U.S. Food and Drug Administration

1. U.S. Food and Drug Administration Notice: Archived Document The content in this document is provided on the FDA’s website for reference purposes only. It was current when produced, but is no longer maintained and may be outdated.

2. Epidemiological Evidence of Pathogen Load Effects Scott A. McEwen DVM DVSc Professor Department of Population Medicine Ontario Veterinary College University of Guelph

3. Overview • Plausible mechanisms of pathogen load effects • Relevant information from non-food animal species • Characteristics of the epidemiological studies reviewed • Summary of evidence from food animal studies (excluding those with resistance as the outcome)

4. Possible Mechanisms • Increased susceptibility of animals to infection – reduced infectious dose • Treatment before exposure to the pathogen increases susceptibility to infection by suppression of normal flora, diminishing colonization resistance • Treatment during exposure to a resistant pathogen facilitates infection by the selective effect of resistance

5. Possible Mechanisms • Increased duration of shedding &/or concentration in feces • Perhaps due to greater degree of colonization (intra- or extra-intestinal) &/or disruption of normal enteric flora • Decreased prevalence / duration of shedding due to treatment

6. Epidemiological Studies • Observational in nature • Natural exposure / infection • drug treatment as it occurs on real farms • Multiple causal factors can be investigated • Related to agent, host, environment • Can assess hierarchical / group effects • Must design & analyze carefully to avoid biases • Different study designs have strengths & limitations re: causal inferences

7. Related Evidence – Humans and Companion Animals • Several case-control studies in humans showed prior antimicrobial use as a risk factor for salmonellosis, and recently, campylobacteriosis (perhaps diminished “colonization resistance”) • Similar findings in hospitalized horses, dogs • Some evidence for causal role of antimicrobial treatment in clostridial enterocolitis of horses and rabbits • The above observations involve clinical disease, not subclinical infection

8. Example – Human Studies • Case-control study - outbreak due to AM-sensitive strain of Salmonella havana • Antimicrobials taken a mean of 15.9 days before and stopped a mean of 7.9 days before onset • 31% of 35 case-patients had taken antimicrobials within 30 days of onset compared with 13% of age- and neighborhood-matched controls (matched odds ratio 4.3) Pavia AT et al. J Infect Dis. 1990;161:255-60

9. Example – Horses • Case-control study - Salmonella saintpaul infection in hospitalized horses • Horses receiving parenteral antimicrobials were at 10.9 times greater risk of having S. saintpaul isolated than were horses not receiving parenteral antimicrobials Hird DW et al. Am J Epidemiol. 1984 Dec;120:852-64

10. Food Animal Epidemiological Studies - Effect Measures • Most studies measured farm-level culture status (prevalence) • A few assessed individual-animal status or proportion of herd shedding • Duration of infection/shedding, and concentration of pathogen in feces not measured explicitly (although prevalence is a function of incidence and duration of infection) • Most outcomes relatively unrefined (multiple serotypes, etc)

11. Antimicrobial Exposure Measures (Risk Factors) • Most studies also measured treatment at the herd level • Drug used for growth promotion yes/no • Some named specific drugs; others not • Groups treated therapeutically yes/no • A few assessed individual-animal treatment (not drug-specific) • None measured duration of treatment • Most treatment variables relatively unrefined

12. Summary of Study Results – Pathogen Load [+,- = direction of effect; NS = not significant; (# in parentheses = # of studies)]

13. Example 1 – E. coli in Cattle • Longitudinal study of 36 Pacific Northwest dairy herds; monthly fecal culture of heifer cattle • “Tentative” association of E. coli O157 prevalence with feeding of ionophores in heifer rations (p=0.1) Herriott DE et al. J Food Prot. 1998;61:802-807

14. Example 2 – E. coli in Cattle • Cross-sectional study of U.S. feedlots identified factors associated with shedding of E. coli 0157 • 63 of the 100 feedlots had at least one positive sample • No association between positive fecal samples and ionophore use, or with feeding antimicrobials Dargatz DA et al. J Food Prot. 1997;60:466-470

15. Example 3 – Salmonella in Swine • Cross-sectional study of 353 Dutch pig farms – outcome was proportion of seropositive samples • Use of tylosin as an antimicrobial growth promoter in finishing feed associated with higher Salmonella seroprevalence van der Wolf PJ. Vet Microbiol. 2001 78:205-219

16. Example 4 – Salmonella in Broilers • Cross-sectional study of 3923 Danish broiler flocks – 12.6% S. typhimurium +ve • Use of unspecified antimicrobials was associated with reduced risk of Salmonella infection (in flocks from Salmonella-negative parent flocks) • Growth promoters not significantly associated with Salmonella infection Chriel M et al. Prev Vet Med. 1999;40:1-17

17. Summary • A modest number of epidemiological studies assessed the effects of antimicrobials on fecal shedding with enteropathogens; none assessed carcass contamination • Most evaluated Salmonella, fewer Shiga toxin-producing E. coli and Campylobacter • Most studies sought to evaluate a broad range of potential risk factors; none were specifically designed to assess pathogen load

