"Giardiasis Risk from an Unfiltered, Protected Surface Water Source"

Joseph L. Glicker and Roger A. Edwards

This article describes the efforts of water utility personnel to determine whether the level than 50,000 cases of gastroenteritis oc- of endemic giardiasis observed in their service area could be attributed to the surface curret in Portland from late 1954 to early water supply, which is chloraminated but not filtered. Comparison of epidemiological 1955. Although the etiology of this out- data for the county in which the utility is located with data from another county that break was then concluded to be the con- used conventionally treated water showed similar incidence rates, partitioning into risk tact spread of an unknown virus, the ob- categories, and age distributions for giardiasis. There was no correlation between the servation of Giurdiu cysts in the stools of location of a giardiasis case and disinfection contact time within the unfiltered water’s patients exhibiting symptoms common distribution system. to the incident and the citywide distribution of cases today would have likely re- Most identified cases of waterborne Bull Run watershed, supplying water to sulted in a diagnosis of epidemic water- giardiasis in the United States have been Portland, Ore., is an example of this type borne giardiasis. associated with epidemic outbreaks of of system. It is an unEltered surface sup- There have been no known or sus- the disease. These outbreaks are fre- ply that takes water from a protected, pected outbreaks of waterborne giardia- quently associated with specific occur- forested watershed. Disinfection is sis in Portland since this 1954 episode. A rences of significant contamination of the achieved with chloramines, known to be change in the configuration of the water water supply with cysts.’ In addition to the least effective of the commonly used supply system (Figure 1) is likely respon- these epidemics, lower levels of Giardia water disinfectants for inactivating Giar- sible for this improvement. In the early cysts may be continually or periodically &a cysts4 A system like this would be 1960s Bull Run Reservoir 2 was con- transmitted through the water supply, expected to have a relatively high inci- structed within the watershed to increase leading to ongoing low levels of giardia- dence of giardiasis based on the assump- storage capacity. This project converted sis in the community. Such endemic wa- tions that underlie national regulatory the intake of the distribution system from terborne giardiasis may be more difficult trends for drinking water.” a flowing river to the lower third of a large ;~;i~;e~;~~$y still be a serious risk to There is no question that Portland’s lake. The quiescent water in this reser- Bull Run supply has the potential for wa- voir provides little of the energy required This concern for public well-being is terborne transmission of giardiasis. A to keep a cyst in suspension, so cysts heightened when the surface water sup- study conducted by the water bureau in entering this reservoir now have the op plies are unfiltered and rely solely on 1985 found cysts in several streams of the portunity to settle, whereas before they disinfection as a barrier to the trans- watershed, often downstream of known could be swept directly into the distribu- mission of waterborne giardiasis. The beaver habitats. An outbreak of more tion intake. Completion of this reservoir

