Introduction

Human African trypanosomiasis (HAT), or sleeping sickness, is a fatal disease endemic in sub-Saharan Africa, and 10 million people reside in areas at risk1. The most recent study revealed that 977 cases of HAT were reported in 2018, which was down from 2164 in 20162. HAT is caused by the protozoa Trypanosoma spp. and transmitted via the tsetse fly (Glossina spp.)1. The two variants of HAT have been reported, and they are caused by either Trypanosoma brucei gambiense (T. b. gambiense) or Trypanosoma brucei rhodesiense (T. b. rhodesiense). Between these species, T. b. gambiense accounts for more than 95% of all cases of HAT3. T. b. gambiense is endemic to West Africa, whereas T. b. rhodesiense is endemic to East Africa4. The reservoir hosts for T. b. gambiense are humans, whereas the hosts for T. b. rhodesiense are domestic and wild animals5. HAT is classified into early and late stages. After an initial bite by the tsetse fly, parasites enter the hemolymphatic system in the early stage and then multiply and spread through the bloodstream, lymphatic system, and systemic organs4. In the late stage, parasites cross the blood–brain barrier into the CNS, causing various neurological signs and symptoms such as sleep disorder with daytime somnolence and nocturnal insomnia (i.e., sleeping sickness)6, and other symptoms including slurred speech, cerebellar ataxia, tremors, headache, myositis, and myelopathy7. If patients are left untreated, the disease deteriorates progressively into impaired consciousness, incontinence, seizures, and eventually death in most cases3.

Malaria is a major cause of death among children younger than 5 years in Africa, where malaria is highly endemic8. WHO reported an estimated 229 million cases of malaria and 409,000 deaths in 20199. Malaria is caused by one of the six Plasmodium species (P. falciparum, P. vivax, P. malariae, P. ovale curtisi, P. ovale wallikeri, and P. knowlesi), which are transmitted via the bite of Anopheles mosquitoes harboring parasites10. Signs and symptoms of malaria range from asymptomatic to severe complications depending on the causative Plasmodium species, parasitemia level, and immune status of the patient11,12,13,14,15. Severe P. falciparum malaria as defined by the most recent WHO criteria by signs including impaired consciousness, prostration, multiple convulsions, acidosis, hypoglycemia, severe malarial anemia, renal impairment, jaundice, pulmonary edema, significant bleeding, shock, and hyperparasitemia16. P. falciparum and P. knowlesi are the main causes of severe malaria and death, whereas other Plasmodium species less frequently cause severe malaria14,15,17.

As both HAT and malaria are endemic to Africa, the possibility of co-infection by these two diseases cannot be discounted. Therefore, the present study investigated current evidence regarding malaria and HAT co-infection in the literature via a systematic review and meta-analysis. In addition, the characteristics of co-infected patients were qualitatively and quantitatively synthesized to provide evidence-based information for further studies.

Methods

Protocol and registration

The systematic review and meta-analysis protocol was registered at PROSPERO (ID: CRD42021266397) and followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses statement18.

Search strategy

The search strategy involved the use of combinations of the following search terms: “(Malaria OR Plasmodium) AND (Trypanosomiases OR “Sleeping Sickness” OR “African Sleeping” OR Nagana OR Trypanosome OR Nannomona) AND (co-infection OR co-infection OR “Co-infection” OR mixed OR concurrent OR Polymicrobial OR multiple OR dual)” (Table S1). All relevant search terms were retrieved from the Medical Subject Heading in The National Center for Biotechnology Information. The searches were performed in PubMed, Web of Science, and Scopus on July 4, 2021, without restriction on language or publication year. Additional searches of the reference lists of the included studies and Google Scholar were performed to assure that all relevant studies were captured in the search protocol.

Eligibility criteria

The eligibility criteria were related to participants (P), phenomena of interest (I), and context (Co) or PICo. The primary outcome of this study was the pooled prevalence of Plasmodium spp. infection among patients with HAT globally. Therefore, P was patients with HAT, I was Plasmodium spp. infection and Co was worldwide context. Original studies (retrospective or prospective observational, cross-sectional studies, cohort, clinical trials, or case–control studies) that reported Plasmodium spp. infection among patients with HAT was included. Conversely, case studies, case series, letters to the editors, commentary, reviews, systematic review, in vitro studies, and in vivo studies were excluded.

