Volume 9, Number 2—February 2003
Risk Factors for Sporadic Giardiasis: a Case-Control Study in Southwestern England
To investigate risk factors for sporadic infection with Giardia lamblia acquired in the United Kingdom, we conducted a matched case-control study in southwest England in 1998 and 1999. Response rates to a postal questionnaire were 84% (232/276) for cases and 69% (574/828) for controls. In multivariable analysis, swallowing water while swimming (p<0.0001, odds ratio [OR] 6.2, 95% confidence intervals [CI] 2.3 to 16.6), recreational fresh water contact (p=0.001, OR 5.5, 95% CI 1.9 to 15.9), drinking treated tap water (p<0.0001, OR 1.3, 95% CI 1.1 to 1.5 for each additional glass per day), and eating lettuce (p=0.01, OR 2.2, 95% CI 1.2 to 4.3) had positive and independent associations with infection. Although case-control studies are prone to bias and the risk of Giardia infection is minimized by water treatment processes, the possibility that treated tap water is a source of sporadic giardiasis warrants further investigation.
Giardia lamblia, a flagellate waterborne protozoan parasite, is a common cause of gastrointestinal disease in industrialized and unindustrialized countries (1). Most information on risk factors for giardiasis has come from investigation of outbreaks. Water is the most frequently identified route of transmission (1,2), through drinking contaminated tap water (3,4) or recreational exposure in lakes, rivers, or swimming pools (5). Person-to-person spread is well documented in day-care centers and among male homosexuals (6,7), and food may also be a vehicle of infection (8).
The relative contribution of these routes of transmission to sporadic cases of giardiasis is unknown, and further studies have been recommended in the United States (9). In the U.K., recreational exposure to fresh water and swimming pools has been identified as a possible risk factor (10–12), as well as travel to developing countries (11). We set out to determine risk factors for giardiasis in residents of southwest England who had not recently traveled outside the U.K.
Our study was conducted from April 1, 1998, to March 31, 1999 in southwest England, including Cornwall and Isles of Scilly, Devon, Somerset, Dorset, Wiltshire, Hampshire, Isle of Wight, Avon, Gloucestershire, and Herefordshire. Laboratories used for the study were the microbiology laboratories (managed by National Health Service trust or Public Health Laboratory Service) in the study area that routinely screened for Giardia cysts in all stool specimens submitted from persons with history of diarrhea. Study laboratories were sited in Torbay, Exeter, Yeovil, Dorchester, Poole, Bournemouth, Salisbury, Basingstoke, Winchester, Portsmouth, Newport (Isle of Wight), Weston-super-Mare, Bristol/Bath, Frenchay, Cheltenham, and Hereford. All microbiology laboratories in the study area that fulfilled the study criteria (16/24) agreed to participate. Ethical approval for the study was obtained from the Public Health Laboratory Service Ethics Committee, the Southwest Multicentre Research Ethics Committee, and 16 Local Research Ethics Committees.
Case-patients were defined as residents of the study area with a history of diarrhea and Giardia cysts in their stool specimen seen by light microscopy (13) in the study period. Case-patients were excluded if they had traveled outside the U.K. in the 3-week period before onset of diarrhea or if a household member had diarrhea in the 3-week period before the patient’s onset of diarrhea. Controls were persons registered at the same general practice as patients, of the same gender, and in the same broad age band (0–5 years, 6–15 years, >16 years). Controls were excluded if they had traveled outside the U. K. during the 3 weeks before interview or if they had diarrhea in the 3 weeks before interview.
The study questionnaire was derived from an instrument used in a case-control study of cryptosporidiosis in the north of England (S. Goh, pers. comm.). The questionnaire covered personal details, recent illness, travel, water contact, water and food consumption (food history focusing on dairy produce, salads, fruit), and contact with animals, farms, and day nurseries. Adults were asked for details of occupation. The questionnaire was modified from a pilot study; the wording of some questions was simplified, and a question about previous infection or treatment of Giardia infection was added. Patients were asked about exposures in the 3 weeks before onset of illness, and controls were asked about exposures in the 3 weeks before completion of the questionnaire. In questions about drinking unboiled tap water and bottled water, patients and controls were both asked about usual consumption patterns.
Cases were reported on identification to the principal researcher by fax or post. Health authorities, through the consultant in communicable disease control, randomly selected three controls for each case selected from the Health Authority population age-sex register. If the general practitioner had no objection to contacting patients and controls, the questionnaire was sent out with a covering letter. One month after the first questionnaire was posted, nonresponders were sent one reminder.
