To the Editor-in-Chief
Schistosomiasis is a chronic, debilitating parasitic disease of great public health importance, caused by trematode blood flukes of the genus Schistosoma. Three species of schistosomes commonly affect humans, including Schistosoma mansoni, Schistosoma hematobium, and Schistosoma japonicum.[1, 2] The transmission of this neglected tropical disease is determined by the existence and geographic distribution of the intermediate host snails. Thus, schistosomal flukes are considered snail species-specific, and frequently geographically specific, distributed throughout much of the tropics and subtropics. Schistosoma mansoni is endemic to tropical Africa and parts of the Near East, northeastern South America, and the eastern Caribbean islands. Schistosoma hematobium is found in Africa and the Middle East, and S. japonicum is found exclusively in Asia, where it is endemic in China, Indonesia, and the Philippines.[1, 2]
In China, only S. japonicum is endemic. However, more and more imported cases infected with African schistosomes (including S. mansoni and S. hematobium) are reported owing to the sharp growth in China-aided projects in Africa and labor services exported to Africa. Since 1979, a total of 147 cases infected with S. mansoni and 283 cases infected with S. hematobium have been reported until now (Figure 1), including the two S. hematobium-infected workers returning from Tanzania and Angola published recently in the Journal of Travel Medicine. It is estimated that over 1 million Chinese workers are now residing in Africa, who have a high risk of exposure to African schistosomiasis. As described by Drs Lim and Wang,[4, 6] many of the infected individuals are asymptomatic or had minor symptoms, who seldom seek care and have a high rate of missed diagnosis and misdiagnosis due to the lack of knowledge on African schistosomiasis; therefore, it is considered that the actual number of the Chinese infected with African schistosomes may be greatly underestimated.
In 1974, Biomphalaria straminea, a snail intermediate host of S. mansoni, was first discovered as an invasive snail species in a stream in Hong Kong, and this snail species was first found in some ponds, ditches, and rivers of Shenzhen city, southern China in 1981. A further survey in 1983 showed the wide distribution of B. straminea in Shenzhen river systems and demonstrated that the snails were introduced into Shenzhen from Hong Kong via water, which proves that B. straminea is able to survive, reproduce, and form new populations naturally in southern China such as Hong Kong and Shenzhen, and it can spread along the river systems. A recent epidemiology survey revealed that B. straminea as a predominant snail population had widely spread in Shenzhen city, and many snail habitats had been observed. Once the infected cases, as sources of infection of schistosomiasis, are imported to regions where the snail intermediate hosts of African schistosome are present, there is a high likelihood of transmission of African schistosomiasis in China.
In addition to the comprehensive health education suggested by Drs Lim and Wang,[4, 6] we hereby propose some interventions and research needs to tackle the risk of transmission of African schistosomiasis, which is likely to be a new emerging public health problem in China. First, a systematic evaluation of the actual schistosome infections in returners from African countries is required under the organization of health sections, entry-exit inspection and quarantine sections, and commerce sections. Second, a systematic investigation of the intermediate host snails including Biomphalaria spp. and Bulinus spp. should be performed in southern China, and determination of the infectivity of the water body is also needed. Snail control interventions should be implemented in snail habitats to eliminate the snail intermediate hosts. Third, the susceptibility of B. straminea detected in southern China to S. mansoni, the development of S. mansoni within the B. straminea, and the susceptibility of the mature cercaria released from B. straminea to definitive hosts, should be evaluated in a laboratory. Finally, the detection and monitoring of praziquantel resistance has to be enhanced in imported cases of African schistosomiasis because there are subjects infected with African schistsomes returning from Africa in whom standard praziquantel treatment failed to clear the infections.[12-15]
The growth in the bilateral trade between China and African countries will result in an increase in the number of people moving to Africa. In the context of existence of the intermediate host snails in China, it is considered that the continuous growth in imported cases will certainly increase the risk of transmission of African schistosomiasis in China. A surveillance-response system is needed to monitor and tackle the risk.