Assessing the public health threat associated with waterborne cryptosporidiosis: report of a workshop.

Reported by CDC and EPA.

On September 22-23, 1994, the Center for Disease Control and Prevention's National Center for Infectious Diseases (NCID) convened a workshop entitled, Prevention and Control of Waterborne Cryptosporidiosis: An Emerging Public Health Threat," for the purpose of assisting CDC and state public health departments in providing guidance on these issues. Representatives from 40 states and from regulator and public health agencies, water utility companies, and advocacy groups discussed approaches to avoiding unnecessary boil-water advisories (i.e., statements to the public advising persons to boil water before drinking it) and preventing and controlling waterborne cryptosporidiosis. Work groups at the meeting addressed four issues: 1) surveillance systems and epidemiologic study designs; 2) public health responses when oocysts are detected in drinking water; 3) cryptosporidiosis in immunocompromised persons; and 4) water sampling methods and interpretation of results. The work groups defined the problems associated with these issues and developed strategies that could be used initially to manage these problems. The work group conclusions are for considerations by persons and organizations who must assist with these issues and by those who seek to advance understanding of waterborne cryptosporidiosis.

Background

Cryptosporidium parvum has been recognized as a human pathogen since 1976. During 1976-1982, the disease was resorted rarely and occurred predominantly in immunocompromised persons. In 1976-1982 the number of reported cases began to increase as a result of the acquired immunodeficiency syndrome (AIDS) epidemic. Initially, the increase in incidence was limited to immunocompromised persons; however, outbreaks and sporadic infections in immunocompetent persons were identified with the aid of newly developed laboratory diagnostic techniques.

Cryptosporidium is a protozoan parasite transmitted by ingestion of oocysts that have been excreted in the feces of infected humans and animals. The infection can be transmitted through person-to-person or animal-to-person contact, ingestion of fecally contaminated water or food, or contact with fecally contaminated environmental surfaces. Several municipal waterborne outbreaks of cryptosporidiosis (1-7), including the 1993 outbreak in Milwaukee, have focused attention and concern on the potential for waterborne transmissions.

Recent studies indicate that Cryptosporidium oocysts are present in 65%-97% of surface water (i.e., rivers, lakes, and streams) tested throughout the United States (8-10). Because Cryptosporidium is highly resistant to chemical disinfectants used to treat drinking water, physical removal of the parasite from water by filtration is an important component of the municipal water treatment process. However, many cities in the United States do not use filtration as part of their water treatment process, and no current method can guarantee complete removal `d oocysts. The risks for transmission can be reduced by water filtration if the filters are properly operated and maintained.

In the United States, all outbreaks of waterborne cryptosporidiosis detected from 1984 through 1993 occurred in communities where water utilities met state and federal standards for acceptable drinking water quality, and all surface water supplies implicated in those outbreaks had been filtered. These outbreaks indicate that utility compliance with Environmental Protection Agency (EPA) water treatment standards did not adequately protect against waterborne cryptosporidiosis. The EPA turbidity standards have been strengthened since the Milwaukee outbreak, and the finished (i e., tap) water in Milwaukee at the time of the outbreak would not have met the new standards. Nevertheless, recentreports of Cryptosporidium oocysts in fully treated (i.e., disinfected and filtered) municipal water that was meeting these new standards indicate small; numbers of oocysts breached water treatment filters in 27%-54% of the communities evaluated (11, 12).

The health risk associated with drinking filtered or unfiltered tap water contaminated with small numbers of C. parvum oocysts is unknown. Although researchers have recovered small numbers of oocysts from drinking water, current laboratory methods cannot reliably determine if these oocysts are viable or are infectious to humans. Moreover, research has not determined whether a) the number of oocysts usually present in drinking water is sufficient to cause illness in humans, b) immunosuppressed persons are more susceptible to lower doses of oocysts than are immunocompetent persons, or c) strains of C. parvum vary in virulence and infectious dose. The results of a study that used a Cryptosporidium strain derived from calves suggested that the infectious dose of oocysts to healthy human volunteers is small (i.e., a median infectious dose could be as few as 132 oocysts) (13). Other reports based on mathematical modeling algorithms indicate that some persons could become infected with a dose as low as one oocyst (14).

