Risk factors, clinical features and treatment of human infection with Plasmodium knowlesi and other Plasmodium species in Sabah, Malaysia

The simian parasite Plasmodium knowlesi is an emergent public health threat in southeast Asia,  with human infections now increasingly reported where its macaque hosts and An. Leucosphyrus  group vector are present. In Malaysia, P. knowlesi is now the most common cause of human  malaria, and has been demonstrated to cause severe and fatal disease. This thesis aimed to provide further understanding of the epidemiology, clinical and laboratory features, and  treatment of P.knowlesi malaria in both children and adults, with studies conducted in an area of  northwest Sabah, Malaysia. Firstly, the major epidemiological study in this thesis was a case-control analysis of factors  associated with acquiring symptomatic P. knowlesi infection. Key findings demonstrated the  highest risk in farmers, specific activities such as plantation work and clearing vegetation,  sleeping outside, travel, and other environmental and household factors, particularly at the  forest-edge, although peri-domestic transmission was also evident. Intrinsic factors such as G6PD  deficiency in humans were shown to have a protective benefit, in addition to conventional  malaria prevention activities such as insecticide spraying of household walls. The presence of  spatio-temporal case-clustering through analysis of P. knowlesi dhfr sequences was demonstrated, however was not found to be related to drug selection pressure from human-to-human transmission. Secondly, a large prospective study in three district hospitals detailed the clinical and laboratory features in children and adults with knowlesi malaria, highlighting the significant morbidity due  to anaemia and acute kidney injury in children, despite an absence of severe complications in this  group. A lower pyrogenic threshold was seen with lower parasitaemia compared to other  Plasmodium species overall. The risk of severe disease was 6.4% in adults, with independent  predictors of severe disease including age ≥45 years and parasitaemia >15,000/μL. Finally, two major randomised controlled trials for the treatment of uncomplicated knowlesi and  vivax malaria in Malaysia were conducted comparing an ACT, artesunate-mefloquine, against  chloroquine in adults and children. Faster parasite and fever clearance, and decreased anaemia  risk at day 28 was seen in the artesunate-mefloquine arm in both studies. There were no treatment failures in the P. knowlesi study. High-grade P. vivax chloroquine resistance was  demonstrated in the P. vivax study, with a 61% risk of recurrence at day 28. These studies support the use of ACT as the first-line blood stage treatment for malaria due to all Plasmodium species in this co-endemic area, which is now reflected in national Malaysian treatment  guidelines

Anopheles bionomics in a malaria endemic area of southern Thailand

Background Ivermectin mass drug administration (MDA) could accelerate malaria elimination in the Greater Mekong Subregion. This study was performed to characterize the bionomics of Anopheles in Surat Thani province, Thailand. Methods Mosquitoes were collected via human landing collections between February and October 2019. Anopheles mosquitoes were morphologically identified to species. Primary Anopheles malaria vectors were dissected to assess parity status, and a subset were evaluated for molecular identification and Plasmodium detection. Results A total of 17,348 mosquitoes were collected during the study period; of these, 5777 were Anopheles mosquitoes. Morphological studies identified 15 Anopheles species, of which the most abundant were Anopheles minimus (s.l.) (87.16%, n = 5035), An. dirus s.l. (7.05%, n = 407) and An. barbirostris s.l. (2.86%, n = 165). Molecular identification confirmed that of the An. minimus s.l. mosquitoes collected, 99.80% were An. minimus (s.s.) (n = 484) and 0.2% were An. aconitus (n = 1), of the An. dirus (s.l.) collected, 100% were An. baimaii (n = 348), and of the An. maculatus (s.l.) collected, 93.62% were An. maculatus (s.s.) (n = 44) and 6.38% were An. sawadwongporni (n = 3). No Anopheles mosquito tested was Plasmodium positive (0/879). An average of 11.46 Anopheles were captured per collector per night. There were differences between species in hour of collection (Kruskal–Wallis H-test: χ2 =  80.89, P Conclusions The study area in Surat Thani province is an ideal location to evaluate the impact of ivermectin MDA on An. minimus parity

Microgeography and molecular epidemiology of malaria at the Thailand-Myanmar border in the malaria pre-elimination phase