Figure 1. Portland’s water supply

The Htdl Rtln wateshed, which serves Portland, Ore., is an unfiltered surface supply that relies solely on disinfection as a barrier to the transmission of waterborne giardiask. nurses from the respective county health departments. A standard form provided by the State Health Division was used to conduct the investigation. For this article, case reports for 1985-86 were reex- amined by the two county health departments, and the likely primary risk factor of the giardiasis case was indicated as one of eight preestablished categories: diapered baby, day care center, untreated water, foreign travel, personal contact, refugee health, refused cooperation, and unknown. From these surveillance reports, comparisons between the two counties were made for incidence rate, risk factors, and age distribution. Multnomah County’s data for 1985-88 were further examined by census tract to determine whether lo- also created a concern about increased and most of the surrounding metropoli- cation within the water distribution sys- chloramine demand, so the disinfectant tan area. This service area includes al- tem would have a bearing on the risk of dose rate was increased from a total chlo- most all of Multnomah County, in which giardiasis. rine residual of 0.6 to 1.7 mg/L. These Portland is located. It is estimated that 95 Monitoring for Giardia cysts at the raw changes offer greater protection from percent of the population of Multnomah water inlet for the Bull Run supply was waterborne giardiasis than was available County is served unfiltered Bull Run performed by seven separate labora- in 1955. water. County residents not receiving tories using variations of three different Recognition of the threat posed by wa- this municipal supply utilize private or methods. No cyst monitoring was con- terborne giardiasis led to the devclop- small groundwater sources. Because dis- ducted for the Benton County water sup- ment of the Surface Water Treatment ease statistics are kept on a countywide plies. Between 1982 and 1987, approxi- Rule of the National Primary Drinking basis, the giardiasis data for Multnomah mately 20,000 gal of raw Bull Run water Water Regulations5 The premise under- County were investigated. was filtered through either l-urn polypro- lying this rule is that “The US Envi- Benton County, located about 75 mi pylene cartridge filters or 5pm polycar- ronmental Protection Agency [USEPA] south of Portland, was selected for com- bonate membrane filters to produce a believes that all surface waters and parison. Although the total population of total of 69 samples for Giardia analysis. groundwater under direct influence of Benton County is about 12 percent that The samples were processed and exam- surface water are at risk, at least to some of Multnomah County, approximately 60 ined either by the USEPA consensus degree, from contamination by Giurdiu percent of its population is concentrated method7 or by fluorescent antibody (FA) lamblia and other protozoa, viruses, and in the city of Corvallis. The water supply proceduresG’” pathogenic bacteria and that public water for Corvallis is frotn two surface sources, The estimates of the cyst concentra- systems using such source waters both of which are treated in conventional tion resulting from these analyses were should provide minimum levels of treat- filtration plants, producing finished compared with the epidemiologic data by ment to ensure protection from illness water turbidities between 0.02 and 0.75 means of a giardiasis dose-response caused by these contaminants.“This goal ntu from raw water with turbidities rang- model. This model was adapted from a is being demanded by the public and is ing from 0.4 to 30 ntu. The water is pre- general model that describes the infec- eagerly desired by everyone associated oxidized with chlorine; flash mixed with tion risk from waterborne microorga- with the water industry. lime, alum, and polymer; flocculated; set- nisms.” It was selected for use with To ascertain whether the “minimum tled; and filtered through mixed-media giardiasis12 after it was shown to predict level of treatment” now provided for filters at typical rates of 2.7 to 4.6gpm/sq the responses obtained in experiments in Portland’s water contributes to the re- ft. Disinfection is achieved with free chlo- which volunteers wer fed various dos- gion’s endemic level of giardiasis, a study rine. People living within the county but ages of Giardiu cysts. 15This model is the was undertaken that examined the Ore- outside Corvallis rely on private installa- same one used by the USEPA to estimate gon Health Division’s giardiasis surveil- tions, small groundwater supplies, or the national level of endemic giardiasis.” lance records and monitored the raw other filtration facilities. The authors es- It projects the probability of contracting water intake to Portland’s distribution timate that less than 3 percent of the an infection from a single exposure to an system for Giardia cysts. These two data county’s population is supplied unfiltered infectious dosage from the existing cyst bases were then compared to assess the surface water. concentration. An infection is judged risk of endemic giardiasis from Port- Giardiasis has been a reportable dis- solely on the basis of the host’s excretion land’s water and to verify assumptions ease in Oregon since 1982. Investiga- of cysts. It does not have to be a symp- made about the efficacy of water treat- tions of reported cases were carried out tomatic infection.The model assumes ment in preventing transmission of this to determine the likely source of the dis- that all cysts are viable and infectious and disease. ease until 1987, when regulations were are randomly distributed in the water. changed to require only investigation of The probability of exposure is therefore Methods and materials cases involving children less than five governed by a Poisson distribution, With The population served by Portland’s years old. The original giardiasis investi- these assumptions, the model reduces to Bull Run water supply includes the city gations were conducted by registered the equation:

