1. Introduction
Cystic echinococcosis (CE) is one of the most important zoonoses [
1], causing a significant public-health concern and huge economic losses worldwide [
2,
3]. In particular, this disease causes high morbidity and mortality in Xinjiang, China [
4]. Xinjiang is the major endemic area of the disease, which is caused by
Echinococcus granulosus sensu stricto (
s.
s.) [
5]. The Altai region is a key CE epidemic area in Xinjiang, China. Since “Control CE Action Plan (2010–2015)” [
6], CE prevalence among livestock in the Altai region has been rarely reported. Molecular genetic studies, based mainly on partial sequencing of mitochondrial DNA (mtDNA), elucidate the extent of strain variation, resulting in the ‘genotype’ nominature of
E. granulosus s.s. intraspecial variation [
7,
8]. Eight Genotypes of
E. granulosus sensu lato (
s.
l.) have been identified in different geographical areas using mtDNA sequences [
1,
9]. According to “International consensus on terminology to be used in the field of echinococcoses” published in the journal
Parasite in 2020, the new classification was
E. granulosus s.
s. (G1 and G3) [
10,
11],
E. equinus (G4),
E. ortleppi (G5) [
11,
12],
E. canadesis (G6/7 and G8/10) [
13,
14,
15,
16], and
E. felidis (lion strain). The G2 genotype is not a separate strain or even a monophyletic cluster but belongs to the G3 genotype [
17], while the G9 genotype is no longer recognized as a distinct strain, as it is probably a microvariant of the G7 genotype [
10]. The G1 genotype from sheep appears to be the dominant strain occurring in Xinjiang [
6,
18].
The Altai region is located in the northwest Xinjiang Uygur Autonomous Region, which is connected to Kazakhstan and Russia in the northwest and bordered by Mongolia in the northeast. It is an important basis of grassland animal husbandry in Xinjiang and it covers 147.47 million hectares of natural grassland. It is also one of the most important hydatid epidemic areas in Xinjiang. In this area, there are many cattle and sheep in the Altai region. Sheep accounted for more than 60% of the total livestock and cattle are 19% of the total. The disease infection rate of sheep, cattle, camels, and horses has been reported to be 9.8%, 8.4%, 6.8%, and 4.3%, respectively [
5]. Guo et al. found that 3.5% of the sheep and 4.1% of cattle were infected with CE.
Altai city, Habahe county, Fuhai county, and Buerjin county in the Altai region have a large slaughter volume; these four counties were the main investigation points in this study. We focused on the slaughterhouses of the four areas receiving large numbers of livestock to investigate the prevalence and genetic characteristics of
E. granulosus s.
s. strain. This information will improve our understanding of CE in the region and will determine the relative contribution of each livestock species to the distribution and transmission of poliovirus. The study assessed the prevalence of
E. granulosus s.
s. in humans in the Altai region, with infection rates ranging from 0.3% to 3% [
19]. However, the genetic variation in
E. granulosus s.
s. in the Altai region remains unclear. This paper investigates the epidemiology of
E. granulosus s.
s. and identifies the genetic variation in
E. granulosus s.
s. in the Altai area of Xinjiang. The main purpose of this study was to investigate the infection rate of domestic animal cystic echinococcosis in Altay area and to analyze the genotype of the infection. The results of the present study provide a baseline survey for further molecular anti-parasite approaches and for implementing effective local control plans in Altai. We proposed further molecular and biological studies to determine the occurrence of other genotypes/strains of
E. granulosus s.s. in livestock and humans in order to confirm the exact source of this zoonotic infection in the Altai region, Xinjiang.
3. Discussion
The study of interspecific genetic variation in
E. granulosus s.l. species has made an important contribution to the study of the epidemiology, geographic distribution, and phylogeny of
E. granulosus. Since earlier studies in the 1990s were conducted, it was not clear that there was a certain degree of variability within
E. granulosus s.l. Subsequent studies on the genetic variability in isolates causing CE have clarified the taxonomy of the parasite, grouping a number of species under the complex
E. granulosus s.l. The methods described by Nakao et al. [
29] and Yanagida et al. [
26] were used to analyze
E. granulosus s.s., allowing us to produce data sets comparable to other sequences from different geographic regions. In total, the slaughterhouses in the four counties of the Altai region are relatively large, so they were selected for study.
