Risk Factors Associated with Salmonella on Swine Farms

April 26, 2010 Print Friendly and PDF


Risk factors associated with Salmonella on swine farms

Originally published as a National Pork Board Factsheet.

Authors: Julie Funk, DVM, MS, PhD, Department of Veterinary; Preventative Medicine, The Ohio State University; Wondwossen Abebe Gebreyes, DVM, PhD, DACVPM, Department of Population Health and Pathobiology, College of Veterinary Medicine, North Carolina State University


This manuscript reviews on-farm risk factors that have been associated with the prevalence status of Salmonella in swine. Salmonella is the second most common etiological cause of bacterial human food borne illness in the US, and most cases can be attributed to contaminated food products. Reduction of human food borne salmonellosis has become a public health priority both nationally and internationally. Public health concerns, increased stringency of regulatory limits at slaughter, and competition for international market share are likely to increase interest in on-farm Salmonella control.


An estimated 1.5 million cases of non-typhoidal salmonellosis in humans occur yearly in the United States, and nearly all cases are food borne.1 This tremendous burden on public health has led to reduction of human salmonellosis to be a key public health objective.2 Salmonella is a ubiquitous organism, and the gastrointestinal tract of vertebrate animals is considered to be its biological niche. Although Salmonella infection can result in clinical disease, it has long been recognized that swine can be asymptomatic carriers of Salmonella.3-12 The number of farms and pigs positive for Salmonella in the US has been estimated to range from 38.2-83%, and 6-24.6% respectively.13,14

Beyond the potential impact on domestic public health and market stability, contamination of pork products with Salmonella may put the ~ $112 million pork export market at risk.15,16 With the coordination of “farm to table” Salmonella control programs by many European pork producers (among the U.S.’s major competitors for export markets),17 demonstration of effective control measures may be important for maintaining international market share. Yet, wholesale adoption of pre-existing control programs may not be practical in the US, due to differences in production systems, industry structure, and regulatory organization.

Significant strides at decreasing Salmonella in one link of the US pork chain, namely at slaughter and processing, have been made. The Hazard Analysis Critical Control Point/Pathogen Reduction Act18 established performance standards for Salmonella at slaughter and processing plants, which has resulted in decreased product contamination.19 It is expected that the Salmonella standards at slaughter and processing will become more stringent over time, creating pressure from packers and processors for reduction of the prevalence of Salmonella positive swine through on-farm interventions.

The focus of this review is on those on-farm risk factors for Salmonella in swine that have been identified through epidemiological investigations. Promising interventions that have thus far been described predominantly in experimental settings (vaccines, antimicrobial treatments and competitive exclusion) are not included. Additionally, potential risks beyond the “farm gate” (exposure during transport, lairrage, contamination of carcasses during slaughter, etc) are not included in this review for the main purposes of brevity and a focus on potential on-farm interventions. These risk factors may suggest areas for further investigation for Salmonella control on US swine farms.


Good hygiene has long been stressed as important for Salmonella control and although intuitively appealing, is a difficult area to evaluate in research investigations. In a previous review of the literature20, good hygiene was identified as the most important risk factor for Salmonella. Unfortunately, this importance was often based on subjective assessment by the investigator (good vs. bad hygiene farms), with little indication of how hygiene was measured. The main dilemma lies in an objective measurement of “good hygiene” and what level or components of “good hygiene” are important for Salmonella control. Results of epidemiological investigations to date suggest not all practices traditionally consistent with good hygiene are associated with decreased Salmonella prevalence on farms; in fact, it appears that some (e.g. all-in, allout pig flow) may increase Salmonella risk.

Swine farm personnel hygiene practices have been associated with decreased Salmonella risk for swine. Researchers have found that hand washing21 and access to toilets and hand washing facilities22 have been associated with decreased Salmonella prevalence on swine farms. Farms that had areas where clothes and footwear could be changed prior to entry into pig areas were associated with reduced Salmonella seroprevalence in Danish market swine21 but were not identified as being associated with Salmonella seroprevalence in Dutch herds.23 It has also been reported that herds with relatively more humans on site daily were at increased risk having high Salmonella fecal shedding22, suggesting that increased human traffic on farms increases the pigs’ risk of infection. Whether personnel hygiene practices are directly related to Salmonella risk or whether they simply serve as a proxy measure of a pork producer’s overall attitude about hygiene is unclear, but it does suggest that improved personnel hygiene may be an important intervention for Salmonella. The relatively small cost incurred may be offset by decreased transfer of other performance impairing pathogens.24,25

The type of flooring pigs are reared on has been evaluated in epidemiological investigations. The biological premise is that certain flooring types decrease pig contact with fecal material. Swine housed on fenestrated (concrete slats, etc.) flooring were found to have lower Salmonella prevalence than pigs housed on solid26 or flush-gutter flooring.13,27

Apart from the associations with improved personnel hygiene and flooring types, the association between improved hygiene and decreased Salmonella prevalence becomes more tenuous. Pigs housed on partially solid floors with high levels of fecal contamination (as a result of dunging on solid parts of the pen) were, surprisingly, less likely to be Salmonella positive based on fecal culture than pigs housed on floors with lesser amounts of fecal contamination.28 Although it has been described that farms that had an area to change clothing and boots prior to entering or leaving the pig area in combination with all-in/all-out production were nearly three times less likely to be seropositive for Salmonella21, others studies find that all-in/all-out production with cleaning between groups is associated with increased Salmonella prevalence. A study conducted by Bahnson et al.26 reported that “good hygiene” in combination with all-in/allout pig flow was associated with increased Salmonella seroprevalence. In agreement with Bahnson, Stege et al.29 indicated that all-in/all-out flow as well as manure-free cleaning between groups of pigs was associated with increased Salmonella seroprevalence. In another investigation23, pig herds in the Netherlands that were reared in production systems where barns were cleaned and disinfected between groups of pigs were at greater odds to have increased Salmonella seroprevalence than in herd where floors and buildings were just cleaned but no disinfectant was used.