p= 1 me-rN (1) TABLE 1 in which P is the probability that a single Rates of incidence ofgiardiasis per 100,OOOfieofile exposure to an infectious dose of Giardia cysts would result in an infection; N is the State of Oregon Multnomah County Benton County cyst concentration in 2 L of water; and r is a constant, 0.02, derived by Rose” to de- Year Cases Incidence Cases Incidence Cases Incidence scribe the interaction between cyst and 1982 735 27.7 host that occurred in the classical Rendt- 1983 833 31.6 orff feeding experiments.13 The derivation 1984 1,057 39.7 1985 1,223 45.7 241 42.9 39 56.4 of r involved determining a value for Y for 1986 1,157 43.5 220 38.9 39 57.3 each dosage used in Rendtorff’s study and 1987 1,171 43.5 238 42.3 48 69.4 then taking the average of those values. 1988 1,198 43.7 269 47.2 22 31.4 The model assumes that there is no 1985-88 mean 44.1 42.8 53.6 safe level of exposure, or threshold, for the contaminant. It predicts that any given concentration of cysts will cause TABLE 2 infection or disease to some portion of Rates of incidence ofgiardiasis per 100,OOO~eo~le by age group for 1985-86 the exposed population. For example, a cyst concentration of 1 cyst/l,000 gal, or Multnomah County Benton County 3 x 10Acysts/L, is a clean water. But according to the model, it is still not com- Average Total Incidence Average Total Incidence pletely free of risk, as is shown here. Age Population* Cases Rate Population Cases Rate o-9 78,340 111 70.8 8,952 18 100.5 lo-19 65,142 20 15.4 12,094 1 4.1 Cyst concentration = 3 x 10dcysts/L 20-29 91,142 60 32.9 18,254 13 35.6 = 6 x lO”cysts/2 L = N (2) 30-39 115,225 85 36.9 9,467 21 110.9 40-49 60,348 27 22.4 6,778 6 44.3 -rN = -(0.02) (0.0006) = -1.2 x lo-” (3) 50-59 46,643 7 7.5 4,569 1 10.9 60-69 49,350 8 8.1 4,267 3 35.2 >70 > 57,810 >7 6.0 > 4,219 >l 11.8 natural antilogarithm of -1.2 x 10” = 0.999988 (4) Total 564,000 325 68,600 64

P= 1 - 0.999988 = 1.2 x 10m5 or 1 case per 100,000 *The listed figures are the average populations for 1985-86, adjusted by the percentage of the total county population falling into a particular age group. These numbers must be doubled to calculate the people (5) incidence rate because two years of case data are used.

Results Total reported cases of giardiasis from 1982 through 1988 are shown in Table 1 for the state of Oregon, along with the incidence rate per 100,000 people. The table shows an increasing incidence of giardiasis from 1982 to 1984 that is likely due to underreporting before health facilities became aware of this addition to reportable diseases. To avoid any bias during this initial period, only data for 1985-86 are used in the comparison between the counties using filtered and unfiltered water. Incidence data for the two counties are also presented in Table 1. During 1985- Figure 2. Breakdown of giardiasis cases into risk categories for Benton and 88, Benton County had a mean incidence Multnomah counties, 1985-86 rate of 53.6 per 100,000 people, whereas Multnomah County had a mean rate of sons. The breakdown into risk categories not report contact with untreated water, 42.8. The statewide incidence rate since from these investigations is presented in and the travel locations were not consid- 1985 has been 44.1 per 100,000 people, a Figure 2. Corresponding categories for ered to present a risk of giardiasis. These value comparable to the 45.9 rate re- the two counties are all within 5.1 percent destinations were mostly within the ported for Vermont.r4 of each other. United States. The “untreated water” cat- During 1985-86, Benton County inves- Several comments need to be made egory was typically used for people re- tigated 64, or 82 percent, of their 78 re- about the risk categories. The “foreign porting contact with untreated water dur- ported cases, and Multnomah County travel” category was for people reporting ing recreational activities. Newly arrived completed investigations on 84 percent illness after returning from places con- refugees with giardiasis were catego- of their cases, or 388 out of 461 cases. sidered high risk for giardiasis, mainly rized as “refugee health,” regardless of Comparison of giardiasis in the two coun- Third World destinations. People in this other possible risk factors. Cases falling ties during 1985-86 is based on just these category did not have to report drinking into this category were eliminated from investigated cases. Because both the untreated water, because even contact the county comparisons because of their “refugee health” and “refused coopera- with these public water systems is pre- likelihood of being contracted elsewhere tion” categories only occurred in Mul- sumed to be a likely risk factor for and imported into Multnomah County. tnomah County, the 63 cases in these two giardiasis. Many of the people in the “un- Table 2 shows the incidence rates for categories were also removed from the known” risk category reported travel the 1985-86 investigated cases in the two data base used in the county compari- prior to the onset of the disease. They did counties, broken down by age groups.