EPA has proposed a plan to collect data concerning a) the occurrence of several pathogens and chemicals in water and b) the ability of water treatment plants to remove these substances (15). The EPA plan the Information Collect Rule (ICR)-will require utilities in the United States that both obtain water from surface water sources and provide service to [greater than or equal]10,000 persons to test for Cryptosporidium oocysts in source water (and in some cases,finished water) for a period of 12-18 months. Almost all utilities are likely to detect oocysts in their surface source water on some occasions, and 24%-50% of utilities can expect to detect oocysts in their treated water (16). When low levels of oocysts are identified in treated water through testing required by the ICR, public health agencies and other local and state officials could be pressured to issue immediate boil-water advisories or respond in other ways to the perceived public health threat, regardless of whether such measures are necessary. Local and state health departments and water utilities have expressed concern because current data are insufficient to determine the health risks associated with low-level oocyst contamination of fully treated drinking water.

Workshop Objectives

The workshop was held to determine and address the public health concerns associated with waterborne cryptosporidiosis and to assess the potential public health, administrative, and economic implications of the ICR' s Cryptosporidium testing component. Each of the more than 300 participants received background information regarding cryptosporidiosis and the ICR. The work groups had the following four specific objectives:

* To identify surveillance systems and epidemiologic study designs for assessing the public health importance of low levels of Cryptosporidium oocysts or elevated turbidity in public drinking water.

* To provide guidance for public health responses to the detection of Cryptosporidium oocysts in drinking water and to provide methods for notifying the public of potential risks for waterborne transmission.

* To identify and examine options for preventing waterborne transmission of Cryptosporidium to immunocompromised persons who use public water supplies.

* To evaluate and address a) water sampling methods for identifying Cryptosporidium oocysts, b) interpretation of data derived from these methods, c) the status of alternative methods of sampling, arid d) laboratory research priorities.

Work Group Conclusions

These work group conclusions are the summarized suggestions from the proceedings of each work group, as presented by the work group leaders at the conclusion plenary session of the workshop, and they were drafted with multiple opportunities for open input from all participants. The workshop formalized the awareness that current knowledge of Cryptosporidium, particularly waterborne cryptosporidiosis, is minimal. This level of knowledge does not provide a scientifically sound basis for many essential decisions regarding the public health risks associated with the infection. This report provides, on the basis of available information, potential strategies for managing cryptosporidiosis. Moreover, these discussions assist with focusing researchers on possible ways to attain the information needed to better understand the risk factors associated with waterborne cryptosporidiosis, thereby enhancing the eventual development of effective prevention strategies.

Surveillance Systems and Epidemiologic Study Designs

Local, state, and national public health agencies should cooperatively initiate and develop surveillance and epidemiologic investigations to assess the public health significance of low levels of Cryptosporidium oocysts in public drinking water. The work group suggested the following seven approaches to surveillance, which are listed hierarchically by increasing order of the perceived effort and cost:

1) Make cryptosporidiosis reportable to CDC.

2) Monitor sales of antidiarrheal medications.

3) Monitor logs maintained by Health Maintenance Organizations (HMOs) and hospitals for complaints of diarrheal illness.

4) Monitor incidence of diarrhea in nursing homes.

5) Monitor laboratory data for Cryptosporidium.

6) Monitor tap water in selected cities.

7) Make immediate epidemiologic assistance available.

Public Health Responses

Discovering Cryptosporidium oocysts in low levels in finished water should not be the only reason for issuing a boil-water advisory. Additional support for such an advisory should include other data indicating that the water quality is unacceptable. A task force should be created to develop general guidelines for implementing and lifting boil-water advisories and to assist local agencies in deciding when boil-water advisories are necessary. A coordinated local-to-national effort should be made before implementation of the ICR to provide information concerning Cryptosporidium and drinking water to public health officials. water utility officials, healthcare procials, immunosuppressed populations, and the general public. Such information should include appropriate prevention strategies.

Cryptosporidiosis in Immunocompromised Persons

A coalition or task force should be established that will place high priority on educating immunocompromised persons about cryptosporidiosis because of the increased risk for severe disease if they become infected. This group should provide information to immunocompromised persons that explains how to reduce the risk for cryptosporidiosis, regardless of the source of transmission, and about specific measures they can take to further reduce the risk for waterborne transmission. Some suggestions are boiling water before use, using water filters, using bottled water, and determining risk in nonoutbreak settings.

