Hepatitis A risk factors

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: João André Alves Silva, M.D. [2]

Overview

Common risk factors in the development of hepatitis A include travel to endemic areas, poor sanitation, and intravenous drug users.

Risk Factors

Travelers

  • Persons from developed countries who travel to developing countries are at substantial risk for acquiring hepatitis A.[1] Such persons include tourists, immigrants, and their children returning to their country of origin to visit friends or relatives, military personnel, missionaries, and others who work or study abroad in countries that have high or intermediate endemicity of hepatitis A.
  • Hepatitis A remains one of the most common vaccine-preventable diseases acquired during travel.
  • The risk might be higher among travelers staying in areas with poor hygienic conditions, varies according to the region and the length of stay, and appears to be increased even among travelers who reported observing protective measures and staying in urban areas or luxury hotels.
  • In the United States, children account for approximately 50% of reported travel-related cases.[2]
  • Travelers who acquire hepatitis A during their trips also might transmit to others on their return.

MSM

  • Cyclic outbreaks have occurred in urban areas in the United States, Canada, Europe, and Australia and can occur in the context of an outbreak in the larger community.[3][4][5][6][7]
  • Seroprevalence surveys have not consistently demonstrated an elevated prevalence of anti-HAV compared with a similarly aged general population.[8][9]
  • Since 1996, ACIP has recommended hepatitis A vaccination of MSM.[10] Although precise data are lacking, vaccine coverage appears to be low. [5]

Users of Injection and Noninjection Drugs

  • During the last 2 decades, outbreaks have been reported with increasing frequency among users of injection and noninjection drugs in Australia, Europe, and North America.[4][11][12][13][14]
  • In the United States, outbreaks have frequently involved users of injected and non-injected methamphetamine, who have accounted for up to 48% of reported cases during outbreaks.[13][15] Cross-sectional serologic surveys have demonstrated that injection-drug users have a higher prevalence of anti-HAV than the general U.S. population.[8][16]
  • Since 1996, ACIP has recommended hepatitis A vaccination of users of illicit drugs, but vaccine coverage data are not available.[17]

Persons with Clotting-Factor Disorders

  • During 1992-1993, outbreaks of hepatitis A were reported in Europe among persons with clotting-factor disorders who had been administered solvent-detergent-treated, "high-purity" factor VIII concentrates that presumably had been contaminated from plasma donors incubating hepatitis A.[18]
  • HAV is resistant to solvent-detergent treatment.[20]

Persons Working with Nonhuman Primates

  • Outbreaks of hepatitis A have been reported among persons working with nonhuman primates that are susceptible to HAV infection, including Old and New World species.[21][22] Primates that were infected were those that had been born in the wild, not those born and raised in captivity.

Risk for Severe Adverse Consequences of Hepatitis A Among Persons with Chronic Liver Disease

  • Although not at increased risk for HAV infection, persons with chronic liver disease are at increased risk for fulminant hepatitis A.[23][24][25] Death certificate data indicate a higher prevalence of chronic liver disease among persons who died of fulminant hepatitis A compared with persons who died of other causes.[26]

Risk for Hepatitis A in Other Groups and Settings

Food-Service Establishments and Food Handlers

  • Foodborne hepatitis A outbreaks are recognized relatively infrequently in the United States. Outbreaks typically are associated with contamination of food during preparation by an HAV-infected food handler; a single infected food handler can transmit HAV to dozens or even hundreds of persons.[27][28][29][30][31] However, the majority of food handlers with hepatitis A do not transmit HAV. Food handlers are not at increased risk for hepatitis A because of their occupation. However, among the approximately 40,000 adults with hepatitis A reported during 1992--2000 for whom an occupation was known, 8% were identified as food handlers, reflecting the large number of persons employed in the food service industry.[27]
  • Evaluating HAV-infected food handlers is a common and labor-intensive task for public health departments. In a 1992 common-source outbreak involving 43 persons, the estimated total medical and disease control cost was approximately $800,000.[32]
  • Outbreaks associated with food, especially green onions and other raw produce, that has been contaminated before reaching a food-service establishment have been recognized increasingly in recent years. Low attack rates are common, and outbreaks often have been recognized in association with a single restaurant in which no infected food handler was identified on subsequent investigation.[33][34][35]

