By Cheryl Squier BSN, CIC; Yu-sen E. Lin, Ph.D. and Janet E. Stout, Ph.D.

Summary: The number of immunocompromised individuals is increasing within our communities. This places a greater emphasis on maintaining the quality of drinking water. The following article identifies potential waterborne pathogens and specific measures that can be taken to prevent infections.

In developed nations, many assume water is free of pathogens. In spite of this expectation, community water systems are often cited as sources of waterborne infections. In a healthcare system, the potable water may be a source of hospital-acquired infection.

A sink or shower may harbor a variety of microbial “monsters,” particularly in the slime layer or biofilm of plumbing systems. Waterborne disease causing organisms may include bacteria, fungi, protozoan parasites and viruses. Transmission of these pathogens may occur via contact, ingestion, aspiration or aerosolization.

Many of these microorganisms are of little concern for healthy individuals. However, for immunocompromised individuals, these microbes can be “opportunistic” pathogens, capitalizing on the compromised status of their host. Specific populations are at higher risk for waterborne infections, such as the elderly and very young as well as AIDS, transplant, cancer and dialysis patients. In view of this, a state health advisory recommended that immunocompromised patients “bring water to a rolling boil for one minute before use” in their homes.1

Protozoan parasites
Protozoan parasites are often the cause of large community outbreaks related to drinking water. These parasites include Giardia lamblia, Cryptosporidium parvum and Cyclospora. Several animal and human sources have been linked to the introduction of waterborne parasites in drinking water. These included dairy farms (Cryptosporidium) and untreated sewage (Giardia). In addition, wet weather events tend to increase the concentrations and total numbers of parasites in source water.2 The largest U.S. community outbreak of Cryptosporidium occurred in Milwaukee in 1993.3 More than 50 people died. In addition, over 400,000 people became ill. In the HIV infected patient, this parasite may cause a life-threatening infection.

Gram negative bacilli are well established waterborne pathogens, particularly in the healthcare setting. As more complex healthcare is provided in the home environment, infections linked to the home water supply may result. For example, Mycobacterium species, Roseomonas gilardii and Ewingella americana have been linked to infections in patients receiving continuous ambulatory peritoneal dialysis both within the healthcare facility and the home environment.4,5,6 The water supply may contaminate the catheter at the insertion site or the fluid bags used for dialysis.

Salmonella typhimurium, Campylo-bacter jejuni, Shigella typhimurium,Vibrio cholerae and Pleiomonasshiggelloides are other bacteria that can be found in contaminated water systems and have been linked to community acquired infection (see Table 1).

Nontuberculous mycobacteria (NTM) found in water has been cited as a cause of hospital acquired infection. Municipal water systems are a major reservoir for NTM. In one study of U.S. dialysis centers, 83 percent of municipal water supplies were colonized by NTM.7 More recently, Mycobacterium avium in a hospital water system was linked to disease in HIV infected patients.8

According to Victor L. Yu, M.D., chief of Infectious Diseases at the VA Pittsburgh Health Care System, data from the Centers for Disease Control and Prevention (CDC) suggest that at least 29,808 Americans have died of hospital acquired Legionnaires’ disease since 1983. Legionella is a high profile bacterium, which is a source of outbreaks in hotels, apartment buildings and healthcare facilities. Many hospitals routinely test their water for Legionella. Awareness of the presence or absence of Legionella in a healthcare facility assists in identifying cases of hospital acquired Legionnaires’ disease. Allegheny County, Pa., was the first county to establish guidelines for environmental culturing of healthcare facilities.9 The state of Maryland recently issued guidelines for healthcare facilities that can be accessed via the Internet (

In 1999, the CDC proposed that bone marrow transplant centers routinely culture their water supply for Legionella.10 The Joint Commission for the Accreditation of Hospital Organizations considers this of sufficient importance to issue a new standard that will go into effect in 2001. This will require that hospitals “manage pathogenic biological agents in cooling towers, domestic hot water, and other aerosolizing water systems.”11

Fungal disease is typically associated with soil or construction projects. Fungi (Aspergillus/Fusarium) are found in decaying organic material and soils and are generally transmitted via inhalation of spores. Transplant and cancer patients are at particularly high risk for fungal infections. More recently, it has been suggested that fungi may be a source of waterborne infection. In one study, fungi were isolated from a hospital water supply.12 The Fusarium isolated from the water supply and the infected patients were found to be identical.

