There were 116 epidemiological investigations (88 in Chinese and 28 in English), which accounted for 41% of the research on Blastocystis sp. in China, including 98 population surveys and 18 animals. China has carried out three national human parasite investigations. In the first national human parasite survey from 1988 to 1992, it found that the nationally weighted average infection rate of Blastocystis sp. was 1.473(±0.075)%, with the highest infection rate in Sichuan 8.01% [18]. However, there was no data on Blastocystis sp. in the second national survey of important human parasites in 2001ཞ2004. In the third national survey of key human parasites in 2014ཞ2016, the highest weighted average infection rate was 5.69% in Guizhou, followed by Guangxi 4.52% [19].
3.3.1 Infection of Blastocystis sp. in humans
According to the current investigation of Blastocystis sp., the population infection rate was 0.007–48.6%. In 2000, surveyed the outpatient diarrhea patients in Guangzhou hospitals with the highest infection rate was 48.6% reported by Zhang et al. [20], and the lowest infection rate was 0.007% in the Xinjiang Uygur Autonomous Region, 2015 [21]. The general population, students, children, hospitalized and outpatients, patients with diarrhea, and people with other diseases were contained in the population epidemiological survey.
There were 44 epidemiological surveys of Blastocystis sp. infection in the general population, with 36 in Chinese, and 8 in English, of which 6 were genotyped. The percentage of Blastocystis sp. infection in the population in different regions of China was ranged from 0.007–43.26%. The infection status and subtype distribution of Blastocystis sp. in parts of China has been listed in Table 4. The highest infection rate of human Blastocystis sp. was 43.26% in Bama Yao Autonomous County [22]. The lowest infection rate was 0.007% [21]. With the development of molecular detection methods, PCR method was used to identify the subtype of Blastocystis sp. in China. Yan and others identified that the subtypes of Blastocystis sp. infected by humans was ST1ཞST3. ST3 was the main subtype (40.0%), followed by ST1 (37.1%), mixed subtypes of ST1 and ST3 account for 14.3%. This study also detected unknown subtype [23].
The risk factors for Blastocystis sp. infection were diverse. A survey of children aged 7–12 years in the border areas of China and Myanmar found that not washing their hands after using the toilet may be a risk factor for Blastocystis sp. infection [24]. Helicobacter pylori infection and hepatitis B were also risk factors for Blastocystis sp. infection [25]. Drinking unboiled tap water, raising livestock and low immune function were the risk factors of Blastocystis sp. [26]. Female and body mass index < 19 were risk factors for intestinal parasite infection in patients with tuberculosis. Feeding poultry or livestock was important risk factor for Blastocystis sp. infection. Anemia and laboring barefoot in farmland were risk factors for intestinal helminth infection in patients with tuberculosis [27, 28].
There were 8 investigations of Blastocystis sp. infections among students, all of which published in Chinese, with prevalence ranging from 1.1–16.61%, 3 of which were tested for subtype. The highest prevalence of students was 16.61% in 2010 grade freshmen admitted to Guilin Medical University [29]. In 1999, the infection rate of a primary school pupil in Nantang Town, Gan County was 1.1% [30], but the infection rate of Blastocystis sp. in Grade 2002 of Gannan Medical College reached 10.09% [31]. The prevalence of Blastocystis sp. was 14.93% among 2008–2010 grades students in Guangxi Medical University [32]. The comparison between these studies shows that it may be that under the management of parents and teachers, primary school students have developed good hygiene habits such as diligent hand washing. However, there are still regional differences, the prevalence of Blastocystis sp. was 1.41% (6/426) in college students in Dali, Yunnan Province, and the infection subtype was ST1 [33]. A study in Guangxi found that infection subtypes include ST1, ST3, ST4, ST6 and ST7, of which ST3 is the main genotype (32.08%) [34].
