So Many Oocysts, So Few Outbreaks F F lo y d F r o s t , G u n t h e r F . C r a u n , R e b e c c a C a ld e r o n , a n d S t e p h e n A . H u b b s o r ce n tu r ie s p e o p le m ore com m on occu rren ce of disease is have understood that after an infection, “endemic.” With endemic disease, most a person is less likely to become ill from exposures result in asymptomatic infeca subsequent exposure. The ancient Chi- tion s rath er th an illn ess. Th e sign ifinese, realizing that those who survived can ce of en dem ic in fection s is seldom childhood smallpox became immune to recogn ized. the disease, intentionally infected their If a large number of people in a comchildren. Modern vaccination programs munity are immune to illness caused by are variants of this ancient approach. Cryptosporidium oocysts becau se of prior Un fortu n ately, prior in fection does in fection s (correspon din g to h igh levels n ot u n iformly protect people from con - of en dem ic in fection ), few er illn esses tractin g su bsequ en t diseases cau sed by w ill resu lt from each n ew exposu re. all path ogen s. Som e in fection s con fer Even tu ally, su ch a large percen tage of im m u n ity for varyin g periods of tim e; th e com m u n ity’s popu lation becom es others confer little or no immunity at all. im m u n e th at n ew exposu res cau se few Alth ou gh prior in fection w ith cryp- illn esses. Th is ph en om en on w as obtosporidiosis does n ot appear to com - served for Entamoeba histolytica infections pletely protect people from su bsequ en t in India and possibly for Cyclospora infecCryptosporidium in fection s, eviden ce is tion s in Peru . In th e case of both th ese mounting that some immunity or short- path ogen s, little eviden ce of ou tbreaks term protection from cryptosporidiosis exists, despite th e fact th at th e parasites occu rs after in fection . are com m on ly tran sm itted. As required by the Information ColPreven tion of w aterborn e disease lection Ru le, selected US w ater u tili- o u t br e a k s is a p r im a r y o bje ct ive o f ties have begun monitoring for specified drin kin g w ater treatm en t. New York ch em ical con tam in an ts, Cryptosporid- City h as establish ed a su rveillan ce proium , a n d o th e r w a gram to m on itor th e terborn e path ogen s. occu rren ce of en teric Water su ppliers wh o ( in t e st in a l) w a t e r fin d Cryptosporidium born e disease, particCQUIRED oocysts in th eir fin u larly cryptosporidioish e d w a t e r w ill be IMMUNITY MAY EXPLAIN sis. Ot h e r US a n d concerned about how THE RELATIVE ABSENCE Can adian cities h ave t o in t e r p r e t t h e se also im plem en ted or fin d in gs in t h e a b- OF CRYPTOSPORIDIOSIS are con siderin g prose n ce o f a p p a r e n t OUTBREAKS IN COMMUNITIES grams for surveillance cryptoporidiosis ou tof cryptosporidiosis. breaks in th eir com - WHERE OOCYSTS Wh en Cryptosporidium m u n it ie s. Acq u ir e d ARE DETECTED IN THE oocysts are detected im m u n it y m a y e x du rin g ICR m on itorp la in t h e r e la t ive DRINKING WATER. ing, will these surveila bse n ce o f cr yp t o la n ce p r o gr a m s be sporidiosis ou tbreaks in com m u n ities able to estim ate th e risk of waterborn e where oocysts are detected in the drink- cryptosporidiosis? in g water. Exposu re via drin kin g w ater con Cryptosporidiosis ou tbreaks are u n - tam in ated w ith low con cen tation s of u su al clu sters of disease. Th e term for oocysts w ill likely resu lt in en dem ic A infections rather than disease outbreaks. How ever, th is m ay preven t detection of the scattered illnesses associated with con su m ption of drin kin g water con tam in ated with oocysts. Con tin u ed lowle ve l co n t a m in a tion m ay m ain tain su ch a h igh degree of im m u n ity in th e p o p u la t io n t h a t overt ou tbreaks of waterborne cryptosporidiosis are preven ted. Th erefore, su rveillan ce program s th at rely on ly on th e detection of cases of illn ess may n ot be su fficien t to assess th e risk of waterborn e tran sm ission of cryptosporidiosis. Th e effect of im m u n ity sh ou ld be con sidered w h en a su rveillan ce program is plan n ed. Pe o p le w h o d r in k w a t e r fr o m groundwater sources would be