It is uncertain why the association between management practices considered “good hygiene” related to cleaning and disinfection and pig flow practices that are well recognized as important for reduction of production impairing diseases in swine may not have similar effects on Salmonella prevalence.30-32 Contamination of the resident environment of animal housing has been implicated in many studies as a source of Salmonella infection.7,33-39 Salmonella is capable of surviving at least 6 years or more in the environment, 40,41 and the challenges of cleaning and disinfection of animal housing are well documented.42-45 Substandard cleaning and disinfection may allow Salmonella to remain as a contaminant on floors detectable by culture.46 Rough surfaced concrete was more likely to have high levels of residual contamination after cleaning and disinfection than smoother surfaces in swine housing.44 Yet, terminal disinfection either through fogging or fine mist of formaldehyde has been demonstrated to decrease the Salmonella contamination in poultry houses.43,45 Human health risks are associated with formaldehyde use and the benefits of its use as a disinfectant should be considered.

Another hypothesis regarding the unexpected associations between all-in/all-out production with between group cleaning and Salmonella prevalence is that cleaning and disinfection reduces the number of competing microorganisms in the environment, allowing residual Salmonella that may be resistant to the cleaning procedures to survive more readily. Support for this “competitive flora” theory comes from the poultry literature for Salmonella control. Investigators have demonstrated that poultry placed on used litter had lower Salmonella prevalence than those on new clean litter.47-52 It is hypothesized that old litter is a hostile environment for Salmonella as a result of properties of used litter (water activity, pH or other chemical components) or that Salmonella does not compete well with the bacterial microflora in the used litter.

The paradox presented by hygiene as a risk factor for Salmonella will likely continue to be a challenge for epidemiological studies and design of on-farm interventions. Before recommending interventions to swine producers, it is necessary to fully investigate what interventions will be cost-effective.

Importance of sow-to-pig transmission

Many investigators have reported relatively high Salmonella prevalence in breeding gilts and sows.46,53-57 Beyond the food safety risk when the sow ultimately enters the food chain, the importance of vertical transmission from the sow to her offspring has only been minimally addressed. Several authors have demonstrated that piglets can be infected early in life.6,46,58 Efforts to use segregated early weaning to prevent sow to pig transmission has had mixed results and are likely to be at best farm specific in success. 59-61 It is paradoxical that that different Salmonella serotypes are often isolated from sows and their piglets,46,58 which might be explained by sampling error, colostral protection, or differential infection efficiency for serotypes in different age pigs. Recent epidemiological surveys in Denmark62 have suggested that pigs produced from sow herds with high Salmonella seroprevalence are at greater risk for isolation of S. Typhimurium.

More research is necessary to evaluate the importance of sow prevalence on the risk of Salmonella in her offspring. If the sow herd is identified as an important source of Salmonella for growing pigs, it has important implications for the breadth and costs of surveillance and control programs.

The risk posed to swine by other vertebrate species

Since all vertebrates are susceptible to being infected with Salmonella, contact with other species may pose an infection risk to swine herds. The risk posed by having other domestic species on a farm with swine has been variable in the literature. Having other domestic animals on the same farm as finisher pigs has been associated with increased Salmonella prevalence.46 Yet, many other researchers have found no association with the presence of domestic animals other than the target species and Salmonella.23,45,63-65

Domestic cats residing on swine farms have been found to be shedding Salmonella.66 Pests (rodents, wild birds, and other wildlife species) have often been implicated as potential sources of Salmonella for swine. Several investigators have demonstrated that mice and rats on farms can be infected with Salmonella and often with the same serotypes as the domestic species investigated.66-71 Many cross-sectional investigations have isolated Salmonella from free-living birds at prevalence rates from 0 to more than 50%.72-76 There is circumstantial evidence that sea gulls were responsible for two Salmonella outbreaks in Scotland. 72,74 Birds near broiler houses have been found to shed Salmonella at relatively high frequencies.76 Finisher pigs housed in facilities that did not exclude birds were at greater risk to be Salmonella positive at slaughter than those reared in bird-proof facilities.77 Foxes near poultry farms have been identified as shedding Salmonella.71

There are sufficient economic benefits for pest control on farms external to Salmonella control (building damage and control of other diseases) that can off-set the costs of pest control that justifies these interventions and may also result in decreased Salmonella risk for swine.

Risks posed by invertebrate species

It has been recognized that flies66,71 and beetles71,77,78 (both mature and immature stages) can be vectors for Salmonella. In fact recent research suggests that the free living nematode Caenorhabiditis elegans can be persistently infected with Salmonella. 78 Although there have not been epidemiological investigations to discern the attributable risk associated with invertebrate species, it appears that they may at least serve as a potential reservoir and vector on farms.

Risk factors associated with feed

Risk factors associated with feed can be divided into two major categories 1) feed as a source of Salmonella due to contamination or 2) the impact of feed ingredients and physical structure on Salmonella prevalence.

Feed as a source of Salmonella

It is well recognized that animal feeds and feedstuffs can be contaminated with Salmonella (Table 1).9,46,79-81 It has been demonstrated in experimental settings that animals can become infected as a result of consuming Salmonella contaminated feed82, and there is a recently reported clinical outbreak of salmonellosis in swine in which contaminated feed is the implicated source.83 There is no doubt that appropriate process control and decontamination steps are needed during feed processing to reduce contamination of feedstuffs in order to avoid dissemination of contaminated feed to herds. Pelleting of feed has long been recommended as a means of decontaminating pig feeds.84,85 But pelleting must be appropriately conducted in order to be successful, including preventing contamination, especially during pellet cooling.86,87

There is justification to question the relative importance of the role of contaminated feed in the epidemiology of Salmonella on swine farms. Most notably, S. Typhimurium, a Salmonella serotype often associated with food borne disease in humans is infrequently isolated from feeds.29,46,81,88 In fact one reference that compared isolation of Salmonella from feed with prevalence in swine indicated that low seroprevalenceherds were more likely to have Salmonella isolated from feed samples than high seroprevalence herds.29 In a multi-country survey in Europe Salmonella was isolated from feedstuffs in 17.6% of herds and 6.9% of all samples.21 Yet, the Salmonella serotypes isolated from the feeds were not the same serotypes isolated from pigs on those farms.

