Waterborne Disease Outbreaks, 1986-1988
William C. Levine, M.D., M.Sc. William T. Stephenson Division of Bacterial Diseases Center for Infectious Diseases Gunther F. Craun, M.P.H. Health Effects Research Laboratory Environmental Protection Agency Summary
From 1986 to 1988, 24 states and Puerto Rico reported 50 outbreaks of illness due to water that people intended to drink, affecting 25,846 persons. The protozoal parasite Giardia lamblia was the agent most commonly implicated in outbreaks, as it has been for the last 10 years; many of these outbreaks were associated with ingestion of chlorinated but unfiltered surface water. Shigella sonnei was the most commonly implicated bacterial pathogen; in outbreaks caused by this pathogen, water supplies were found to be contaminated with human waste. Cryptosporidium contamination of a chlorinated, filtered public water supply caused the largest outbreak during this period, affecting an estimated 13,000 persons. A large multistate outbreak caused by commercially produced ice made from contaminated well water caused illness with Norwalk-like virus among an estimated 5,000 persons. The first reported outbreak of chronic diarrhea of unknown cause associated with drinking untreated well water occurred in 1987. Twenty-six outbreaks due to recreational water use were also reported, including outbreaks of Pseudomonas dermatitis associated with the use of hot tubs or whirlpools, and swimming-associated shigellosis, giardiasis, and viral illness. Although the total number of reported water-related outbreaks has been declining in recent years, the few large outbreaks due to Cryptosporidium, Norwalk-like agent, Shigella sonnei, and Giardia lamblia caused more cases of illness in 1987 than have been reported to the Water-Related Disease Outbreak Surveillance System for any other year since CDC and the Environmental Protection Agency began tabulating these data in 1971. INTRODUCTION
Since 1971, CDC--in collaboration with the Environmental Protection Agency (EPA)--has tabulated data on waterborne-disease outbreaks separately from those for foodborne-disease outbreaks and has compiled these data in annual surveillance reports. The Water-Related Diseases Activity has the following goals: 1) to determine trends in the incidence of water-related diseases in the United States, 2) to characterize the epidemiology of water-related diseases, 3) to disseminate information on prevention and control of water-related diseases to appropriate public health personnel, 4) to train personnel in federal, state, and local health departments in epidemiologic techniques used in the investigation of water-related disease outbreaks, and 5) to collaborate with local, state, and other federal and international agencies in initiatives concerning the prevention of water-related diseases.
In addition to outbreaks associated with water intended for drinking, the Water-Related Disease Surveillance Report cites reports of 1) outbreaks of illness associated with exposure to recreational water and 2) epidemiologic investigation of outbreaks of gastroenteritis on ocean-going passenger vessels that call on U.S. ports. METHODS Definition of Terms
A waterborne disease outbreak is an incident in which 1) two or more persons experience a similar illness after consumption or use of water intended for drinking and 2) epidemiologic evidence implicates the water as the source of illness. In addition, a single case of chemical poisoning constitutes an outbreak if laboratory studies indicate that the water has been contaminated by the chemical. Only outbreaks associated with water intended for drinking are included.
Community public water systems (municipal systems) are defined as public or investor-owned water systems that serve large or small communities, subdivisions, or trailer parks with at least 15 service connections or 25 year-round residents. Noncommunity public water systems (semipublic water systems) are those of institutions, industries, camps, parks, hotels, or service stations that may be used by the general public. Individual systems (private water systems), which are generally wells and springs, are those used by one or several residences or by persons traveling outside populated areas. These definitions correspond to those in the Safe Drinking Water Act (Public Law 93-523) of l974.
Disease outbreaks associated with water used for recreational purposes meet the same criteria used for waterborne outbreaks associated with drinking water. However, outbreaks associated with recreational water involve exposure to or unintentional ingestion of fresh or marine water, but they exclude wound infections caused by water-related organisms. Sources of Data
State health departments report water-related disease outbreaks to CDC on a standard reporting form. In addition, the Health Effects Research Laboratory of EPA contacts all state water-supply agencies annually to obtain information about waterborne disease outbreaks. This report includes information from both sources. Representatives from CDC and EPA review and summarize outbreak data and also work together to investigate and evaluate waterborne disease outbreaks. Also, on request by state health departments, CDC and EPA offer epidemiologic assistance, provide consultation in the engineering and environmental aspects of water treatment, and, when indicated, collect large-volume water samples to identify viruses, parasites, and bacterial pathogens.