These rates have been adjusted for the been broken down into ranges around seconds of travel time later in the three difference in the makeup of the age cate- the three temperatures used in a previ- supply conduits. Contact time to the first gories in the two counties. The O-9 and ous study of chloramine inactivation of significant number of customers is about 30-39 age groups are shown to have the Giardia cysts in Bull Run water.15 The 5 hours in winter and 3 hours in summer. greatest incidence in both counties. mid-range of 6.6-14’C occurs twice in the Contact time for most of the customers is Analysis of the census tract data uti- year during periods of different water de- three to five days including distribution lizes the 102 defined geographical tracts mand and thus appears twice in the cal- storage. However, several individuals re- set up for national census counts that lie culations. The mean daily flow of water ceive water after only 45 min in summer within Multnomah County and the ser- supplied to Portland was determined for and 90 min in winter. Total chlorine resid- vice area for Bull Run water. These data each temperature range, and that mean uals at the first customer typically range were examined by comparing the 1985- was multiplied by the conservative cyst from 1.2 to 1.6 mg/L. Measurement is by 86 records with those for 1987-88 to concentration of 0.2 cyst/gal to deter- amperometric titration.7 check for any patterns of clustering that mine the number of cysts entering the The level of disinfection practiced at might be attributed to insufficient disin- distribution system. Portland generates a chloramine C x T fection. Census tracts for just the “un- The disinfection process at two differ- value17 of 300-500 for the first significant known” and “refused cooperation” cases ent points along the conduits is pre- number of users but only 50-150 for the were provided for 1985-86 because those sented in the remainder of Table 4. The first customer. The USEPA recommends were the case types most likely to be estimate of risk at the first customer is chloramine C x Tlevels of 1,500 for water caused by endemic waterborne giardia- determined and then the risk at Mt. temperatures of 15°C and 3,000 when sis. In 1987-88, cases were not investi- Tabor. Contact time was calculated as the temperatures are 3°C.” These USEPA gated, so all reported cases except refu- combination of flows in two or three con- C x Tvalues, however, are based on stud- gee health were listed by census tract. To duits thatwould satisfy the water demand ies using preformed monochloramines. review these data, the 1987-88 records (or mean daily flow in millions of gallons Disinfection by chloramines produced were combined and ranked by incidence per day) in the least time. The total chlo- under actual field conditions is generally rate, with the first-ranked census tract rine residuals were estimated from the recognized as being more effective than having the highest incidence rate and the Meyer article15 for a chloramine dose of that by preformed monochloramines.‘7 last tract having the lowest rate. Census 15 lb of chlorine/100 mil gal. Interpola- Typically, field chloramination produces tracts with giardiasis cases in both 1985 tion of the data in the Meyer article was a mixture of chlorine species with differ- and 1986 were then flagged on the 1987- again used to estimate percentage inacti- ent disinfection capabilities, each in equi- 88 list. These flagged locations ranked vation based on the achieved C x Tvalue. librium with the others, depending on tenth, seventeenth, thirty-fourth, thirty- The final cyst concentration was then the pH, relative concentrations of applied sixth, fortieth, fifty-third, eighty-fifth, and number of cysts resisting inactivation, di- chlorine and ammonia, and the method nintieth out of the 90 census tracts that vided by the mean flow per day for the of application. Meyer15 found that by sim- had cases. The seventeenth-ranked cen- period. The daily exposure probability is ulating field chloramination practices, sus tract is on the east end of the water calculated per the infection model in Eq 99.8 percent of cysts could be