Feed components and physical structure: The dry, the fine and the pelleted

Epidemiological investigations, predominantly from Europe, have repeatedly demonstrated that feed composition and structure are associated with Salmonella prevalence. Among factors that have been identified are feeding wet vs. dry diets to pigs, acidified diets (feed and/or water), feed particle size, feed form (pelleted vs. meal diets), heat-treated vs. non-heat-treated feeds, as well as actual feed ingredients. Many investigators have reported that swine herds that fed dry vs. wet diets were at increased risk to have high Salmonella seroprevalence.21,23,29,89-91 Yet US pigs fed from wet/dry feeders were at increased risk of being positive by lymph node culture.77 It is important to note that wet feeding in Europe often includes a fermentation step or addition of organic acids to prevent feed spoilage. In fact, trough feeding (adding water to feed with no preservation step) was associated with an increased risk of having a Salmonella positive culture from pooled fecal samples in swine herds in the Netherlands.90 In clinical trials of feeding wet fermented feeds to swine, the results have been mixed. Fermented liquid feed did not decrease the prevalence of Salmonella as compared to dry feed in a clinical trial on an English herd.92

Whey feeds and acidifiers

Feeding whey in diets is typically done using a liquid whey product and is often the liquid used in fermented feed. In 1987, van Schie93 reported that swine farms that fed liquid whey as part of the diet had a lower prevalence of Salmonella than those farms that used water to moisten the feed. In another study21, herds feeding whey were at decreased risk of being seropositive for Salmonella Investigators hypothesized that a component of this effect may be related to the acidic pH of whey.

Interventions using organic acids in order to decrease the pH (mimicking the effect of whey) of pig feedstuffs or water have had varied results in clinical intervention trials. Among those studies that suggested that organic acids may be efficacious, addition of organic acids (formic acid or a combination of formic and proprionic acids) either in feed or water in concert with other management changes, appeared to reduce Salmonella seroprevalence.94 Lactic acid added to dry rations (either pelleted or meal based) reduced the isolation frequency of Salmonella from pooled pen samples in a naturally infected herd as compared to pigs fed these diets without lactic acid.95

Other researchers have not demonstrated the same level of success with organic acids. In an on-farm clinical trial on a herd that had a chronically high Salmonella prevalence, formic acid added to the water of finisher pigs had no effect on seroprevalence.96 When organic acids (commercially prepared formic and proprionic blend) were added to dry feeds in herds already infected with Salmonella at the time of organic acid administration, it did not reduce the Salmonella seroprevalence or isolation frequency from feces.97 Wingstrand et al.98 were only able to reduce Salmonella prevalence in 1 of 2 trials by adding organic acids to dry feeds. Feed acidification did not alter the Salmonella prevalence in British herds in before and after comparisons of fecal isolation of Salmonella.92 In a multinational epidemiological investigation of European herds there was no association identified between addition of organic acids to the diet and Salmonella seroprevalence.21

It is ambiguous whether organic acids are an efficacious intervention for Salmonella control on swine farms. At best, the current status suggests that it is of variable benefit, particularly if formic acid or a combination of formic and proprionic acids are used. The positive results demonstrated with lactic acid may suggest an area of promise. Although organic acids can be corrosive to metal and concrete, addition of organic acids to dry feeds or water sources may be more easily accomplished within the current US feed manufacturing and delivery infrastructure than implementing fermented liquid feeding systems. Further evaluation of the effects of acidic pH and fermented feeds on Salmonella prevalence is needed to evaluate their effects within commercial production settings.

Pelleting of diets and particle size

Many epidemiological studies have found that pigs fed pelleted rations were at increased risk of high Salmonella seroprevalence compared to those fed diets in meal form.21,29,99 As to the biological mechanism that might result in increased Salmonella prevalence in pigs fed pelleted feeds as compared to those fed meal feeds, investigators hypothesized that it may be a result of the smaller particle size, heat treatment, or the pelleted form. For finisher pigs, it appears that the pelleted form and the fineness of grind, (but not heat treatment), are associated with increased Salmonella prevalence.100-101 Unfortunately, treatments that resulted in the best Salmonella protection were also the treatment with the poorest production performance (feed efficiency in particular). No effect on Salmonella prevalence was identified with feed form (pellet vs. meal) in nursery age pigs.95 Nursery pigs produced from sow herds that were fed pellets were at increased risk for high fecal prevalence of Salmonella, and the finishers from these pellet fed sow herds were at increased risk of high seroprevalence.62,102 However, no significant difference in Salmonella seroprevalence or fecal shedding in sows was found between those fed meal or pelleted diets.103

In efforts to identify a feed that will reduce Salmonella concentrations in finishers without the concurrent reduction in production performance as seen with meal feeds relative to pelleted diets, investigators have evaluated the effect of different ingredients in pelleted form on Salmonella prevalence. A pelleted feed containing 10% beet pellets and a meal-form wheat based feed both decreased Salmonella concentrations as compared to a pelleted wheat based ration, but the pigs fed the 10% beet pellets had the added advantage of having growth performance equivalent to that of pigs fed the standard wheat based pellet diet.104 In another experimental investigation105, different wheat to barley ratios in pelleted rations on Salmonella seroprevalence were compared. Salmonella seroprevalence decreased with increasing barley content in the feed, and production performance was not significantly different from the standard treatment pellet that contained 100% wheat.

Although the identification of candidate ingredients that can alter Salmonella seroprevalence while maintaining production performance is promising, there is still more data needed to evaluate optimal concentrations in the diets, as well as the consideration of the economic implications of these ingredients relative to those used in the US. There is a need to evaluate different ratios of the predominant cereal grains used in US swine feeds.

Environmental Temperature/Season

Groups of finisher pigs in North Carolina USA with high Salmonella prevalence were at greater odds to have been sampled in winter and spring (approximately late November through late June).22 In the same study, pigs reared during periods of large variability in daily high temperature were at greater risk to be high prevalence.22 These results are similar to others who reported increased seroprevalence during the Fall and Winter in Danish swine.106 Baum et al.107 conversely, found that US herds tested in summer and fall had higher seroprevalence—but different herds were represented in each season, so it may only represent herd differences as opposed to seasonal variation.

