will eliminate pathogen contamination of fresh and minimally processed foods, the application of effective food safety interventions must be at the farm and additional interventions need to be thereafter at subsequent stages of food processing, packaging, distribution, retail, and home or foodservice establishments. Combinations of interventions may be needed throughout the food continuum to provide continuous reduction in pathogen contamination and ultimately the incidence of foodborne illnesses.”
In summary, there is a great body of scientific research on controlling salmonellae in food animals. There are also examples of successes in controlling certain Salmonella strains that are important to animal welfare and commerce. What is lacking are the financial and regulatory incentives to control Salmonella strains that affect human health.
Baumler A.J., Hargis B.M., Tsolis R.M. 2000. Tracing the origins of Salmonella outbreaks, Science 287: 50-52.
Callaway, T.R., Edrington, T.S., Anderson, R.C., Harvey, R.B., Geneovese, K.J., Kennedy, C.N., Venn, D.W., Nisbet, D.J., 2008. Probiotics, prebiotics and competitive exclusion for prophylaxis against bacterial disease. Animal Health Research Reviews 9, 217-225.
Doyle, Michael P. and Marilyn C. Erickson. 2012. Opportunities for mitigating pathogen contamination during on-farm food production. International Journal of Food Microbiology 152: 54-74.
Foley, Steven L., Rajesh Nayak, Irene B. Hanning, Timothy J., Johnson, Jing Han and Steven C. Ricke. 2011. Population Dynamics of Salmonella enterica Serotypes in Commercial Egg and Poultry Production. Appl. Environ. Microbiol. 2011, 77(13):4273-4279.
Pomeroy BS, Nagaraja KV, Ausherman LT, Peterson IL, Friendshuh KA. 1989, Studies on feasibility of producing Salmonella-free turkeys. Avian Dis. 1989 Jan-Mar;33(1):1-7.
Silliker, J.H. 1982.The Salmonella problem: current status and future direction. Journal of Food Protection. May 1982. v. 45 (7).
Velge, Philippe; Axel Cloeckaert, Paul Barrow. 2005. Emergence of Salmonella epidemics: The problems related to Salmonella enterica serotype Enteritidis and multiple antibiotic resistance in other major serotypes. Vet. Res. 36 (2005) 267-288.© Food Safety News