Child Care Centers

  • Outbreaks among children attending child care centers and persons employed at these centers have been recognized since the 1970s, but their frequency has decreased as overall hepatitis A incidence among children has declined in recent years.[36][37][38]
  • Because infection among children is typically mild or asymptomatic, outbreaks often are identified only when adult contacts (typically parents) become ill.[37][39]
  • Poor hygiene among children who wear diapers and the handling and changing of diapers by staff contribute to the spread of HAV infection; outbreaks rarely occur in child care centers in which care is provided only to children who are toilet trained.
  • Although child care centers might have been the source of outbreaks of hepatitis A in certain communities, disease in child care centers more commonly reflects extended transmission from the community. Despite the occurrence of outbreaks when HAV is introduced into child care centers, results of serologic surveys do not indicate a substantially increased prevalence of HAV infection among staff at child care centers compared with prevalence among control populations.[40]

Health-Care Institutions

  • Outbreaks have occasionally been observed in neonatal intensive-care units because of infants acquiring infection from transfused blood and subsequently transmitting hepatitis A to other infants and staff.[41][42][43]
  • Outbreaks of hepatitis A caused by transmission from adult patients to health-care workers are typically associated with fecal incontinence, although the majority of hospitalized patients who have hepatitis A are admitted after onset of jaundice, when they are beyond the point of peak infectivity.[44][45]
  • Data from serologic surveys of health-care workers have not indicated an increased prevalence of HAV infection in these groups compared with that in control populations.[46]

Institutions for Persons with Developmental Disabilities

  • Historically, HAV infection was highly endemic in institutions for persons with developmental disabilities.[47] As fewer children have been institutionalized and as conditions in institutions have improved, the incidence and prevalence of HAV infection have decreased, although outbreaks can occur in these settings.

Schools

  • In the United States, the occurrence of cases of hepatitis A in elementary or secondary schools typically reflects disease acquisition in the community.
  • Child-to-child disease transmission in the school setting is uncommon; if multiple cases occur among children at a school, the possibility of a common source of infection should be investigated.[48][49]

Workers Exposed to Sewage

  • Data from serologic studies conducted outside the United States indicate that workers who had been exposed to sewage had a possible elevated risk for HAV infection; however, these analyses did not control for other risk factors (e.g., socioeconomic status).[50][51][52]
  • In published reports of three serologic surveys conducted among U.S. wastewater workers and appropriate comparison populations, no substantial or consistent increase in the prevalence of anti-HAV was identified among wastewater workers.[53][54][55] No work-related instances of HAV transmission have been reported among wastewater workers in the United States.