In an effort to increase reporting, CDC sent a letter in January 1989 to all state and territorial epidemiologists and laboratory directors requesting any additional reports of outbreaks investigated during the 3-year period 1986 to 1988. States not responding or reporting any outbreak during this period were contacted again by telephone.
Vessel masters of passenger cruise ships must report all persons who visited the ship's physician because of diarrheal illness during each voyage as a part of their request for permission to enter a port. If greater than or equal to 3% of passengers visit the ship's physician with gastrointestinal illness on a 1-week voyage, a quarantine officer will board and inspect the ship, and an epidemiologic investigation may be conducted. Interpretation of the data
Data in this report should be interpreted with caution. The number of waterborne disease outbreaks reported to CDC and EPA represents a fraction of the total number that occur. Since investigations were sometimes incomplete or conducted long after the outbreak, the quality of epidemiologic evidence implicating water varied; however, the waterborne hypothesis was the most logical explanation in these outbreaks. The likelihood of an outbreak's coming to the attention of health authorities varies considerably from one locale to another and depends largely upon consumer awareness, physician interest, and disease surveillance activities of state and local health and environmental agencies. Large interstate outbreaks and outbreaks of serious illness are most likely to come to the attention of health authorities. The quality of investigation conducted by state or local health departments varies considerably according to the department's interest in waterborne diseases and its budgetary, investigative, and laboratory resources. Additionally, a few outbreaks involving large numbers of persons may vastly alter the relative proportion of cases attributed to various etiologic agents. Therefore, this report should not be used to draw firm conclusions about the true incidence of waterborne disease outbreaks, or about the relative incidence of waterborne diseases of various etiologies. RESULTS
For the 3-year period 1986-1988, state and local health departments reported 50 outbreaks due to water that people intended to drink, causing illness in 25,846 persons. Twenty-two outbreaks were reported in 1986, 15 in 1987, and 13 in 1988. The individual outbreaks are listed by year in Tables 1-3. Each outbreak is summarized by agent and type of water system (Table 4). Outbreaks occurred in all months except December; 26% occurred in July (Figure 1).
Each of 24 states and Puerto Rico reported at least one outbreak during this period. Thirty-four percent of outbreaks were reported from three states: Pennsylvania (7), Colorado (6), and Vermont (4).
Giardia lamblia was identified as the causative agent in nine (18%) outbreaks, eight of which were associated with deficiencies in community water systems. Six of these outbreaks were associated with unfiltered surface water systems in which the only treatment was chlorination. The largest Giardia outbreak reported to CDC affected greater than 500 people; increased turbidity of the water and improper operation of the chlorinator may have contributed. Cryptosporidium in a chlorinated and filtered water supply caused one large parasitic disease outbreak (1). Shigella sonnei was the most common bacterial agent, implicated in four outbreaks. The largest of these was associated with contamination of a reservoir after heavy rains and loss of electricity. Thirty persons were hospitalized among the estimated 1,800 who became ill. One outbreak of gastroenteritis due to Salmonella newport and other species of Salmonella was caused by improper placement of a sewer line, possibly contaminating untreated well water. Another Salmonella-related outbreak was caused by ingestion of water from a storage tank that had possibly been contaminated by irrigation water; the one patient's isolate serotyped was identified as S. dublin. One Campylobacter-related outbreak occurred and was possibly caused by contamination of water mains through a cross-connection. Norwalk-like viral agents were detected serologically or by identification of Norwalk antigen in stool in three outbreaks (2,3). In 24 of the 50 outbreaks, the etiology of acute gastrointestinal illness (AGI) could not be determined. Also reported was an outbreak of chronic gastrointestinal illness (CGI) of unknown etiology; the outbreak was associated with drinking untreated ground water at a rural restaurant (4).
The 1987 listing includes one multistate outbreak of Norwalk-like viral gastroenteritis due to contaminated ice (2). At least four clusters of illness were traced to ice produced by a manufacturer in Pennsylvania, including illness following a university football game in Pennsylvania, a museum fund-raising event in Delaware, and at least two catered events. Water used to make the ice originated in wells that had been flooded by a creek during heavy rains in early September; high concentrations of fecal coliforms were found in both the ice and well water. The ice (60-300 tons), produced the week after the flood and distributed to Pennsylvania, Delaware, and New Jersey, may have caused illness in greater than 5,000 people; this estimate is based on distribution records and on the attack rates found in the Pennsylvania investigations. Residents with private wells flooded by the same creek were also thought to have been affected. The manufacturer recalled the remaining ice.
