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Cryptosporidiosis: An Outbreak Associated with Drinking Water Despite State-of-the-Art Water Treatment

Susan T. Goldstein, MD; Dennis D. Juranek, DVM, MSc; Otto Ravenholt, MD, MPH; Allen W. Hightower, MS; Debra G. Martin, RN; June L. Mesnik, BA; Sean D. Griffiths, BA; Angela J. Bryant, BS; Rick R. Reich, BA; and Barbara L. Herwaldt, MD, MPH
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From the Centers for Disease Control and Prevention, Atlanta, Georgia; and the Clark County District Health Department, Las Vegas, Nevada. Acknowledgments: The authors thank June Constantino, Daniel Maxon, Fran Courtney, Clare Schmutz, and others of the Divisions of Epidemiology, Environmental Health, and Nursing, Clark County District Health Department, Las Vegas, Nevada; J.T. Monscvitz and staff at the Southern Nevada Water System, Las Vegas, Nevada; Robert Sullivan, Linda Blish, Ron Zegers, and staff at the Las Vegas Valley Water District, Las Vegas, Nevada; Penny Williams, Lauresa Benham, and Ida Burnett-Spurlin at laboratory A, Las Vegas, Nevada; Drs. Jerry Cade, Kathryn Crooks, and Walter Herron and the physicians and staff of the Wellness Center and the AIDS inpatient unit, University Medical Center, Las Vegas, Nevada; Margaret R. Hurd, Michael J. Arrowood, Mary E. Bartlett, and Patrick J. Lammie, Division of Parasitic Diseases, Centers for Disease Control and Prevention; Dr. David Dorway and staff of Children's Clinic, Las Vegas, Nevada; staff of Desert Allergy and Pediatric Center, Las Vegas, Nevada; and Bertha Warwick and staff of Social Service, Las Vegas, Nevada. Requests for Reprints: Barbara L. Herwaldt, MD, MPH, Centers for Disease Control and Prevention, Division of Parasitic Diseases, Mailstop F-22, 4770 Buford Highway NE, Atlanta, GA 30341-3724. Current Author Addresses: Dr. Goldstein: Centers for Disease Control and Prevention, Division of Viral and Rickettsial Diseases, Hepatitis Branch, Mailstop G-37, 1600 Clifton Road NE, Atlanta, GA 30333.

Copyright ©2004 by the American College of Physicians

Ann Intern Med. 1996;124(5):459-468. doi:10.7326/0003-4819-124-5-199603010-00001
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Objective: To determine the magnitude and source of an outbreak of cryptosporidiosis among persons with human immunodeficiency virus (HIV) infection and to determine whether the outbreak extended into the immunocompetent population.

Design: Matched case–control study and environmental investigation.

Setting: Clark County, Nevada.

Participants: Adults with HIV infection (36 case-patients with laboratory-confirmed Cryptosporidium parvum infection and 107 controls), matched by physician or clinic and by CD4+ cell count category.

Measurements: Potential risk factors for infection, death rates, and data on water quality.

Results: Review of surveillance and microbiology records identified 3 cases of cryptosporidiosis in 1992 (the first year that cryptosporidiosis was reportable in Nevada), 23 cases in 1993, and 78 cases in the first quarter of 1994. Of the 78 laboratory-confirmed cases in the first quarter of 1994, 61 (78.2%) were in HIV-infected adults. Of these 61 adults, 32 (52.5%) had died by 30 June 1994; at least 20 of the 32 (62.5%) had cryptosporidiosis listed on their death certificates. In the case–control study, persons who drank any unboiled tap water were four times more likely than persons who drank only bottled water to have had cryptosporidiosis (odds ratio, 4.22 [95% CI, 1.22 to 14.65]; P = 0.02). For persons with CD4+ cell counts less than 100 cells/mm3, the association between tap water and cryptosporidiosis was even stronger (odds ratio, 13.52 [CI, 1.78 to 102.92]; P = 0.01). Additional data indicate that this outbreak also affected persons who were not infected with HIV. No elevated turbidity values or coliform counts and no Cryptosporidium oocysts were found in testing of source (Lake Mead) or finished (treated) water during the study period, but so-called presumptive oocysts were intermittently found after the investigation in samples of source water, filter backwash, and finished water.

Conclusions: A cryptosporidiosis outbreak was associated with municipal drinking water, despite state-of-the-art water treatment and water quality better than that required by current federal standards. This outbreak highlights the importance of surveillance for cryptosporidiosis and the need for guidelines for the prevention of waterborne-Cryptosporidium infection among HIV-infected persons.


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Figure 1.
Number of cases of Cryptosporidium infection reported to the Clark County District Health Department by month of diagnosis, January 1993 to December 1994 (n = 148).Cryptosporidium

The outbreak period was 1 December 1993 to 30 June 1994; the study period was 1 January to 30 April 1994; and the interview period was mid-May to mid-June 1994. Because the months of onset of illness for persons whose cases were diagnosed after the study period were unknown, we estimated the duration of the outbreak period. During the study period, 78 persons with laboratory-confirmed infection were identified.

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Figure 2.
Flow diagram of the review of laboratory records, the case–control studies, and the community health survey.
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Figure 3.
Maps of Clark County showing the residences of persons with cryptosporidiosis and the water distribution system. Top.nBottom.

Geographic distribution of the residences of persons with laboratory-confirmed cryptosporidiosis ( = 73). Location of residence was known for 57 of the 61 adults with human immunodeficiency virus (HIV) infection (93.4%), for all 4 adults without HIV infection, for both HIV-infected children, and for 10 of the 11 immunocompetent children (90.9%). Cacti denote sparsely populated areas. Map not drawn to scale. Lake Mead and the major water pipelines serving Clark County. Water from Lake Mead originates in the Colorado River. It is disinfected and filtered in the treatment plant and then pumped into the distribution system. Clark County is divided into five geographic areas, all of which are served by the treatment plant. Figure not drawn to scale.

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Figure 4.
Month of onset of diarrhea among adults with human immunodeficiency virus (HIV) infection and cryptosporidiosis, immunocompetent children with cryptosporidiosis, and county employees with diarrheal illness: Clark County, December 1993 to April 1994. Top.Middle.Bottom.

The epidemic curve for 51 of the 61 HIV-infected adults (83.6%) with laboratory-confirmed cryptosporidiosis. Of the other 10 HIV-infected adults, 8 (13.1%) had an unknown date of onset; 1 (1.6%) became ill in August 1993; and 1 (1.6%) became ill in September 1993. The epidemic curve for all 11 immunocompetent children with laboratory-confirmed cryptosporidiosis. The epidemic curve for 75 of the 91 county employees (82.4%) who reportedly had a diarrheal illness during the study period.

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