Distinct patterns of epidemiological characteristics and spatiotemporal distribution of HFMD were seen in Dali. Cases were highest in summer, among preschool children and in diaspora children. Areas showing the highest incidence rates were Yongping, Binchuan and Dali City. No global spatial autocorrelation was found but local spatial autocorrelation showed aggregation of cases on 4/1-7/31 and 10/1-12/31 and in Dali City, Yangbi Yi, Binchuan, Yongping, Yunlong, Nanjian, Weishan and Heqing county.
A year-on-year increase in incidence and positive test rates were seen in Dali between 2013 and 2021, reaching a peak in 2019, thereafter declining from 2020 to 2021. The mean annual incidence rate was 150.90/100000, lower than that of Yunnan Province in the same period. HFMD incidence in Dali may have declined due to the emergence of novel coronavirus 2019 (COVID-19) and the implementation of flow restriction policies, strengthening of health management and resource occupation.
Several pathogens were responsible for HFMD transmission in Dali with variations in dominant pathogens each year, consistent with the findings of George GM et al [16]. EV71 was predominant in 2013, 2014 and 2017, CA16 in 2015, 2016 and 2019 and other EVs in 2018 and 2021, in agreement with the study of Xu Y et al [17]. Inactivated EV71 vaccine was launched in Yunnan Province in 2016 and EV71 continued to be the dominant strain in Dali in 2017, contrary to the report of Huang L et al [18]. Vaccine coverage in Dali may have been poor, consistent with the study of He Zuo et al [19]. EV71 was seen to decline after 2018, perhaps due to the establishment of an immune barrier to this strain, causing decreased prevalence and allowing a different strain to become dominant, increasing HFMD incidence. Other EVs have gradually become dominant, a situation which requires careful monitoring.
Cases of HFMD in Dali were concentrated in the summer, consistent with other findings at home and abroad [20-22]. The HFMD season gradually changed from spring and summer to summer and autumn between 2013 and 2019. EV71 tends to predominate in spring, CA16 in summer and autumn and other EVs in winter. The variation in dominant pathogens with season may be related to climate, temperature, precipitation and relative humidity. Meteorological factors have previously been shown to influence HFMD incidence [23-25]. The onset season seen in 2020-2021 was different from that of previous years, consistent with the Japanese experience of HFMD [26]. HFMD incidence was concentrated in autumn and winter, perhaps due to COVID-19, and was related to population mobility, social economy and policy factors [27]. Therefore, limiting assembly of susceptible populations and implementing prevention and control policy measures may regulate the seasonal distribution of HFMD and reduce morbidity.
Incidence and positive tests were highest for children under 6 years old. Few cases were seen below the age of 1 with a peak at 1 year followed by a decline with increasing age. More boys were infected than girls, consistent with the findings of Flipo R et al [28] and showing a similar age distribution to that of HFMD in Korea [29]. Immunity conferred by breastfeeding accounts for the few cases seen in infants. Post-breastfeeding, preschool children accounted for the highest number of cases since self-birth immunity development is incomplete and personal hygiene lax. Numbers of cases among children of school-age declined year by year, reflecting increased immunity and personal hygiene awareness. Diaspora children, childcare children and students were the most susceptible populations. Outsourcing of nursery cleaners, absence of domestic helpers in the home and class sizes of more than 22 children have all been shown to be independent risk factors for HFMD [30]. Therefore, efforts should focus on prevention and control among preschool children, informing their guardians and the heads of childcare institutions, to improve awareness of HFMD prevention and strengthen health management.
The top three incidence rates were found in Yunlong, Binchuan and Dali City, in line with the report of Liu Yanjun et al [31]. The top three positive test rates were found in Xiangyun, Binchuan and Heqing counties. Each county showed a different composition of pathogens, with EV71 predominating in Nanjian, CA16 in Midu and other EVs in Yunlong. High population mobility, high economic level and rich medical resources all contribute to high incidence, diagnosis and reporting rates. Prevention and control measures are recommended for counties and districts with high incidence.
Global spatial autocorrelation analysis revealed a positive spatial correlation between HFMD incidence in Dali in 2017 but no spatial correlation for other years. HFMD thus appears to be randomly distributed in Dali, contrary to the findings of Li Jialin et al [32]. Geographical location may not be a decisive factor in the spatial distribution of HFMD. Local spatial autocorrelation analysis showed some correlation in areas adjacent to Dali from 2013 to 2019, consistent with the findings of other regions [33-34]. It is recommended to pay attention to the prevention and control of HFMD in adjacent areas. Significant spatiotemporal aggregation in Dali was shown by spatiotemporal scan analysis. Clusters were apparent in Dali City, Yangbi, Binchuan, Yongping, Yunlong, Nanjian, Weishan and Heqing and in the time spans of 4/1-7/31 and 10/1-12/31. Cluster area dynamics show changes with time, indicating the instability of onset cluster area over time. Focus on areas with high incidence and high aggregation of HFMD should not exclude scrutiny of non-high aggregation areas. Rational allocation of resources for timely detection and control of cluster outbreaks would improve prevention and control of HFMD.
Limitations
Data was obtained by passive surveillance which may lead to underestimation and bias in assessing HFMD incidence. Pathological testing data for 2020 was unavailable. Where other EVs were identified as dominant pathogens no further information was available to clarify the specific pathogen type. Causes of HFMD prevalence were not further tested or verified.