Our study is the first prospective epidemiologic study of sporadic cryptosporidiosis that has investigated independent risk factors for C. hominis and C. parvum infections. No significant differences were found between initial symptoms, severity of illness, or duration of hospital stay in persons infected with either C. hominis or C. parvum infections. No significant differences were found between median duration for C. hominis and C. parvum; however, the variation of duration for C. hominis was significantly higher than for C. parvum infections. This finding suggests that C. hominis infections may be less predictable in terms of duration and more prone to extremes than C. parvum.
The main risk factors identified—travel abroad, contact with a patient, and touching cattle—are broadly similar to those identified by Robertson and colleagues (6
). Strongly significant negative factors were eating ice cream and eating raw vegetables. Factors significant at the 0.05 level were toileting contact with a child <5 years of age and number of glasses of unboiled water drunk at home. Eating tomatoes was negatively associated.
Health authority of residence was strongly significant in all models. However, given that we found that the ability to recruit controls differed between health authorities, much of this difference may be artifactual. Nevertheless, health authority of residence was retained in all models in the event that other risk factors varied in relation to locality of residence. The issue of geographic variation in cryptosporidiosis will be included in a subsequent report.
With regard to the main hypotheses under investigation, travel outside the United Kingdom, contact with other people with diarrhea, and contact with animals were all strongly associated with Cryptosporidium
infection. Robertson et al. (6
) also identified travel outside Australia as a risk factor. However, they suggested that OR may be inflated because of ascertainment bias of patients, which applies to our study as well. A general practitioner may be more likely to request a fecal sample from a patient with diarrhea who has traveled abroad. In addition, previous research indicates that most laboratories in the North West of England and Wales routinely screen for Cryptosporidium
oocysts if the patient is known to have traveled outside of the United Kingdom (14
). When analysis is restricted to cases where the species was known, travel outside the United Kingdom was significant for C. hominis
infection but not for C. parvum
. The relationship between C. hominis
infection and overseas travel has been noted previously (15
The risk for infection increased significantly upon contact with cattle when all patients were compared to controls, and for C. parvum
alone but not for C. hominis
alone. Previous research has associated farm animal contact with outbreaks of Cryptosporidium
; moreover, calf contact and lamb contact have been identified as risk factors for sporadic infection (6
). Several outbreaks have also been associated with farm visits within the United Kingdom. The risk for contact with other farm animals was not significant. The association with C. parvum
but not C. hominis
is also consistent with previous findings (15
). Our study was conducted during an epidemic of foot and mouth disease, when access to the countryside and contact with farm animals were severely restricted for a large period (17
), a fact that makes the cattle association even more dramatic. No significant association was found between ownership of or contact with domestic pets and sporadic infection. Although some researchers have suggested pets may present a risk (18
), other studies indicate that pets are not a major risk factor for acquiring Cryptosporidium
). Indeed, previous research has found various types of domestic animal contact to be protective (6
One variable, number of glasses of unboiled water drunk at home, was significant in the model with all patients. This water consumption variable was the only one to be included in one of the multivariable models. The Australian study also found no association with drinking publicly supplied water (6
). However, one of the two water catchment areas in this study was highly protected, with no livestock farming. The nature of the water catchment areas in Australia might preclude generalizing its results to other parts of the world. Interpreting this finding is difficult. Few of the drinking water variables associated with risk from water consumption were significant in the single variable analysis (). Neither drinking unboiled tap water nor use of a water filter was significant, which suggests that drinking water from public supplies was not an important risk factor (20
). In the single variable analysis, number of glasses of bottled water drunk was also associated with risk for infection, although whether or not persons drank bottled water was not associated with risk. We suggest that the significant association with amount of unboiled water drunk may be an artifact attributable to recall bias either because the patient believes that his or her illness was waterborne (21
) or because the person has been drinking more water as a result of illness. Our findings suggest that drinking tap water does not appear to be of major importance for sporadic disease.
The remaining risk factor included in the major hypotheses we tested, use of swimming pools, did not achieve significance, although number of times one swam in a toddler pool almost reached significance in the model for C. hominis
. Use of a toddler pool and number of times swum in a swimming pool, but not use of a swimming pool, were significant in the single variable analyses (). Swimming pool use has previously been associated with many outbreaks of Cryptosporidium
in the United Kingdom and elsewhere, and use of a toddler pool has been associated with sporadic cases (6
). The importance of swimming pool exposure as a risk factor for sporadic cryptosporidiosis was suggested by Hunter and Quigley (22
). They demonstrated a protective effect of swimming pool use in an outbreak associated with drinking water and suggested that this finding was due to immunity from an increased risk for sporadic disease in persons who go swimming.
In addition to the main hypotheses, a number of other associations were detected. These included a negative association with eating raw vegetables and tomatoes in the model with all patients and C. parvum only, a negative association with eating fresh fruit for C. hominis, a negative association with eating ice cream in the model with all patients, and an association with toileting children <5 years of age in the all-case model and diaper-changing contact in the C. hominis model. Also in the C. hominis model, spending time sleeping or sitting outside on the ground was associated with infection, the number of persons 5–15 years of age living in the same home was negatively associated with infection, and usually washing raw fruit and vegetables before eating had a protective effect.
The negative association with eating raw vegetables is also consistent with previous studies, which have suggested a protective effect from eating raw vegetables (6
). Whether this represents the effect of immunity through repeat exposure by this route or through another mechanism is unclear (22
).If the immunity hypothesis is correct, the fact that eating raw vegetables was strongly negatively associated with C. parvum
, but not C. hominis
, infection would suggest contamination of raw vegetables with animal-derived fecal material.
The negative association with ice cream was unexpected. In the single variable analysis, consuming other dairy products such as uncooked soft cheese, uncooked hard cheese, and cream were also negatively associated with illness. Unpasteurized milk products have previously been associated with Cryptosporidium
infections, and consuming such products was identified as a risk factor for sporadic cases of infection in Adelaide (6
). However, in the United Kingdom, unpasteurized milk is not used in ice-cream production, so this association is difficult to explain. We investigated the possibility that this finding was due to the different times of the year that patients and controls were recruited. However, in all but 1 month, controls were more likely to report ice cream consumption than patients were. A recently published case-control study on risk factors for giardiasis in the South West of England also reported a negative association with ice cream consumption (24
Associations of toileting contact with children <5 years (all patients) and diaper-changing contact (C. hominis) were independent of whether the children were being helped to use the toilet or having their diapers changed had diarrhea. This observation would suggest that asymptomatic carriage of C. hominis may be common in very young children even in the absence of symptoms. Asymptomatic carriage of C. hominis may be the main reservoir of infection.
In conclusion, we showed that the main risk factors for C. parvum infection (contact with cattle) and C. hominis (travel abroad and changing diapers) differ. We also showed that when the case group includes both C. parvum and C. hominis as well as cases in which the species is not known, the risk factors vary again (travel abroad and contact with a case-patient). Although restricting analysis to cases where species is known reduces the power of the study by having fewer cases, analyses conducted on populations of patients that contain two pathogens with different epidemiologic features may mask species specific risk factors. Future studies of the epidemiology of and risk factors for cryptosporidiosis should ensure that strains are speciated adequately.