BACKGROUND: Endemic malaria in Thailand continues to only exist along international borders. This pattern is frequently attributed to importation of malaria from surrounding nations. A microgeographical approach was used to investigate malaria cases in a study village along the Thailand–Myanmar border. METHODS: Three mass blood surveys were conducted during the study period (July and December 2011, and May 2012) and were matched to a cohort-based demographic surveillance system. Blood slides and filter papers were taken from each participant. Slides were cross-verified by an expert microscopist and filter papers were analysed using nested PCR. Cases were then mapped to households and analysed using spatial statistics. A risk factor analysis was done using mixed effects logistic regression. RESULTS: In total, 55 Plasmodium vivax and 20 Plasmodium falciparum cases (out of 547 participants) were detected through PCR, compared to six and two (respectively) cases detected by field microscopy. The single largest risk factor for infection was citizenship. Many study participants were ethnic Karen people with no citizenship in either Thailand or Myanmar. This subpopulation had over eight times the odds of malaria infection when compared to Thai citizens. Cases also appeared to cluster near a major drainage system and year–round water source within the study village. CONCLUSION: This research indicates that many cases of malaria remain undiagnosed in the region. The spatial and demographic clustering of cases in a sub-group of the population indicates either transmission within the Thai village or shared exposure to malaria vectors outside of the village. While it is possible that malaria is imported to Thailand from Myanmar, the existence of undetected infections, coupled with an ecological setting that is conducive to malaria transmission, means that indigenous transmission could also occur on the Thai side of the border. Improved, timely, and active case detection is warranted

Natural human Plasmodium infections in major Anopheles mosquitoes in western Thailand.

BackgroundThe 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.MethodsAdult 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.ResultsThe 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.ConclusionsThis 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

Evaluation of CDC light traps for mosquito surveillance in a malaria endemic area on the Thai-Myanmar border.

BackgroundCenters for Disease Control and Prevention miniature light traps (CDC-LT) baited with CO2 are a routine tool for adult mosquito sampling used in entomological surveys, and for monitoring and surveillance of disease vectors. The present study was aimed at evaluating the performance of baited and unbaited CDC-LT for indoor and outdoor trapping of endemic mosquito species in northwestern Thailand.MethodsCDC-LT (n = 112) with and without dry ice baits were set both indoors and outdoors in 88 selected houses for stretches of 5 consecutive nights per month in 7 villages in Tha Song Yang district, Tak province between January 2011 and March 2013. Individual traps were repeatedly placed in the same location for a median of 6 (range 1-10) times. Mosquitoes were identified by morphological characteristics and classified into blood-fed, empty, male/female and gravid. Absolute mosquito numbers were converted to capture rates (i.e., mosquitoes per trap and year). Capture rates were compared using multilevel negative binomial regression to account for multiple trap placements and adjust for regional and seasonal differences.ResultsA total of 6,668 mosquitoes from 9 genera were collected from 576 individual CDC-LT placements. Culex was the predominant captured genus (46%), followed by anopheline mosquitoes (45%). Overall, CO2 baited traps captured significantly more Culex (especially Culex vishnui Theobald) and Anopheles mosquitoes per unit time (adjusted capture rate ratio (aCRR) 1.64 and 1.38, respectively). Armigeres spp. mosquitoes were trapped in outdoor traps with significantly higher frequency (aCRR: 1.50), whereas Aedes albopictus (Skuse) had a tendency to be trapped more frequently indoors (aCRR: 1.89, p = 0.07). Furthermore, capture rate ratios between CO2 baited and non-baited CDC-LT were significantly influenced by seasonality and indoor vs. outdoor trap placement.ConclusionThe present study shows that CDC-LT with CO2 baiting capture significantly more Culex and Anopheles mosquitoes, some of which (e.g., Cx. vishnui, Cx. quinquefasciatus Say, An. minimus s.l. Theobald, An. maculatus s.l. Theobald) represent important disease vectors in Thailand. This study also shows significant differences in the capture efficiency of CDC-LT when placed indoors or outdoors and in different seasons. Our study thus provides important guidelines for more targeted future vector trapping studies on the Thai-Myanmar border, which is an important cross-border malaria transmission region in Thailand

Microgeography and molecular epidemiology of malaria at the Thailand-Myanmar border in the malaria pre-elimination phase