Infrequently, viruses are identified as sources of community waterborne outbreaks and in at least one case a small round structure virus (SRSV) was linked to hospital acquired infection.13 Viral outbreaks usually present as gastrointestinal illnesses. Since physicians don’t routinely order viral cultures on patients with diarrhea, viral outbreaks may be missed. Other viral agents that have been linked to waterborne disease are Hepatitis A and Norwalk virus.

Approaches to prevention
Although chlorine is the primary disinfectant of choice in water treatment practice, many waterborne pathogens are resistant to chlorine and are often found in finished water. These chlorine-resistant pathogens include viruses, parasites and bacteria that can cause hepatitis, gastroenteritis, cryptosporidiosis and Legionnaires’ disease.

In the past decade, some water treatment advancements have improved disinfection efficiency. Enhanced coagulation process and rapid sand filtration have been used to effectively remove a significant percentage of Cryptosporidium and Cyclospora. Post-treatment or on-site disinfection are also available to enhance biological safety of drinking water. For example, the Pittsburgh Water and Sewer Authority studied post-treatment options in an uncovered reservoir to remove Giardia cysts and Cryptosporidium oocysts.

Pilot testing compared ozonation and membrane filtration. All tested membrane filter systems acted as absolute barriers to Giardia cysts (5 log removal) and Cryptosporidium oocysts (6 log removal); whereas ozone inactivated 0.1-to-0.5 log10 units using doses of 2.9 to 6.6 mg/L of ozone.14 The current Surface Water Treatment Rule sets Maximum Contaminant Level Goals (MCLGs) for Legionella, Giardia and viruses to zero because any exposure to these contaminants represents some health risk. However, MCLGs—unlike maximum contaminant levels (MCLs)—aren’t enforceable and on-site treatment may be required to remove these pathogens. For example, many hospitals have used on-site treatment with copper-silver ionization, chlorination or chlorine dioxide to control Legionella from the water distribution systems.15 An epidemiological study also suggested that fewer Legionnaires’ disease outbreaks were reported in municipalities using monochloramine as a residual disinfectant vs. free chlorine.16 However, the effectiveness of chloramines as microbiocidal agents is still somewhat controversial.17

With increasing survival rates among neonates, transplant, HIV and cancer patients and an increasing aging population, drinking water quality will become even more critical. Some changes in disinfection practice (i.e. from free chlorine to mono-chloramine) may result in changes in microbial populations in the finished water.


