Risk factors associated with malaria infection along China-Myanmar border: a case-control study

Abstract

Background: The World Health Organization (WHO) has certificated China malaria free, but imported malaria is a continuous challenge in preventing reintroduction of malaria in the border area of China. Understanding risk factors of malaria along China-Myanmar border is benefit for preventing reintroduction of malaria in China and achieving the WHO’s malaria elimination goal in the Greater Mekong Subregion (GMS).

Methods: This is a case-control study with one malaria case matched to two controls, in which cases were microscopy-confirmed malaria patients and controls were feverish people with microscopy-excluded malaria. A matched logistic regression analysis (LRA) was used to identify risk factors associated with malaria infection.

Results: From May 2016 through October 2017, the study recruited 223 malaria cases (152 in China and 71 in Myanmar) and 446 controls (304 in China and 142 in Myanmar). All the 152 cases recruited in China were imported malaria. Independent factors associated with malaria infection were overnight out of home in one month prior to attendance of health facilities (adjusted odd ratio [AOR] 13.37, 95% confidence interval [CI] 6.32-28.28, P<0.0001), staying overnight in rural lowland and foothill (AOR2.73, 95% CI 1.45-5.14, P=0.0019), staying overnight at altitude <500m (AOR5.66, 95% CI 3.01-10.71, P<0.0001) and streamlets≤100m (AOR9.98, 95% CI 4.96-20.09, P<0.0001) in the border areas of Myanmar; and people lacking of knowledge of malaria transmission (AOR2.17, 95% CI 1.42-3.32, P=0.0004).

Conclusions: Malaria transmission is highly focalized in lowland and foothill in the border areas of Myanmar. The risk factors associated with malaria infection are overnight staying out of home, at low altitude areas, proximity to streamlets and lack of knowledge of malaria transmission. To prevent reintroduction of malaria transmission in China and achieve the WHO’s malaria elimination goal in the GMS Subregion, cross-border collaboration is continuously necessary, and health education is sorely needed for people in China to maintain their malaria knowledge and vigilance, and in Myanmar to improve their ability to apply personal protection.

Background

Malaria is an anopheline mosquito-borne parasitic disease and has closely been accompanied with human being from beginning human history [1]. However, malaria continuously remains one of main global public health threats. Due to service disruptions during the coronavirus disease 2019 (COVID-19) pandemic, there were an estimated 241 million malaria cases in 2020, increased from 227 million in 2019, and malaria deaths increased by 12% compared with 2019, to an estimated 627 thousand [2]. The goal of the World Health Organization (WHO) is to eliminate malaria in all countries of in the Greater Mekong Subregion (GMS) by 2030 [3]. Cross-border malaria transmission keeps a regional impediment towards malaria elimination in the GMS [4]. China Yunnan is a unique province with malaria ecology and vector system similar to those of five other countries in the GMS. The investigation of risk factors of border malaria can provide scientific evidences in planning malaria elimination in the GMS. After the national malaria programme and its partners dedicated efforts for seven decades, the WHO certified China malaria-free status on June 30, 2021 [5]. However, Yunnan Province in southwestern China shares 4060 km of border with Myanmar (1997 km), Laos (710 km) and Vietnam (1353 km). The interruption of malaria transmission in the north of Vietnam has been reported [6, 7]. A rapid progress, malaria control towards subnational elimination goals has also reported in the northern Laos [8]. Imported malaria threat from Vietnam and Lao PDR is therefore slight to China [9]. Malaria was also effectively controlled in most parts of the China-Myanmar border [10]. However, Yunnan borders Myanmar’s five local ethnic special regions that are out of the central governmental management of Myanmar Union. Some parts of the special regions, such as Kachin Special Region II (KR2) and Kongkang Autonomous Regions, are currently in military conflicts. Health services of the Ethnic Health Organizations are too weak to be able to control malaria effectively in the border area of Myanmar. Malaria is still high endemic in some border areas of Myanmar. Movement of both the human population and anopheline mosquitoes infected with Plasmodium spp. can cause cross-border malaria transmission [11]. Knowing risk factors of malaria infection well along the China-Myanmar border area is helpful in preventing reintroduction of malaria transmission.