Cool weather ventilation is a compromise between maintaining adequate air exchange while conserving heat which may result in periods where ventilation is not optimal. Unpredictability of weather conditions makes proper setting of ventilation systems difficult. Improper ventilation or temperature stress might be a biological explanation for the association with Salmonella prevalence. What makes further evaluation of this risk factor promising is that there are production performance and pig health advantages to maintaining proper ventilation for swine buildings, which could help off-set extra costs associated with improvement in ventilation engineering and management.

Stocking density/marketing group effects

Groups of finisher pigs categorized as having high Salmonella prevalence were more likely to be stocked at higher pig densities (less space allowance per pig) at the time of sampling than low prevalence groups in a study of US swine.22 In this study initial stocking density was standardized by farm standard operating procedures, so the variation at the time of finisher sampling was accounted for by how many pigs had been removed for marketing prior to sampling. One potential explanation for this finding is that transmission/shedding of Salmonella is reduced when pigs are at lower densities due either to decreased pig-to-pig contact or decreased stress. Alternatively, if the initial infection occurs at approximately the same time for all pigs in a barn, pigs that remain on the farm longer (because they were initially a lighter weight or grew more slowly and therefore were sold to market later) had more time to recover from the infection prior to slaughter. There are known impacts related to stocking density for growth performance in swine108-111 but the data regarding animal density and marketing-group as risk factors for Salmonella shedding in swine are sparse. Linton et al.33 identified higher prevalence of infection in pens with higher pig density— but this result was not confirmed on subsequent sampling in the same herd. Morrow et al.112 also described that pigs in older marketing groups had a decreased isolation prevalence of Salmonella from cecal contents at slaughter. Conversely, Bahnson and Fedorka-Cray113 reported that the last group sold from a batch of pigs was at greater risk of high prevalence based on lymph-node culture at slaughter. Further investigation is needed to evaluate the effect of stocking density and marketing group on Salmonella prevalence in swine. Potential interventions could include altering stocking density in finisher units or segregation of different marketing groups at slaughter according to Salmonella risk. It is also critical to evaluate the effect of timing of sampling in order to standardize the measurement of Salmonella prevalence for future research investigations, since the determination of a farm’s Salmonella status can be altered based on timing of sampling.

Herd Health Status

Several authors have described decreased risk for Salmonella if the herd is considered to be of high health status, typically defined by membership in Specific Pathogen Free (SPF) Programs or membership in quality assurance programs that verify certain management practices are conducted.62,89,90,102 There have also been reports that herds that have experienced diarrhea outbreaks during the growing phase were at increased risk for Salmonella infection.91,114 Groups of finisher pigs with high Salmonella prevalence were more likely to have above median feed conversion rates as compared to low prevalence groups in a study of US swine.22

These associations with health status may reflect the overall expertise and management skills of the pork producer, and it is difficult to hypothesize an exact mechanism since so many management factors may be different in high health herds as compared to conventional herds. The promising aspect of these associations is that if management practices that allowed for high health status designation on swine farms were also associated with decreased Salmonella risk, economically there would be rewards for producers due to improved production performance if market benefits are not available for Salmonella control.


It is evident that the epidemiology of Salmonella on swine farms is complex and that research regarding Salmonella control on swine farms is still needed. This complexity, despite the body of effort described in this review, is going to require significant resources in order to further elucidate the epidemiology of Salmonella and the efficacy of proposed interventions on swine farms. Perhaps just as critical, evaluation of the cost effectiveness of any intervention on the farm, or at any level of the “farm to fork” continuum must also be considered in order to best utilize resources for the reduction of Salmonella contamination. Coordinated efforts throughout the pork chain will be critical to achieving reduced Salmonella contamination of pork.


1. Mead PS, Slutsker L, Dietz V, McCaig LF, Bresee JS, Shapiro C, Griffin PM, Tauxe RV. Food-related illness and death in the United States. Emerg Inf Dis. [serial online] 1999; 5: [33 screens]. Available from: URL:http://www.cdc.gov/ncidod/EID/eid.htm.

3. McDonagh VP and Smith HG. The significance of the abattoir in Salmonella infection in Bradford. J Hyg. 1958; 56:271-279.

4. Newell KW, McClarin R, Murdock CR, MacDonald WN, Hutchinson HL. Salmonellosis in Northern Ireland, with special reference to pigs and Salmonella contaminated pig meal. J Hyg Camb. 1959; 57: 92-105.

5. Kampelmacher EH, Guinee PAM, Hofstra K, van Keulen A. Studies on Salmonella in slaughter-houses. Zbl Vet Med B. 1961; 8:1025-1041.

6. Guinee PAM, Kampelmacher EH, Hofstra K, van Keulen A. Salmonellae in young piglets in the Netherlands. Zbl Vet Med B. 1964; 12:250-256.

7. Williams LP and Newell KW. Sources of Salmonellas in market swine. J Hyg Camb. 1968; 66:281-293.

8. Lee JA, Ghosh AC, Mann PG, Tee GH. Salmonellas on pig farms and in abattoirs. J Hyg Camb. 1972; 70:141-150.

9. Ghosh AC. An epidemiological study of the incidence of Salmonellas in pigs. J Hyg Camb 1972; 70:151-160.

10. Harvey RWS, Price TH, Morgan J. Salmonella surveillance with reference to pigs-Cardiff abattoir, 1968-1975. J Hyg Camb. 1977; 78:439-448.

11. Williams DR, Hunter D, Binder J, Hough E. Observations on the occurrence of Salmonella Choleraesuis and other Salmonellas in two herds of feeder pigs. J Hyg Camb. 1981; 86:369-377.

12. Oosterom J. and Notermans S. Further research into the possibility of Salmonella-free fattening and slaughter pigs. J Hyg Camb. 1983; 91:59-69.

13. Davies PR, Morrow WEM, Jones FT, Deen J, Fedorka-Cray PJ, Harris IT. Prevalence of Salmonella in finishing swine raised in different production systems in North Carolina, USA. Epidemiol Infect. 1997; 119:237-244

16. Davies PR. Food safety and its impact on domestic and export markets. Swine Health Prod. 1997; 5:13-20.

17. Alban L, Stege H, Dahl J. The new classification system for slaughter pig herds in the Danish Salmonella surveillance program. Prev Vet Med. 2002; 53:133-146.

