Prevalence of Signs of Severity Identified in the Thai Population with Malaria: A Systematic Review and Meta-Analysis

Understanding the prevalence of signs of severity identified in the Thai population with malaria could aid clinical management and disease control efforts, decrease mortality, and promote malaria elimination in Thailand. This systematic review aimed to collate the evidence regarding signs of severity identified in the Thai population with malaria. MEDLINE, Web of Science, and Scopus were searched for potentially relevant studies. The quality of the included studies was assessed using the Joanna Briggs Institute critical appraisal tools. The pooled prevalence of signs of severity among patients with severe malaria and the pooled proportion of each sign of severity among all signs of severity were estimated using random-effects models. Heterogeneity among included studies was assessed using Cochran’s Q test. A subgroup analysis was performed to evaluate whether differences in pooled estimates between different study sites. Publication bias was assessed by visualizing funnel plot asymmetry and using Egger’s test. Among 741 studies identified by literature searching, 12 studies of a total of 2900 patients with severe malaria, in 7 Thai hospitals, met the eligibility criteria. Results of meta-analyses showed that the signs of the severity of malaria with the highest prevalence in Thailand were jaundice (54%), hyperparasitemia (47%), impaired consciousness/coma (21%), acidosis (18%), renal impairment (13%), shock (10%), convulsions (9%), severe anemia (8%), pulmonary edema/acute respiratory distress syndrome (ARDS) (8%), hypoglycemia (4%), and bleeding/disseminated intravascular coagulation (DIC) (2%). The signs of the severity of malaria that made up the highest proportion of all signs of severity identified in the Thai population with malaria were hyperparasitemia (33%), jaundice (33%), impaired consciousness/coma (12%), acidosis (9%), renal impairment (7%), severe anemia (6%), convulsions (5%), shock (5%), pulmonary edema/ARDS (3%), bleeding/DIC (1%), and hypoglycemia (1%). The present study revealed the prevalence of signs of severity identified in the Thai population with malaria. Jaundice, hyperparasitemia, and impaired consciousness/coma were the most common signs of severity identified. These results may inform the management of patients with severe malaria and promote malaria-elimination efforts in Thailand.


Background
Malaria is a life-threatening disease caused by members of the protozoan genus Plasmodium that are transmitted to humans via the bites of infected female Anopheles mosquitoes [1]. Globally, approximately 229,000 cases of malaria and 409,000 deaths were reported in 2019 [1]. Five Plasmodium species (P. falciparum, P. vivax, P. malariae, P. ovale, and P. knowlesi) most commonly infect humans [2,3]. In some countries in South East Asia

Search Strategy
MEDLINE, Web of Science, and Scopus were searched for relevant studies using a combination of search terms "("severe malaria" OR "complicated malaria") AND (Thai OR Thailand OR Siam)" from inception to 8 August 2021 (Table S1). Additional searching for relevant studies using Google Scholar was performed to ensure that all potentially relevant studies were included in the search results.

Eligibility Criteria
Eligibility criteria were based on a PICo (participants, outcome of interest, context) framework. Participants were patients with severe P. falciparum malaria in all age groups according to WHO guidelines [5]. The outcome of interest was the prevalence of signs of severity identified in the Thai population with malaria. The context was Thailand. Therefore, the inclusion criteria for studies were as follows: (1) studies enrolled patients with severe P. falciparum malaria; (2) studies must be conducted in Thailand; (3) any study design was considered; and (4) studies reported signs of severity among patients with malaria. The exclusion criteria were as follows: (1) duplicate studies using the same groups of participants; (2) case reports of severe malaria; (3) studies of uncomplicated malaria; (4) studies that do not provide data on signs of severity; (5) short reports, (6) studies with no full-text available; and (7) review articles.

Study Selection
Potentially relevant studies were selected according to the eligibility criteria. After duplicate studies were removed, the titles and abstracts of the remaining studies were screened, and irrelevant studies were excluded. Next, the full texts of the remaining studies were examined for eligibility, and studies that did not meet the eligibility criteria were excluded. The reason for exclusion was noted. The studies that met the eligibility criteria were included for further analysis. Study selection was performed by two authors independently. Disagreements between authors in study selection were resolved by discussion.

Data Extraction
The following data were extracted from each study into a standardized spreadsheet: name of the first author, publication year, year the study was conducted, study sites, study design, number of participants, age groups included, percentage of male participants, number of patients who died, number of patients with each sign of severity, and the total number of signs of severity. Data were extracted by two authors. Disagreements between authors in data extraction were resolved by discussion.