In malaria elimination setting, malaria is increasingly imported, caused by Plasmodium vivax, and clustered demographically in adult men with shared epidemiological risk factors [12]. In high endemic situation, risk factors can be investigated through representative cross sectional surveys. However, in the situation of low malaria transmission, the sectional survey is unlikely to adequately identify cases and risk factors because the sectional surveys are unlikely to adequately detect cases [13]. Case-control study is a well-established tool to investigate rare diseases and identify associated demographic, behavioral, and clinical risk factors, being particularly appropriate for rare diseases. As yet, this method has not been extensively used to the epidemiological study of malaria, which has mostly been investigated in high-endemic settings [14]. To further understand risk factors of malaria infection along the China-Myanmar border, a case-control study was conducted to investigate factors associated malaria infection in five prefectures of Yunnan Province in China and a hospital in the KR2 of Myanmar.

Methods

Study site

In order to recruit enough number of malaria cases for this study, the study covered border areas of five prefectures, namely, Dehong, Baoshan, Lincang, Pu'er and Xishuangbanna in China and the KR2 based on the epidemiology of malaria [10–11, 14–26] (Fig. 1). The Laiza City and nearby areas in the KR2 of Myanmar is the hottest spot of malaria along the China-Myanmar border [11]. In China, more than 80% of malaria cases in Yunnan Province in recent five years were imported from the Laiza City and nearby areas. Yingjiang County of Dehong Prefecture neighbouring the Laiza City and nearby areas reported more than one third of Yunnan’s imported malaria cases in recent five years [25, 26]. The subjects of this study were enrolled from 37 health facilities in China’s five border prefectures and Myanmar’s Laiza City Hospital (Fig. 1). Microscopy for malaria parasites is well performed in all these 38 health facilities. The study sites selected in China were former hyperendemic areas where are at a high risk of reestablishing malaria transmission due to the presently existing vectors.

Laboratory diagnosis and structure interview

Microscopy for malaria parasites is one of their normal test items in the selected health facilities. A finger or earlobe prick in febrile patients was performed to generate blood smears. The blood slides were stained with Giemsa for microscopy. The malaria cases were recruited from microscopy-confirmed malaria patients between May 2016 and October 2017, meanwhile, the feverish people with microscopy-excluded malaria were enrolled as controls after informed consent obtained. The blood slides were reread by an expert microscopist with the WHO malaria microscopy level one certificate for the secondary confirmation. A paper-based questionnaire was administered to the subjects in Mandarin in China’s health facilities and in Kachin ethnical language in the Laiza City Hospital of Myanmar. The questionnaire has 36 questions, covering demographics, occupation, activities, travel, housing condition, local ecology, socio-economic status, behavior, malaria awareness and knowledge, and preventive measures. In the questionnaire, details of principal household components were recorded to construct a family wealth index (FWI) (Table 1) [27–30].

Results

Risk factors of malaria cases enrolled in China

The malaria case group and the control group were similar in terms of demographics, proportions living in towns, residence proximity to forest, vector control measures, knowledge of malaria prevention, and engagement in trading, road- building and mining, sight-seeing and lumbering activities in one month prior to enrolling into the study (P > 0.10) (Additional file Table 2). The matched univariate and multivariate LRA identified five independent risk factors associated with malaria infection from subjects recruited in China. Two of the risk factors were overnight in Myanmar (adjusted odd ratio [AOR] 30.31, 95% confidence interval [CI] 15.42–59.58, P < 0.0001) and overnight out of home (AOR 69.71, 95%CI 8.59-261.47, P < 0.0001) in the one month prior to enrolling into the study. Three of the risk factors were staying overnight in rural lowland and foothill (AOR 3.05, 95%CI 1.59–5.84, P = 0.0008), staying overnight at altitude < 500m (AOR 4.59, 95%CI 2.35–8.97, P < 0.0001) and staying overnight nearby streamlets (≤ 100m) (AOR 8.08, 95%CI 2.23–29.25, P = 0.0015) for malaria cases in Myanmar in the one month prior to attendance of health facilities (Table 2).

