Giardia is one of the most widespread protozoa causing diarrhoea in the world with approximately 200 million symptomatic individuals, and 500,000 new cases reported each year (WHO, 1996). Giardia is especially common in areas where poor sanitary conditions and defective water treatment prevail. In the United Kingdom (UK), the United States (US) and Mexico, giardiasis seasonality has been shown to have a peak incidence during late summer. However, no seasonal pattern has been observed during day care outbreaks (Ortega and Adam, 1997).
This waterborne pathogen has a prevalence of 2 to 5 % and 20 to 30 % in industrialised and developing countries respectively. Several studies have shown that children were more frequently infected than adults, particularly those
malnourished. The prevalence of Giardia infection in infants, and children under 10 years old, vary between 15 to 20 % in developing countries (Noor Azian et al., 2007, Ortega and Adam, 1997).
Recent studies using PCR suggest that the proportion of reported giardiasis represents 5% of common waterborne diseases; and that campylobacteriosis and salmonellosis are the most common waterborne diseases in Europe with proportions of 55% and 35% respectively (Fig 1.4 A) (ECDC, 2013, ECDC, 2012).
These studies also show that reported cases of Giardia infection accounted for 8%
of all reported cases of gastro-intestinal infections in the UK (Fig 1.4 B.) (PHE, 2013, ECDC, 2013, ECDC, 2012). In the US, similar studies show that approximately 20 % of all the reported cases of common gastro-intestinal diseases between 2004 and 2013 were giardiasis (Fig 1.4 C) (CDC, 2013). Giardiasis appears to have a very low incidence rate in European countries where Giardia infection were reported and monitored (Fig 1.5). The highest and lowest incidence rates were observed in Bulgaria, with an incidence rate of 234.1 reported cases per million per year, and in Lithuania, with an incidence rate of 0.843 reported cases per million per year, respectively (Fig 1.5) (ECDC, 2013, ECDC, 2012, ECDC, 2010a). When compared to the other European countries with an incidence rate for giardiasis, the UK has one of the highest incidence rates observed, with 58.9 reported cases per million per year (Fig 1.5) (ECDC, 2013, ECDC, 2012, ECDC, 2010a).
Assemblage A and B are associated with human infections; their distribution varies considerably among studies and within the same country. Feng and Xiao reviewed the analysis of over 4,000 human isolates, from different geographical areas, by PCR amplification of DNA extracted from faeces. According to them, assemblage B appears to be slightly more common in both industrialised and developing countries than assemblage A, in the case of endemic giardiasis. Indeed, 708 cases in developed countries and 1,589 cases in developing countries have been reported as assemblage B infections; for assemblage A, 482 and 1,096 cases have been reported in developed and developing countries respectively (Feng and Xiao, 2011). Even with the data summarised in this chapter, there is only a small amount of data available to epidemiologically link Giardia genotypes/subtypes – in source and/or drinking water- and human endemic giardiasis. Data on human
Fig 1. 4: Proportion of giardiasis and other common gastrointestinal diseases in Europe and the US and proportion of Giardia infection in the UK Giardia (ECDC, 2013, ECDC, 2012, CDC, 2013, PHE, 2013). A. Proportion of waterborne diseases in Europe (2006-2011); B. Proportion of common gastrointestinal infection in the UK (2000-2012); C. Proportion of common gastro-intestinal diseases in the USA (2004-2013).
