New insights into malaria vector bionomics in Lao PDR: a nationwide entomology survey

Abstract

Background In Laos, the malaria burden remains high despite a significant reduction of cases during the last decade. In the context of the disease elimination by 2030, a nationwide entomological survey was conducted to better understand the distribution, abundance and behaviour of major malaria vectors (Anopheles spp.) in the country. Methods Mosquito collections were implemented in ten villages from ten provinces during the rainy and dry seasons of 2014 and 2015 by using human landing catch (HLC) and cow bait collection (CBC) methods. After morphological identification in the field, molecular identification of the sibling species of Anopheles mosquitoes from the Funestus, Leucosphyrus, and Maculatus groups were determined using PCR specific alleles. A screening of Plasmodium falciparum and Plasmodium vivax infections in the vectors was carried out by quantitative PCR assays. Results A total of 14,146 adult mosquitoes representing 25 different Anopheles species were collected and morphologically identified. Molecular identification revealed the presence of 12 sibling species within the main primary vector groups, including Anopheles maculatus, Anopheles rampae, Anopheles sawadwongporni, Anopheles pseudowillmori, Anopheles dravidicus, Anopheles minimus, Anopheles aconitus, Anopheles pampanai, Anopheles harrisoni, Anopheles dirus, Anopheles baimaii, Anopheles nemophilous. Anopheles maculatus and An. minimus were predominant during both the dry and rainy seasons, but showed highly zoophilic preferences (Zoophilic index of 98% and 95%, respectively). Overall, 22% of the total malaria vectors were collected between 10:00 PM and 5:00 AM indoors when people are sleeping. Twenty-seven percent of primary and secondary vectors were collected outdoors before 10:00 PM or after 5:00 AM, times when people are usually awake and outdoors. Only two specimens were positive for P. falciparum, one An. aconitus from Phongsaly and one An. minimus from Vientiane Province Conclusions The results indicate that people living in rural areas in Laos are constantly exposed to malaria vectors throughout the year and specifically outdoors. The use of LLINs/IRS remains important but innovative tools and new strategies are needed to address locally, the early and outdoor malaria transmission. Lack of expertise in general entomological methods may further exacerbate the situation.

Anopheles ecology, genetics and malaria transmission in northern Cambodia

“…Nevertheless, on the rare occasions when thorough sampling and screening occurs, the results highlight a high diversity of malaria vectors including presumed zoophilic vectors 28 , 29 , which is expected since most of the Anopheles mosquitoes in the Greater Mekong Subregion often exhibit zoophilic or opportunistic feeding behavior 16 , 17 , 25 . In addition, zoophilic preferences are also reported in populations of the main malaria vectors such as A. dirus or A. minimus 21 , 23 , 30 – 32 . Thus, in an era of malaria elimination in the Greater Mekong Subregion, embedded in a fast-changing environment, limiting screening to anthropophilic and/or classical primary vectors is likely to underestimate the importance of behaviourally opportunistic mosquitoes that could maintain residual malaria transmission, thereby decreasing the effectiveness of malaria elimination programs.…”

“…The complexity of malaria transmission in the Greater Mekong Subregion is further amplified by a high diversity of vector species, including several sibling species only distinguishable molecularly, exhibiting a large spatial heterogeneity in distribution and behavioural patterns both between and within species and species complexes 21 – 23 . Consequently, a deep understanding of the vectors involved in malaria transmission is extremely difficult without molecular species identifications, even though many studies have relied solely on morphological identifications likely due to the cost involved.…”

Mosquitoes (Diptera: Culicidae) from Villages and Forest Areas of Rural Communes in Khanh Hoa and Binh Phuoc Provinces, Vietnam

“…Many Anopheles samples were collected during the first hour of collection (between 6 pm and 7 pm), indicating that peak host-seeking behavior may occur earlier than 6 pm, possibly during the day time, especially in dark environments, such as in forest areas in Khanh Hoa and Binh Phuoc. This may not be an isolated observation as biting activity during the day was observed in Anopheles species in northern Cambodia ( Vantaux et al 2021 ), and in Laos ( Marcombe et al 2020 ). Herein, only CBT was conducted, thus, we were not able to infer the anthrophophily of Anopheles species in the malaria transmission context.…”

Characterizing the spatial distribution of multiple malaria diagnostic endpoints in a low-transmission setting in Lao PDR

“…The mean MFI values for the antigens used to define historic and recent exposure to P. falciparum and P. vivax are as follows: PfMSP119 541.26 (±1334.88); PfAMA1 723. 33 Table 4 presents the results of the PCR and serological exposures from the active survey including the number of individuals and households sampled per district and the number positive for PCR and serological exposure to P. vivax and P. falciparum.…”