References

  1. ↑ Steffen R, Kane MA, Shapiro CN, Billo N, Schoellhorn KJ, van Damme P (1994). "Epidemiology and prevention of hepatitis A in travelers". JAMA : the Journal of the American Medical Association. 272 (11): 885–9. PMID 8078167. Unknown parameter |month= ignored (help); |access-date= requires |url= (help)
  2. ↑ CDC. Hepatitis surveillance. Report no. 61. Atlanta, GA: US Department of Health and Human Services, CDC. 2006.
  3. ↑ Nainan OV, Armstrong GL, Han XH, Williams I, Bell BP, Margolis HS (2005). "Hepatitis a molecular epidemiology in the United States, 1996-1997: sources of infection and implications of vaccination policy". The Journal of Infectious Diseases. 191 (6): 957–63. doi:10.1086/427992. PMID 15717272. Retrieved 2012-02-29. Unknown parameter |month= ignored (help)
  4. ↑ 4.0 4.1 Bell BP, Shapiro CN, Alter MJ, Moyer LA, Judson FN, Mottram K, Fleenor M, Ryder PL, Margolis HS (1998). "The diverse patterns of hepatitis A epidemiology in the United States-implications for vaccination strategies". The Journal of Infectious Diseases. 178 (6): 1579–84. PMID 9815207. Retrieved 2012-02-29. Unknown parameter |month= ignored (help)
  5. ↑ 5.0 5.1 Cotter SM, Sansom S, Long T, Koch E, Kellerman S, Smith F, Averhoff F, Bell BP (2003). "Outbreak of hepatitis A among men who have sex with men: implications for hepatitis A vaccination strategies". The Journal of Infectious Diseases. 187 (8): 1235–40. doi:10.1086/374057. PMID 12696002. Retrieved 2012-02-29. Unknown parameter |month= ignored (help)
  6. ↑ CDC. Hepatitis A among homosexual men-United States, Canada, and Australia.MMWR 1992;41:155, 161-4.
  7. ↑ Friedman MS, Blake PA, Koehler JE, Hutwagner LC, Toomey KE (2000). "Factors influencing a communitywide campaign to administer hepatitis A vaccine to men who have sex with men". American Journal of Public Health. 90 (12): 1942–6. PMC 1446451. PMID 11111274. Unknown parameter |month= ignored (help); |access-date= requires |url= (help)
  8. ↑ 8.0 8.1 Villano SA, Nelson KE, Vlahov D, Purcell RH, Saah AJ, Thomas DL (1997). "Hepatitis A among homosexual men and injection drug users: more evidence for vaccination". Clinical Infectious Diseases : an Official Publication of the Infectious Diseases Society of America. 25 (3): 726–8. PMID 9314468. Retrieved 2012-02-29. Unknown parameter |month= ignored (help)
  9. ↑ Katz MH, Hsu L, Wong E, Liska S, Anderson L, Janssen RS (1997). "Seroprevalence of and risk factors for hepatitis A infection among young homosexual and bisexual men". The Journal of Infectious Diseases. 175 (5): 1225–9. PMID 9129091. Retrieved 2012-02-29. Unknown parameter |month= ignored (help)
  10. ↑ CDC. Prevention of hepatitis A through active or passive immunization: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 1996;45(No. RR-15):1-30.
  11. ↑ Harkess J, Gildon B, Istre GR (1989). "Outbreaks of hepatitis A among illicit drug users, Oklahoma, 1984-87". American Journal of Public Health. 79 (4): 463–6. PMC 1349976. PMID 2929804. Unknown parameter |month= ignored (help); |access-date= requires |url= (help)
  12. ↑ Hutin YJ, Bell BP, Marshall KL, Schaben CP, Dart M, Quinlisk MP, Shapiro CN (1999). "Identifying target groups for a potential vaccination program during a hepatitis A communitywide outbreak". American Journal of Public Health. 89 (6): 918–21. PMC 1508638. PMID 10358687. Unknown parameter |month= ignored (help); |access-date= requires |url= (help)
  13. ↑ 13.0 13.1 Vong S, Fiore AE, Haight DO, Li J, Borgsmiller N, Kuhnert W, Pinero F, Boaz K, Badsgard T, Mancini C, Nainan OV, Wiersma S, Bell BP (2005). "Vaccination in the county jail as a strategy to reach high risk adults during a community-based hepatitis A outbreak among methamphetamine drug users". Vaccine. 23 (8): 1021–8. doi:10.1016/j.vaccine.2004.07.038. PMID 15620475. Retrieved 2012-02-29. Unknown parameter |month= ignored (help)
  14. ↑ CDC. Hepatitis A among drug abusers. MMWR 1988;37:297-300, 305