20. Berends, BR, Urlings HAP, Snijders JMA, Van Knapen F. Identification and quantification of risk factors in animal management and transport regarding Salmonella spp. in pigs. Int J Food Microbiol. 1996; 30:37-53.

21. Lo Fo Wong, D.M.A., 2001. Epidemiology and control options of Salmonella in European pig herds, PhD Thesis, Royal Veterinary and Agricultural University, Copenhagen, Denmark.

22. Funk JA, Davies PR, Gebreyes WA. Risk factors associated with Salmonella enterica prevalence in three-site production systems in North Carolina, USA. Berl Münch Tierärztl Wschr. 2001; 114:335-338.

23. van der Wolf PJ, Wolbers WB, Elbers ARW, van der Heijden, HMJF, Koppen JMCC, Hunneman WA, van Schie FW, Tielen MJM. Herd level husbandry factors associated with the serological Salmonella prevalence in finishing pig herds in The Netherlands., Vet Microbiol., 2001; 78:205-219.

24. Amass SF, Stevenson GW, Anderson C, Groye LA, Dowell C, Vyverberg BD, Kanitz C, Ragland D. Investigation of people as mechanical vectors for porcine reproductive and respiratory syndrome virus. Swine Health Prod. 2000; 8:161-166.

25. Otake S, Dee SA, Rossow KD, Deen J, Joo HJ, Molitor TW, and Pijoan C. Transmission of porcine reproductive and respiratory syndrome virus by fomites (boots and coveralls). Swine Health Prod. 2002; 10:59-65.

27. Davies PR, Morrow WEM, Jones FT, Deen J, Fedorka-Cray PJ, Gray JT. Risk of shedding Salmonella organisms by market-age hogs in a barn with open-flush gutters. J Am Vet Med Assn 1997; 210:386-389.

30. Clark LK, Scheidt AB, Armstrong CH, Knox K, Mayrose VB. The effect of all-in/all-out management on pigs from a herd with enzootic pneumonia. Vet Med. 1991; 86:946-951.

31. Clark LK, Hill MA, Kniffen TS, VanAlstine W, Stevenson G, Meyer KB, Wu CC, Scheidt AB, Knox K, Albregts S. An evaluation of the components of medicated early weaning. J Swine Health Prod. 1994; 2:5-11.

32. Harris DL. Alternative approaches to eliminating endemic diseases and improving performance of pigs. Vet. Rec. 1988; 123:422-423.

33. Linton AH, Heard TW, Grimshaw JJ, Pollard P. Computer-based analysis of epidemiological data arising from salmonellosis in pigs. Res Vet Sci 1970; 2:523-532.

34. Zecha BC, McCapes RH, Dungan WM, Holte RJ, Worcester WW, Williams JE. The Dillon beach project-a five-year epidemiological study of naturally occurring Salmonella infection in turkeys and their environment. Avian Dis. 1977; 21:141-159.

35. Lahellec C, Colin P, Bennejean G, Paquin J, Guillerm A, Debois JC. Influence of resident Salmonella on contamination of broiler flocks. Poultry Sci. 1986; 65:2034-2039.

36. Bailey JS. Control of Salmonella and Campylobacter in Poultry Production. A Summary of Work at Russell Research Center. Poultry Sci. 1993; 72:1169-1173.

37. Caldwell DJ, Hargis BM, Corrier DE, Vidal L, DeLoach JR.. Evaluation of persistence and distribution of Salmonella serotype isolation from poultry farms using drag-swab sampling. Avian Dis. 1995; 39:617-621.

38. Baggesen DL, Wegener HC, Bager F, Stege H, Christensen J. Herd prevalence of Salmonella enterica infections in Danish slaughter pigs determined by microbiological testing. Prev Vet Med. 1996; 26:201-213.

39. Hoover NJ, Kenney PB, Amick JD, Hypes, WA. Preharvest sources of Salmonella colonization in turkey production. Poultry Sci. 1997; 76:1232-1238.

40. McLaren IM and Wray C. Epidemiology of Salmonella Typhimurium infection in calves: persistence of Salmonellae on calf units. Vet Rec. 1991; 129:461-462.

41. Plym-Forshell L and Eskebo I. Survival of Salmonellas in urine and dry faeces from cattle-an experimental study. Acta Vet Scand. 1996; 37:127-131.

42. Davies RH and Wray C. Observations on disinfection regimens used on Salmonella enteritidis infected poultry units. Poultry Sci. 1995; 74:638-647.

43. Davies RH and Wray C. Studies of contamination of three broiler breeder houses with Salmonella enteritidis before and after cleansing and disinfection. Avian Dis. 1996; 40:626-633.

44. Madec F, Humbert F, Salvat G, Maris P. Measurement of the residual contamination of post-weaning facilities for pigs and related risk factors. J Vet Med B. 1999; 46:37-45.

45. Rose N, Beaudeau F, Drouin P, Toux JY, Rose V, Colin P. Risk factors for Salmonella enterica subsp. enterica contamination in French broiler-chicken flocks at the end of the rearing period. Prev Vet Med. 1999; 39:265-277.

46. Funk JA, Davies PR, Nichols MA. Longitudinal study of Salmonella enterica in two, three-site production systems. Vet Microbiol. 2001; 83:45-60

47. Botts CW, Ferguson LC, Birkeland JM, Winter AR. The influence of litter on the control of Salmonella infections in chicks. Am J Vet Res. 1952; 13:562-565.

48. Tucker JF. Survival of salmonellae in built-up litter for housing of rearing and laying fowl. Br Vet J. 1967; 123:92-103.

49. Olesiuk OM, Snoeyenboes GH, Smyser CF. Inhibitory effect of used litter on Salmonella Typhimurium transmission in the chicken. Avian Dis. 1971; 15:118-124.