Study selection

Study selection was performed independently by two authors (KUK, MK). Discrepancies between the authors were resolved through discussion until a consensus was reached. After duplicates were removed, the remaining studies were screened for titles and abstracts. After non-relevant studies were excluded, the full text of the remaining studies was examined according to the eligibility criteria. Then, studies that did not meet the eligibility criteria were excluded for specific reasons. Finally, studies that met the eligibility criteria were included for further data extraction.

Data extraction

Studies were extracted into a standardized pilot datasheet under the following categories: name of the first author, year of publication, study site, the year when the study was conducted, study design, number of participants enrolled, characteristics of participants, age, male ratio, number of infecting Plasmodium spp. among patients with HAT, number of patients with HAT, malaria diagnostic method, and HAT diagnostic method.

Risk of bias

The risk of bias among the included studies was assessed using the checklist for analytical cross-sectional studies developed by the Joanna Briggs Institute (JBI)19. The JBI tool assessed eight characteristics of the included studies: inclusion criteria of the participants, study subjects and setting, measurement of exposure, measurement of the condition, identification of the confounding factor, measurement of outcome, and appropriateness of statistical analysis. Scores of 7–8, 4–6, and < 4 indicated a low, moderate, and high risk of bias, respectively.

Data syntheses

Data syntheses included qualitative and quantitative syntheses. For quantitative synthesis, statistical analysis was used to determine the pooled prevalence of Plasmodium spp. infection among patients with HAT. Subgroup analyses were performed by types of HAT (gambiense or rhodesiense HAT) to identify the source of heterogeneity regarding prevalence among the included studies. The heterogeneity of the outcome among the included studies was assessed using Cochran’s Q and the I2 statistic for consistency. Cochran’s Q less than 0.05 or I2 greater than 50% indicated substantial heterogeneity of the outcome among the included studies. Pooled analysis of the prevalence was performed using a random-effects model20. Publication bias was assessed if the number of the included studies was at least 1021. All quantitative analyses were performed using Stata version 14 (StataCorp, College Station, TX, USA). For qualitative synthesis, a narrative synthesis of the impact of Plasmodium spp. infection on clinical and outcome characteristics of patients with HAT was performed when the included studies reported qualitative data.

Results

Search results

Three hundred twenty-seven studies were identified in PubMed (179 studies), Web of Science (94 studies), and Scopus (54 studies). After 95 duplicates were removed, 232 studies were screened for titles and abstracts. After 218 non-relevant studies were excluded, the full text of 14 studies was examined. After screening the full text of the studies, six animal studies, two review articles, and one article in which no patients with HAT had malaria were excluded. Five studies22,23,24,25,26 were included in the systematic review. An additional four studies6,27,28,29 were identified from searches of reference lists. Finally, nine studies6,22,23,24,25,26,27,28,29 were included in the systematic review and meta-analysis (Fig. 1).

Figure 1
figure 1

Study flow diagram demonstrated the selection of relevant studies.