Water Sampling Methods and Interpretation of Results

Current methods are limited with regard to detecting oocysts in source and finished drinking water. These technical limitations restrict the ability of public health officials to practically interpret data on the occurrence and public health importance of Cryptosporidium in drinking water. Research should be accelerated to develop alternative, dependable methods for detecting Cryptosporidium in drinking water.

In November 1994, as a result of the workshop, NCID initiated the Working Group on Waterborne Cryptosporidiosis. This group is a coalition of agencies and organizations that meets biweekly by teleconference to discuss concerns about cryptosporidiosis. The group has convened smaller task forces that are developing information to help local and state public health departments, healthcare providers, water utilities, and regulatory agencies address many of the strategies proposed. Additional information concerning the Working Group on Waterborne Cryptosporidiosis can be obtained by calling [phone] or [phone].

References

(1.) D'Antonio RG, et al. "A waterborne outbreak of cryptosporidiosis in normal hosts," Ann Intern Med 1985; 103:886-8. (2.) Gallaher MM, et al. "Cryptosporidiosis and surface water," Am J Public Health 1989; 79:39-42. (3.) Hayes EB, et al. "Large community outbreak of cryptosporidiosis due to contamination of a filtered public water supply,: N Engl J Med 1989; 320:1372-6. (4.) CDC. Surveillance for waterborne disease outbreaks--United States, 1986- 1988. MMWR 1990; 39(No. SS-1):1-21. (5.) CDC. Surveillance for waterborne disease outbreaks-United States, 1989-1990. MMWR 1991; 40(No. SS-3):1-21. (6.) CDC. Surveillance for waterborne disease outbreaks-United States, 1991-1992. MMWR 1993; 42(No. SS-5):1-22. (7.) MacKenzie WR, et al. "A massive outbreak in Milwaukee of Cryptosporidium infection transmitted through the public water supply," N Engl J Med 1994, 331:161=7. (8.) Rose JB. "Occurrence and significance of Cryptosporidium in water," J of Am Water Waters Assoc, 1988; 80:53-801. (9.) Rose JB, Gerba CP, Jakubowski W. "Survey of potable water supplies for Cryptosporidium and Giardia," Environ Science and Tech 1991; 25:1393-400. (10.) LeChevallier MW., Norton WD, Lee RG. "Occurrence of Giardia and Cryptosporidium spp. in surface water supplies," Appl Environ Microbiol 1991; 57:2610-6. (11.) LeChevallier MW., Norton WD, Lee RG. "Giardia and Cryptosporidium spp. in filtered drinking water supplies," Appl Environ Microbiol 1991; 57:2617-21. (12.) LeChevallier MW., Moser RH. "Occurrence of Giardia and Cryptosporidium in raw and ffinished drinking water," J Am Water Works Assoc (in press). (13.) Dupont HL, et al. "The infectivity of Cryptosporidium parvum in healthy volunteers," N Engl J Med 1995; 332:855-9. (14.) Haas CN, Rose JB. "Reconciliation of microbial risk models and outbreak epidemiology: the case of the Milwaukee outbreak," In: Proceedings of the American Water Works Association 1994 Annual Conference: Water Quality, Denver: American Water Works Assoc., 1994:517-23. (15.) EPA. "National primary drinking water regulations: monitoring requirements for public drinking water supplies: Cryptosporidium, Giardia, viruses, disinfection, byproducts, water treatment plant data and other information requirements," Federal Register 1994; 59:6332-444. (16.) LeChevallier MW, Moser RH, Monitoring of Giardia and Cryptosporidium in the American water system. Voorhees, NJ, American Water Works Service Co., Inc., 1993.

Reported by David G. Addiss, M.D., M.P.H., Michael J. Arrowood, Ph.D., Mary E. Bartlett, Daniel G. Colley, Ph.D., Dennis D. Juranek, D.V.M., M.Sc., Jonathan E. Kaplan, M.D.,, Robert Perciasepe, M.S., Jame K. Elder, Stig E. Regli, and Paul S. Berger, Ph.D.