inactivated service area but is not wholly within its 1 and is then multiplied by the number of with chloramine C x T values of 385 at boundaries. Of those flagged locations days of the year for each temperature 18’C, whereas at 3’C, a C x T of 650 remaining, the thirty-sixth-ranked cen- range to yield the relative risk. Total risk would inactivate 99.4 percent of the cysts sus tract had the shortest disinfection for the water year is the sum of the rela- present. These findings indicate that contact time. It was the area of the Mt. tive risks. An annual risk calculated in field chloramination can reduce the Tabor reservoirs, which are the terminus this manner is highly dependent on the C x T value that is needed for Giardia of the conduits from Bull Run (Figure 1). largest relative risk. At the first cus- cyst inactivation by nearly an order of Examination of the 69 raw water sam- tomer, the total risk is 4.6 x 10-l, or 4,600 magnitude from the level required for ples collected at the inlet to Portland’s cases per 10,000 people. At the Mt. Tabor preformed monochloramines. distribution system produced a total of 15 reservoirs the total risk is 7.0 x 10m3, or Monitoring for cysts at the raw water Giurdia cysts in 13 samples. Results of 70 cases per 10,000 people. intake has shown that some cysts are the analyses are summarized in Table 3. entering Portland’s distribution system, Based on the cyst recovery efficiencies of Discussion so some percentage of giardiasis cases in the laboratories reporting positive sam- The study of the reported giardiasis the community should be attributed to ples, an estimate of the cyst concen- cases was conducted in order to test the the water supply under the assumptions tration in Portland’s raw water ranges hypothesis that a low but noticeable num- of USEPA’s C x T values. This disease from 0.013 to 0.105 cyst/gal (0.34 to 2.77 ber of cases are being transmitted incidence should be observable as an cysts/100 L). This range is equivalent to through the unfiltered water supply in overall excess of case rate, an excess in one cyst in lo-77 gal (one cyst in 38-291 Portland. This supply is unique in that the number of cases that cannot be attrib- L). Because cyst recovery efficiencies for since the turn of the century, its 107-sq- uted to other causes, or unexplained dif- this procedure are so low, it could be mi area has been closed to public entry. ferences in the distribution of cases argued that cysts might well have been Although carefully controlled logging of among the age groups of the Multnomah present in all samples. If all negative sam- the watershed was started in 1958, there County cases when compared with an ples were assumed to have one cyst, then are no recreational uses, habitation, over- area using filtered water that complies this recovery calculation gives a more night stays, septic tanks, or other human with the USEPA cyst removal and inacti- conservative cyst concentration range of sanitary discharges within the drainage vation criteria. 0.02-0.25 cyst/gal (0.53-6.60 cysts/100 area. Since 1977 management of the wa- None of these observations were sup- L), or one cyst per 4-50 gal (one cyst in tershed has been governed by a federal ported by the giardiasis data. Benton 15-189 L). law16 that makes the continued produc- County has a higher incidence rate than Using a cyst concentration of 0.2 tion of “. . . pure, clear raw potable water” Multnomah County (Table 1). Within the cyst/gal, the infections predicted by the the primary management goal. risk categories of “diapered baby,” “day risk assessment model for two disinfec- Chloramines are used as a disinfectant care,” and “personal contact,” the initial tant contact times are presented in Table because of their ability to retain a total exposure could have been via the water 4. The first part of the table reflects the chlorine residual over the 30 mi separat- supply. Benton County had comparable conditions that might be expected based ing the watershed and the city. Chlorine rates for all these categories except “day on historical records. The water year has is added first, with ammonia added a few care” (Figure 2), and this exception