Quality of the Included Studies
The quality of the included studies was assessed using the Joanna Briggs Institute (JBI) critical appraisal tools [18]. For clinical trials, the JBI critical appraisal checklist for randomized controlled trials (13 criteria) was used. For observational studies, the JBI critical appraisal checklist for studies reporting prevalence data (nine criteria) was used. A score of more than 75% of the total indicated a high-quality study, whereas a score less than 75% of the total indicated a moderate-or low-quality study. Moderate-or low-quality studies were retained for qualitative synthesis.

Data Synthesis
The pooled prevalence of signs of severity identified in the Thai population with malaria and the pooled proportion of each sign of severity among all signs of severity were estimated using random-effects models assuming that the outcomes of the included studies were heterogeneous. Heterogeneity among the included studies was assessed using Cochran's Q test. Values of p < 0.01 or I 2 more than 50% indicated substantial heterogeneity among the included studies. If the outcome of the included studies was homogenous (no substantial heterogeneity), data were pooled using fixed-effects models. A subgroup analysis was performed to evaluate differences in pooled estimates between different study sites. Publication bias was assessed by visualizing funnel plot asymmetry and using Egger's test. If the funnel plot was asymmetrical or Egger's test returned a value of p < 0.05, contour-enhanced funnel plots were evaluated to identify potential sources of funnel plot asymmetry. All analyses were performed using Stata version 14.2 (StataCorp, College Station, TX, USA).

Search Results
A total of 741 studies were identified from the 3 databases: 299 from MEDLINE, 142 from Scopus, and 300 from Web of Science. After 326 duplicates were removed, the titles and abstracts of the 415 remaining studies were screened, and 351 irrelevant studies were excluded. The full texts of 64 studies were examined, and 53 studies were excluded for the following reasons: 30 studies used the same group of participants, 7 studies were case reports of severe malaria, 5 studies were of uncomplicated malaria, 5 studies did not report data on signs of severity, 3 studies were conducted in other countries, 1 short report did not provide data on signs of severity, 1 study had no full-text available, and 1 study was a review article. Eleven studies [14,[19][20][21][22][23][24][25][26][27][28] met the eligibility criteria and were included in the meta-analysis. One additional study [29] was identified through another source (Google Scholar) and was included in the meta-analysis. Finally, 12 studies [14,[19][20][21][22][23][24][25][26][27][28][29] were included in the qualitative and quantitative syntheses (Figure 1).

Methodological Quality of the Included Studies
For clinical trials, one study was a high-quality study, while three studies were moderate-or low-quality studies [22,23,27]. For observational studies, all eight studies had scores of more than 75% of the total indicated high-quality studies [14,[19][20][21][24][25][26]28,29]. Moderate-or low-quality studies were retained in the present systematic review for quantitative and qualitative syntheses.

Jaundice
The meta-analysis of the results of nine studies that reported jaundice as a sign of severity, including a total of 2233 patients, showed that the pooled prevalence of jaundice among patients with severe malaria in Thailand was 54% (9 studies, 1213/2233 cases; 95% confidence interval (CI): 36-72%, I 2 : 99.02%). The lowest prevalence of jaundice (20%, 95% CI: 10-37%) was reported in a study conducted at the Hospital for Tropical Diseases in 1994 [23]. The highest prevalence of jaundice (98%, 95% CI: 93-99%) was reported in a study conducted at the Hospital for Tropical Diseases in 1991 [29] (Figure 3).
Meta-analysis of the results of nine studies, including a total of 3552 signs of severity, showed that the pooled proportion of jaundice among all signs of severity was 33% (nine studies, 1213/3552 signs of severity; 95% CI: 24-43%, I 2 : 97.07%). The lowest proportion of jaundice (16%, 95% CI: 8-31%) was reported in a study conducted at Mae Sot Hospital from 2003 to 2005 [19]. The highest proportion of jaundice (54%, 95% CI: 47-61%) was reported in a study conducted at the Hospital for Tropical Diseases in 1999 [21] (Supplementary Figure S1).

Methodological Quality of the Included Studies
For clinical trials, one study was a high-quality study, while three studies were moderate-or low-quality studies [22,23,27]. For observational studies, all eight studies had scores of more than 75% of the total indicated high-quality studies [14,[19][20][21][24][25][26]28,29]. Moderate-or low-quality studies were retained in the present systematic review for quantitative and qualitative syntheses.