18. Summary • Given the exploratory nature of these studies, and the comparatively unrefined exposure and outcome variables used, important associations may have gone undetected • Future epidemiological studies should be specifically designed to assess pathogen load effects • Such studies are inherently post-approval

19. Conclusions • Most studies found no evidence of a pathogen load effect • Some found evidence of a protective effect; fewer found a positive effect • Overall, current epidemiological evidence suggests that undesirable pathogen load effects of antimicrobials used in Europe and North America, if they exist, are probably minor

20. Bibliography (For reference - not to be shown at meeting) • Busato A, Hofer D, Lentze T, Gaillard C, Burnens A. Prevalence and infection risks of zoonotic enteropathogenic bacteria in Swiss cow-calf farms. Vet Microbiol. 1999 Sep 29;69(4):251-63. • Chriel M, Stryhn H, Dauphin G. Generalised linear mixed models analysis of risk factors for contamination of Danish broiler flocks with Salmonella typhimurium. Prev Vet Med. 1999;40:1-17. • Dargatz, D.A., Wells, S.J., Thomas, L.A., Hancock, D.D., Garber, L.P. Factors associated with the presence of Escherichia coli O157 in feces of feedlot cattle. J. Food Prot. 1997;60:466-470 • Evans SJ, Sayers AR. A longitudinal study of campylobacter infection of broiler flocks in Great Britain. Prev Vet Med 2000 Aug 10;46(3):209-23. • Herriott DE, Hancock DD, Ebel ED, Carpenter LV, Rice DH, Besser TE. Association of herd management factors with colonization of dairy cattle by Shiga toxin-positive Escherichia coli O157. J Food Prot. 1998 Jul;61(7):802-7. • Hird DW, Pappaioanou M, Smith BP. Case-control study of risk factors associated with isolation of Salmonella saintpaul in hospitalized horses. Am J Epidemiol. 1984 Dec;120(6):852-64. • Irwin, R.J., McEwen, S.A., Clarke, R.C. and Meek, A.H. 1989 The prevalence and antimicrobial resistance patterns of verocytotoxin and non-verocytotoxin-producing Escherichia coli and Salmonella in Ontario broiler chickens. Can. J. Vet. Res. 53:411-418. • Losinger WC, Garber LP, Smith MA, Hurd HS, Biehl LG, Fedorka-Cray PJ, Thomas LA, Ferris K. Management and nutritional factors associated with the detection of Salmonella sp. from cattle fecal specimens from feedlot operations in the United States. Prev Vet Med. 1997 Aug;31(3-4):231-44. • Losinger WC, Wells SJ, Garber LP, Hurd HS, Thomas LA. Management factors related to Salmonella shedding by dairy heifers. J Dairy Sci 1995 Nov;78(11):2464-72 • Pavia AT, Shipman LD, Wells JG, Puhr ND, Smith JD, McKinley TW, Tauxe RV. Epidemiologic evidence that prior antimicrobial exposure decreases resistance to infection by antimicrobial-sensitive Salmonella. J Infect Dis. 1990 Feb;161(2):255-60 • Refregier-Petton J, Rose N, Denis M, Salvat G. Risk factors for Campylobacter spp. contamination in French broiler-chicken flocks at the end of the rearing period. Prev Vet Med 2001 Jul 19;50(1-2):89-100 • Skov MN, Angen O, Chriel M, Olsen JE, Bisgaard M. Risk factors associated with Salmonella enterica serovar typhimurium infection in Danish broiler flocks. Poult Sci. 1999 Jun;78(6):848-54. • Stege H, Christensen J, Nielsen JP, Willeberg P. Data-quality issues and alternative variable-screening methods in a questionnaire-based study on subclinical Salmonella enterica infection in Danish pig herds. Prev Vet Med. 2001 Jan 17;48(1):35-54 • Uhaa IJ, Hird DW, Hirsh DC, Jang SS. Case-control study of risk factors associated with nosocomial Salmonella krefeld infection in dogs. Am J Vet Res. 1988 Sep;49(9):1501-5. • van der Wolf PJ, Bongers JH, Elbers AR, Franssen FM, Hunneman WA, van Exsel AC, Tielen MJ. 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 Jul 1;67(4):263-75. • van der Wolf PJ, Wolbers WB, Elbers AR, van der Heijden HM, Koppen JM, Hunneman WA, van Schie FW, Tielen MJ. Herd level husbandry factors associated with the serological Salmonella prevalence in finishing pig herds in The Netherlands. Vet Microbiol. 2001 Feb 12;78(3):205-19. • Wilson, J.B., McEwen, S.A., Clarke, R.C., Leslie, K.E., Waltner-Toews, D. and Gyles, C.L. 1993 Risk factors for bovine infection with verocytotoxigenic E.coli. Prev.Vet. Med. 16: 159-170. • Wray C, Todd JN, Hinton M. Epidemiology of Salmonella typhimurium infection in calves: excretion of S typhimurium in the faeces of calves in different management systems. Vet Rec. 1987 Sep 26;121(13):293-6.