50. Turnbull PCB and Snoeyenboes GH. The roles of ammonia, water activity and pH in the salmonellacidal effect of long-used poultry litter. Avian Dis. 1973; 17:72-86.

51. Gustafson RH and Kobland JD. Factors influencing salmonella shedding in broiler chickens. J Hyg. 1984; 92:385-394.

52. Opara OO, Carr LE, Russek-Cohen E, Tate CR, Mallinson ET. Miller RG, Stewart LE, Johnston RW, Joseph SW. Correlation of water activity and other environmental conditions with repeated detection of Salmonella contamination on poultry farms. Avian Dis. 1992; 36:664-671.

53. Keteran K, Brown J, Shotts EB. Salmonella in the mesenteric lymph nodes of healthy sows and hogs. Am J Vet Res. 1982; 43:706-707.

54. Davies PR, Funk JA, Morrow M. Fecal shedding of Salmonella by gilts before and after introduction to a swine breeding farm. Swine Health Prod. 2000; 8:25-29.

59. Dahl J, Wingstrand A, Nielsen B, Baggesen DL. Elimination of Salmonella typhimurium infection by the strategic movement of pigs. Vet Rec. 1997; 140:679-681.

60. Fedorka-Cray PJ, Harris DL, Whipp SC. Using isolated weaning to raise Salmonella-free swine. Vet Med. 1997; 92:375-382.

61. Nietfeld JC, Feder I, Kramer TT, Schoneweis D, Chengappa MM. Preventing Salmonella infection in pigs with offsite weaning. Swine Health Prod. 1998; 6:27-32.

63. Renwick SA, Irwin RJ, Clarke RC, McNab WB, Poppe C, McEwen SA. Epidemiological associations between characteristics of registered broiler chicken flocks in Canada and the Salmonella culture status of floor litter and drinking water. Can Vet J 1992; 33:449-458.

64. Henken AM, Frankena K, Goelema JO, Graat EAM, Noordhuizen JPTM. Multivariate epidemiological approach to salmonellosis in broiler breeder flocks. Poultry Sci. 1992; 71:838-843.

65. Kabagambe EK, Wells SJ, Garber LP, Salman MD, Wagner B, Fedorka-Cray PJ. Risk factors for fecal shedding of Salmonella in 91 US dairy herds in 1996. Prev Vet Med. 2000; 43:177-194.

66. Barber DA, Weigel RM, Isaacson RE, Bahnson PB, Jones CJ. Distribution of Salmonella in swine production ecosystems. J Food Protect. 2002; 65:1861-8.

67. Henzler DJ, and Opitz HM. The role of mice in the epizootiology of Salmonella enteritidis infection on chicken layer farms. Avian Dis. 1992; 36:625-631.

68. Guard-Petter J, Henzler DJ, Mahbubur Rahman M, Carlson RW. On-farm monitoring of a mouse-invasive Salmonella enterica Serovar Enteritidis and a model for its association with the production of contaminated eggs. Appl Environ Microbiol 1997; 63:1588-1593.

69. Davies RH and Wray C. Mice as carriers of Salmonella enteriditis on persistently infected poultry units. Vet Rec. 1995; 137:337-341.

70. Davies R and Breslin M. Environmental contamination and detection of Salmonella enterica serovar enteriditis in laying flocks. Vet Rec. 2001; 149:699-704.

71. Liebana E, Garcia-Migura L, Clouting C, Clifton-Hadley FA, Breslin M, Davies RH. Molecular fingerprinting evidence of the contribution of wildlife vectors in the maintenance of Salmonella Enteriditis in layer farms. J Appl Microbiol. 2003; 94:1024-1029.

72. Johnston WS, Maclachlan GK, Hopkins GF. The possible involvement of seagulls (Larus larus) in the transmission of salmonella in dairy cattle. Vet Rec. 1979; 105:526-527.

73. Butterfield J, Coulson JC, Kearsey SV, Monaghan,P, McCoy JH, Spain GE. The herring gull Larus argentatus as a carrier of salmonella. J Hyg. 1983; 91:429-436.

74. Coulson JC, Butterfield J, Thomas C. The herring gull Larus argentatus as a likely transmitting agent of Salmonella Montevideo to sheep and cattle. J. Hyg. 1983; 91:437-443.

75. Fenlon DR. A comparison of salmonella serotypes found in the feces of gulls feeding at a sewage works with serotypes present in the sewage. J Hyg. 1983; 91:47-52.

76. Craven SE, Stern NJ, Line E, Bailey JS, Cox NA, Fedorka-Cray P. Determination of the incidence of Salmonella spp., Campylobacter jejuni, and Clostridium perfringens in wild birds near broiler chicken houses by sampling intestinal droppings. Avian Dis. 2000; 44:715-720.

77. Goodwin MA and Waltman WD. Transmission of Eimeria, viruses, and bacteria to chicks: darkling beetles (Alphitobius diapernus) as vectors of pathogens. J Appl Poultry Sci. 1996; 5:51-55.

78. Aballay A, Yorgey P, Ausubel M. Salmonella typhimurium proliferates and establishes a persistent infection in the intestine of Caenorhabditis elegans. Curr Biol. 2000; 10:1539-1542.

80. Fedorka-Cray PJ, Hog, A, Gray JT, Lorenzen K, Velasquez J, Von Behren P. Feed and feed trucks as sources of Salmonella contamination in swine. Swine Health Prod. 1997; 5:189-193.

81. Harris IT, Fedorka-Cray PJ, Gray JT, Thomas LA, Ferris K. Prevalence of Salmonella organisms in swine feed. J Am Vet Med Assn. 1997; 210:382-385.

82. Williams Smith H. The effect of feeding food naturally contaminated with salmonellae. J. Hyg Camb. 1960; 58:381-389.

84. Kampelmacher EH, Guinee PAM, van Keulen A. Prevalence of salmonellae in pigs fed decontaminated and normal feeds. Zbl Vet Med. 1965; 12:258-267.

85. Edel W, Guinee PAM, van Schothorst M, Kampelmacher EH. Salmonella infections in pigs fattened with pellets and unpelleted meal. Zbl Vet Med B. 1970; 14:393-401.