  1. Pennsylvania Department of Health Cryptosporidiosis Fact Sheet (Guidance for People with Severely Weakened Immune Systems), Pennsylvania Department of Health, Dec. 1, 1995.
  2. States, S., D. Stadterman, L. Ammon, et al., “Protozoa in river water: sources, occurrence, and treatment,” Journal of the American Water Works Association, 1997, 89: 74-83.
  3. MacKenzie,W.R., H.J. Neil, M.S. Hoxie, et al., “A massive outbreak in Milwaukee of cryptosporidium infection transmitted through the public water supply,” New England Journal of Medicine, 1994, 331(3):161-167.
  4. Vera, G., S.Q. Lew, “Mycobacterium fortuitum peritonitis in two patients receiving continuous ambulatory peritoneal dialysis,” American Journal of Nephrology, 1999, 19:586-596.
  5. Kati, C., E. Bibashi, E. Kokolina, et al., “Case of peritonitis caused by Ewingella americana in a patient undergoing continuous ambulatory peritoneal dialysis,” Journal of Clinical Microbiology, 1999, 37(11):3733-4.
  6. Sandoe, J.A., J. Malnick, K.W. Loudon, “A case of peritonitis caused by Roseomonas gilardii in a patient undergoing continuous anbulatory peritoneal dialysis,” Journal of Clinical Microbiology, 1997, 35(8):2150-2.
  7. Carson, L.A., L.A. Bland, L.B. Cusik, et al., “Prevalence of non-tuberculosis mycobacteria in water supplies of hemodialysis centers,” Applied Environmental Microbiology, 1998, 54:3122-3125.
  8. von Reyn, C.F., J.N. Maslow, T.W. Barber et al., “Persistent colonisation of potable water as a source of Mycobacterium avium infection in AIDS,” Lancet, 1994, 343:1137-1141.
  9. Allegheny County Health Department, Approaches to Prevention and Control of Legionella Infection in Allegheny County Health Care Facilities (Second Edition), Allegheny County Health Department, 1997, Pittsburgh, 1-15.
  10. Centers for Disease Control, Guidelines for Prevention of Opportunistic Infections in Bone Marrow Transplant Recipients, Federal Register, Thursday, Sept. 2, 1999, 64.
  11. Joint Commission for Accreditation of Hospitals Organization, Management of the Environment of Care, EC Standard 1.7, 2001.
  12. Anaissie, E.J., and H. Boutati, “Fusarium, a significant emerging pathogen in patients with hematological cancer: ten years experience,” 37th Interscience Conference on Antimicrobial Agents and Chemotherapy, Toronto 1997, Abstract J-87.
  13. Schvoerer, E., F. Bonnet, V. Dubois, et al., “A hospital outbreak of gastroenteritis possibly related to the contamination of tap water by a small round structured virus,” Journal of Hospital Infections, 1999, 43:149-154.
  14. States, S., M. Scheuring, R. Evans, et al., “Membrane filtration as post-treatment,” Journal of the American Water Works Association, 2000, 92:52-68.
  15. Lin, Y.E., R.D. Vidic, J. E. Stout, et al., “Legionella in water distribution systems,” Journal of the American Water Works Association, 1998, 90:112-121.
  16. Kool, J.L., D. Bergmire-Sweat, J.C. Butler et al., “Hospital characteristics associated with colonization of water systems by Legionella and risk of nosocomial Legionnaires’ disease: a cohort study of 15 hospitals,” Infectious Control Hospital Epidemiology, 1999; 20:798-805.
  17. Lin, Y.E., V.L. Yu, R.D. Vidic, et. al. Discussion of monochloramine and Legionnaires’ disease, Journal of the American Water Works Association, 2000, 92:88-90.
  18. Surveillance for Waterborne-Disease Outbreaks—United States, 1995-1996. Morbidity and Mortality Weekly Report, 1998, 47:27.
  19. United States Environmental Protection Agency, “25 years of the safe drinking water act: history and trends,” EPA 816-R-99-007, Dec. 1999, p. 30.
  20. Daley, Melvin, “Waterborne Pathogens,” Emerging Pathogens Guidebook, VA National Engineering Service Center, St. Louis, 1998.

About the authors
Cheryl Squier, BSN, CIC, is an infection control nurse at the VA Pittsburgh Healthcare System. She has published in the area of HIV and infection control. She has a review article on nosocomial waterborne infection “in-press” in Current Infectious Diseases.

Yu-sen E. Lin, Ph.D., is an environmental engineer with the Infectious Disease Section, Department of Medicine, University of Pittsburgh and the VA Medical Center Special Pathogens Laboratory in Pittsburgh. He has written numerous articles in journals including Water Research and the Journal of American Water Works Association.

Janet E. Stout, Ph.D., is a microbiologist and director of the VA Medical Special Pathogens Laboratory in Pittsburgh. She is also an assistant professor at the University of Pittsburgh School of Medicine. She has written numerous articles on Legionnaires’ disease and other waterborne pathogens that have been published in the New England Journal of Medicine, Journal of the American Medical Association and the Journal of Infectious Diseases.


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