A required sample size of 192 cases was estimated by using Generalised Linear Interactive Modelling (GLIM) (14). This size was based on a case-control ratio of 1:3, with 95% confidence and 80% power to detect an odds ratio (OR) of at least 3 for the variable under investigation. Data were double entered onto a microcomputer, and single variable conditional logistic regression was carried out by using GLIM (15). Variables with p<0.2 in the single variable analysis were then included in a multivariable conditional logistic regression analysis. A series of models were fitted; clearly nonsignificant terms were eliminated to maximize the number of subjects that could be used in the regression modeling. As a large number of observations were “don’t know” or “missing” for the “swallowing water while swimming” variable, an extra level was included for this variable. This addition resulted in a final main effect model that included 655 subjects (95% of the 689 available). All the 2-way interactions were tested, including those involving age and sex.
Questionnaires were returned from 232/276 case-patients and 574/828 controls, response rates of 84% and 69%, respectively. Two additional patients were not included because their general practitioners did not agree to participate in the study. After the exclusion criteria were applied, data from 192 cases and 492 controls were available for analysis, a ratio of 1:2.5. Of the 40 case-patients excluded, 18 had traveled outside the U.K., 15 were possible secondary case-patients, and 7 did not report diarrhea. Of the 82 excluded controls, 43 reported diarrhea, 38 had traveled outside the U.K., and reasons for exclusion were missing for one. The incidence of giardiasis fulfilling the study criteria was 4.7/100,000 during the 12-month study period. Seasonal variation was not marked, and no outbreaks were detected during the study. Cases were most frequent in the 30- to 39-year age group. Although frequency matching had been used to obtain controls with similar age distribution to the case-patients, some differences were observed in those remaining for analysis: 17% of the patients and 22% of the controls were <16 years of age. Eighty-seven (44%) of the case-patients were male, and 109 (56%) were female. Among the nonresponders, a higher proportion were male (33/44,) but the age distribution was similar. Social class distribution was similar in adult case-patients and controls (p=0.1).
By definition, all case-patients (192) had diarrhea. One hundred and forty-eight (77%) case-patients had abdominal pain, 73 (38%) reported vomiting, 53 (28%) had a fever, 12 (6%) noted blood in stools, and 90 (47%) reported other symptoms, including tiredness, weight loss, nausea, and headache. The median reported duration of illness was 21 days (range 1–305). Thirteen patients reported that they were still unwell at the time of completing the questionnaire.
In the single variable analysis, a dose-response relationship existed between reported number of glasses of tap water usually drunk per day and risk of illness (p<0.0001, Table 1). No evidence of a dose-related response existed for those who drank only bottled water. The homes of >98% of patients and controls were on public water supply (provided by one of the seven statutory water companies in the study area). Most case-patients (73%) were supplied by the same water treatment works as their matched controls. Home water supply was purely surface water for 40% of cases and 41% of controls, and purely ground water for 39% of cases and 33% of controls; data on water supply at work were not available. Several exposures relating to swimming in chlorinated water and fresh water contact were significantly associated with illness (Table 2). In a wide range of questions on food consumption, eating lettuce was the only variable significantly (p<0.05) associated with infection (OR 2.3, 95% confidence interval [CI] 1.3 to 4.1, p=0.003). Visiting a farm was more frequent among case-patients (OR 2.2, 95% CI 1.2 to 4.1, p=0.01), but none of the associations with specified animal exposures (dogs, cats, horses, cows, sheep) was statistically significant. Case-patients were more likely to have contact with a nursery (OR 2.2, 95% CI 1.2 to 4.2, p=0.01).
In the multivariable analysis, swallowing water while swimming, recreational contact with fresh water, dose response to drinking tap water, eating lettuce, and not eating ice cream were independently associated with illness (Table 3). For swallowing water while swimming, the “don’t know/missing” category had an OR of 5.3 (95% CI 1.9 to 14.8). If analysis of the water swallowing variable was restricted to those who specified chlorinated water, the association remained highly significant in the model (p=0.006, OR 3.9, 95% CI 1.3 to 11.4). Attributable fractions were estimated as 14% for swallowing water, 9% for fresh water contact, 35% for drinking three or more glasses of tap water per day, and 40% for eating lettuce.
We found an association between drinking tap water and giardiasis in persons who had not recently traveled outside the U. K. This finding had a high level of statistical significance and was unexpected. U.K. public water supplies are thought to be at low risk for transmitting Giardia infection because of the lower resistance of Giardia cysts to chlorination compared with Cryptosporidium oocysts (16,17), their easier removal by filtration (18), and the few outbreaks of giardiasis attributed to drinking water (19). A causal link cannot, however, be excluded, as Giardia cysts have been frequently found in U. K. surface waters (D. Sartory, pers. comm.), and outbreaks attributed to chlorinated water in the United States are well documented (3,20). The association with drinking water might have been overestimated because of changes in water consumption after illness or because of bias. Information bias is a recognized problem in case-control studies; such bias was not found, however, in a case-control study conducted during and after a giardiasis outbreak in Nevada (21). The matched design of our study reduced the risk of error from confounding factors but also resulted in close matching on home water supply such that assessing the effect of drinking water quality indicators on outcome was not possible.