There were 8 investigations about infection in children of Blastocystis sp., including 7 published in Chinese and 1 in English. However, there was no DNA sequence analysis performed. According to these studies, the prevalence was 0.4–35.9%. The highest prevalence of children with diarrhea was 35.9% in Jiangxi Provincial Children's Hospital [35]. Moreover, the prevalence of children with repeated abdominal pain was 35.4% in the First Affiliated Hospital of Gannan Medical University [36]. The prevalence of children with diarrhea was 3.1% and 8.9% in four sentinel hospitals in Kunming, Yunnan Province and the First Affiliated Hospital of Fujian Medical University, respectively [37, 38]. Cao et al performed fecal microscopy on children treated in the outpatient and inpatient departments of Shenzhen Children's Hospital, and found that the infection rate of Blastocystis sp. was 0.4% [39]. Children with diarrhea have a higher infection rate, but the infection rate in Jiangxi is the highest.
There were 11 investigations of Blastocystis sp. infection in inpatients or outpatients, all of which were published in Chinese with 1 sequence analysis. The prevalence was 3.7–36.6%. The highest prevalence in hospitalized patients was 36.6% [40]. The infection rate of patients in the First Affiliated Hospital of Guangxi Medical University is 22.78%, and the infection rate has a significant difference in gender [41]. The prevalence of some patients who attended the five hospitals were 16.27% and 16.77%, respectively in 2005 and 2013, in Nanning, Guangxi [42]. The infection rate of Blastocystis sp. in hospitals in Guangxi region was relatively high, and the infection rate has not changed much over time. It may be that the local population can see the doctor in time after experiencing symptoms such as diarrhea, resulting in a higher rate of infection of Blastocystis sp. in local hospitals. The subtypes of patients with Blastocystis sp. infection in two hospitals in Guangxi are ST1ཞST4, ST6 and ST7, of which ST2 is the main genotype (35.09%) [43]. The prevalence of hospitalized patients was 9.47% in Tengchong City, Yunnan Province [44]. From 2014 to 2017, among the samples sent to the National Institute of Parasitic Diseases for testing, the infection rate of Blastocystis sp. was 3.7% [45].
13 (12 published in Chinese and 1 in English) surveys of Blastocystis sp. in patients with diarrhea, one of which did sequence analysis, and the prevalence was 0.42–48.6%. The infection rate of patients with diarrhea in outpatient clinics of hospitals in Guangzhou was 48.6% (84/138), which was the highest infection rate of diarrhea patients. Those diagnosed with Blastocystis sp. infection mainly complained of abdominal pain or diarrhea, 19% of them were associated with colitis, and some showed symptoms of fatigue and anorexia [20]. The lowest prevalence of Blastocystis sp. of patients with diarrhea was 0.42% in Fuzhou area. Blastocystis sp. was one of the main causes of diarrhea in local children. The infection rate of Blastocystis sp. in patients with diarrhea is significantly different in seasons, the highest in summer [46]. Blastocystis sp. was one of the common pathogens in patients with diarrhea. The prevalence was 4.8% in patients with diarrhea over 5 years old in Kunming, Yunnan, and it was highest in summer [47]. But Zhang et al. found that the infection rate of Blastocystis sp. in spring was the highest, followed by summer. In the four sentinel hospitals in Kunming, Yunnan Province the prevalence of diarrhea patients was 4.2%, and the subtypes were ST1 and ST2, of which ST1 occupied the predominate subtype (97.9%) [48]. The infection rate of patients with diarrhea was 18.54% in Guangxi. Most of the patients with Blastocystis sp. have a history of drinking raw water or eating outside for a long time, and some of them have or are raising animals. It was found that the mixed infection of Blastocystis sp. and other parasites accounted for 31.87%, of which 73.8% were co-infected with liver flukes. And patients with mixed liver fluke infection have eaten sashimi [49].