Jaundice
The meta-analysis of the results of nine studies that reported jaundice as a sign of severity, including a total of 2233 patients, showed that the pooled prevalence of jaundice among patients with severe malaria in Thailand was 54% (9 studies, 1213/2233 cases; 95% confidence interval (CI): 36-72%, I 2 : 99.02%). The lowest prevalence of jaundice (20%, 95% CI: 10-37%) was reported in a study conducted at the Hospital for Tropical Diseases in 1994 [23]. The highest prevalence of jaundice (98%, 95% CI: 93-99%) was reported in a study conducted at the Hospital for Tropical Diseases in 1991 [29] (Figure 3).
Meta-analysis of the results of 12 studies, including a total of 4324 among all signs of severity, showed that the pooled proportion of impaired consciousness among all signs of severity was 12% (12 studies, 482/4324 signs of severity; 95% CI: 8-16%, I 2 : 96.55%). The lowest proportion of impaired consciousness was reported in a study conducted at the Hospital for Tropical Diseases in 1999 (1%, 95% CI: 0-3%) [27]. The highest proportion of impaired consciousness was reported in a study conducted at Sangklaburi Hospital and Mae Sot Hospital (35%, 95% CI: 30-40%) [21] (Supplementary Figure S4). Meta-analysis of the results of nine studies, including a total of 3552 signs of severity, showed that the pooled proportion of jaundice among all signs of severity was 33% (nine studies, 1213/3552 signs of severity; 95% CI: 24-43%, I 2 : 97.07%). The lowest proportion of jaundice (16%, 95% CI: 8-31%) was reported in a study conducted at Mae Sot Hospital from 2003 to 2005 [19]. The highest proportion of jaundice (54%, 95% CI: 47-61%) was reported in a study conducted at the Hospital for Tropical Diseases in 1999 [21] (Supplementary Figure S1).
Meta-analysis of the results of 11 studies, including a total of 3630 signs of severity, showed that the pooled proportion of renal impairment among all signs of severity was 7% (11 studies, 270/3630 signs of severity; 95% CI: 4-9%, I 2 : 90.32%). The lowest proportion of renal impairment was reported in a study conducted at the Hospital for Tropical Diseases in 1999 (2%, 95% CI: 1-5%) [27]. The highest proportion of renal impairment was reported in a study conducted at Ramathibodi Hospital and Pramongkutklao Hospital from 1992 to 1994 (32%, 95% CI: 19-50%) [ Meta-analysis of the results of 12 studies, including a total of 4324 among all signs of severity, showed that the pooled proportion of impaired consciousness among all signs of severity was 12% (12 studies, 482/4324 signs of severity; 95% CI: 8-16%, I 2 : 96.55%). The lowest proportion of impaired consciousness was reported in a study conducted at the Hospital for Tropical Diseases in 1999 (1%, 95% CI: 0-3%) [27]. The highest proportion of impaired consciousness was reported in a study conducted at Sangklaburi Hospital and Mae Sot Hospital (35%, 95% CI: 30-40%) [21] (Supplementary Figure S4).
Meta-analysis of the results of nine studies, including a total of 4026 signs of severity, showed that the pooled proportion of severe anemia among all signs of severity was 6% (nine studies, 273/4026 signs of severity; 95% CI: 3-9%, I 2 : 95.57%). The lowest proportion of severe anemia was reported in a study conducted at the Hospital for Tropical Diseases from 2006 to 2021 (1%, 95% CI: 0-4%) [26]. The highest proportion of severe anemia was reported in a study conducted at Shoklo Hospital in 1994 (14%, 95% CI: 12-17%) [14] (Supplementary Figure S9).
Meta-analysis of the results of seven studies, including a total of 1141 signs of severity, showed that the pooled proportion of pulmonary edema/ARDS among all signs of severity was 3% (seven studies, 66/1141 signs of severity; 95% CI: 1-5%, I 2 : 83.21%). The lowest proportion of pulmonary edema/ARDS was reported in a study conducted at the Hospital for Tropical Diseases in 1991 (1%, 95% CI: 0-3%) [29]. The highest proportion of pulmonary edema/ARDS was reported in a study conducted at Ramathibodi Hospital and Pramongkutklao Hospital from 1992 to 1994 (19%, 95% CI: 9-36%) [20] (Supplementary Figure S13). Meta-analysis of the results of nine studies, including a total of 4026 signs of severity, showed that the pooled proportion of severe anemia among all signs of severity was 6% (nine studies, 273/4026 signs of severity; 95% CI: 3-9%, I 2 : 95.57%). The lowest proportion of severe anemia was reported in a study conducted at the Hospital for Tropical Diseases from 2006 to 2021 (1%, 95% CI: 0-4%) [26]. The highest proportion of severe anemia was reported in a study conducted at Shoklo Hospital in 1994 (14%, 95% CI: 12-17%) [14] (Supplementary Figure S9).

