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During 2016–2019, Aedes surveillance was carried out in approximately 436 counties and regions from 23 PLADs in China every year.
When comparing the BI among the Class I PLADs, similar seasonal fluctuations of Aedes larvae were observed in all PLADs from 2016–2019. In Yunnan, the BI exceeded 5 from the second half of June to the first half of October in 2016 and 2019. For Hainan, the BI exceeded 5 in all years and the BI were between 5 and 20 sporadically throughout the year. Concerning Fujian, the BI exceeded 5 from the first half of March to the second half of November and were higher than 10 from the second half of April to second half of September in 2017 and 2019. For Guangxi, the BI were higher than 5 from the second half of May to the first half of October in 2019. In Zhejiang, the BI exceeded 5 from the second half of April to the first half of November in all surveillance years.
However, all BI in Guangdong did not exceed 5 from 2016 to 2019. The risk indicated by BI surveillance was inconsistent with the actual dengue outbreak in Guangdong (Figure 1).
Excluding May and October in Shanghai and May, June, and October in Jiangsu, the BI in all Class II PLADs during the study months from 2016–2019 were higher than 5. Except Shanghai and Jiangsu, all other PLADs were at the risk of dengue outbreak or epidemic (Table 1).
Province Month BI Risk range* 2016 2017 2018 2019 Shanghai May 2.62 4.51 2.92 2.81 No risk June 4.95 5.65 6.01 3.93 No–Low risk July 6.18 7.58 7.32 6.38 Low risk August 5.49 7.76 7.12 4.97 No–Low risk September 4.66 6.79 6.09 3.93 No–Low risk October 3.23 4.96 2.47 2.45 No risk Chongqing May 6.25 5.99 7.44 5.78 Low risk June 11.10 9.45 12.30 9.23 Low–Medium risk July 10.34 12.33 11.64 10.68 Medium risk August 9.80 9.45 9.40 8.39 Low risk September 7.73 8.54 8.85 13.55 Low–Medium risk October 5.95 5.09 5.05 8.35 Low risk Jiangsu May 2.40 1.27 3.07 2.50 No risk June 4.86 3.43 3.45 2.92 No risk July 5.99 4.66 3.07 4.92 No–Low risk August 7.13 4.74 3.64 4.07 No–Low risk September 4.78 5.00 2.89 3.47 No–Low risk October 3.99 1.94 1.21 1.68 No risk Anhui May 3.82 5.11 7.48 4.75 No–Low risk June 9.06 8.31 11.27 7.59 Low–Medium risk July 14.60 14.99 14.24 12.06 Medium risk August 11.96 18.14 14.75 12.24 Medium risk September 9.57 16.69 11.67 11.69 Low–Medium risk October 5.70 6.85 6.63 5.00 Low risk Jiangxi May 14.10 0.95 17.50 15.38 No–Low risk June 23.60 15.90 13.56 14.34 Medium–High risk July 12.48 7.90 11.00 11.38 Low–Medium risk August 12.67 9.62 7.55 10.56 Low–Medium risk September 8.76 8.89 8.00 7.69 Low risk October 15.33 4.63 6.70 2.86 No–Medium risk Henan May 10.10 14.70 10.50 4.69 No–Medium risk June 21.39 17.36 16.06 11.57 Medium–High risk July 31.88 26.22 24.90 22.92 High risk August 32.40 31.91 23.01 26.38 High risk September 32.07 23.62 13.49 18.91 Medium–High risk October 14.41 13.09 9.50 9.68 Low–Medium risk Hubei May 10.90 7.11 7.06 6.21 Low–Medium risk June 13.42 10.83 7.83 8.08 Low–Medium risk July 19.83 13.54 8.22 8.03 Low–Medium risk August 13.76 9.85 5.88 10.22 Low–Medium risk September 8.64 17.98 7.54 3.99 No–Medium risk October 7.22 5.87 3.53 7.14 No–Low risk Hunan May 17.90 6.90 13.93 12.60 Low–Medium risk June 17.90 31.20 13.83 26.47 Medium–High risk July 9.65 12.20 13.09 26.53 Medium–High risk August 10.95 4.14 13.23 16.48 No–Medium risk September 10.95 12.61 11.53 13.91 Medium risk October 3.44 5.55 9.78 4.98 No–Low risk Sichuan May 4.90 10.15 12.24 9.14 No–Medium risk June 7.60 15.39 11.95 12.89 Low–Medium risk July 8.33 18.23 15.96 28.31 Low–High risk August 13.36 12.62 14.02 11.64 Medium risk September 7.56 6.87 7.75 11.48 Low–Medium risk October 8.98 3.26 3.78 8.57 No–Low risk * No risk (BI<5): dengue transmission negated; Low risk (5≤BI<10): sporadic dengue occurrence; Medium risk (10≤BI<20): dengue outbreak risk; High risk (BI≥20): dengue epidemic risk. Table 1. Breteau index (BI) in class II areas in China (2016–2019).
As for the BI in Class III PLADs, the BI exceeded 5 in Hebei while it did not exceed 5 in Liaoning and Tianjin during the period of surveillance from 2016 to 2019. The BI exceeded 5 in most months in Shandong and Shaanxi and for a few months in Shanxi (Figure 2).
The MOI was another key index that was utilized by some PLADs for the surveillance of Aedes larvae. This indicator was more sensitive than BI in some PLADs, especially in Guangdong. The MOI exceeded 5 from the first half of May to the first half of October in Guangdong and from the second half of May to the second half of September in Guangxi for almost all study years (Figure 3).
Figure 3.MOI of Guangdong and Guangxi in Class I areas of China (2016–2019). BI=breteau index; MOI=mosq-ovitrap index.
For Guizhou, the MOI exceeded 5 from June to September in all studied years and exceeded 5 in May and October in 2016. In Hunan, the MOI exceeded 5 from June to September in most study years. Beijing, as a representative of Class III area, had a relatively high MOI in August and was at the risk level between dengue transmission to epidemic risk, especially from 2017 to 2019 (Table 2).
Class Province Month MOI Risk level* 2016 2017 2018 2019 II Guizhou May 5.29 4.61 4.71 4.13 No–Low risk June 7.81 7.59 6.73 5.45 Low risk July 8.87 7.43 9.28 8.11 Low risk August 11.03 5.75 9.01 10.08 Low–Medium risk September 9.05 5.42 5.84 7.86 Low risk October 5.74 4.09 3.96 4.34 No–Low risk Hunan May 3.12 1.35 1.44 0.78 No June 9.83 3.77 2.28 5.11 No–Low risk July 8.89 5.59 5.18 6.04 Low risk August 6.49 5.18 3.40 6.06 No–Low risk September 6.49 6.72 3.81 4.49 No–Low risk October 3.52 3.76 0.86 1.58 No III Beijing June 3.74 2.34 1.27 1.06 No July 3.57 7.45 3.99 3.05 No August 4.87 10.76 8.71 9.30 No–Medium risk September 5.87 3.69 4.86 1.16 No–Low risk * No risk (MOI<5), dengue transmission negated; Low risk (5≤MOI<10), sporadic dengue occurrence; Medium risk (10≤MOI<20), dengue outbreak risk; High risk (MOI≥20), dengue epidemic risk. Table 2. Mosq-ovitrap index (MOI) in class II and III areas in China (2016–2019).
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