Five outbreaks were attributed to chemical intoxications. One was caused by back-siphoning of sodium hydroxide into a public water supply. Patients seen in emergency rooms had mouth ulcers, dermatitis, and first-degree skin burns. At several homes, the tap water had a pH of 12. Another outbreak due to an alkaline community water supply (pH 8.5-11) caused malaise, headaches, and dry, cracking, pruritic skin. Large amounts of soda ash and lime had been used to soften the water, without subsequent adjustment of the pH. Hyperfluoridation of a water supply resulted in illness characterized by nausea, vomiting, diarrhea, abdominal cramps, and pruritus. The pH of the water was 3.1, and copper levels were elevated because of the effects of acidity on the copper pipes. Ethylene-glycol contamination of a public water supply due to a cross-connection with a heating system also caused an outbreak (5). Symptoms included excessive fatigue, sleepiness, unsteadiness when walking, and dizziness. Water from the spigot had an ethylene-glycol concentration of 9%. One fatal case of methemoglobinemia was reported, involving an infant who was fed powdered formula; the formula had been prepared with well water contaminated with nitrates (6).
Pathogens reportedly were isolated from water samples during the investigation of five outbreaks: Giardia cysts in three, Cryptosporidium oocysts in one, and Salmonella in one. Coliform organisms were detected during the investigation of water systems in 32 outbreaks; in four outbreaks, coliforms were not detected. In the remaining investigations of outbreaks, no information was reported concerning testing of water for coliforms.
Most reported outbreaks were associated with community and noncommunity public water systems, as has been the case for most years since 1971 (Table 5). Deficiencies in water treatment were identified in 23 (46%) outbreaks, and untreated ground water was implicated in 16 (32%) (Table 6). Well water was the most common source of outbreaks, accounting for 28 (56%) over the 3-year period.
Data on 26 outbreaks associated with recreational water use for the period 1986-1988 are shown in Table 7. The most common illness was Pseudomonas dermatitis; all 10 of these outbreaks were associated with use of whirlpools or hot tubs. The two outbreaks of giardiasis were associated with swimming in pools; three outbreaks of shigellosis affected persons swimming in lakes; and one outbreak in which a Norwalk-like agent was confirmed also affected persons swimming in a lake. Four other swimming-related outbreaks of AGI were of unknown etiology. One outbreak of aseptic meningitis involved four persons who swam in a family pool filled with unchlorinated water from a nearby creek. An outbreak of a febrile enterovirus-like illness was associated with swimming in an inadequately chlorinated wading pool (7). Pontiac fever affected 14 persons who had used a hotel whirlpool; symptoms were fever, shortness of breath, chest pain, myalgia, and joint pain. Six of eight patients tested had elevated antibodies to Legionella pneumophila serogroup 6. An outbreak of leptospirosis among persons swimming in a stream was reported from Hawaii.
Since the previous surveillance summary was published (1988) (8), we have obtained 10 additional reports from previous years: one from 1983, two from 1984, and seven from 1985. One of the 10 was associated with recreational water use. These outbreaks are included in Table 8, and those associated with water used for drinking are included in Table 5.
From 1986 to 1988, CDC personnel investigated 10 outbreaks of diarrheal illness on cruise ships calling on U.S. ports. Three were investigated in 1986, two in 1987, and five in 1988. The mean size of the outbreak was 323 passengers (range 112-477). One ship had outbreaks on two consecutive 1-week cruises (9). Norwalk-like agents were identified as the cause of two outbreaks. No specific pathogens were identified in the eight others, but many had features consistent with Norwalk-like viral illness. Two outbreaks were possibly associated with meals served on board; in one, a "marinated calamari and vegetable julienne" dish was suspect. The ship's water was the probable vehicle in one outbreak. Illness on two cruises was possibly caused by food or water consumed offshore. DISCUSSION
The number of reported outbreaks associated with water intended for drinking was the same in 1986 as in the previous year, although the number was smaller in 1987 and 1988. The number of outbreaks reported in 1988 was the smallest ever reported to the Waterborne Outbreak Surveillance System. Additional outbreaks for these years may be reported to CDC after this summary is published.