© 2015 Parker et al.; licensee BioMed Central. Background: Endemic malaria in Thailand continues to only exist along international borders. This pattern is frequently attributed to importation of malaria from surrounding nations. A microgeographical approach was used to investigate malaria cases in a study village along the Thailand-Myanmar border. Methods: Three mass blood surveys were conducted during the study period (July and December 2011, and May 2012) and were matched to a cohort-based demographic surveillance system. Blood slides and filter papers were taken from each participant. Slides were cross-verified by an expert microscopist and filter papers were analysed using nested PCR. Cases were then mapped to households and analysed using spatial statistics. A risk factor analysis was done using mixed effects logistic regression. Results: In total, 55 Plasmodium vivax and 20 Plasmodium falciparum cases (out of 547 participants) were detected through PCR, compared to six and two (respectively) cases detected by field microscopy. The single largest risk factor for infection was citizenship. Many study participants were ethnic Karen people with no citizenship in either Thailand or Myanmar. This subpopulation had over eight times the odds of malaria infection when compared to Thai citizens. Cases also appeared to cluster near a major drainage system and year-round water source within the study village. Conclusion: This research indicates that many cases of malaria remain undiagnosed in the region. The spatial and demographic clustering of cases in a sub-group of the population indicates either transmission within the Thai village or shared exposure to malaria vectors outside of the village. While it is possible that malaria is imported to Thailand from Myanmar, the existence of undetected infections, coupled with an ecological setting that is conducive to malaria transmission, means that indigenous transmission could also occur on the Thai side of the border. Improved, timely, and active case detection is warranted

Infectivity of symptomatic and asymptomatic Plasmodium vivax infections to a Southeast Asian vector, Anopheles dirus

International audiencePlasmodium vivax is now the predominant species causing malarial infection and disease in most non-African areas, but little is known about its transmission efficiency from human to mosquitoes. Because the majority of Plasmodium infections in endemic areas are low density and asymptomatic, it is important to evaluate how well these infections transmit. Using membrane feeding apparatus, Anopheles dirus were fed with blood samples from 94 individuals who had natural P. vivax infections with parasitemias spanning four orders of magnitude. We found that the mosquito infection rate was positively correlated with blood parasitemia and that infection began to rise when parasitemia was >10 parasites/ll. Below this threshold, mosquito infection is rare and associated with very few oocysts. These findings provide useful information for assessing the human reservoir of transmission and for establishing diagnostic sensitivity required to identify individuals who are most infective to mosquitoes

Microgeography and molecular epidemiology of malaria at the Thailand-Myanmar border in the malaria pre-elimination phase

© 2015 Parker et al.; licensee BioMed Central. Background: Endemic malaria in Thailand continues to only exist along international borders. This pattern is frequently attributed to importation of malaria from surrounding nations. A microgeographical approach was used to investigate malaria cases in a study village along the Thailand-Myanmar border. Methods: Three mass blood surveys were conducted during the study period (July and December 2011, and May 2012) and were matched to a cohort-based demographic surveillance system. Blood slides and filter papers were taken from each participant. Slides were cross-verified by an expert microscopist and filter papers were analysed using nested PCR. Cases were then mapped to households and analysed using spatial statistics. A risk factor analysis was done using mixed effects logistic regression. Results: In total, 55 Plasmodium vivax and 20 Plasmodium falciparum cases (out of 547 participants) were detected through PCR, compared to six and two (respectively) cases detected by field microscopy. The single largest risk factor for infection was citizenship. Many study participants were ethnic Karen people with no citizenship in either Thailand or Myanmar. This subpopulation had over eight times the odds of malaria infection when compared to Thai citizens. Cases also appeared to cluster near a major drainage system and year-round water source within the study village. Conclusion: This research indicates that many cases of malaria remain undiagnosed in the region. The spatial and demographic clustering of cases in a sub-group of the population indicates either transmission within the Thai village or shared exposure to malaria vectors outside of the village. While it is possible that malaria is imported to Thailand from Myanmar, the existence of undetected infections, coupled with an ecological setting that is conducive to malaria transmission, means that indigenous transmission could also occur on the Thai side of the border. Improved, timely, and active case detection is warranted