Another new finding was an increased risk for giardiasis associated with eating lettuce (green salad). A true risk is certainly plausible, as irrigation and fertilization practices may lead to contamination with Giardia and other organisms (22,23). Outbreaks of giardiasis in the United States have been attributed to eating salad contaminated by food handlers (24). The negative association between illness and eating ice cream is unlikely to represent a true protective effect. This finding may have arisen from study biases, but preference for ice cream could simply be a marker for those who are less likely to eat salads, which are at higher risk of Giardia contamination. An apparently false protective effect of soft drinks has been observed in outbreaks of cryptosporidiosis attributed to tap water (25).
The association between giardiasis and swallowing water while swimming is not surprising; several outbreaks of giardiasis have been linked with exposure to chlorinated water in swimming pools (3,5). If those who did not know if they swallowed water were more likely to have actually swallowed water, this could explain the high OR in this category. A higher risk of exposure to recreational fresh water accords with results of other studies (10,12,26). By contrast with studies from the United States (26,27), contact with children’s nurseries was not found to be an independent risk factor. In New Zealand, where the attack rate among young children is relatively high, an increased risk was found in adults who changed infants’ diapers (28), and information on household contact with young children should be sought in further studies. No risk was identified from contact with animals. One case-control study in the east of England showed associations with exposure to farm animals and pets, particularly pigs, dogs, and cats (29); other studies have not found such associations (26,30). The microbiologic evidence for zoonotic transmission is similarly conflicting (2). Although no questions were included on sexual behavior, the similar numbers of male and female cases in the study does not support a hypothesis that male homosexual behavior was an important method of transmission in this population.
The incidence of laboratory-reported cases of Giardia infection in England and Wales was 8.2/105 in 1998–2000 (31), and a study of infectious intestinal disease in England in 1993–95 suggested that the true incidence rate of giardiasis was around 50/105 (32). Giardia infection may be asymptomatic in children, but infection in adults usually leads to acute severe gastrointestinal illness that may be prolonged (median duration 3 weeks in our study) and may lead to chronic malabsorption and weight loss. In marked contrast to most other gastrointestinal infections, giardiasis can be successfully treated with drugs such as metronidazole (1). Higher awareness of non–travel-associated infection is needed in primary care and diagnostic microbiology laboratories so that correct treatment can be given and disease incidence measured more accurately. A causal link between exposure to treated drinking water and sporadic giardiasis would have considerable public health importance but cannot be established on the basis of this study. Further case-control studies, in conjunction with assessment of drinking water quality that includes examination for Giardia cysts, would help to resolve this issue.
Dr. Stuart is a regional epidemiologist in southwest England, working for the National Communicable Disease Surveillance Centre. He has a research interest in the epidemiology of infectious diseases.
We are grateful for the help and cooperation received during this study from laboratory, health authority, and primary care staff in southwest England. We particularly thank Nick Andrews for statistical support; case-patients and controls for their kind assistance in completing questionnaires, and the water companies in the study area for supplying water quality data.
This work was supported by a grant from the Public Health Laboratory Service.
- Hill DR. Giardia lamblia. In: Mandell GL, Bennett JE, Dolin R, editors. Mandell, Douglas and Bennett’s principles and practice of infectious diseases. 4th ed. Edinburgh: Churchill Livingstone; 1995. p. 2487–93.
- Flanagan PA. Giardia—diagnosis, clinical course and epidemiology: a review. Epidemiol Infect. 1992;109:1–22.
- Levy DA, Bens MS, Craun GF, Calderon RL, Herwaldt BL. Surveillance for waterborne-disease outbreaks—United States, 1995–1996. MMWR Morb Mortal Wkly Rep. 1998;47:6–7.
- Moorehead WP, Guasparini R, Donovan CA, Mathias RG, Cottle R, Baytalan G. Giardiasis outbreak from a chlorinated community water supply. Can J Public Health. 1990;81:358–62.
- Porter JDH, Ragazzoni HP, Buchanon JD, Waskin HA, Juranek DD, Parkin W. Giardia transmission in a swimming pool. Am J Public Health. 1988;78:659–62.
- Rauch AM, Van R, Bartlett AV, Pickering LK. Longitudinal study of Giardia lamblia in a day care center population. Pediatr Infect Dis J. 1990;9:186–9.
- Meyers JD, Kuharic HA, Holmes KK. Giardia lamblia infection in homosexual men. Br J Vener Dis. 1977;53:54–5.
