In our retrospective analysis of human CE cases, the number was observed to be doubled in the recent ten years (Figure 1). However, the true reason for the increasing number of sporadic human CE cases is not clear. Dogs, pigs and sheep have been confirmed previously to be infected with E. granulosus s.l. in Heilongjiang Province [7,8]. The ecological environment might be a main factor. In the recent decade, the national and international travels and livestock trades have been increasing as well as the number of imported dogs because of the pet raising. Thus, we can not rule out the possibility that some CE patients were infected in a known endemic region of China or other countries.
In our analysis, CE patients were observed to reach a peak in the age group of 41–50 years. It was in agreement with the result in Turkey, where most of the cases appeared in 41–50 (22.68%) years . A similar result has been reported in Tunisia that most of the CE cases were aged 30–44 years, followed by the second age-group ranging from 45–59 years . In general, the peaks of human CE cases were not identical to one another in different studies, however, the prevalence of E. granulusos infection generally increased in age. The results may be associated with the fact that E. granulosus infections remain silent for years before the enlarging cysts cause symptoms in the affected organs although causative infection may often be acquired early in life.
In the analysis of gender distribution of CE patients in the present study, the females had a higher occurrence of CE patients (60.66%, 111/183) than males (39.34%, 72/183). 58.6% female CE patients were from the two age groups of 31–40 and 41–50 years. Similar results were observed in case analyses of human CE in Tunisia, Jordan, Iran and China [2,10-12]. The phenomenon may be related to the fact that adult females expose to the E. granulosus-infected dogs and the contaminated environment more frequently than adult males due to home activities, such as feeding dogs and milking livestock. However, analyses of surgical cases of human CE in Kyrgystan and Turkey drew an opposite conclusion, and Torgerson et al. pointed out that men were more likely to seek treatment once clinical signs presented based on the fact of no gender bias in CE cases detected in the ultrasound study [9,13]. In Jordan, males younger than 15 years of age showed significantly higher surgical incidence of CE cases than females of comparable age; whereas the number of female cases of different age groups over 15 years of age was consistently higher than that of males . The reason for distribution difference of CE in age and gender is unclear. It may be related to the virulence of different genotypes of E. granulosus, the infective dose of the parasite and health status of hosts.
In our analysis, 45.36% (83/183) of CE patients were farmers. In the rural areas of Heilongjiang Province, with the development of modern agriculture, especially in recent years, some farmers were liberated from their complex manual work and began to keep the economic animals, such as cattle, sheep and pigs. Meanwhile, domestic dogs are kept in large numbers by local farmers to guard property and livestock. The presence of dogs and sheep in the same area provides the possibility to complete the life cycle of E. granulosus. In the epidemiology of human CE, dogs are generally considered to be the major definitive host transmitting E. granulosus to humans. The poor hygiene practices also increase the opportunity of dogs and sheep infected with the parasite, respectively through the feeding of infected offal from sheep and eating food and drinking water contaminated by eggs from dogs. The farmers have opportunity to be infected with the eggs of E. granulosus by farming activities involving livestock and dogs, and/or by home slaughtering practices. Meanwhile, local fur industry might lead to CE infection of workers. In the present analysis, the number of CE cases was found to be larger in countrysides than in cities. This might result from the different living environments of urban and rural inhabitants.
Hydatid cysts of E. granulosus are the most common in livers and lungs although they can be found in any part of the human body. In the present analysis, the livers were also found to be the most affected organ (95.08%, 174/183). The result was similar to previous findings that the majority of cases presented as isolated liver disease [14-16]. However, with an exception in South Africa, the lungs were the most commonly affected organs, accounting for 11 of the 14 CE patients having pulmonary CE alone and the remaining three having pulmonary and abdominal CE; meanwhile, it was pointed out that the strain of E. granulosus causing disease, genetic predisposition or co-infections with HIV and/or TB might play a role . The report of more pulmonary cases in children might be attributable to a lower tolerance of hydatid cyst development in pulmonary tissue in children, and thus they were more likely to present for treatment .
Variability in measurements of cyst size has been documented in human CE cases. Our retrospective analysis revealed that the mean diameters were 7.4 cm, 5.2 cm, 13.5 cm and 4.9 cm for the hepatic, pulmonary, splenic and cerebral cysts, respectively. The largest cyst was from a case of hepatic CE with 30 cm in diameter. In Tunisia, the mean diameter of the hepatic cysts was 8.3 cm . In Libya, the diameter of the hepatic cyst was high up to 40 cm . Intra-abdominal CE cyst localization was found to be related to bigger cyst size by analyzing clinical data from 91 CE patients in Turkey . However, a study of CE in slaughtered animals showed that the cysts in the lungs was larger than those in the livers . So far, there has been no reasonable explanation about why CE cysts appeared markedly different in size. Whether they are related to the affected organs is unclear besides the living time of cysts in hosts.
In our analysis, occurrences of CE patients with a single cyst and multiple cysts were 75.96% (139/183) and 24.04% (44/183), respectively. Similarly, in Tunisia, 70.7% (29/41) of hepatic CE patients had a single cyst, and 29.3% (12/41) had multiple cysts ; in Kyrgystan, a single cyst was found in 83.57% (529/633) of CE patients while multiple cysts in 16.43% (104/633) . In general, CE patients with a single cyst are more than those with multiple cysts.
The coincidence rates of the three methods were tried to compare against the results of histopathology in 119 hepatic CE cases. CT showed the largest coincidence rate (96.64%) than ultrasound and ELSIA for hepatic CE patients. Two previous studies described that a positive diagnosis was obtained before operation in 97.11% (34/35) and 90% (27/30) hepatic CE patients based on Ct and ultrasound, respectively [21,22]. The results above might be related to the fact that CT gives more accurate description of the cyst characteristics compared to ultrasound as well as the cyst location. Suwan found that sonography was superior to CT in the characterization of cyst content, but CT was superior to sonography in detecting gas within the cysts and minute calcifications [23,24]. Ultrasound is gradually used in the epidemiology of human CE because of its convenience and it is often combined with serological test to diagnose CE. In Libya, 69% (233/339) out of the ultrasound-positive CE cases were antibody seropositive while 11.2% (12/106) out of ultrasound-negative people were antibody seropositive . It is known that surgical operation of CE is different from that of general cysts. The spillage of viable protoscoleces during the surgery can lead to post-operative reoccurrence of CE while the release of antigenic hydatid fluid into bloodstream can cause anaphylactic shock and even death. Thus, it is important to improve the accurate diagnosis rate before operation by the combined use of different methods in the diagnosis of human CE.