Convulsions
Meta-analysis of the results of three studies, including a total of 1479 patients, showed that the pooled prevalence of convulsions among patients with severe malaria in Thailand was 9% (three studies, 121/1479 cases; 95% CI: 2-19%, I 2 : 98.41%). The lowest

Publication Bias
Publication bias was assessed by visualizing funnel plots and using Egger's test. Funnel plot asymmetry was observed when the effect size and standard error of the effect size for the pooled prevalence of impaired consciousness were plotted using the data from 12 studies ( Figure 19). Egger's test showed significant small-study effects (p = 0.014), indicating that the asymmetrical distribution of the funnel plot was related to small-study effects. A contour-enhanced funnel plot indicated that the asymmetrical distribution of the included studies arose from the heterogeneity of the included studies (p < 0.01; Figure 20). Publication bias and heterogeneity among the outcomes of included studies might have contributed to the asymmetrical distribution of the funnel plot. 5-7%, I 2 : 0%). A lower pooled prevalence of mortality was observed in Bangkok (two studies; 1%, 95% CI: 0-2%, I 2 : 99.01%) ( Figure 18).

Publication Bias
Publication bias was assessed by visualizing funnel plots and using Egger's test. Funnel plot asymmetry was observed when the effect size and standard error of the effect size for the pooled prevalence of impaired consciousness were plotted using the data from 12 studies ( Figure 19). Egger's test showed significant small-study effects (p = 0.014), indicating that the asymmetrical distribution of the funnel plot was related to small-study effects. A contour-enhanced funnel plot indicated that the asymmetrical distribution of the included studies arose from the heterogeneity of the included studies (p < 0.01; Figure  20). Publication bias and heterogeneity among the outcomes of included studies might have contributed to the asymmetrical distribution of the funnel plot.

Discussion
Although P. vivax (95%) and P. falciparum (4-5%) were the most common causes of malaria in Thailand in the year 2020 [13], nevertheless, due to Thailand's National Malaria

Discussion
Although P. vivax (95%) and P. falciparum (4-5%) were the most common causes of malaria in Thailand in the year 2020 [13], nevertheless, due to Thailand's National Malaria