86. Davies RH and Wray C. Distribution of Salmonella contamination in ten animal feed mills. Vet Microbiol. 1997; 51:159-169.

87. Himathongkham S, das Gracas Pereira M, Riemann H. Heat destruction of Salmonella in poultry feed: effect of time, temperature, and moisture. Avian Dis. 1996; 40:72-77.

90. van der Wolf PJ, Bongers JH, Elbers ARW, Franssen FMMC, Hunneman WA, van Exsel ACA, Tielen MJM. Salmonella infections in finishing pigs in The Netherlands: bacteriological herd prevalence, serogroup and antibiotic resistance of isolates and risk factors for infection. Vet. Microbiol. 1999; 67:263-275.

93. van Schie FW and Overgoor. An analysis of the possible effects of different feed upon the excretion of salmonella bacteria in clinically normal groups of fattening pigs. Vet Q. 1987; 9:185-188.

106. Christensen J and Rudemo M. Multiple change-point analysis applied to the monitoring of Salmonella prevalence in Danish pigs and pork. Prev Vet Med. 1998; 36: 131-143.

108. Randolph JH, Cromwell GL, Stahly TS, Kratz DD. Effects of group size and space allowance on performance and behavior of swine. J Anim Sci. 1981; 53:922-927.

109. Kornegay ET, Lindemann MD, Ravindran V. Effects of dietary lysine levels on performance and immune response of weanling pigs housed at two floor space allowances. J Anim Sci. 1993; 71:552-556.

110. Kornegay ET, Meldrum JB, Chickering WR. Influence of floor space allowance and dietary selenium and zinc on growth performance, clinical pathology, and adrenal weights of weanling pigs. J Anim Sci. 1993; 71:3185-3198.

111. Hyun Y, Ellis M, Riskowski G, Johnson RW. Growth performance of pigs subjected to multiple concurrent environmental stressors. J. Anim. Sci. 1998; 76:721-727.

112. Morrow WE, See MT, Eisemann JH, Davies PR, Zering K. Effect of withdrawing feed from swine on meat quality and prevalence of Salmonella colonization at slaughter. J Am Vet Med Assoc. 2002; 220:497-502.

114. Møller K, Jensen TK, Jorsal SE, Leser TD, Carstensen B. Detection of Lawsonia intracellularis, Serpulina hyodysenteriae, weakly beta-hemolytic intestinal spirochetes, Salmonella enterica, and haemolytic Escherichia coli from swine herds with and without diarrhea among growing pigs. Vet Microbol. 1998; 62:59-72.


2. Anonymous. Healthy People 2010. United States Departments of Health and Human Services. Downloaded on April 14, 2002 from: http://www.cdc. gov/nchs/hphome.htm#Healthy%20People%202010.

14. Anonymous. Shedding of Salmonella by finisher hogs in the U.S. 1997. Info Sheet N223.196, United States Department of Agriculture, Animal and Plant Inspection Service, Veterinary Services, National Animal Health Monitoring System.

15. Anonymous. Yearly US Pork Export Data-2001. US Meat Export Federation. Downloaded on April 14th , 2002 from: http://www.usmef.org/exportstats/ pork/1201.leadingpork.cfm.

18. Pathogen reduction; hazard analysis and critical control point (HACCP) systems; final rule. Federal Register 1996; 61:38805-38855.

19. Anonymous. Progress Report on Salmonella Testing of Raw Meat and Poultry Products, 1998-2001. Downloaded on May 30, 2003 from: http://www.fsis.usda.gov/OPHS/haccp/salm4year.htm

26. Bahnson PB, Mateus-Pinella N, Omran LM, Grass J, Fransen L, Fedorka-Cray PJ. Risk factors for high levels of antibody to Salmonella spp. Among market weight pigs. Proceedings of the 4th International Symposium on the Epidemiology and Control of Salmonella and other food borne pathogens in Pork. Leipzig, Germany. 2001:250-252.

28. Funk JA, Davies PR, Nichols MA, Morrow WEM. Evaluation of the association between pen fecal accumulation and prevalence of Salmonella enterica shedding in swine. Proceedings of the 3rd International Symposium on the Epidemiology and Control of Salmonella in Pork. Washington D.C. 1999:126-130.

29. Stege H, Christensen J, Baggesen DL, Nielsen JP. Subclinical Salmonella infection in Danish finishing pig herds: The effect of Salmonella contaminated feed. Proceedings of the 2nd International Symposium on the Epidemiology and Control of Salmonella in Pork. Copenhagen, Denmark. 1997:81-84.

55. McKean JD, Hurd HS, Larsen S, Rostagno M, Griffith RW, Wesley I. Impact of commercial pre-harvest processes on the prevalence of Salmonella enterica in cull sows. Proceedings of the 4th International Symposium on the Epidemiology and Control of Salmonella and other food borne pathogens in Pork. Leipzig, Germany. 2001:295-297.

56. Nielsen JP, Jensen A, Wingstrand A. Age related sero-prevalence in 4-7 month old breeding animals, gilts and sows from Danish breeding and multiplying herds. Proceedings of the 4th International Symposium on the Epidemiology and Control of Salmonella and other food borne pathogens in Pork. Leipzig, Germany. 2001:281-284.

57. von Altrock A. Results of the German investigation in the EU project “Salmonella in pork (SALINPORK)”: investigations on farms. Proceedings of the 4th International Symposium on the Epidemiology and Control of Salmonella and other food borne pathogens in Pork. Leipzig, Germany. 2001:198-201.

58. McCracken R, O’ Carrol JA, Funk JA, Davies PR. Salmonella shedding by sows and suckling piglets. Proc AASP. Quebec City, Quebec. 1997:297 -299.

62. Kranker S, Dahl J, Wingstrand A. Bacteriological and serological examination and risk factor analysis of Salmonella occurrence in sow herds, including risk factors for high Salmonella seroprevalence in receiver finishing herds. Proceedings of the 4th International Symposium on the Epidemiology and Control of Salmonella and other food borne pathogens in Pork. Leipzig, Germany. 2001:230-236.

