Occurrence, genotyping, and health risk of Cryptosporidium and Giardia in recreational lakes in Tianjin, China

Highlights

• Protozoan incidence, genotypes and risks were studied simultaneously.

• All detected protozoa except, C. fragile, are potentially infectious to humans.

• Genotypes, age, sex, and disease severity were considered in the QMRA.

• Protozoan disease burden violated the WHO threshold of 10⁻⁶ DALYs pppy.

• Fraction of the population exposed and exposure frequency most affected the risk.

Abstract

Cryptosporidium and Giardia are critical parasites in the etiology of diarrhea worldwide, and often cause waterborne outbreaks. The presence of Cryptosporidium and Giardia in recreational lakes was investigated with molecular characterization, and a comprehensive quantitative microbial risk assessment (QMRA) of protozoan infections was performed, considering multiple exposure pathways, differences in age, sex, and disease severity, and the genotypes of the protozoa. Forty-three (82.7%) and 51 (98.1%) water samples were positive for Cryptosporidium oocysts and Giardia cysts, respectively, with average counts of 3.65 oocysts/10 L and 12.58 cysts/10 L, respectively. Six Cryptosporidium species and three Giardia lamblia assemblages were confirmed with molecular analyses. The protozoan concentration was significantly associated with water turbidity, but not with the total coliform numbers. Swimming in the lakes entailed the highest incidence risk of 5.72 × 10⁻⁴ per person per year (pppy) (95% confidence interval (CI): 0.03–43.33 × 10⁻⁴) for Cryptosporidium and 4.04 × 10⁻⁴ pppy (95% CI: 0.01–32.66 × 10⁻⁴) for Giardia, whereas wading entailed the lowest risk (2.20 × 10⁻⁴ and 1.70 × 10⁻⁴ pppy, respectively). The annual burdens attributable to recreational-water-associated cryptosporidiosis and giardiasis were 3.44 (95% CI: 0.04–23.51) and 1.81 (95% CI: 0.01–12.96) disability-adjusted life years per 1,000,000 individuals per year, respectively. Children were more likely to have an individual disease burden than adults, and males were more likely than females. Sensitivity analysis highlighted the great importance of controlling the proportion of exposed individuals and reducing the frequency of exposure. The methodology and results of this study will allow us to better evaluate and reduce the burden of Cryptosporidium and/or Giardia infections associated with recreational water use in China and other countries.

Introduction

Cryptosporidium and Giardia are critical parasites in the etiology of diarrhea worldwide, and they often cause waterborne outbreaks. During the period from 2011 to 2016, at least 239 and 142 waterborne outbreaks were attributed to Cryptosporidium and Giardia, respectively, and most of them occurred in developed countries (Efstratiou et al., 2017; Rosado-Garcia et al., 2017). Although few waterborne outbreaks of cryptosporidiosis or giardiasis were reported in developing countries, it is clear that this distorted picture is attributable to the lack of surveillance systems, including a failure to identify any of the parasites or to provide reports of outbreak events (Efstratiou et al., 2017). In the most populous developing country, China, Cryptosporidium and Giardia are not usually detected because neither of these protozoans is listed as a routine inspection item in cases of diarrhea (Wang et al., 2011b). Therefore, the incidence of cryptosporidiosis and giardiasis among the Chinese is probably underestimated. Nevertheless, these parasites have been widely detected in the feces of farm, wild, laboratory, and zoo animals, as well as house pets in China (Karanis et al., 2007; Li et al., 2005; Lv et al., 2009; Wang et al., 2011a; Xu et al., 2016; Zhang et al., 2016). They have also been detected in wastewater (Liu et al., 2011), reclaimed water (Zhang et al., 2015), and source water (Hu et al., 2014). However, the occurrence and risk of these protozoa in recreational waters in China have been unclear until now, which makes it difficult to properly manage recreational waters and to develop reasonable standards of water quality.

The species/genotypes of Cryptosporidium and Giardia present in the environment is an important factor affecting the risk of human infection, and have often been ignored in previous studies that have assessed the risk these protozoa pose. Some Cryptosporidium and Giardia species are host-specific and have been only found in animals (Laishram et al., 2012; Moore et al., 2016; Ryan et al., 2014; Zahedi et al., 2016). Of the nearly 20 species/genotypes of Cryptosporidium reported in humans, C. parvum, C. hominis, C. ubiquitum, and C. meleagridis are the commonest causative agents of cryptosporidiosis (Ryan et al., 2014). Similarly, G. lamblia (also known as G. duodenalis and G. intestinalis) is the only known Giardia species that causes human disease. It is divided at the genetic level into eight assemblages (A–H) with different host specificities, two of which (A and B) are the main etiological agents of human giardiasis (Ryan and Cacciò, 2013). Therefore, the identification of environmental protozoan species or assemblages should help to control the risks they pose to public health.

Quantitative microbial risk assessment (QMRA) is a useful method of predicting the human health risk of pathogens, and it has been widely used to assess the infection risk of Cryptosporidium and/or Giardia in various types of water (de Man et al., 2014; Ehsan et al., 2015; Pintar et al., 2010; Schoen and Ashbolt, 2010; Zhang et al., 2015). For instance, the risk of recreational-lake-related Cryptosporidium infection in Canada was estimated to be as high as 10 infections per 1000 swims (95% confidence interval (CI): 2–30) or 1.0 × 10⁻² per swim per person for children and 4 infections per 1000 swims (95% CI: 4–10) for adults (Pintar et al., 2010). Lower risks were identified in Belgium, where the infection risk associated with recreational lakes was 2.79–5.74 × 10⁻⁵ for Cryptosporidium and 0.704–1.46 × 10⁻⁴ for Giardia per swim per person (Ehsan et al., 2015). However, as mentioned earlier, risk assessments of these parasites rarely take into account information on the species/genotypes that are infectious to humans, which may lead to an overestimation of the risk (Lapen et al., 2016). Furthermore, most previous studies have focused on the annual individual probability of infection, without considering the severity of the disease, which has made it difficult to compare the risks of different symptoms or illnesses.

To provide greater insight into the importance of recreational water in the transmission of these pathogenic parasites, we undertook the present study to determine the occurrence, genotypes, and health risks of Cryptosporidium and Giardia in recreational water during the high-occurrence season for waterborne cryptosporidiosis and giardiasis (Efstratiou et al., 2017; Hopkins et al., 2013; Loganthan et al., 2012; Perkins and Trimmier, 2017; Rosado-Garcia et al., 2017). Compared with previous studies of the risks presented by these parasites, this study has the following characteristics: (i) different factors, including multiple exposure routes, age, and sex, were considered when assessing exposure; (ii) the gastroenteritis burden was calculated separately by its severity category, i.e., differences in duration and disability; and (iii) the fraction of (oo)cysts infectious to humans was considered. The methodology used and our results will be helpful in better evaluating and reducing the burden of protozoan infection associated with recreational water in China and other regions.