There were 14 investigations of Blastocystis sp. combined with other diseases, 9 published in Chinese and 5 in English. The prevalence was 3.7–20.7%, of which 3 did DNA sequence analysis. These investigations included HIV patients, tuberculosis patients, cancer patients and chronic disease patients co-infection with Blastocystis sp.. There were 8, 2, 2 and 2 surveys respectively. The average infection rate was 12.30% [26, 50–56], 6.1% [27, 28], 13.1% [57, 58], 17.8% [59, 60], respectively. Among the co-infections, patients with chronic diseases had the highest rate of Blastocystis sp. infection, followed by cancer patients, and tuberculosis patients had the lowest. It has been found that the highest prevalence of Blastocystis sp. among HIV patients was 20.7% in Guangxi Province in 2015 [50]. The lowest prevalence was 3.7% among HIV seropositive individuals in Tengchong, Yunnan. The ST1, ST3, ST4, ST7, and ST12 subtypes were found, among which ST12 infection was first discovered in China. CD4 + cell count ≤ 500 cells/µl, and an HIV-RNA viral load ༞ 50 copies/ml were the influencing factors for Blastocystis sp. infection among HIV-seropositive individuals [26]. Tian et al suggested that HIV and Blastocystis sp. co-infection made IL-2 levels higher than those without HIV infection, altered the Th1/Th2 balance, and accelerated the transformation of HIV infection to AIDS [51]. The prevalence of HIV and Blastocystis sp. co-infection was 16.23% (49/302) in rural areas of China [52]. The prevalence of Blastocystis sp. among HIV positives was 19.57% in the suburbs of Fuyang City, Anhui Province [54]. The infection rate of Blastocystis sp. was 6.2% among tuberculosis patients without HIV infection. However, the infection rate was 7.6% in healthy group, there was no difference between the two groups [28]. The infection rate of Blastocystis sp. was 7.1% (27/381) in cancer patients. The subtypes ST1 and ST3 in cancer patients appeared the symptom of diarrhea, of which 66.7% (8/12) was ST1 and 40% (6/15) was ST3. Zhang and others detected Blastocystis sp. in cancer patients: lung, stomach, colorectal, liver, esophagus, breast and hematologic. The infection rate of Blastocystis sp. in the first three cancer patients was relatively high [57]. The infection rate of Blastocystis sp. was 18.29% in chronic disease patients. These patients have abdominal pain, diarrhea, vomiting and other symptoms. Low immune status and poor nutritional status of patients with chronic diseases are the factors that cause Blastocystis sp. infection [60].
3.3.3 Investigation on the distribution of Blastocystis sp. in different populations and animals
Surveys of Blastocystis sp. of human were conducted in 18 provinces, among which the survey in the general population was conducted in 15 provinces (Fig. 3). Calculate the average infection rate of each province, and the infection rate range is 0.82%ཞ21.59%. The highest prevalence was 21.59% in Guangxi Zhuang Autonomous Region, followed by Hunan Provinces (18.4%), Sichuan Province (8.01%), Yunnan Province (7.82%), Fujian Province (5.99%), Shaanxi Province (3.46%), and Zhejiang Province (the lowest prevalence of 0.82%). Investigation of Blastocystis sp. infection among students in 3 provinces of Guangxi, Jiangxi and Yunnan, among inpatients or outpatients in 3 provinces of Guangxi, Yunnan, and Shanghai, among children in 4 provinces of Shenzhen, Fujian, Jiangxi, and Yunnan, among patients with diarrhea in 8 provinces including Fujian and Anhui et al., and among patients with other diseases in 5 provinces including Yunnan and Anhui et al.
Investigations of Blastocystis sp. among animal have conducted on 12 provinces of China. Based on the number of positive animals and the number of inspections in each province, the rate of Blastocystis sp. infection was calculated from 2.54–79.41% (Fig. 4). The highest prevalence of animal was 79.41% in Guangzhou [71], and the lowest animal prevalence was 2.54% in Liaoning Province [62]. The prevalence of Blastocystis sp. of primates was 79.41% [71], 75.59% [61], and 19.33% [72] in Guangzhou, Shaanxi, and Guilin investigated, respectively. The infection of Blastocystis sp. in birds was investigated in three provinces, Sichuan, Heilongjiang and Shaanxi (Table 5). Surveys of mammal Blastocystis sp. have conducted in 10 provinces/cities including Qinghai and Shaanxi (Table 6). The lowest prevalence of the mammal was 2.54% in Liaoning and the highest was 61.74% in Shaanxi Province.