77. Bahnson PB, Fedorka-Cray PJ, Mateus-Pinella N, Fransen L, Grass J, and Gray J. Herd-level risk for Salmonella culture positive status in slaughtered pigs. Proceedings of the 4th International Symposium on the Epidemiology and Control of Salmonella and other food borne pathogens in Pork. Leipzig, Germany. 2001:244-249.

79. McChesney DG, Kaplan G, Gardner P. FDA survey determines Salmonella contamination. Feedstuffs. 1995; 67(7):20, 23.

83. Neumann EJ and Kniffen TS. Clinical salmonellosis related to contaminated feedstuffs in a large swine production system. Proceedings of the 3rd International Symposium on the Epidemiology and Control of Salmonella in Pork. Washington D.C. 1999:158-160

88. Baggesen DL, Sorensen LL, Krause M, Gerner-Smidt P. The occurrence of Salmonella enterica serotypes in animal feed, pork, and man. Proceedings of the 2nd International Symposium on the Epidemiology and Control of Salmonella in Pork. Copenhagen, Denmark. 1997:56-62.

89. Dahl J. Cross-sectional epidemiological analysis of the relation between different herd factors and Salmonella seropositivity. Proc ISVEE. 1997:04.23.1-04.23.3.

91. Beloeil PA, Eveno E, Gerault P, Fravalo P, Rose V, Rose N, Madec F. An exploratory study about contamination of pens of finishing pigs by ubiquitous Salmonella. Proceedings of the 3rd International Symposium on the Epidemiology and Control of Salmonella in Pork. Washington D.C. 1999:101-105.

92. McLaren I, Davies RH, Bedford S. Observations of the effect of “on-farm” interventions in relation to Salmonella infection. Proceedings of the 4th International Symposium on the Epidemiology and Control of Salmonella and other food borne pathogens in Pork. Leipzig, Germany. 2001:72-74.

94. Dahl J, Wingstrand A, Baggesen DL, Nielsen B, Thomsen LK. The effect of a commercial organic acid preparation on seroprevalence and shedding of Salmonella in finishing pigs. Proc IPVS. Bologna, Italy. 1996:178.

95. Jørgensen L, Kjærsgaard HD, Wachmann H, Jensen BB, Knudsen KEB. Effect of pelleting and use of lactic acid in feed on Salmonella prevalence and productivity in weaners. Proceedings of the 4th International Symposium on the Epidemiology and Control of Salmonella and other food borne pathogens in Pork. Leipzig, Germany. 2001:109-111.

96. Dahl J, Wingstrand A, Baggesen DL, Nielsen B. Salmonella reduction at the farm level. Proc IPVS. Bologna, Italy. 1996:181.

97. Wingstrand A, Jørgensen L, Christensen G, Thomsen LK, Dahl J, Borg Bensen B. Reduction of subclinical Salmonella infection by feeding coarse ground feed and adding formic acid to water. Proc IPVS. Bologna, Italy. 1996:180.

98. Wingstrand A, Dahl J, Thomsen LK, Jørgensen L, Jensen BB, 1997. Influence of dietary administration of organic acids and increased feed structure on Salmonella Typhimurium infection in pigs. Proceedings of the 3rd International Symposium on the Epidemiology and Control of Salmonella in Pork. Washington D.C. 1999:170-172.

99. Bush EJ, Wagner B, Fedorka-Cray PJ. Risk factors associated with shedding of Salmonella by U.S. finishing hogs. Proceedings of the 3rd International Symposium on the Epidemiology and Control of Salmonella in Pork. Washington D.C. 1999:106-108.

100. Jørgensen L, Dahl J, Wingstrand A. The effect of feeding pellets, meal, and heat treatment on the Salmonella prevalence in finishing pigs. Proceedings of the 3rd International Symposium on the Epidemiology and Control of Salmonella in Pork. Washington D.C. 1999:308-312.

101. Kjeldsen N, and Dahl J. The effect of feeding non-heat treated, non-pelleted feed compared to feeding pelleted, heat-treated feed on the Salmonella prevalence of finishing pigs. Proceedings of the 3rd International Symposium on the Epidemiology and Control of Salmonella in Pork. Washington D.C. 1999:313-316.

102. Kranker S and Dahl J. Bacteriological and serological examination and risk factor analysis of Salmonella occurrence in sow herds, including risk factors for high Salmonella seroprevalence in receiver finishing herds. Proceedings of the 4th International Symposium on the Epidemiology and Control of Salmonella and other food borne pathogens in Pork. Leipzig, Germany. 2001:237-243.

103. Kjærsgaard HD, Jørgensen L, Wachmann H, Dahl J Effect on Salmonella prevalence by feeding sows meal or pelleted feed. Proceedings of the 4th International Symposium on the Epidemiology and Control of Salmonella and other food borne pathogens in Pork. Leipzig, Germany. 2001:115-117.

104. Hansen CF, Knudsen KEB, Jensen BB, Kjærsgaard D. Effect of meal feed, potato protein concentrate, barley, beet pellets, and zinc gluconate of Salmonella prevalence, gastro-intestinal health and productivity in finishers. Proceedings of the 4th International Symposium on the Epidemiology and Control of Salmonella and other food borne pathogens in Pork. Leipzig, Germany. 2001:103-105.

105. Jørgensen L, Kjærsgaard HD, Wachmann H, Jensen BB, Knudsen KEB. Effect of wheat bran and wheat: barley ratio in pelleted feeds on Salmonella prevalence and productivity in finishers. Proceedings of the 4th International Symposium on the Epidemiology and Control of Salmonella and other food borne pathogens in Pork. Leipzig, Germany. 2001:112-114.

107. Baum DH, Harris DL, Nielsen B, Fedorka-Cray PJ1998. Risk Factors Associated with Increased Seroprevalence of Salmonella in Finishing Swine. Proc IPVS. Birmingham, England. 1998:79.

113. Bahnson PB and Fedorka-Cray PJ. The associations between herd characteristics and Salmonella in slaughter age pigs. Proceedings of the 2nd International Symposium on the Epidemiology and Control of Salmonella in Pork. Copenhagen, Denmark. 1997:145-147.

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This work is supported by the USDA National Institute of Food and Agriculture, New Technologies for Ag Extension project.