Prevalence
Giardia prevalence in Denmark, Finland, Norway and Sweden has been estimated to 2.97% in the asymptomatic adult population, and 5.81% in the symptomatic population [
9]. In New Zealand, the incidence shows a bimodal pattern, peaking in the 1-4 and 25-44 age groups. The incidence in the youngest age group is nearly double that of the other age groups [
10]. A similar pattern has been shown in Vermont [
11]. In Bergen, however, statistics from the outbreak showed a peak prevalence for women aged 20-30 years and only 1% of the laboratory confirmed giardiasis cases were children less than 5 years of age [
2], giving an estimated prevalence of 0.6%. It must be noted that this prevalence is based only on children seeking medical care for symptomatic disease, and they are not true prevalence data. This estimate has included only children living in the area with contaminated water, and is therefore a maximum estimate. The present data revealed a prevalence of
Giardia infection 1.2% for the total population of children aged 1-5 years one year after the epidemic. The fact that the prevalence one year after the outbreak is higher than during the outbreak supports the hypothesis that children were asymptomatic, and therefore under diagnosed during the outbreak.
Prevalence of
Giardia in day care centres are known to be increased compared to the general population. A study from Vermont found a rate of 300 infected per 100,000 (0.3%) in day care and 195 per 100,000 (0.2%) among children not attending day care [
11]. In Denver the prevalence among day care attendees were 16% compared to 9% among non attendees [
12]. The prevalence in the general public of the Nordic countries is lower compared to these endemic areas [
9], but no good estimates on the child population are available. The increased risk may be related to wearing diapers and eating at the day care centre [
10]. The present study did not find any difference in frequency of
Giardia infection between children attending or not attending a day care centre. This suggests that children in day care centres were not at higher risk than the general population during this outbreak. Several factors may have lead to this. Children in day care centres might not drink much water, and therefore be less exposed to primary infection. An epidemiologic study from the outbreak showed increased risk for illness if water intake was in excess of the general public [
2]. The authors of that study also points to a study of Norwegian food- and drinking habits that shows children drink less water than adults. Hygienic efforts like hand washing and proper diaper-changing techniques have shown to reduce the spread of pathogens by secondary transmission [
12]. The Chief medical officer of infection control emphasized the importance of personal hygiene in limiting the transmission of the parasite in his information to the public. Beyond that, no specific information was given to day care centres during or after the epidemic.
The exposed children experienced more diarrhoea and flatulence than unexposed children. When excluding the
Giardia positive children the symptom score was unaffected. This may be a sign of undiagnosed giardiasis during the past year, which we could not detect at the time of the study. The
Giardia positive children, however, had a low symptom score. This is consistent with a range of studies suggesting giardiasis is often asymptomatic in children [
1,
5].
Previous studies have investigated whether secondary transmission is an important part of the epidemiology of
Giardia, as asymptomatic children excrete infective cysts in their stools [
6]. A longitudinal study of
Giardia lamblia infection in a day care centre population found that only 22% of infected children had symptoms attributable to
Giardia infection [
5]. Secondary transmission rates from children with giardiasis to household contacts are suggested between 17 and 47% [
5,
13,
14]. One study from New Zealand found housewives and nursing mothers at increased risk of infection, suggesting this is due to person to person transmission from children [
15]. In our study this was an important matter of investigation. The aim was to settle whether asymptomatic children may be a relevant reservoir of secondary infections maintaining an increased
Giardia incidence after the epidemic. The reported incidence of the general population was increased for six months after
Giardia was eradicated from the water supply. We could not discover any difference between
Giardia positive and negative children in terms of day care attendance or giardiasis in the family. The present study therefore does not support the theory that asymptomatic children are a reservoir for secondary transmission.
Findings in the groups
The exposed and unexposed groups were demographically similar except for household size. Children in the exposed group were part of smaller households than the unexposed group (table
2). This is expected, as larger households might move out of the city centre leading to smaller families living in the city centre and thus being more exposed to water from the contaminated reservoir. We did not, however, find any difference in household size when comparing the
Giardia positive children to the rest of the children (p = 0.791) or to the rest of the exposed group (p = 0.658). The prevalence of
Giardia, measured by the antigen test, was higher in the exposed group, but not significantly different from the unexposed group. There might be a difference we could not detect due to the small size of the final unexposed group.
Study limitations
Children in the exposed group scored significantly higher on diarrhoea and flatulence compared to the unexposed group. An important explanation to consider is the recall bias, as parents in the exposed group would possibly worry about, and to a greater extent pay attention to changes in their children's stools. However, if recall bias was the only explanation, one would expect to find similar bias and differences between the groups also in other relevant symptoms than diarrhoea. Due to a similar distribution of other symptoms, we consider this a true difference in the frequency of diarrhoea between the exposed and unexposed children.
Diarrhoea and flatulence may also be indicative for post infectious IBS found in adults after the same outbreak [
16]. Our findings may suggest that children in the exposed group had been infected by
Giardia during the outbreak, and suffer from
Giardia sequele similar to those seen in adults. However, the questionnaires asked about symptoms for the last year. This period covers a possible
Giardia infection and also possible persistent symptoms. The study is therefore not designed to consider long term consequences of
Giardia infection, and further studies are needed on this subject.
In assessing our results, one has to consider the diagnostic test used in the present study. The immunocard STAT! antigen test was chosen because of its good sensitivity (81-93.5%) shown in previous studies [
17,
18]. A study of the test sensitivity in an adult patient population with symptoms persisting after the infection was also performed in our setting, showing a lower sensitivity of 60% [
19]. This may have lead to underestimating of
Giardia positive children in the present study. However, if the antigen test detected only 60% of the true
Giardia positive cases in the study, the prevalence would still be low (2%) and similar to the expected prevalence in a Nordic setting.
The classification of exposed and unexposed children is based on parents' answers in a questionnaire. This may lead to a classification bias, but we consider the answer based re-classification to lead to less bias than classification based on residency alone.