- Osterholm MT, Forfang JC, Ristinen TL, Dean AG, Washburn JW, Godes JR, An outbreak of foodborne giardiasis. N Engl J Med. 1981;304:24–8.
- Centers for Disease Control and Prevention. Giardiasis surveillance—United States, 1992–1997. MMWR Morb Mortal Wkly Rep. 2000;49:2.
- Gray SF, Rouse AR. Giardiasis—a cause of traveller`s diarrhoea. Commun Dis Rep Rev. 1992;2:R45–7.
- Gray SF, Gunnell DJ, Peters TJ. Risk factors for giardiasis: a case-control study in Avon and Somerset. Epidemiol Infect. 1994;113:95–102.
- Neal KR, Slack RCB. Risk factors for Giardia infection—more than just drinking the water. Recreational use of water in the United Kingdom and abroad is a route of infection. Proceedings of the Society for Social Medicine Conference, 1997. York, U.K.: Society for Social Medicine;1997.
- Farthing MJG. Giardiasis. In: Gilles HM editor. Protozoal disease. London: Arnold; 1997. p. 562–84
- Andrews NJ, Swan AV. SAMG: A sample size investigation system using GLIM4. GLIM Newsletter. NAG Ltd;1994;23.
- Francis B, Green M, Payne C, eds. The GLIM System release 4 manual. Oxford: Clarendon Press; 1993.
- Jarroll EL, Bingham AK, Meyer EA. Effect of chlorine on Giardia lamblia cyst viability. Appl Environ Microbiol. 1981;41:483–7.
- Korich DG, Mead JR, Madore MS, Sinclair NA, Sterling CR. Effects of ozone, chlorine dioxide, chlorine, and monochloramine on Cryptosporidium parvum oocyst viability. Appl Environ Microbiol. 1990;56:1423–8.
- Steiner TS, Thielman NM, Gjuerrant RL. Protozoal agents: what are the dangers for the public water supply? Annu Rev Med. 1997;48:335.
- Jephcott AE, Begg NT, Baker IA. Outbreak of giardiasis associated with mains water in the United Kingdom Lancet 1986;1(8483):730–2.
- Isaac-Renton JL, Philion JJ. Factors associated with acquiring giardiasis in British Columbia residents. Can J Public Health. 1992;83:155–8.
- Navin TR, Juranek DD, Ford M, Minedew DJ, Lippy EC, Pollard RA. Case-control study of waterborne giardiasis in Reno, Nevada. Am J Epidemiol. 1985;122:269–75.
- Takayanagui OM, Febronio LH, Bergamini AM, Okino MH, Silva AA, Santiago R, [Monitoring of lettuce crops of Ribeirao preto, SP, Brazil]. [article in Portuguese]. Rev Soc Bras Med Trop. 2000;33:169–74.
- Kapperud G, Rørvik LM, Hasseltvedt V, Høiby EA, Iversen BG, Staveland K, Outbreak of Shigella sonnei infection traced to imported iceberg lettuce. J Clin Microbiol. 1995;33:609–14.
- Rose JB, Slifko TR. Giardia, Cryptosporidium and Cyclospora and their impact on foods: a review. J Food Prot. 1999;62:1059–70.
- Willocks L, Crampin A, Milne L, Seng C, Susman M, Gair R, A large outbreak of cryptosporidiosis associated with a public water supply from a deep chalk borehole. Commun Dis Public Health. 1998;1:239–43.
- Dennis DT, Smith RP, Welch JJ, Chute CG, Anderson B, Herndon JL, Endemic giardiasis in New Hampshire: a case-control study of environmental risks. J Infect Dis. 1993;167:1391–5.
- Chute CG, Smith RP, Baron JA. Risk factors for endemic giardiasis. Am J Public Health. 1987;77:585–7.
- Hoque ME, Hope VT, Scragg R, Kjellström T, Lay-Yee R. Nappy handling and risk of giardiasis. Lancet. 2001;357:1017–8.
- Warburton ARE, Jones PH, Bruce J. Zoonotic transmission of giardiasis: a case control study. Commun Dis Rep Rev. 1994;4:R32–5.
- Mathias RG, Riben PD, Osei WD. Lack of an association between endemic giardiasis and a drinking water source. Can J Public Health. 1992;83:382–4.
- Public Health Laboratory Service. Giardia lamblia laboratory reports England and Wales 1986–2000. PHLS website 2001. Available from: URL: http://www.phls.org.uk/topics_az/giardia/data_all.htm
- Wheeler JG, Cowden JM, Sethi D, Wall PG, Rodrigues LC, Tompkins DS, Study of infectious intestinal disease in England: rates in the community, presenting to GPs, and reported to national surveillance. BMJ. 1999;318:1046–50.
Comments to the Authors
West Nile Virus RNA
in Tissues from Donor
Transmission to Organ