Risk factors of malaria cases enrolled in Myanmar

The case group and control group were similar in terms of demographics, FWI, nationality, country overnight in one month prior to enrolling into the study, proximity of vegetation, screens of windows and doors, types of bed nets used, knowledge of malaria prevention (P > 0.10) (Additional file Table 2). The matched univariate and multivariate LRA identified four independent risk factors associated with malaria infection from subjects enrolled in Myanmar, namely, staying overnight out of home in one month prior to attendance of health facilities (AOR 2.72, 95%CI 1.40–5.29, P = 0.0033), staying overnight in Laiza city (AOR 3.44, 95%CI 1.55–7.64, P = 0.0023), staying overnight at altitude < 500m (AOR 2.81, 95%CI 1.21–6.51, P = 0.0161) and streamlets ≤ 100m (AOR4.63, 95%CI 1.04–20.57, P = 0.0441) (Table 3).The Laiza city is located in a mountain valley with an altitude from 230 to 260m and nearby a river and a few of streamlets. Staying in the Laiza city and nearby areas is at a high risk of malaria infection.

Among overall combined subjects enrolled in China and Myanmar, the case malaria group and the control group were similar in terms of demographics, education, knowledge and awareness in malaria prevention (P > 0.10). The matched univariate and multivariate LRA identified six independent risk factors associated with malaria infection from the overall subjects enrolled in this study. Five of the risk factors were as same as identified from the subsample recruited in China, namely, staying overnight in Myanmar in one month prior to attendance of health facilities (AOR11.25, 95%CI 5.32–23.79, P < 0.0001), staying overnight out of home(AOR13.37, 95%CI 6.32–28.28, P < 0.0019), especially staying in rural lowland and foothill (AOR2.73, 95%CI 1.45–5.14, P = 0.0441), staying overnight at altitude < 500m (AOR5.66, 95%CI 3.01–10.71, P < 0.0001) and proximity (≤ 100m) of streamlets(AOR9.98, 95%CI 4.96–20.09, P < 0.0001). Due to increased sample size, the variable of no knowledge of malaria transmission, namely, the malaria cases lacking knowledge of mosquitoes transmitting malaria as compared to the controls, was identified as one of risk factors associated with malaria infection (AOR2.17, 95%CI 1.42–3.32, P = 0.0004).

Discussion

Risk factors of malaria infection 

The low altitude leads to hot temperature and adequate precipitation and then abundant breeding sites of anopheline mosquitoes in the China-Myanmar border area. All results of former investigations documented that people in low altitude areas were at a high risk factor of malaria infection. A retrospective case-control study suggested that independent risk factors associated with malaria infection were overnight in the lowland, foothill and half-hill areas, and near anopheline mosquito breeding sites in the China-Myanmar border area. [14]. A cross-sectional study reported that independent risk factors for slide positivity were age, lower altitude, lack of knowledge about malaria transmission and symptoms, inaction against mosquito bites and delayed treatment-seeking in the Salween river valley of Shan Special Region II (SR2), northern Myanmar [16]. In this study, the comparison of the multivariate LRA results between the subjects recruited in China and Myanmar indicated that the risk factors associated with malaria infection were mainly overnight out of home in the one month prior to illness, staying in rural lowland and foothill, staying at altitude <500m and streamlets≤100m in the border areas of Myanmar. For the subsample enrolled in China, the overnight in Myanmar was a part of overnight out of home. For the subsample enrolled in Myanmar, the Laiza city and nearby areas was altitude <500m and streamlets≤100m, staying in the Laiza city and nearby areas was therefore at a high risk of malaria infection. The LRA result of the overall sample indicated that no knowledge of malaria transmission was a risk factor of malaria infection due to the increased sample size. The proportion of case-patients with knowledge of malaria transmission (67.3%) was lower than that of controls (77.6%) (P=0.0055). This indicates that health education on malaria should be necessary for people in both China and Myanmar. The public health in China should maintain people’s malaria knowledge and vigilance, and remind people using personal protection against malaria infection when they are in the endemic areas of other countries. Cross-border workers should be educated on preventive measures for malaria through effective behavior change communication [25].