expected to h ave less im m u n ity to cryptosporidiosis. Becau se th ey are likely to h ave m ore cases of sym ptom atic illn ess in respon se to occu rren ces of con tam in ation in their water supplies, outbreaks of waterborn e disease in th ese com m u n ities sh ou ld be easier to recogn ize. Half of the reported cryptosporidiosis ou tbreaks associated w ith pu blic an d in dividu al drin kin g w ater system s in th e Un ited States were cau sed by con tamin ated grou n dwater. Attack rates in ou tbreaks associated w ith grou n dw ater can be particu larly h igh . In Yakim a, Wash ., seven of ten fam ily m em bers became ill when their well became contam in ated in 1993. People drin kin g u n treated grou n dwater in a sm all com m u n ity n ear Walla Walla, Wash ., in 1994 also su ffered a h igh attack rate— 80 percen t. A deep grou n dw ater sou rce w as im plicated in a 1992–93 ou tbreak of cryptosporidiosis in North Warrin gton , En glan d, wh ere in vestigators su ggested Copyright (C) 1997 American Water Works Association 8 JOURNAL AWWA that low levels of immunity in the comm u n ity m ay h ave con tribu ted to h igh rates of illn ess. Alm ost 40 percen t of the identified cases occurred in children u n der age five. An d th e attack rate for laboratory-con firm ed cases w as eigh t tim es greater th an th at observed in two 1988 ou tbreaks associated with su rface water supplies in Swindon/Oxfordshire, En glan d, an d Ayrsh ire, Scotlan d. Waterborne outbreaks are also likely to occu r wh en a com m u n ity switch es fr o m a so u r ce t h a t is fr e e o f Cryptosporidium to a source in which oocysts are presen t. For exam ple, u n til 1993 Kitchener–Waterloo, Ont., Canada, had u sed on ly grou n dwater sou rces. Th en th e u tility began u sin g a n ew ly con stru cted filtration plan t th at obtain ed w ater from an oocyst-con tam in ated su rface su pply, an d a cryptosporidiosis o u t br e a k o ccu r r e d . Se ve r a l o t h e r m u n icipalities th at h ad draw n w ater from th e sam e sou rce for several years did n ot report an ou tbreak at th at tim e. During the year following the outbreak, n o ca se s o f cr yp t o sp o r id io sis w e r e reported in Kitch en er–Waterloo. Du rin g th e secon d year, cases occu rred prim arily in in fan ts an d you n g ch ildren . Th is is con sisten t with th e expectation th at if con tam in ation of th e water con fers immunity, most cases of illness will occu r in you n g ch ildren , visitors, an d n ew residen ts. If en dem ic Cryptosporidium in fection protects residen ts from illn ess, visitors w ou ld be expected to h ave a h igh er risk of becom in g ill. In several ou tbreaks of w aterborn e cryptosporidiosis, disproportion ately m ore illn ess occu rred in visitors th an in residen ts. In Co llin gw o o d , On t ., w h e r e m o r e th an h alf th e laboratory-con firm ed cases in a 1996 waterborn e ou tbreak of cryptosporidiosis occu rred in visitors, ph ysician s argu ed in a local n ewspaper t h a t t h e y w e r e n o t se e in g illn e sse s con sisten t with an ou tbreak. It is likely th at m ore residen ts w ere in fected bu t did n ot seek m edical care becau se th ey su ffered n o sym ptom s or w ere on ly m ildly ill. A sim ilar cryptosporidiosis ou tbreak occu rred in Jackson Cou n ty, Ore., in 1992. Initial surveillance implicated the water system in Medford, wh ich u sed h igh -qu ality sprin g w ater as a sou rce of su pply. Bu t in vestigators even tu ally focu sed on Talen t, a n eigh borin g town with h igh ly con tam in ated sou rce water an d a m argin al filtration plan t, after a h igh attack rate was observed am on g visitors to a weddin g party th ere. Oth er eviden ce also su ggests th at It is u n likely th at m on itorin g water visitors m ay be at h igh risk for cryp- su pplies for th e presen ce of oocysts will t o sp o r id io sis. In To r ba y, En gla n d , h elp u s predict h ealth effects. Th e 1993 in creased in ciden ces of cryptosporidio- cr yp t o sp o r id io sis o u br e a k in Cla r k sis recu rred every Au gu st for several Cou n ty, Nev., w h ich th e Cen ters for years, corresponding with a large influx Disease Con trol an d Preven tion con of tou rists. In addition to in creasin g clu