Discussion
Although P. vivax (95%) and P. falciparum (4-5%) were the most common causes of malaria in Thailand in the year 2020 [13], nevertheless, due to Thailand's National Malaria Elimination Strategy 2017-2026 (NMES), malaria incidence and mortality in Thailand decreased from 2012 to 2020 [30]. The NMES introduced the strategy of robust surveillance and response approach for elimination for each malaria case to deploy needed interventions [31]. Under the strategy, Thai people in several rural communities can access early diagnosis and effective antimalarial treatment. For better-sustained malaria elimination, continuous malaria surveillance and continuous monitoring of patients with malaria are needed to achieve the goal of national malaria control programs and ensure that malaria in Thailand is eliminated, and previous information on clinical status and signs of severity among patients with severe malaria in the past decade is crucial.
The most recent study in Thailand that reported patients with severe malaria was conducted between 2006 and 2012, and those patients were likely to have been referred for treatment to the Hospital for Tropical Diseases in Bangkok [26]. The Hospital for Tropical Diseases was established in 1961 by the late Professor Emeritus Chamlong Harinasuta and the late Professor Khunying Tranakchit Harinasuta and offers services for the treatment of malaria and tropical diseases [32]. Previous studies showed that referrals, no prior history of malaria, body temperature above 38.5 • C, white blood cell count > 10 × 10 9 /mL, presence of schizonts or gametocytemia in peripheral blood smears, and albumin levels of < 3.5 g/dL were associated with increased risks of severe malaria in Thailand [26,33,34]. Furthermore, patients with severe malaria had longer hospital stays and were at increased risk of acquiring other infections than those with uncomplicated malaria [26].
The present study showed that jaundice was the most frequent sign of severity in patients with severe malaria (54%), followed by hyperparasitemia (47%) and impaired consciousness (21%). In South Asian countries, such as India, jaundice (44%) and severe anemia (20%) were reported as predominant signs of severity [35,36]. The results of the present meta-analysis that showed the high proportion of jaundice (54%) was in accordance with the previous study in India, showing most adults with P. falciparum malaria had jaundice (58.90%) [37]. The difference in the proportions of signs of severity among patients with P. falciparum malaria in India could be explained by the difference in the age groups of patients. Signs of severity such as jaundice and acute renal failure were common in adults, whereas severe anemia was a predominant sign of severity in children [35]. For severe malaria in South East Asian countries, there are differences in the prevalence of signs of severity. In Singapore, disseminated intravascular coagulation (DIC) (48.5%), renal impairment (36.3%), and hyperparasitemia (36.3%) were commonly found in P. falciparum malaria, whereas shock (50%) and DIC (37.5%) were commonly found in P. vivax malaria [38]. In Malaysia, severe malaria was mostly caused by P. knowlesi (29%), P. vivax (16%), and P. falciparum (11%) [39]. In patients with severe P. knowlesi infections, acute kidney injuries (AKI) (45.6%), jaundice (42%), and hyperparasitaemia (32.5%) were commonly observed [3]. Jaundice (69%), hypotension (46%), and respiratory distress were commonly observed in P. falciparum malaria, whereas hypotension (71%) and jaundice (29%) were frequently observed in P. vivax infections [39]. In Cambodia, the most common signs of severity caused P. falciparum (and a small number of P. vivax, mixed infection) included prostration (68%), impaired consciousness (65%), and respiratory distress (48%) [40]. In Myanmar, jaundice (46%), coma (20.8%), and acute kidney injury (19%) were the most frequent signs of severity [41]. Meanwhile, cerebral malaria and jaundice were the most frequently severe complications and signs of severity reported in patients with P. falciparum, respectively [42,43]. Therefore, the most frequent sign of severity observed by the metaanalysis, jaundice (54%), was in accordance with the studies that reported jaundice as the highest prevalence among patients with severe P. falciparum malaria (69%) in Malaysia [39] and Myanmar [41].
Although P. vivax is the most common Plasmodium species found in Thailand, patients with severe P. vivax in Thailand were rarely reported [44]. Therefore, data on signs of severity among patients with P. vivax in Thailand are not available for inclusion in the present meta-analysis. In Africa, where malaria is endemic, such as Ethiopia, severe anemia (42%) and hyperpyrexia (21%) were the most common signs of severity in patients with P. vivax malaria [45]. Another study conducted in the same areas demonstrated that prostration (31%), hyperpyrexia (24%), and severe anemia (13.8%) were predominant signs of severity [46]. In Asian countries, such as India, hepatic dysfunction (29%), renal dysfunction (21%), and cerebral malaria (16.1%) were common in patients with P. vivax malaria [47]. The meta-analysis of 49 studies that reported severe P. vivax malaria demonstrated that severe anemia, jaundice, and respiratory distress were frequently observed [8].
The present study had several limitations. First, the prevalence and proportions of each sign of severity were heterogeneous among the included studies. Therefore, random-effect models were applied for all meta-analyses. With the limitation of the high heterogeneity between studies, careful interpretations of the meta-analysis results were recommended. Second, the period under analysis ends in 2012 due to the limited number of studies reporting signs of the severity of malaria in Thailand because the prevalence of malaria in Thailand has continuously decreased in recent decades. In addition, no severe malaria in Thailand has been reported since 2014 [48]. Third, data from a single hospital in Bangkok is not representative of the entire country. In addition, other hospitals in other regions of Thailand did not report severe malaria in the literature. Therefore, those patients could not be included in the systematic reviews. Additionally, information on severe malaria in Thailand was limited. Fourth, there were limitations in the definitions of signs of severity, such as impaired consciousness, which was later defined by the WHO criteria for severe malaria [5]. Therefore, studies conducted before the introduction of the WHO criteria for severe malaria [14,20,23,27,29] did not report the impaired consciousness but reported "cerebral malaria", which might cover one or more signs of severity, such as prostration, impaired consciousness, and convulsion. These differences in definitions of signs of severity might affect the pooled prevalence of the signs of severity among patients with severe malaria in Thailand.

Conclusions
The present study revealed that jaundice, hyperparasitemia, and impaired consciousness/coma were the most common signs of severity identified in the Thai population with malaria. Data from the meta-analysis results are essential in managing patients with severe malaria in Thailand, with particular attention to the management of signs of severity, such as monitoring and managing jaundice, monitoring patients' parasite status, or giving parenteral antimalarial treatment for patients with hyperparasitemia, and close observation of unconscious patients. The effective management of patients with malaria will reduce the signs of severity and prevent mortality which promotes malaria-elimination efforts in Thailand.  Table S1, search terms; Table S2, quality of the included studies. Institutional Review Board Statement: The reporting of this systematic review and meta-analysis followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA). We reported the results of our study according to PRISMA. The protocol of systematic review was registered at PROSPERO ID: CRD42021283237.

Informed Consent Statement: Not applicable.
Data Availability Statement: All data in this study were provided in the main manuscript and supplementary files.