Fig 1. 5: Giardiasis incidence rate per country in Europe between 2006 and 2011 (ECDC, 2013, ECDC, 2012, ECDC, 2010a). Incidence rates for giardiasis were calculated via the following formula: 𝑵𝒖𝒎𝒃𝒆𝒓 𝒐𝒇 𝒄𝒂𝒔𝒆𝒔 𝒂𝒓𝒊𝒔𝒊𝒏𝒈 𝒊𝒏 𝒕𝒉𝒆 𝒄𝒐𝒖𝒏𝒕𝒓𝒚 𝒐𝒗𝒆𝒓 𝟔 𝒚𝒆𝒂𝒓𝒔
𝑨𝒗𝒆𝒓𝒂𝒈𝒆 𝒑𝒐𝒑𝒖𝒍𝒂𝒕𝒊𝒐𝒏 𝒏𝒖𝒎𝒃𝒆𝒓 𝒐𝒇 𝒕𝒉𝒆 𝒄𝒐𝒖𝒏𝒕𝒓𝒚 𝒃𝒆𝒕𝒘𝒆𝒆𝒏 𝟐𝟎𝟎𝟔−𝟐𝟎𝟏𝟏 , for each European country with available data on number of Giardia infection between 2006 and 2011. Incidence rates were expressed in Number of reported cases per million per year.
0 20 40 60 80 100 120 140 160 180 200 220 240
Austria Belgium Bulgaria Cyprus Czech Rep Estonia Finland Germany Hungary Ireland Latvia Lithuania Luxembourg Malta Poland Romania Slovakia Slovenia Spain Sweden UK
Incidence Rate (number of reported cases per million per year)
giardiasis epidemiology in outbreaks are very limited. However, a systematic review analysed 14 publications on Giardia sporadic infections, outbreaks and case-control study from 22 countries and suggested that assemblage B was more present, during outbreaks, than assemblage A or assemblages A/B co-infection, with 60, 35 and 5%
respectively (Caccio and Ryan, 2008). Even if assemblage B seems overall more present that assemblage A, no strong conclusions can be yet drawn from current data available. The fact that genetic exchange was shown to be possible in vitro suggests that Giardia may be capable of genetic recombination among member of the species complex (Caccio and Ryan, 2008, Poxleitner et al., 2008, Cooper et al., 2007, Ramesh et al., 2005); as a consequence, its virulence may vary according to the genetic exchange occurring during infection. Many questions still remain unanswered and more investigations are needed to understand Giardia infection epidemiology.
Giardiasis is associated with wide symptomatology, ranging from the absence of symptoms to acute or chronic diarrhoea, malabsorptive and allergic manifestations, weight loss, vomiting and childhood failure to thrive; but whether the severity of the disease is influenced by host, parasite, or host-parasite factors remains still largely unknown (Feng and Xiao, 2011, Caccio et al., 2005). The study of the association between Giardia assemblages and virulence has produced some inconsistent and contradictory results so far. Some studies have linked assemblage A to mild intermittent diarrhoea and assemblage B to severe, acute or persistent diarrhoea (Feng and Xiao, 2011, Caccio and Ryan, 2008, Caccio et al., 2005, Homan and Mank, 2001). Other studies have linked assemblage A to severe giardiasis and assemblage B to asymptomatic to mild giardiasis (Feng and Xiao, 2011, Read et al., 2002). Another study, realised in south-west London between 1999 and 2005, has shown similar epidemiological and clinical features. But differences between both assemblages, in term of fever intensity, have been observed; assemblage A being more frequently associated with fever than assemblage B, and yet, assemblage B is predominant in the urban UK setting (Breathnach et al., 2010). At present, data available for assemblage virulence are inconclusive (Nash, 2013, Feng and Xiao, 2011, Breathnach et al., 2010, Caccio and Ryan, 2008, Caccio et al., 2005, Homan and Mank, 2001). The exact contribution of Giardia genetic variability to
symptomatology and virulence is still under debate and requires further investigation.
Mixed infections have also been reported in several studies and countries, the percentage of mixed infections ranges from 2 % to 21 % (Gelanew et al., 2007).
Most of these infections are assemblage A/B; some studies have identified assemblages A or B with C or D (canine-specific) infections (Traub et al., 2009).
Mixed infections of assemblages A and F (cat-specific) have been identified in humans in Ethiopia (Gelanew et al., 2007). However, none of these results could be confirmed by analysis of either tpi or ssu rRNA genes (Feng and Xiao, 2011, Traub et al., 2009, Caccio and Ryan, 2008, Gelanew et al., 2007). These mixed infections are endemic and have only been observed in small local communities.