d e d w a s w a t e r bo r n e , o ccu r r e d dem an d, wh ich m ay h ave overloaded despite a state-of-th e-art w ater treatth e filtration facilities, th e tou rists m ay m en t facility an d exten sive water qu ala lso r e p r e se n t a it y m o n it o r in g t h a t h igh ly su sce p t ible detected n o oocysts. Likepopu lation with low wise, detection of oocysts URVEILLANCE im m u n ity to th e disin a fin ish ed water su pply e a se . A 1 9 9 3 o u t h as n ot predicted an ou tPROGRAMS THAT RELY break at a Min n esota br e a k o f illn e ss, e ve n ONLY ON THE DETECTION resort affected on ly when disease surveillance OF CASES OF ILLNESS visitors an d h ad on e program s w ere in itiated of th e h igh est attack to identify outbreaks more MAY NOT BE SUFFICIENT rates of reported US effectively. It is u n likely TO ASSESS THE RISK outbreaks. Of 38 peothat the enhanced disease p le w h o co n su m e d su rveillan ce program s in OF WATERBORNE w ater at th e resort, t h e Un it e d St a t e s a n d TRANSMISSION OF 2 6 ( o r 6 8 p e r ce n t ) Can ada will h elp explain becam e ill; on ly on e t h e h e a lt h e ffe ct s o f CRYPTOSPORIDIOSIS. (5 percen t) of 20 visoocysts detected in water it o r s w h o h a d n o t systems. In fact, these procon su m ed th e water gram s m ay in dicate th at becam e ill. th e h igh est risk of illn ess occu rs with If prior Cryptosporidium infections pro- in frequ en tly con tam in ated, h igh -qu altect again st sym ptom atic illn ess, wh at ity sou rce water. sh ou ld ou r goal be? We m u st critically Th e fa ilu r e o f su r ve illa n ce p r o evaluate arguments that acquired immu- gram s to iden tify ou tbreaks of disease nity is an acceptable alternative to effec- m ay be du e to h igh levels of im m u tive w ater treatm en t th at rem oves or n ity in th e com m u n ity. To determ in e in activates oocysts. Water su ppliers w h eth er a h igh degree of im m u n ity sh ou ld con tin u e to redu ce con su m ers’ exists in a com m u n ity, serological su rexposu re to Cryptoporidium in drin kin g veys sh ou ld be in clu ded in disease su rwater. veillan ce activities. Epidem iologic stu dEn dem ic in fection s are n ot with ou t ies th at in clu de serological testin g can p u blic h e a lt h o r e co n o m ic co n se - better assess th e risk of w aterborn e qu en ces. Even if on ly a sm all percen t- disease ou tbreaks an d can h elp u tility age of en dem ic in fection s resu lt in dis- m an agers an d operators evalu ate th eir ease, the cumulative number of illnesses su ccess in redu cin g th e risk of en dem ic over a year or tw o can be large. En - in fection . Serological su rveys w ill also dem ic illn esses are sim ply m ore diffi- provide baselin e data an d in form ation cu lt to detect an d to attribu te to a par- abou t ch an ges in a com m u n ity’s seroticu lar sou rce, su ch as a water su pply, logical respon se, w h ich m ay be assorecreation al activity, food item , or day ciated w ith w ater qu ality ch an ges or care cen ter. In addition , im m u n osu p- operation al problem s at th e treatm en t pressed popu lation s—in clu din g people plan t. with AIDS, patien ts u n dergoin g certain kinds of medical treatment, and the very Floyd Frost is director of the Southwest Cenelderly—may be at special risk of severe ter for Managed Care Research, 2425 Ridgeillness from either epidemic or endemic crest Dr., Sou th east, A lbu qu erqu e, N M in fection . 87108. Gunther F. Craun is president of It wou ld be n ice to elimin ate water- Gunther F. Craun and Associates, 101 West born e cryptosporidiosis with a tech n o- Frederick St., Su ite 104, Stau n ton , VA logical fix. However, if th e h ypoth esis 24401. Rebecca Calderon is chief of the US of acqu ired im m u n ity is correct, th e Environmental Protection Agency’s Epiproblem of cryptosporidiosis ou tbreaks demiology and Biomarkers Branch, Research will only intensify as we optimize treat- Triangle Park, NC 27711. Stephen A. Hubbs ment processes, improve drinking water is a vice-president at Louisville Water Comqu ality, an d con trol oth er sou rces of pany, 3018 Frankfort Ave., Louisville, KY in fection . 40206. S Copyright (C) 1997 American Water Works Association 10 JOURNAL AWWA