We studied the prevalence of CE in sheep (4.52%) and cattle (4.84%) in the Altai region of Xinjiang. The prevalence reported is within the range reported for Xinjiang in previous studies. These studies include the work of Meng et al. [
5] (10.7% in sheep, 7.4% in cattle) and Guo et al. [
6] (4.6% in sheep and 4.5% in cattle). The present study examined 2898 sheep and 703 cattle and the data suggest an endemic steady state with a reduced infection pressure in these hosts. This conclusion is consistent with the results of Guo et al. [
6], who reached the same conclusion for CE prevalence in northern Xinjiang. Four selected regions had infection rates ranging from 2.58% to 5.84% in sheep and 2.78% to 6.02% in cattle, with no difference between regions. The prevalence of sheep was 4.52%, which was lower than a previous sheep study in Xinjiang (9.8%) by Meng et al. [
5]. The sheep surveyed in this study were mostly less than 10 months old and, similar to a previous study, the infection rates increased as the sheep aged [
5]. Since cattle are generally slaughtered after 2 years of feeding, the growth period of cattle is longer than that of sheep, which leads to a higher infection rate of cattle than that of sheep. Due to the insufficient positive sample size, the statistical difference is not significant. Through the implementation of the national prevention and control of echinococcosis (2010–2015) [
30], infection from livestock in these places has also decreased somewhat. Earlier studies reported somewhat greater infection rates compared to the present study. The difference in infection rates may be attributed to sample size, unhygienic measures used at the slaughterhouse, and abiotic climatic factors that favored the onset of infection. The prevalence of parasites in slaughtered sheep and cattle is a feasible and economical way to assess the infection rates of the four regions. Although this sampling technique has the limitation that it is not a population-based random sampling survey and is unable to cover all slaughtered animals at the sites investigated, the advantages of a survey on slaughtered sheep and cattle are apparent.
We looked at the teeth of each animal to estimate its age and divided each species into groups based on age. The infection rate is generally higher in older livestock compared to younger individuals. The higher prevalence of old age may be mainly due to age-related diseases and the chronic nature of the disease [
31]. The age variation can be associated with differences in exposure to infection because aged animals may have been exposed to more infective periods. Similar trends in the prevalence of CE infection in sheep and cattle clearly indicate a higher risk of disease in older livestock compared to younger ones [
29,
32].
Here, it can be seen from the results that, first of all, females are usually numerically more represented than males in ultrasound screening, which increases the possibility of gender selection bias in sampling [
33], which is related to the small sample size. The relationship between sex and echinococcosis has been studied in many studies [
34,
35,
36], but inconclusively. Secondly, our study was similar to the result of Khan et al. [
36]. People in the 21–50 age group were the main labor force and had a higher probability of exposure to pathogens than other groups, and the risk of infection was correspondingly higher. In addition, the samples in this study were all from hepatic echinococcosis samples, without pulmonary echinococcosis samples. Finally, the relationship between ethnic groups and echinococcosis was not described here due to the limited sample size.
The overall results of this study clearly indicate that overall nucleotide diversity and haplotype diversity are low. Tajima’s D was negative in all analysis sequences, indicating Altai population expansion and/or purification selection. In the
cox1 sequence comparison, the negative Fu’s F value was significantly higher, indicating the presence of a rare haplotype, which could explain the expansion or hitchhiking of the
E.
granulosus s.s. population. The Fu’s F test was developed based on the distribution of haplotypes or alleles. A slight deviation from neutrality was recorded in the Fuhai genotypes, with low negative values but no significant deviation from neutrality was recorded in the Habahe and Buernjin genotypes (
Table 4). There were significant differences between Altai and Fuhai, but no significant differences between Habahe and Buernjin, indicating that the genotypes of Altai and Fuhai are in a state of expansion that is greater than that of Habahe and Buernjin. Fst values do not support the differentiation in the subpopulation of
E. granulosus s.s. in the Altai region (
Table 5,
Table 6 and
Table 7). After multiple investigations of genetic variation in subpopulations (
Table 5,
Table 6 and
Table 7) of
E. granulosus s.s., values remained low in variation and actually differed in pathogenicity, biological characteristics, and host responses.
