Impact of climate variability on Plasmodium vivax and Plasmodium falciparum malaria in the high risk area of Yunnan Province, China

Abstract

Background: Malaria remains a public health problem in the remote and poor area of Yunnan Province, China. Yunnan faces an increasing risk of imported malaria infections from Mekong-River neighboring countries. Aims: This study aimed to identify the high risk area of malaria transmission in Yunnan Province, and to estimate the effects of climatic variability on the transmission of Plasmodium vivax and Plasmodium falciparum in the identified area. Methods: We identified spatial clusters of malaria cases using spatial cluster analysis at a county level in Yunnan Province, during 2005-2010, and estimated the weekly effects of climatic factors on P.vivax and P.falciparum based on a dataset of daily malaria cases and climatic variables. A distributed lag nonlinear model (DLNM) was used to estimate the impact of temperature, relative humidity and rainfall on both types of malaria parasite up to 10-week lags. Results: The results show that the primary cluster area was identified in western Yunnan along China-Myanmar border. The relative risk (RR) in malaria caseswith a 1 °C increase in minimum temperature was statistically significant from lag 4 to lag 9 weeks with the highest RRs of 1.03 (95% CI, 1.01, 1.05) for P.vivax at a 7 week lag and1.07 (95% CI, 1.04, 1.11) for P.falciparum at a 6-week lag; the RR with a 10-mm increment in rainfall was statistically significant from lag 2 to lag 4 weeks and from lag 9 to lag 10 weeks with the highest RRs of1.03 (95% CI, 1.01, 1.05) forP.vivax at a 2-week lag and 1.04 (95% CI, 1.01, 1.06) for P.falciparum at a lag of 2 weeks; and the RR with a 10% rise in relative humidity was statistically significant from lag 3 to lag 8 weeks with the highest RRs of 1.24 (95% CI: 1.10, 1.41) for P.vivax at a 5-week lag. Conclusions: Our findings suggest that the China-Myanmar border is a high risk area for malaria transmission. Climatic factors appeared to be major determinants for malaria transmission in this area. The estimated lag effects for these factors and malaria are consistent with the life cycle of malaria parasite.