TABLE 3 Results of monitoringfor Giardia cysts at the raw water intake to Portland’s distribution system, 1982-87

Ongerth, USEPA Meyer, James M. ROSE!, University Region X, Oregon Health Montgomery, Riggs, University of Seattle, Sciences Pasadena, Berkeley, of Water Parameter Washington Wash. University c&i. C&f. Arizona Bureau

Method Polyclonal Direct Direct Direct Polyclonal Monoclonal Monoclonal FA USEPA USEPA USEPA FA FA FA Samples analyzed 17 2 9 1 1 2 37 Positive samples 7 0 2 0 1 1 2 Cysts found 8 0 3 0 1 1 2 Volume examined-gal 380 1,400 3,800 350 350 85 15,000 Recovery efficiency--percent* 20 6 6 6 15 15 1 Cyst concentration-numbers/gait 0.105 0.02 0.013 0.05 0.019 0.078 0.013 Adjusted cyst concentration- 0.24 0.02 0.04 0.05 0.02 0.16 0.25 number/gal+

*Based on direct recovery tests for these samples if available; otherwise at the low end of reported ranges tcalculated as the number of cysts found divided by the volume examined divided by the recovery efficiency FDetermined by assuming at least one cyst was found in all samples TABLE 4 Risk assessment&r giardiasis in Portland’s water sufiply

Symbol for Parameter Parameter s 6.5”C 6.6-14’C 14OC 14-6.6OC

Historical means A Number of days in year within temperature range 147 56 57 105 B Daily mean flow for temperature range--mgd 109.0 107.3 121.8 138.7 C Incoming cysts/day [B (0.2 cyst/gal)] 21.8 x 10s 21.5 x lo6 24.4x lo6 27.7 x lo6 Disinfection at the first customer D Disinfectant contact time-w& 88 88 104 89 E Total chlorine residual--mg/l 1.5 1.4 1.4 1.4 E InactivationCxT(DxE) of 13230 12340 146 125 cysts--gercent15 82 41 H Remaining cysts/day [C (100-G)] 15.3 x 10s 12.9 x 10s 4.39 x 10s 16.3 x lo6 : RemainingDaily exposure cysts/gal probability* (H/B) 1.41.5 x 10-l10-3 1.21.3 x 10-l10” 3.63.8 xx lo-’lOA 1.2 x 10-l 1.3 x 10-3 K Relative risk (/ x A) 2.2 x 10-l 7.3 x 10-z 2.2 x 10-z 1.4 x 10-l

L Annual risk (ZK) 4.6 x 10-l = 4,600 cases per 10,000 people Disinfection at the Mt. Tabor reservoirs M Disinfection contact time--min 331 331 386 344 N Total chlorine residual-mg/l 1.4 1.3 1.2 1.3 0 CxT(MxN) 463 430 463 447

i InactivationRemaining cysts/dayof cysts---percent [C (100-p)] 98.043.6 x lo4 99.843.0 x 103 >99.824.4 x lo3 99.855.4 x 103 R Remaining cysts/gal (Q/B) 4.0 x lo3 4.0 x lOA 2.0 x 1o-4 4.0 x 1o-4 S Daily exposure probability* 4.2 x 10” 4.2 x 10” 2.1 x 10-s 4.2 x 1O-6 T Relative risk (S x A) 6.2 x 10” 2.4 x lo4 1.2 x lOA 4.4 x 10-4 U Annual risk (27) 7.0 x 10m3 = 70 cases per 10,000 people

*From the model P = l-e?N, in which r = 0.02 and N = the cyst concentration/21

might be explained by a difference in land’s distribution system (Figure 1). vide clear evidence of inadequate disin- need or availability. The incidence of “un- Some water districts draw water from the fection at this point in the distribution known” cases in the two counties was conduits between the watershed and system. also comparable. Waterborne giardiasis Portland. These districts get the least The census tracts that did have cases would be expected to either affect the benefit from disinfection contact time during 1987-88 seemed randomly scat- population uniformly or preferentially in- and so would be expected to experience tered throughout the service area. The fect either the elderly because of their the highest rate of endemic giardiasis appearance of giardiasis cases in the Mt. reduced ability to respond to stress or the among all the users of this unfiltered Tabor area supports the theory that cysts very young because they have not had an water if the disinfection treatment was are surviving the disinfection treatment opportunity to develop antibodies from a allowing passage of infectious Giardia because the reservoirs at Mt. Tabor re- previous exposure. Such patterns are not cysts. There is only one census tract ceive water from the watershed. From evident in the comparison. In both coun- within these districts that is included this point, water is then distributed to the ties, there are high incidence rates for among the tracts with unexplained service area within Portland’s city limits. the young and for active adults, and low giardiasis cases in both 1985 and 1986, Still, the lack of cases occurring consis- rates among the elderly (Table 2). Ac- and half of its area is outside the water tently upstream of this location suggests cording to this comparison of counties service boundary. The easternmost that a mechanism other than insufficient using different water treatments, there is tracts with no unexplained cases in 1985- contact time is responsible for the cases no indication that the unfiltered water is 86 that are within Multnomah County but recorded within the county. contributing to endemic giardiasis. outside Portland ranked eleventh, thirty- The risk assessment for giardiasis in The census tract data must be re- fifth, fifty-sixth, and fifty-seventh on the Portland’s water supply presented in viewed with some knowledge of Port- 1987-88 list. These rankings do not pro- Table 4 uses the giardiasis dose-re-