  15. ↑ 15.0 15.1 Hutin YJ, Sabin KM, Hutwagner LC, Schaben L, Shipp GM, Lord DM, Conner JS, Quinlisk MP, Shapiro CN, Bell BP (2000). "Multiple modes of hepatitis A virus transmission among methamphetamine users". American Journal of Epidemiology. 152 (2): 186–92. PMID 10909956. Retrieved 2012-02-29. Unknown parameter |month= ignored (help)
  16. ↑ Ivie K, Spruill C, Bell B. Prevalence of hepatitis A virus infection among illicit drug users, 1993--1994 [Abstract no. A010]. Antiviral Therapy 2000;5(Suppl 1):A.7.
  17. ↑ CDC. Prevention of hepatitis A through active or passive immunization: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 1996;45(No. RR-15):1-30.
  18. ↑ Mannucci PM, Gdovin S, Gringeri A, Colombo M, Mele A, Schinaia N, Ciavarella N, Emerson SU, Purcell RH (1994). "Transmission of hepatitis A to patients with hemophilia by factor VIII concentrates treated with organic solvent and detergent to inactivate viruses. The Italian Collaborative Group". Annals of Internal Medicine. 120 (1): 1–7. PMID 7504424. Unknown parameter |month= ignored (help); |access-date= requires |url= (help)
  19. ↑ Mah MW, Royce RA, Rathouz PJ, Wang JG, White GC, Janco RL, Hanna WT, Davis PC, Johnson CA, Poon MC (1994). "Prevalence of hepatitis A antibodies in hemophiliacs: preliminary results from the Southeastern Delta Hepatitis Study". Vox Sanguinis. 67 Suppl 1: 21–2, discussion 23. PMID 8091730. |access-date= requires |url= (help)
  20. ↑ Soucie JM, Robertson BH, Bell BP, McCaustland KA, Evatt BL (1998). "Hepatitis A virus infections associated with clotting factor concentrate in the United States". Transfusion. 38 (6): 573–9. PMID 9661691. Unknown parameter |month= ignored (help); |access-date= requires |url= (help)
  21. ↑ Hinthorn DR, Foster MT, Bruce HL, Aach RD (1974). "An outbreak of chimpanzee associated hepatitis". Journal of Occupational Medicine. : Official Publication of the Industrial Medical Association. 16 (6): 388–91. PMID 4836166. Unknown parameter |month= ignored (help); |access-date= requires |url= (help)
  22. ↑ Dienstag JL, Davenport FM, McCollum RW, Hennessy AV, Klatskin G, Purcell RH (1976). "Nonhuman primate-associated viral hepatitis type A. Serologic evidence of hepatitis A virus infection". JAMA : the Journal of the American Medical Association. 236 (5): 462–4. PMID 180303. Unknown parameter |month= ignored (help); |access-date= requires |url= (help)
  23. ↑ Akriviadis EA, Redeker AG (1989). "Fulminant hepatitis A in intravenous drug users with chronic liver disease". Annals of Internal Medicine. 110 (10): 838–9. PMID 2712463. Unknown parameter |month= ignored (help); |access-date= requires |url= (help)
  24. ↑ Vento S, Garofano T, Renzini C, Cainelli F, Casali F, Ghironzi G, Ferraro T, Concia E (1998). "Fulminant hepatitis associated with hepatitis A virus superinfection in patients with chronic hepatitis C". The New England Journal of Medicine. 338 (5): 286–90. doi:10.1056/NEJM199801293380503. PMID 9445408. Retrieved 2012-02-29. Unknown parameter |month= ignored (help)
  25. ↑ Keeffe EB (1995). "Is hepatitis A more severe in patients with chronic hepatitis B and other chronic liver diseases?". The American Journal of Gastroenterology. 90 (2): 201–5. PMID 7847285. Unknown parameter |month= ignored (help); |access-date= requires |url= (help)
  26. ↑ Williams I, Bell B, Kaluba J, Shapiro C. Association between chronic liver disease and death from hepatitis A, United States, 1989--92 [Abstract no. A39]. IX Triennial International Symposium on Viral Hepatitis and Liver Disease. Rome, Italy, April 21--25, 1996
  27. ↑ 27.0 27.1 Fiore AE (2004). "Hepatitis A transmitted by food". Clinical Infectious Diseases : an Official Publication of the Infectious Diseases Society of America. 38 (5): 705–15. doi:10.1086/381671. PMID 14986256. Retrieved 2012-02-29. Unknown parameter |month= ignored (help)
  28. ↑ Weltman AC, Bennett NM, Ackman DA, Misage JH, Campana JJ, Fine LS, Doniger AS, Balzano GJ, Birkhead GS (1996). "An outbreak of hepatitis A associated with a bakery, New York, 1994: the 1968 "West Branch, Michigan' outbreak repeated". Epidemiology and Infection. 117 (2): 333–41. PMC 2271694. PMID 8870631. Unknown parameter |month= ignored (help); |access-date= requires |url= (help)
