Natural human Plasmodium infections in major Anopheles mosquitoes in western Thailand

Abstract

Background: The Thai-Myanmar border is a remaining hotspot for malaria transmission. Malaria transmission in this region continues year-round, with a major peak season in July-August, and a minor peak in October-November. Malaria elimination requires better knowledge of the mosquito community structure, dynamics and vectorial status to support effective vector control.

Methods: Adult Anopheles mosquitoes were collected using CDC light traps and cow bait in 7 villages along the Thai-Myanmar border in January 2011 - March 2013. Mosquitoes were determined to species by morphological characters. Plasmodium-positivity was determined by circumsporozoite protein ELISA.

Results: The 2986 Anopheles mosquitoes collected were assigned to 26 species, with Anopheles minimus sensu lato (s.l.) (40.32%), An. maculatus s.l. (21.43%), An. annularis s.l. (14.43%), An. kochi (5.39%), An. tessellatus (5.26%), and An. barbirostris s.l. (3.52%) being the top six most abundant species. Plasmodium-infected mosquitoes were found in 22 positive samples from 2906 pooled samples of abdomens and heads/thoraxes. Four mosquito species were found infected with Plasmodium: An. minimus s.l., An. maculatus s.l., An. annularis s.l. and An. barbirostris s.l. The infectivity rates of these mosquitoes were 0.76, 0.37, 0.72, and 1.74%, respectively. Consistent with a change in malaria epidemiology to the predominance of P. vivax in this area, 20 of the 22 infected mosquito samples were P. vivax-positive. The four potential vector species all displayed apparent seasonality in relative abundance. While An. minimus s.l. was collected through the entire year, its abundance peaked in the season immediately after the wet season. In comparison, An. maculatus s.l. numbers showed a major peak during the wet season. The two potential vector species, An. annularis s.l. and An. barbirostris s.l., both showed peak abundance during the transition from wet to dry season. Moreover, An. minimus s.l. was more abundant in indoor collections, whereas An. annularis s.l. and An. barbirostris s.l. were more abundant in outdoor collections, suggesting their potential role in outdoor malaria transmission.

Conclusions: This survey confirmed the major vector status of An. minimus s.l. and An. maculatus s.l. and identified An. annularis s.l. and An. barbirostris s.l. as additional vectors with potential importance in malaria transmission after the wet season.

Fig. 1

Mosquito collection sites. The seven villages Mae Usu (MU), Tae Nu Ko (TN), Mae Plu (MP), Tha Song Yang (TS), Suan Oi (SO), Tala Oka (TO) and Nong Bua (NB) are along the Thai-Myanmar border. Pie charts represent the abundance of 9 mosquito genera collected in each village

Fig. 2

Abundance of four major Anopheles mosquitoes. a Mosquitoes collected in indoor traps. b Mosquitoes collected in outdoor traps. c Relative abundance of mosquitoes collected in indoor and outdoor traps during the hot, wet, and dry seasons. Absolute numbers of collected mosquitoes were shown on top of each bar. Shading represents rainfall (mm.) during 2012–2013

Fig. 3

Illustration of monthly cumulative abundance of four major anopheline species (An. minimus s.l., An. maculatus s.l., An. annularis s.l., and An. barbirostris s.l.), Plasmodium positivity, and malaria incidence in the 7 villages. Shading represents rainfall (mm.) during March 2012–March 2013. Star, circle, triangle, and arrow head indicate the Plasmodium-positive mosquitoes of the respective species