I NTERPRETATION

The theory that the symptoms were explained by chronic infection not identified by analysis of stool samples was not confirmed. At the university hospital an extended antibiotic regime was given to 25 patients with persisting symptoms and negative stool analysis. They received treatment with either albendazole/metronidazole or tetracycline/folate. Initially four patients reported substantial improvement, but symptoms later recurred in all four. One year later 24 still reported abdominal symptoms(189).

The outbreak in Bergen was caused by Giardia assemblage B(190). There are indications that different strains differ in their ability to produce disease in man(29, 191), but there are no conclusive data on the importance of different assemblages.

The mechanisms for prolonged symptoms and loss of function are unclear. In medically unexplained physical symptoms possible important factors have been divided into three groups; premorbid disposition or vulnerability, factors that trigger the condition (for instance an infection) and factors that sustain it.

We have little data on premorbid vulnerability. In Paper I we found that among the patients returning to their GP with recurring symptoms a significantly higher proportion had seen a physician for abdominal complaints the previous two years.

This was an objective measure since the data were extracted from the medical records. However, the number of patients was low. The larger cohort was asked two years after the outbreak about abdominal problems prior to their Giardia-infection.

There was no association between the response to that question and current abdominal symptoms(192).

We were reluctant to retrospectively introduce questions on psychological factors and adverse life events in mailed questionnaires. We feared that participants would find these issues of limited relevance, and possibly lead to lower response rate. The results would also be vulnerable to recall bias as discussed above. However, in paper II we did include the neuroticism-part of the short-scale Eysenck Personality Questionnaire (EPQ-N) at 12 months. This consists of 12 questions to be answered by “yes” or “no”, giving a total score between 0 and 12(193). Of the 118 persons invited to participate, 99 returned the questionnaire and 95 completed the EPQ-N (80.5% response rate).

The interpretation of the results will be restricted by the cross-sectional design and the low number of patients, but we did not find any correlation between EPQ-N score and persistent symptoms.

Acute giardiasis is the triggering factor in this group of patients, but it is intriguing that this non-invasive infection of the small intestine causes persisting systemic symptoms and impaired function of the large bowel.

Duodenal inflammation persisted for a period after successful treatment in some patients referred to the hospital(42), and in another group some degree of visceral hypersensitivity was demonstrated(194).

Lactose intolerance has been suggested as a cause of lasting symptoms, but this has not been backed by research. Reduced lactase activity during infection has been shown in experimental studies of giardiasis in gerbils(195), and also in Giardia-patients(196). However, lactose intolerance does not seem to play a role in the development of postinfectious IBS after bacterial gastroenteritis(197), nor does subjective milk intolerance predict lactose intolerance in IBS patients(198). In Paper III we found no correlations to support that lactose intolerance is the mechanism for post-giardiasis IBS. Also, lactose intolerance could hardly explain the fatigue. And finally, if this was the explanation, patients would be likely to experience

exacerbation of symptoms related to intake and avoid milk. The symptoms should therefore subside.

The mechanisms of parasite pathogenicity and the host response in giardiasis are largely unknown. During the acute infection there are various degrees of

inflammation with villous abnormalities, epithelial dysfunction and disruption of tight junctions, but in many patients with symptomatic giardiasis there is no detectable microscopic changes. Both mast cells and T-cells play a role in the immune response, as do B-cells which produce IgA and IgG anti-Giardia antibodies. However, most of what is known about disease mechanism is based on studies in animals, and has not been confirmed in humans(199). The results of infection are hypersecretion, malabsorption and reduced disaccharidase activity causing diarrhoea and other gastrointestinal symptoms(200). Fatigue has also been identified as a common feature of acute giardiasis(33). This means that a large proportion of patients suffered from long term symptoms that resemble those of the acute infection. It is not clear why symptoms persist. A broad approach to the pathogenesis is needed, and focus should be on both pathogen and host factors. The immune response may be important, in particular T-lymphocytes. Their role has been linked to acute giardiasis(199). In

postinfectious IBS T-lymphocyte infiltration has been reported(138) and the conclusion of a systematic review was a trend towards increased T-cell activity in patients with CFS(201). Another approach to the understanding of both IBS and chronic fatigue has been to focus on the hypothalamus-pituitary-adrenal axis (HPA axis)(202, 203). There is no obvious one-dimensional connection between giardiasis, or infections in general, and the HPA axis, but this scope opens for a more multi-dimensional understanding of the pathogenesis of unexplained somatic disorders, including immunological, endocrinological, psychological and even cultural influences.

We have not investigated patient factors that might sustain symptoms, like anxiety, depression, hypochondriasis, adverse life events and patients coping ability and strategies.

The municipality of Bergen claimed full responsibility for the outbreak and decided to give compensation to those affected. The conclusion was reached in the beginning of April 2005, after a period when several patients had voiced their discontent in the media and in meetings with politicians. This could possibly lead to aggravation of symptoms, both because the dissatisfaction with the authorities could counteract health promoting mechanisms and because symptoms could have consequences for the claims. On the other hand, having somewhere to place responsibility and blame could move focus from what had happened during the outbreak towards future improvement and coping. Also the benefit of exaggerating the symptoms would be limited as compensation was restricted to any verified economical loss. Although these considerations will apply in all outbreaks of disease caused by contamination of public water we have not seen this addressed in other studies published after

waterborne outbreaks.

Finally, in the effort to contribute with novel findings one should bear in mind the possible pitfalls of assumed causality. In order to avoid this several criteria for causality have been proposed. The list presented by Bradford Hill in 1965 is still

accepted as a valid framework(204). There are nine aspects that should be considered before inferring that an association is explained by causation:

1. Strength. The stronger the association the more likely causation will be, but he warns against dismissing causality barely on the grounds of a wearer

association.

2. Consistency. An association observed several times by different observers, in different settings and populations, is more likely to be causal.

3. Specificity. If one factor is associated to a specific disease or other restrictions concerning outcome this will be an argument for causality. However, there are also good examples of non-specificity, for instance smoking increasing the risk of several diseases, and several risk factors increasing the risk of coronary heart disease.

4. Temporality. The cause must precede the effect.

5. Biological gradient. When possible one should try to find evidence of a dose-response curve, and if found this suggests causality.

6. Plausibility. Although dependent on the state of current knowledge a proposed causation is more likely if biologically plausible.

7. Coherence. A theory of causal relationship should be coherent with other knowledge of the disease and the potential causal factors.

8. Experiment. Introducing a causal factor would not be ethically possible, but removing the factor could yield support for causality.

9. Analogy. If similar situations or factors have caused serious effects it could be argued that analogy should be deemed sufficient to infer causality, as a precautionary principle.

The criteria relating to specificity or biological gradient are not met. The association between acute giardiasis and postinfectious complications is consistent in several studies following the outbreak in Bergen, but should be confirmed in other settings and populations. Based on studies on postinfectious complications among patients suffering from other infections and possible mechanisms, one could argue that

causality to some extent is supported by analogy, and maybe also plausibility and coherence. The studies in this thesis were based on a “natural experiment” as the outbreak happened by accident, and though this design is weaker than a proper experiment it should be given consideration in the interpretation of the results. At least it suggests that the infection preceded IBS and chronic fatigue. All these indications and the strength of the association constitute substantial support for causality between acute giardiasis and postinfectious IBS and chronic fatigue.