The epidemiology and molecular epidemiology of Giardiasis in North West England

Giardiasis, cause by the parasitic protozoan Giardia duodenalis, is one of the most common infectious gastrointestinal diseases in humans worldwide. However, its true population burden and epidemiology and in particular its zoonotic transmission potential are still poorly understood. Furthermore, G. duodenalis is not a uniform parasite but a complex of seven genetic assemblages or cryptic species (named A to G) that infect humans and a variety of domesticated and wild animals, and that can only be distinguished using molecular genotyping methods. Although there is some evidence that the two Giardia assemblages infecting humans (namely A and B) may differ in their virulence and major transmission routes, data are still scarce. In the UK, several studies suggested that giardiasis is considerably under-diagnosed and a few data are available on the genetic diversity of the parasite causing infection and disease in this country. We investigated the burden, clinical outcomes, risk factors and molecular diversity of giardiasis in North West England using both a descriptive and analytical approach. In Chapter 2, we analysed the self-reported clinical and exposure data collected over four years from clinical cases of giardiasis in Central Lancashire, as part of an enhanced surveillance program on the illness. The resulting average disease rate of 22.5 cases/100,000 population was high when compared to the available national figures. Giardiasis was particularly abundant in adults in their 30s and children under five, and the disease rate in males was significantly higher than in females. Furthermore, the clinical picture of the cases confirmed the high morbidity associated with this infection particularly in terms of the length of illness and severity of symptoms. Only 32% of the cases reported foreign travel during the exposure window. The results suggested the presence of a hidden burden of disease in adults and males, and indicated that local transmission of Giardia can be more common than expected. In Chapter 3, we performed a case-control study to determine the significant risk factors for symptomatic giardiasis in North West England, by recruiting clinical cases of Giardia and age and sex matched controls from Central and East Lancashire and Greater Manchester. The multivariable logistic regression analysis done on 118 cases and 226 controls revealed that overall travelling abroad (particularly to developing countries) was an important risk factor for the illness (OR 9.59). Following the exclusion of participants that reported foreign travel, four risk factors were significant for the acquisition of giardiasis: going to a swimming pool (OR 2.67), changing nappies (OR 3.38), suffering irritable bowel syndrome (OR 3.66) and drinking un-boiled water from the tap (OR 8.17). The results indicated the important role of swimming pools and contact with children in nappies for the transmission of the parasite. In Chapter 4, whole faecal DNA was extracted from the faecal samples of the cases part of the surveillance and case-control studies and the Giardia assemblages and sub-assemblages causing infection were determined using PCR amplification and DNA sequencing of up to four parasite genes (beta-giardin, glutamate dehydrogenase, triose-phosphate isomerase and small-subunit ribosomal RNA). The majority of infections (64%) were caused by assemblage B, followed by assemblage A (33%), whereas mixed-assemblage infections were rare (3%). The majority of the assemblage A isolates belonged to the sub-assemblage AII and showed completed identity with previously described isolates, and six multi-locus genotypes were identified. The level of genetic sub-structuring as revealed by phylogenetic analysis was significantly higher in assemblage B isolates compared with A isolates: a higher proportion of novel assemblage B sequences was detected compared to what was observed in assemblage A isolates. A high number of assemblage B sequences showed heterogeneous nucleotide positions that prevented the unambiguous assignment to a specific sub-assemblage. Up to 17 different assemblage B multi-locus genotypes were found. The molecular genotyping results showed that Giardia assemblage B was responsible for the majority of the clinical infections and confirmed the occurrence of a high diversity of parasite multi-locus genotypes. In Chapter 5, we integrated the epidemiological and the molecular data generated by the enhanced surveillance and case-control studies and we studied the clinico-epidemiological differences between cases infected with Giardia assemblage A or B. Our results showed a difference in the age prevalence between the two assemblages, with assemblage A being more common in older cases. Cases infected with assemblage B reported a series of symptoms more frequently than cases infected with assemblage A, as well as reporting a longer illness. Although the exposure profile of the cases largely overlapped between the two assemblages, two different types of exposures were reported more frequently in the two groups of cases: keeping a dog in assemblage A cases and the presence in the household of children and children at nursery in assemblage B cases. The results suggested that assemblage A could have a major zoonotic reservoir, whereas assemblage B could be transmitted more commonly via the human-to-human route.