  29. ↑ Lowry PW, Levine R, Stroup DF, Gunn RA, Wilder MH, Konigsberg C (1989). "Hepatitis A outbreak on a floating restaurant in Florida, 1986". American Journal of Epidemiology. 129 (1): 155–64. PMID 2910057. Unknown parameter |month= ignored (help); |access-date= requires |url= (help)
  30. ↑ Massoudi MS, Bell BP, Paredes V, Insko J, Evans K, Shapiro CN (1999). "An outbreak of hepatitis A associated with an infected foodhandler". Public Health Reports (Washington, D.C. : 1974). 114 (2): 157–64. PMC 1308455. PMID 10199718. |access-date= requires |url= (help)
  31. ↑ Latham RH, Schable CA (1982). "Foodborne hepatitis A at a family reunion use of IgM-specific hepatitis a serologic testing". American Journal of Epidemiology. 115 (5): 640–5. PMID 6282115. Unknown parameter |month= ignored (help); |access-date= requires |url= (help)
  32. ↑ Dalton CB, Haddix A, Hoffman RE, Mast EE (1996). "The cost of a food-borne outbreak of hepatitis A in Denver, Colo". Archives of Internal Medicine. 156 (9): 1013–6. PMID 8624166. Retrieved 2012-02-29. Unknown parameter |month= ignored (help)
  33. ↑ Amon JJ, Devasia R, Xia G, Nainan OV, Hall S, Lawson B, Wolthuis JS, Macdonald PD, Shepard CW, Williams IT, Armstrong GL, Gabel JA, Erwin P, Sheeler L, Kuhnert W, Patel P, Vaughan G, Weltman A, Craig AS, Bell BP, Fiore A (2005). "Molecular epidemiology of foodborne hepatitis a outbreaks in the United States, 2003". The Journal of Infectious Diseases. 192 (8): 1323–30. doi:10.1086/462425. PMID 16170748. Retrieved 2012-02-29. Unknown parameter |month= ignored (help)
  34. ↑ Dentinger CM, Bower WA, Nainan OV, Cotter SM, Myers G, Dubusky LM, Fowler S, Salehi ED, Bell BP (2001). "An outbreak of hepatitis A associated with green onions". The Journal of Infectious Diseases. 183 (8): 1273–6. doi:10.1086/319688. PMID 11262211. Retrieved 2012-02-29. Unknown parameter |month= ignored (help)
  35. ↑ Wheeler C, Vogt TM, Armstrong GL, Vaughan G, Weltman A, Nainan OV, Dato V, Xia G, Waller K, Amon J, Lee TM, Highbaugh-Battle A, Hembree C, Evenson S, Ruta MA, Williams IT, Fiore AE, Bell BP (2005). "An outbreak of hepatitis A associated with green onions". The New England Journal of Medicine. 353 (9): 890–7. doi:10.1056/NEJMoa050855. PMID 16135833. Retrieved 2012-02-29. Unknown parameter |month= ignored (help)
  36. ↑ CDC. Hepatitis surveillance. Report no. 61. Atlanta, GA: US Department of Health and Human Services, CDC. 2006.
  37. ↑ 37.0 37.1 Hadler SC, Webster HM, Erben JJ, Swanson JE, Maynard JE (1980). "Hepatitis A in day-care centers. A community-wide assessment". The New England Journal of Medicine. 302 (22): 1222–7. doi:10.1056/NEJM198005293022203. PMID 6245363. Retrieved 2012-02-29. Unknown parameter |month= ignored (help)
  38. ↑ Venczel LV, Desai MM, Vertz PD, England B, Hutin YJ, Shapiro CN, Bell BP (2001). "The role of child care in a community-wide outbreak of hepatitis A". Pediatrics. 108 (5): E78. PMID 11694662. Retrieved 2012-02-29. Unknown parameter |month= ignored (help)
  39. ↑ Shapiro CN, Hadler SC (1991). "Hepatitis A and hepatitis B virus infections in day-care settings". Pediatric Annals. 20 (8): 435–41. PMID 1945541. Unknown parameter |month= ignored (help); |access-date= requires |url= (help)
  40. ↑ Jackson LA, Stewart LK, Solomon SL, et al. Risk of infection with hepatitis A, B or C, cytomegalovirus, varicella or measles among child care providers. Pediatr Infect Dis J 1996;15:584-9.
  41. ↑ Rosenblum LS, Villarino ME, Nainan OV, Melish ME, Hadler SC, Pinsky PP, Jarvis WR, Ott CE, Margolis HS (1991). "Hepatitis A outbreak in a neonatal intensive care unit: risk factors for transmission and evidence of prolonged viral excretion among preterm infants". The Journal of Infectious Diseases. 164 (3): 476–82. PMID 1651359. Retrieved 2012-02-29. Unknown parameter |month= ignored (help)
  42. ↑ Klein BS, Michaels JA, Rytel MW, Berg KG, Davis JP (1984). "Nosocomial hepatitis A. A multinursery outbreak in Wisconsin". JAMA : the Journal of the American Medical Association. 252 (19): 2716–21. PMID 6492350. Unknown parameter |month= ignored (help); |access-date= requires |url= (help)