50 MANAGEMENT AND OPERATIONS JOURNAL AWWA Copyright (C) 1991 American Water Works Association sponse model as a validity check for the Conclusions Disinfectants: A Review. Jour. AWWA, estimate of cyst concentration in raw Bull Within the constraints of the data ex- 76:5:74 (May 1984). Run water, and the Meyer determination amined, this study provides no evidence 5. National Primary Drinking Water Regula- tions; Final Rule. Fed Reg, 54:124:27486 of the disinfection capability of chloram- to support the hypothesis that current (1989). ines for Giardia.15 The 7.0 x 10d3 annual operation of the unfiltered Bull Run sys- 6. VEAZIE, L.; BROWNIEE, I.; & SEARS, HJ. An infection rate predicted by the model for tem in Portland leads to a greater inci- Outbreak of Gastroenteritis Associated Mt. Tabor is equivalent to 7,900 cases in dence of giardiasis transmission than with Giardia lamblia. Symposium on Water- two years for the population of Mul- does a well-operated filtered water sup- borne Transmission of Giardiasis (W. tnomah County. This total is obviously ply. The age and case risk factors do not Jakubowski and J.C. Hoff, editors). EPA- much greater than the incidence of cases indicate that any endemic giardiasis is 600/9-79-001. USEPA, Cincinnati, Ohio actually detected. being transmitted through the Bull Run (1979). The difference between the review of water system. Rather, these distributions 7. Standard Methods for the Examination of Water and Wastewater. APHA, AWWA, and morbidity records and the predictions of show that endemic giardiasis is being WPCF, Washington, D.C., (16th ed., 1985). the risk model is not unexpected. In a transmitted through personal contact 8. RIGGS. I.L. ET AL. Detection of Giardia previously published account of the and through contact with untreated lamblid by Immunofluorescence. Appl. giardiasis model,5 the risk estimates pro- water during recreational activities. Most Envir. Microbial., 45:2:698 (1983). duced were judged to be acceptable, al- cases occur in diapered children, pre- 9. ROSE, J.B. ETAL. Detection of Cryptosporid- though, as is the case here, they were sumably by hand-to-mouth contact, and ium and Giardia in Environmental Waters. greater than the reported disease inci- in active adults in contact with untreated Proc. 1986 AWWA WQTC, Portland, Ore. dence. The morbidity reports are just the water. Other cases occur by direct con- 10. ONGEHTH, J.E. Giardia Cyst Concentration in River Water. Jour. AWWA, 81:9:81 (Sept. reported incidence of disease within a tact with these primary source cases. In- 1989). community. Unreported, undiagnosed, cidence rates of giardiasis within Mult- 11. HAAS, C.H. Estimation of Risk Due to Low mild, or asymptomatic cases will not nomah County are lower than in Benton Doses of Microorganisms: A Comparison show up in these records. Undetected County, even before the “refugee health” of Alternative Methodologies. Amer. Jour. cases are important in a community be- cases occurring in Multnomah County Epidemiol., 118:4:573 (1983). cause although a mild or asymptomatic are discounted. The locations within 12. ROSE, J.B. Risks of Waterborne Giardiasis. infection may be of little concern to the Multnomah County of both the total and Proc. 1989 AWWA WQTC. Philadelphia, host, that person could serve as a source “unknown” risk cases occurring each Pa. 13. RENDTORFF, R.C. The Experimental Trans- of disease for someone with a lower tol- year show no signs of clustering that mission of Human Intestinal Protozoan Par- erance. The risk model provides an esti- could be attributed to a marginally disin- asites. ILGiardia lamblia Cysts Given in mate of these undetected cases, but it fected, unfiltered water supply. Capsules. Amer. Jour. Hygiene, 59:209 must be understood that this estimate is The dose-response model used by the (1954). for just one location, whereas the morbid- USEPA to estimate the number of cases 14. BIRKHEAD, G. & VOGT, R.L. Epidemiologic ity records represent incidence in the of endemic giardiasis is seen to be a prac- Surveillance for Endemic Giardia lamblia population at large. The population is ex- tical aid when it is applied in conjunction Infection in Vermont. Amer. Jour. posed to a range of disinfection times with morbidity records. Care is required Ebidemiol.. 