The decrease in reported outbreaks may be due to an actual decrease in number of occurrences or to a decrease in the recognition or reporting of outbreaks. At the Workshop on Methods for Investigation of Waterborne Disease Outbreaks in Denver in October 1988, representatives from many state health departments, departments of natural and environmental resources, CDC, and EPA addressed this issue. The participants made recommendations to improve reporting; these recommendations include designating Waterborne Outbreak Surveillance Coordinators in each state, at CDC, and at EPA to assist with the gathering and reporting of data on waterborne outbreaks (10-12).
Despite the smaller number of outbreaks reported in recent years, some individual outbreaks were large. As a result, more cases of water-related illness were reported in 1987 than in any other year since CDC and EPA began tabulating these data in 1971. The number of cases reported in 1988 was also larger than in 3 of the 4 previous years. Several large outbreaks due to Cryptosporidium, Norwalk-like viruses, Shigella sonnei, and Giardia lamblia are responsible for this reported increase in the total number of ill persons.
The cryptosporidiosis outbreak that occurred in Georgia in 1987 was the largest outbreak ever reported to the Waterborne Outbreak Surveillance system. An estimated 13,000 people became ill with gastroenteritis after consuming water from a filtered, chlorinated public water supply (1). Cryptosporidium oocysts were isolated from patients' stool specimens and from samples of treated public water. Unlike a previously reported cryptosporidiosis outbreak associated with intermittent contamination of a water supply (13), this water system met current state and federal drinking water standards. These standards may not be sufficient to prevent outbreaks due to this pathogen; the investigation showed that although treated water met turbidity standards, filters allowed passage of particulate matter likely accompanied by oocysts. Recommended improvements in chemical flocculation and filtration practices resulted in clearance of oocysts from the water.
The outbreak of Norwalk-like viral gastroenteritis associated with commercially manufactured ice in 1987 (2) demonstrates the potential for large outbreaks when a commercial product becomes contaminated; this potential applies to water and ice as well as to food. Production of commercial ice, like bottled water, is not consistently monitored by state or federal agencies.
Shigella sonnei was the bacterial agent most commonly associated with waterborne outbreaks for the period 1986-1988, causing four outbreaks with 2,733 persons ill. Each of the drinking water outbreaks was associated with deficiencies in water supplies, leading to contamination with human waste. Large outbreaks of shigellosis can lead to secondary transmission in day-care centers, temporary encampments, and other settings.
Giardia was the agent most commonly implicated in outbreaks, as it has been for the last 10 years. Filtration is necessary to remove Giardia from water; chlorination alone is insufficient without high concentrations and long contact times (14,15). To prevent waterborne transmission of Giardia and other infectious agents, EPA has prepared criteria for filtration and disinfection of all public water systems using surface-water sources (16,17).
The first reported outbreak of chronic diarrhea associated with drinking untreated well water occurred in 1987 (4). This outbreak was listed as CGI. Patients with CGI during this outbreak had diarrhea of unknown cause lasting greater than or equal to 4 weeks. Examination of stools did not detect pathogenic viruses, bacteria, or parasites. Patients had clinical and pathologic characteristics similar to those of patients in an outbreak associated with drinking raw milk that occurred in Brainerd, Minnesota, in 1984 and 1985 (18); the term "Brainerd diarrhea" has been used to describe this illness. Because the disease can be debilitating and the cause is unknown, rapid identification, investigation, and control of outbreaks of chronic diarrhea are particularly important.
Outbreaks of AGI of unknown etiology accounted for 48% of all outbreaks reported. Incubation period, duration of illness, and symptoms for many of the outbreaks suggest that they were caused by 27-nanometer Norwalk-like viruses. In four outbreaks in which patients were culture-negative for common bacterial pathogens, fewer than 50% of the patients vomited, and the mean or median duration of illness was greater than or equal to 4 days; these outbreaks may have been due to Escherichia coli. Increased availability of testing for viral serology, detection of viral antigen in stool, and identification of E. coli toxins would aid in determining the causes of these outbreaks.
Swimming-associated outbreaks due to Shigella, Giardia, Norwalk-like viruses, and other enteroviruses continue to occur; outbreaks due to unintentional ingestion of these pathogens during swimming have now been well documented (7,19-26). Provision of temporary toilet facilities to adequately serve summer crowds is the single measure most likely to prevent such outbreaks. EPA has published guidelines for evaluating the quality of recreational waters (27,28).