  43. ↑ Noble RC, Kane MA, Reeves SA, Roeckel I (1984). "Posttransfusion hepatitis A in a neonatal intensive care unit". JAMA : the Journal of the American Medical Association. 252 (19): 2711–5. PMID 6492349. Unknown parameter |month= ignored (help); |access-date= requires |url= (help)
  44. ↑ Goodman RA (1985). "Nosocomial hepatitis A". Annals of Internal Medicine. 103 (3): 452–4. PMID 4026088. Unknown parameter |month= ignored (help); |access-date= requires |url= (help)
  45. ↑ Papaevangelou GJ, Roumeliotou-Karayannis AJ, Contoyannis PC (1981). "The risk of nosocomial hepatitis A and B virus infections from patients under care without isolation precaution". Journal of Medical Virology. 7 (2): 143–8. PMID 6267188. |access-date= requires |url= (help)
  46. ↑ Gibas A, Blewett DR, Schoenfeld DA, Dienstag JL (1992). "Prevalence and incidence of viral hepatitis in health workers in the prehepatitis B vaccination era". American Journal of Epidemiology. 136 (5): 603–10. PMID 1442723. Unknown parameter |month= ignored (help); |access-date= requires |url= (help)
  47. ↑ Szmuness W, Purcell RH, Dienstag JL, Stevens CE (1977). "Antibody to hepatitis A antigen in institutionalized mentally retarded patients". JAMA : the Journal of the American Medical Association. 237 (16): 1702–5. PMID 139479. Unknown parameter |month= ignored (help); |access-date= requires |url= (help)
  48. ↑ Hutin YJ, Pool V, Cramer EH, Nainan OV, Weth J, Williams IT, Goldstein ST, Gensheimer KF, Bell BP, Shapiro CN, Alter MJ, Margolis HS (1999). "A multistate, foodborne outbreak of hepatitis A. National Hepatitis A Investigation Team". The New England Journal of Medicine. 340 (8): 595–602. doi:10.1056/NEJM199902253400802. PMID 10029643. Retrieved 2012-02-29. Unknown parameter |month= ignored (help)
  49. ↑ Niu MT, Polish LB, Robertson BH, Khanna BK, Woodruff BA, Shapiro CN, Miller MA, Smith JD, Gedrose JK, Alter MJ (1992). "Multistate outbreak of hepatitis A associated with frozen strawberries". The Journal of Infectious Diseases. 166 (3): 518–24. PMID 1323618. Retrieved 2012-02-29. Unknown parameter |month= ignored (help)
  50. ↑ Lerman Y, Chodik G, Aloni H, Ribak J, Ashkenazi S (1999). "Occupations at increased risk of hepatitis A: a 2-year nationwide historical prospective study". American Journal of Epidemiology. 150 (3): 312–20. PMID 10430237. Retrieved 2012-02-29. Unknown parameter |month= ignored (help)
  51. ↑ Glas C, Hotz P, Steffen R (2001). "Hepatitis A in workers exposed to sewage: a systematic review". Occupational and Environmental Medicine. 58 (12): 762–8. PMC 1740082. PMID 11706141. Retrieved 2012-02-29. Unknown parameter |month= ignored (help)
  52. ↑ Poole CJ, Shakespeare AT (1993). "Should sewage workers and carers for people with learning disabilities be vaccinated for hepatitis A?". BMJ (Clinical Research Ed.). 306 (6885): 1102. PMC 1677505. PMID 8388287. Unknown parameter |month= ignored (help); |access-date= requires |url= (help)
  53. ↑ Trout D, Mueller C, Venczel L, Krake A (2000). "Evaluation of occupational transmission of hepatitis A virus among wastewater workers". Journal of Occupational and Environmental Medicine / American College of Occupational and Environmental Medicine. 42 (1): 83–7. PMID 10652693. Retrieved 2012-02-29. Unknown parameter |month= ignored (help)
  54. ↑ Weldon M, VanEgdom MJ, Hendricks KA, Regner G, Bell BP, Sehulster LM (2000). "Prevalence of antibody to hepatitis A virus in drinking water workers and wastewater workers in Texas from 1996 to 1997". Journal of Occupational and Environmental Medicine / American College of Occupational and Environmental Medicine. 42 (8): 821–6. PMID 10953820. Retrieved 2012-02-29. Unknown parameter |month= ignored (help)
  55. ↑ Venczel L, Brown S, Frumkin H, Simmonds-Diaz J, Deitchman S, Bell BP (2003). "Prevalence of hepatitis A virus infection among sewage workers in Georgia". American Journal of Industrial Medicine. 43 (2): 172–8. doi:10.1002/ajim.10174. PMID 12541272. Retrieved 2012-02-29. Unknown parameter |month= ignored (help)

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