Asia was the EV71-associated-HFMD stricken area. In Southeast Asia, including Taiwan Province of China, several sub-genotypes of EV71 strains co-circulated in a same region. However, the mainland China showed entirely different epidemic patterns. As currently known, except for single cases, EV71 strains were all belonged to C4 sub-genotype. Rare non-C4 EV71 isolated from different years and areas were reported (11–13). We had reported the first imported B5 strains in Xiamen (isolated in 2009) that displayed a close genetic relationship with the 2006–2008 Taiwan strains and Singapore strains (15). Five years later, the second imported B5 strain which shared the highest nucleotide identity with Vietnam strains in 2011–2013 was reported in Chongqing (11). Two C2 strains were respectively isolated in Shandong (in 1998) and Beijing (in 2015) (12, 13).
For high sensitivity, detection reagents used in mainland China were usually developed for specifically detecting EV71 C4 strains. The prevalence and risk factors of non-C4 strains were not clear. What’s more, EV71 vaccine was designed based on C4 strain and cross protections against non-C4 EV71 strains was unknown. As very few researchers systematically concerned prevalence of non-C4 EV71 strains in mainland China, deep surveillance of non-C4 EV71 strains would provide important information. This study reported the prevalence of non-C4 strains through a 10-year epidemic study in Xiamen.
In this study, we firstly systematically reported non-C4 EV71 cases in a same region of mainland China. 32 non-C4 EV71 strains, covering B3, B5, C2, C3 and India orphan strain, had been identified. The non-C4 strains accounted for 2.12% (32/1512, far greater than other districts) of EV71 strains. The non-C4 strains had closed relationships with South-east Asia strains. There might be two reasons. On one hand, a deep, comprehensive and long-term surveillance system for pathogen spectrum of HFMD had been performed in Xiamen for 10 years. As all sub-genotypes of HEV could be identified under our system, non-C4 EV71 strains wouldn’t be ignored. On another hand, as Xiamen city is an important tourist city and a key transportation hub between mainland China and Southeast Asia, Xiamen may stands at the forefront of EV71 epidemic changes. Epidemic patterns of HEV in Xiamen might be affected by imported HEV strains from Southeast Asia. A study in Taiwan showed large epidemics occurred just after minorities of strains were identified a few years ago. This suggested that large epidemics caused by non-C4 EV71 strains can occur at any time and enough attention still should be given.
This study provided new understanding of EV71 epidemic patterns in mainland China, and also provides important information for formulating EV71 prevention and control strategies, and developing vaccine and diagnostic reagents.