Outbreaks of Pseudomonas dermatitis associated with use of hot tubs, whirlpool baths, and swimming pools are preventable if the water is maintained at a pH of 7.2-7.8 with free residual chlorine levels of at least 1.0 mg/L (29). Pontiac fever due to aerosolized antigens of Legionella pneumophila is another risk of whirlpool use (30,31). CDC health and safety guidelines for public spas and hot tubs are available (32).
Large outbreaks aboard passenger cruise ships continue to occur, although less often than in the past, demonstrating the importance of diarrheal disease surveillance and the Vessel Sanitation Program (33,34).
Illnesses reported by the Waterborne Outbreak Surveillance System probably represent only a small proportion of all illness associated with waterborne-disease agents, not only because of underreporting, but also because many sporadic cases occur. An association of sporadic cases of giardiasis with use of unfiltered municipal or nonmunicipal water has been demonstrated (35), as has an association of sporadic cryptosporidiosis with ingestion of untreated surface water (36). Illnesses occurring after years of chronic exposure to low-level toxins are not detectable by this system, nor are outbreaks or sporadic illnesses due to opportunistic pathogens that may be widespread in chlorinated water systems but may cause illness with insidious onset and long incubation periods in persons who are immunocompromised (37,38).
Despite these limitations, the data summarize reported waterborne disease outbreaks, identifying disease agents, types of water systems involved, and associated deficiencies. Monitoring these patterns is important because new diseases, such as cryptosporidiosis and Brainerd chronic diarrhea, can emerge, possibly requiring new means of control. Maintaining the capabilities of local and state health departments to investigate outbreaks and to identify their causes remains important in controlling and preventing outbreaks of waterborne disease.
outbreak of cryptosporidiosis due to contamination of a filtered public water supply. N Engl J Med 1989;320:1372-6.
2. CDC. Outbreak of viral gastroenteritis--Pennsylvania and Delaware. MMWR 1987;36:709-11.
3. CDC. Viral gastroenteritis--South Dakota and New Mexico. MMWR 1988;37:69-71.
4. Parsonnet J, Trock SC, Bopp CA, et al. Chronic diarrhea associated with drinking untreated water. Ann Intern Med 1989;110:985-91.
5. CDC. Ethylene glycol intoxication due to contamination of water systems. MMWR 1987;36:611-4.
6. Johnson CJ, Bonrud PA, Dosch TL, et al. Fatal outcome of methemoglobinemia in an infant. JAMA 1987;257:2796-7.
7. Lenaway DD, Brockmann R, Dolan GJ, Cruz-Uribe F. An outbreak of an enterovirus-like illness at a community wading pool: implications for public health inspection programs. Am J Public Health 1989;78:889-90.
8. CDC. Water-related disease outbreaks, 1985. MMWR 1988;37(SS-2):15-31.
9. CDC. Gastroenteritis outbreaks on two Caribbean cruise ships. MMWR 1986;35:383-4. 10. Craun GF, McGoldrick JL. Workshop on methods for investigation of waterborne disease outbreaks: summary and recommendations. Cincinnati, Ohio: US Environmental Protection Agency (in press). 11. Craun GF, ed. Surveillance and investigation of waterborne disease outbreaks. Cincinnati, Ohio: US Environmental Protection Agency (in press). 12. Craun GF, ed. Waterborne diseases in the United States. Boca Raton: CRC Press, 1986. 13. D'Antonio RG, Winn RE, Taylor JP, et al. A waterborne outbreak of cryptosporidiosis in normal hosts. Ann Intern Med 1985;103:886-8. 14. Kent GP, Greenspan JR, Herndon JL, et al. Epidemic giardiasis caused by a contaminated public water supply. Am J Public Health 1988;78:139-43. 15. Jakubowski W. Purple burps and the filtration of drinking water supplies. Am J Public Health 1988;78:123-5. 16. Environmental Protection Agency. National primary drinking water regulations: filtration and disinfection; turbidity, Giardia lamblia, viruses, Legionella and heterotrophic bacteria--proposed rule. Federal Register 1987; 52:42178-222. (40 CFR parts 141, 142). 