129:4:762 (1989). 15. &YER, E.k hi AL.. Inactivation of Giardia greater than that assumed for the risk in using the model to assure that the muris Cysts by Chloramines. Water Rex model and, therefore, incurs less risk concentration of source water cysts en- Bull., 25:2:335 (1989). than that calculated by the model. Table tered into the calculations is defensible. 16. Bull Run Watershed Management Act. US 4 demonstrates the significance of this The incidence rates reported to health Code Nov. 23, 1977; Public Law 95-200. effect by showing the substantial reduc- authorities and predicted by the model 17. HOFF, J.C. Strengths and Weaknesses of tion of risk between two points in the will together likely bracket the level of Using CT Valuesto Evaluate Disinfection distribution system with different contact giardiasis in a community. Practice. Proc. 1987 AWWA Seminar on times. Assurance of Adecmate Disinfection, Kan- Acknowledgment sas City, MO. Of all the calculations included in the 18. Guidance Manual for Comnliance with the use of the model, the assignment of a cyst The authors thank Jan Kolden, disease Filtration and Disinfection Requirements concentration is the least certain. When control manager of the Multnomah for Public Water Supplies Using Surface 0.2 cyst/gal was chosen as the concen- County Health Department, for ideas on Water Sources. USEPA, Washington, D.C. tration of cysts entering the distribution study design, facilitating case investiga- (Oct. 1989). system, the goal was to make an upper- tion, and review of the article. They also bound estimate of the conditions occur- thank Dianne Soucie of the Multnomah About the authors: ring in Portland’s water. This concentra- County Health Department and Susan Joseph L. Glicker is di- tion, corrected for recovery efficiency Spangler of the Benton County Health rector of water quality and with all samples assumed to be posi- Department for their review of giardiasis and environmentalpol- tive, is meant to be conservative to com- cases; Darrel McVey of the city of icy for the Portland, pensate for the inefficiencies of the meth- Corvallis for information on the Corvallis Ore., Water Bureau, ods. Although conservative, it is still water treatment plants; and Portland 1120 S. W. 5th Ave., within the range of 0.03-0.49 cyst/gal Water Bureau employees Edie Fabian for Portland, OR 97204, cited elsewhere for pristine water- cyst monitoring and Doug Bloem and where he has workedfor sheds.5*‘0 The model’s estimates of cyst George Mickelson for data summaries. 11 years. Glicker is a graduate of the Uni- concentrations in other published ac- References versity of Illinois at Champaign with a counts, however, preferentially use raw bachelor’s degree in engineering and a data uncorrected for recovery effi- 1. CRALJN, G.F. Surface Water Supplies and master’s degree in biochemistry. He cur- Health.Jour. AWWA, 80:2:40 (Feb. 1989). ciency.5”2 Using lower estimates of cyst rently serves on committees of AWWA as 2. LAPHAM,SC. ETAL.A Prospective Study of concentration in the source water, as well as the Association of Metropolitan would be justified by accepted use of the Giardiasis and Water Supplies in Colorado. Amer. low. Public Health. 77:3:354 (1987). Water Agencies. Roger A. Edwards* is a model and the range of concentrations 3. CHUTE: C.G.; SMITH, R.P:; & BARON, J.k. water microbiologist for the Portland, Ore., found during monitoring for cysts in raw Risk Factors for Endemic Giardiasis. Amer. Water Bureau, 10991 SE. Lusted Rd., water, would generate proportionally Jour. Public Health, 77:5:585 (1987). Sandy, OR 97055. lower estimates of endemic giardiasis 4. WOLFE, R.L.; WARD, N.R.; & OLSON, B.H. among water users. Inorganic Chloramines as Drinking Water *To whom correspondence should be addressed