Malaria along China-Myanmar border and other parts in Southeast Asia

In most areas of the Southeast Asia, the year-round high rainfall and temperatures, and abundant malaria vectors lead to persistent and intense malaria transmission. Most parts of the GMS are low latitude and altitude, the year-round high temperatures and rainfall that are suitable for fertility of malaria vectors and persistent malaria transmission [32]. Forests are traditionally considered as a major determinant of malaria risk in the GMS [33, 34]. In Vietnam a high proportion of malaria cases and deaths were reported in the central mountainous and forested areas [35]. In Myanmar, most of malaria cases were reported to occur in forest or forest fringe areas, and loggers and gem miners were at high risk of malaria [36, 37]. However, the China-Myanmar border areas with altitude from 200m to 5128m are mostly mountainous. The low altitude areas have high temperatures, more mosquito breeding sites and malaria vectors [14]. After malaria was effectively controlled along the China-Myanmar border [10], the major malaria hot spots are the Laiza and its nearby areas of the KR2 [18-19], the Salween river valley of the SR2 [15, 16] and the small golden triangle at China-Myanmar-Laos border in the Mekong River Valley [4, 11] in the northern Myanmar. All the three malaria hot spots are at low altitude. The two former investigations [14, 16] and this study did not identify malaria in association with forest in the northern Myanmar. This suggests that the heterogeneity and complexity of malaria should be recognized and considered in planning control and elimination programmes in the GMS [38].

Malaria epidemiological characteristics along China-Myanmar border 

In eliminating settings, malaria cases are increasingly male, adult, clustered geographically, imported among migrant and other hard-to reach groups, and caused by P. vivax [12, 14]. All of 152 case-patients recruited in China were imported among migrant, and most of them were male, adult and P. vivax malaria. In the chi square test, the proportions of male and aged ≥16 year old case-patients recruited in China were significantly higher than the proportions of these case-patients recruited in Myanmar (male, x²=7.4081, P=0.0065; age, x²=12.1295, P=0.0060). However, the proportions of P. vivax between two subsamples were not significant (x²=1.0111, P=0.3146). When the high percentage of P. vivax malaria indicated the success in control of P. falciparum malaria [10], it documents the difficulty in control of P. vivax malaria [18]. P. vivax malaria is one of challenges for malaria elimination in the GMS.

Implications

The WHO certification of malaria elimination requires applicant countries to provide evidence that 1) local malaria transmission has been fully interrupted, resulting in zero indigenous human malaria cases for at least the past three consecutive years (36 months), and 2) an adequate program for preventing reintroduction of malaria transmission is fully functional throughout the country [39, 40]. The last indigenous malaria case in China was reported from Yingjiang County in the Yunnan border area in April 2016 [41]. Malaria elimination in the Yunnan border area has strongly contributed to the remarkable achievement that the WHO certified China malaria-free status on June 30, 2021 [5, 11]. The results of this study helped public health decision-makers planning cost-effective strategies of malaria elimination gearing to the high risk location and populations [11]. However, the Yunnan border area is still challenged by reintroduction of malaria transmission. The threat of imported malaria from the border areas of Myanmar will continuously exist for a long time. With understanding the local risk factors of malaria infection, the ‘‘3+1’’ strategy for intensive surveillance, rapid response and border collaboration for malaria elimination was developed to reduce the threat of imported malaria for malaria elimination and prevention of malaria transmission reintroduction in the Yunnan border area [11]. Reduced border collaboration of malaria had ever led to slightly resurgent malaria in some border areas of Myanmar since 2014 [11, 15, 18]. The Laiza and nearby areas of the KR2 with a population of approximately 30 thousand persons is one of the malaria hotspot areas along China-Myanmar border due to the risk factors presented in this paper and the weak health services [11]. The number of malaria cases increased from 518 in 2013 to 2,367 in 2016 in the Laiza and nearby areas. The strengthened collaborative interventions between China and Myanmar during 2017-2019 reduced the number of malaria cases to 274 in 2019 [11]. However, reduced collaborative interventions due to the coronavirus disease 2019 pandemic led to malaria resurgence again. A total of 1,532 cases were reported in the Laiza and nearby areas of KR2 in 2021. This led to most of malaria cases in Yunnan were imported from the Laiza and nearby areas in 2021. The Yunnan borders Myanmar’s five special regions that are out of the central governmental management of Myanmar Union. The military conflicts and the weak health services cannot effectively control malaria in the five special regions. In cross-border collaboration of malaria control, the risk factors presented in this paper can be considered to plan cost-effective strategies.