17. Environmental Protection Agency. National primary drinking water regulations: filtration and disinfection; turbidity, Giardia lamblia, viruses, Legionella and heterotrophic bacteria--final rule. Federal Register 1989;54:27486-541. (40 CFR parts 141, 142). 18. Osterholm MT, MacDonald KL, White KE, et al. An outbreak of a newly recognized chronic diarrhea syndrome associated with raw milk consumption. JAMA 1986;256:484-90. 19. Makintubee S, Mallonee J, Istre GR. Shigellosis outbreak associated with swimming. Am J Public Health 1987;77:166-8. 20. Rosenberg ML, Hazlet KK, Schaefer J, Wells JG, Pruneda RC. Shigellosis from swimming. JAMA 1976;236:1849-52. 21. Sorvillo FJ, Waterman SH, Vogt JK, England B. Shigellosis associated with recreational water contact in Los Angeles County. Am J Trop Med Hyg 1988;38:613-7. 22. Porter JD, Ragazzoni HP, Buchanon JD, Waskin HA, Juranek DD, Parkin WE. Giardia transmission in a swimming pool. Am J Public Health 1988;78:659-62. 23. Greensmith CT, Stanwick RS, Elliot BE, Fast MV. Giardiasis associated with the use of a water slide. Pediatr Infect Dis J 1988;7:91-4. 24. Kappus KD, Marks JS, Holman RC, et al. An outbreak of Norwalk gastroenteritis associated with swimming in a pool and secondary person-to-person transmission. Am J Epidemiol 1982;116:834-9. 25. Harter L, Frost F, Grunenfelder G, Perkins-Jones K, Libby J. Giardiasis in an infant and toddler swim class. Am J Public Health 1984;74:155-6. 26. Turner M, Istre GR, Beauchamp H, Baum M, Arnold S. Community outbreak of adenovirus type 7a infections associated with a swimming pool. South Med J 1987;60:712-5. 27. Dufour AP. Health effects criteria for fresh recreational waters. Research Triangle Park, North Carolina: US Environmental Protection Agency, 1984; EPA publication no. 600/1-84-004, Office of Research and Development, Health Effects Research Laboratory. 28. Cabelli VJ. Health effects criteria for marine recreational waters. Research Triangle Park, North Carolina: US Environmental Protection Agency, 1983; EPA publication no. 600/1-80031, Office of Research and Development, Health Effects Research Laboratory. 29. CDC. Outbreak of Pseudomonas aeruginosa serotype 0:9 associated with a whirlpool. MMWR 1981;30:329-31. 30. Spitalny KC, Vogt RL, Orciari LA, Witherell LE, Ethkind P, Novick LF. Pontiac fever associated with a whirlpool spa. Am J Epidemiol 1984;120:809-17. 31. Mangione EJ, Remis RS, Tait KA, et al. An outbreak of Pontiac fever related to whirlpool use, Michigan 1982. JAMA 1985;253:535-9. 32. CDC. Suggested health and safety guidelines for public spas and hot tubs. Atlanta: CDC, 1981 DHHS publication no. 99-960. 33. Addiss DG, Yashuk JC, Clapp DE, Blake PA. Outbreaks of diarrheal illness on passenger cruise ships, 1975-85. Epidem Infect 1989;103:63-72. 34. CDC. Vessel sanitation scores. MMWR 1988;37:114-7. 35. Birkhead G, Vogt RL. Epidemiologic surveillance for endemic Giardia lamblia infection in Vermont: the roles of waterborne and person-to-person transmission. Am J Epidemiol 1989;129:762-8. 36. Gallaher MM, Herndon JL, Nims LJ, Sterling CR, Grabowski DJ, Hull HF. Cryptosporidiosis and surface water. Am J Public Health 1989;79:39-42. 37. du Moulin GC, Stottmeier KD, Pelletier PA, Tsang AY, Hedley-Whyte J. Concentration of Mycobacterium avium by hospital hot water systems. JAMA 1988;260:1599-601. 38. Laussucq S, Baltch AL, Smith RP, et al. Nosocomial Mycobacterium fortuitum colonization from a contaminated ice machine. Am Rev Respir Dis 1988;138:891-4.
Disclaimer All MMWR HTML documents published before January 1993 are electronic conversions from ASCII text into HTML. This conversion may have resulted in character translation or format errors in the HTML version. Users should not rely on this HTML document, but are referred to the original MMWR paper copy for the official text, figures, and tables. An original paper copy of this issue can be obtained from the Superintendent of Documents, U.S. Government Printing Office (GPO), Washington, DC 20402-9371; telephone: (202) 512-1800. Contact GPO for current prices.**Questions or messages regarding errors in formatting should be addressed to email@example.com.
Page converted: 08/05/98
This page last reviewed 5/2/01