In this study, nine Haps of E. granulosus s.s. were identified in human CE cases, while five Haps were present only in humans. Nevertheless, the cox1 gene of E. granulosus s.s. shows considerable genetic variation, and the Fst index of cox1 gene from humans is obviously higher than that from livestock. That study indicated that the E. granulosus human subpopulations are distinct from the subpopulations of other hosts. The statistically distinct Fst values of the human E. granulosus isolates as compared to livestock indicate the presence of surplus variable alleles, further indicating the limited gene flow, which probably suggests that human Haps may come from sheep and cattle, which may be an infection from a definitive host infection with the same Haps.
G1 was the most common genotype found. G1 genotypes have a greater worldwide distribution than the G3 genotype [
37], while the G3 genotype was the least common. In this study, the G1 genotype has 17 Haps and 166 sequences, suggesting a high rate of G1 variation. This is the first documentation that the two strains occur simultaneously in this area. Earlier studies reported that G1 and G3 genotypes of
E. granulosus s.
s. infect humans and livestock in Xinjiang [
6]. However, we found two strains (G1 and G3) in the Altai region. The G1 genotype is the predominant epidemic
E. granulosus s.
s., which is similar to previous results [
6,
24,
38] and is also responsible for the majority of human and livestock infections in many countries, such as China, Iran, Brazil, Italy, and Turkey [
9]. All sequences in the present study, designated as Haps (Hap1–Hap22), form a clear clade together with the G1 and G3 genotypes expressing
E. granulosus s.
s. strains in the G1 and G3 complex. These results provide support for considering the G1 and G3 complex as a discrete species [
39]. Both known strains (G1 and G3) of
E. granulosus s.
s. were found in four regions, indicating that G1 and G3 were widely distributed and had no regional differences. Hap3 (65.00%), detected in 117 isolates, was the major haplotype and showed complete sequence homology with previous reports from China [
6], Turkey [
40], Argentina [
27], Australia [
41], Iran [
26], and Kyrgyzstan [
22].
G1 was the most common genotype found. G1 genotypes have a greater worldwide distribution than the G3 genotype [
37], while the G3 genotype was the least common. In this study, the G1 genotype has 17 Haps and 166 sequences, suggesting a high rate of G1 variation. This is the first documentation that the two strains occur simultaneously in this area. Earlier studies reported that G1 and G3 genotypes of
E. granulosus s.
s. infect humans and livestock in Xinjiang [
6]. However, we found two strains (G1 and G3) in the Altai region. The G1 genotype is the predominant epidemic
E. granulosus s.
s., which is similar to previous results [
6,
24,
38] and is also responsible for the majority of human and livestock infections in many countries, such as China, Iran, Brazil, Italy, and Turkey [
9]. All sequences in the present study, designated as Haps (Hap1–Hap22), form a clear clade together with the G1 and G3 genotypes expressing
E. granulosus s.
s. strains in the G1 and G3 complex. These results provide support for considering the G1 and G3 complex as a discrete species [
39]. Both known strains (G1 and G3) of
E. granulosus s.
s. were found in four regions, indicating that G1 and G3 were widely distributed and had no regional differences. Hap3 (65.00%), detected in 117 isolates, was the major haplotype and showed complete sequence homology with previous reports from China [
6], Turkey [
40], Argentina [
27], Australia [
41], Iran [
26], and Kyrgyzstan [
22].
The study could have epidemiological significance and it could have a direct impact on public health in the Altai region. Because of scarce data about E. granulosus s.s. in Altai, genotype identification should be the first step for controlling CE infections in this endemic area. This research could increase information about the global cycle of different genotypes of E. granulosus s.s. The confirmation of (G1–G3) complex of E. granulosus s.s. in livestock and humans in the Altai of Xinjiang reinforces the need for greater attention to the essential genetic approach for assessing all species and variants associated with echinococcosis.