Limitations

This study has certain weaknesses, which might be the limitations of the case-control study itself too. 1) To mitigate the confounding effect, controls were matched with age, sex, healthy status and health facilities. The matching criteria make the study lose the chance to identify whether they are an independent risk or confounding factors of malaria infection. 2) Because the Laiza and nearby areas are the main malaria hot spot that exported most of malaria cases (>80%) detected in Yunnan Province, this study only recruited the subjects from the Laiza city hospital in Myanmar. Due to malaria scarceness, the subjects were enrolled from 37 health facilities in China. The large difference in the number of health facilities involved in this study between China and Myanmar is one of weakness. However, the case-patients recruited in China were imported from most parts of the border areas of Myanmar. This would not lead to selection bias. 3) Owing to a few of non-malaria febrile patients with overnight history in Myanmar, among the 304 controls recruited in China, 259 (85.2%) had no overnight history in Myanmar in the one month before attendance of health facilities. The high proportion of non-malaria febrile patients without overnight history in Myanmar might influence assessment of other potential risk factors. However, this can document that overnight in Myanmar is an obvious risk factor for Chinese people. 4) The subjects were only recruited in the health facilities. If some patients just bought anti-malarial drugs from the drug stores for malaria treatment and the self-medication worked, they might not seek diagnosis and treatment in the health facilities. This occasion might lead to exclusion of them from the study and selection bias. However, with reduction of malaria burden, anti-malarial drugs are disappearing from drug stores because of losing chance of making money. This leads to anti-malarial drugs mainly available in the public health facilities in China. The chance of the selection bias should be small. 5) Some participants declined to answer certain questions that they thought of as sensitive, this might cause responding bias. 6) To avoid recall bias, most of variables were defined to collect information in the one month prior to attendance of health facilities. This may miss to collect enough valuable information.

Conclusion

All malaria cases were infected in Myanmar. The Laiza and nearby areas are the dominant reservoirs of malaria parasites along China-Myanmar border. P. vivax is the dominant malaria parasites, and most malaria patients are male and adult. The independent risk factors of malaria infection were overnight out of home, staying in rural lowland and foothill, low altitude areas and nearby local vector breeding sites in the border areas of Myanmar; and subjects lacking knowledge of malaria transmission. To prevent reintroduction of malaria transmission in China and achieve the WHO malaria elimination goal in the GMS, cross-border collaboration is continuously necessary. Health education is sorely needed for people in China to maintain their malaria knowledge and vigilance, and in Myanmar to promote their ability to apply personal protection.

Abbreviations

AOR: Adjusted odds ratio; CI: Confidence interval; COVID-19: Coronavirus disease 2019; WHO: World Health Organization; GMS: Greater Mekong Subregion; KR2: Kachin Special Region II; FWI: Family wealth index; LRA: Logistic regression analysis; SR2: Shan Special Region II.

Declarations

Acknowledgements

We thank all participants for their contribution of time and patience in the study. We would like to thank Dr Liu Hui from Yunnan Institute of Parasitic Diseases for her coordination of the investigation and Ms Li Huiqiong from Yunnan Simao Sanmu Group for the entry of the data. The opinions expressed are those of the authors and do not necessarily reflect those of the above-mentioned organizations and people.

Authors’ contributions

JWX conceived the study idea, analyzed the data and wrote the manuscript. JWX, DWD, CW, XWZ and JXL contributed towards the study design, data interpretation and rigorous manuscript review. All authors read, discussed and approved the final manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (No. 81560543 and 81673113).

Availability of data and materials

Not applicable.

Ethics approval and consent to participate

The ethical approval of study (YIPDEA 201502) was granted by the Ethics Committee of the Yunnan Institute of Parasitic Diseases in China. The study was also approved by the Department of Health of the Kachin Independence Organization, Myanmar. The ethical approval documented a verbal consent procedure as sufficient because microscopy for malaria parasites is the golden standard for malaria diagnosis and one of normal test items in these health facilities, and the study was interviewed-based. According to the World Medical Association Declaration of Helsinki, the purpose and procedures of the study were explained and disclosed to the participants before obtaining their verbal informed consent. For the child participants, a statement that verbal informed consent was obtained from their parent or guardian. The questionnaire was administered following a verbal informed consent. Participation was entirely voluntary, and participants could pass on a question, take a break or withdraw their consent from the study without providing any explanation at any time. Their continued consent was assumed if they did not refuse to answer questions.

Consent for publication

Not applicable.

Competing interests

The authors declare that they have no competing interests

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