Full size image

Characteristics of the included studies

All included studies were published between 2001 and 2019, and among them, seven studies6,23,24,26,27,28,29 were conducted between 1999 and 2014. The included studies were conducted in Angola22, Kenya23, Uganda24,26, Tanzania and Uganda25, Southern Sudan27, Sudan28, and Democratic Republic of Congo29, whereas one study was conducted in several countries (Angola, Central African Republic, Côte d’Ivoire, Democratic Republic of Congo, Equatorial Guinea, Republic of Congo, and Southern Sudan)6 (Fig. 2). Two studies22,25 were clinical trials, two23,24 were retrospective studies, three6,28,29 were cohort studies, one27 was a cross-sectional study, and one26 was a case–control study. Four studies23,24,26,27 enrolled patients with early- and late-stage HAT, whereas the other studies6,22,25,28,29 enrolled only patients with late-stage HAT. All included studies used the microscopic method to examine malaria parasites. Four studies6,22,27,28 used the agglutination test for trypanosomiasis as a serological screening test followed by microscopy (direct in blood, lymph aspirate, or cerebrospinal fluid or trough concentration by hematocrit centrifugation technique), whereas other studies used microscopy23,24,29 or microscopy/molecular methods25,26 to examine Trypanosoma spp. (Table 1). Eight studies22,23,24,25,26,27,28,29 reported the number of Plasmodium spp. infections among patients with HAT, whereas only one study reported that malaria trophozoites were detected in half of the patients tested for malaria in the initial blood smear6. Among 1555 patients with HAT in eight studies22,23,24,25,26,27,28,29, 707 patients were infected by Plasmodium spp.. Among these patients, four studies6,22,27,28 reported cases of Plasmodium spp. infection (534 cases) among patients with gambiense HAT (1181 cases), and the other studies23,24,25,26 reported Plasmodium spp. infection (173 cases) among patients with rhodesiense HAT (374 cases).

Figure 2
figure 2

modified by the authors from https://mapchart.net/world.html. Authors are allowed to use, edit, and modify any map created with mapchart.net for publication freely by adding the reference to mapchart.net.

Epidemiology of Human African trypanosomiasis (HAT) and malaria co-infection in the African continent. HAT human African trypanosomiasis; T. b. Trypanosoma brucei; (a) Angola; (b) Democratic Republic of Congo; (c) Angola, Central African Republic, Cˆote d’Ivoire, Democratic Republic of Congo, Republic of Congo, Equatorial Guinea, and South Sudan; (d) Kenya; (e) Uganda; (f) Tanzania and Uganda; (g) South Sudan; (h) Uganda; (i) Sudan. Map was retrieved and

Full size image
Table 1 Characteristics of the included studies.
Full size table

Risk of bias

The risk of bias among the included studies was assessed using the checklist for analytical cross-sectional studies developed by the JBI. A score of 7 points was given for five studies23,24,27,28,29, which were considered to have a low risk of bias, whereas four studies6,22,25,26 were given scores of 5–6 points and categorized as having a moderate risk of bias (Table S2).

Prevalence of Plasmodium spp. infection among patients with HAT

The prevalence of Plasmodium spp. infection (692 cases) among patients with HAT was estimated using the data (n/N) of seven non-case–control studies that enrolled a total of 1523 patients with HAT22,23,24,25,27,28,29. Among the individual studies, Two studies by Kagira et al.23 conducted in Kenya and by Blum et al.29 conducted in the Democratic Republic of Congo reported Plasmodium spp. infection rates of 100% among patients with HAT. Meanwhile, two studies22,25 conducted in Angola, Tanzania and Uganda showed the high prevalence of Plasmodium spp. infection among patients with HAT at 88% and 83%. Lower rates were recorded in other studies (18–30%)24,27,28. Overall, the pooled prevalence of Plasmodium spp. infection among patients with HAT was 50% (95% confidence interval [CI] = 28–72%, I2 = 98.1%, seven studies; Fig. 3).

Figure 3
figure 3

Pooled prevalence of malaria in human African trypanosomiasis. ES effect size, CI confidence interval; black diamond symbol, point estimate, Dashed line: pooled prevalence of malaria in human African trypanosomiasis; I2, level of heterogeneity; p = 0.00 or less than 0.05, significant heterogeneity.

Full size image

Subgroup analysis by HAT type revealed that the pooled prevalence of Plasmodium spp. infection among patients with gambiense HAT was 46% (95% CI = 14–78%, I2 = 96.62%, four studies), whereas that among patients with rhodesiense HAT was 44% (95% CI = 40–49%, I2 = 98.3%, three studies, Fig. 4). The study by Kagira et al.23 reported a prevalence of Plasmodium spp. infection of 100% among patients with rhodesiense HAT. Meanwhile, the study by Kagira et al. Blum et al.29 reported a prevalence of Plasmodium spp. infection of 100% among patients with gambiense HAT.

Figure 4
figure 4

Subgroup analysis of the pooled prevalence of malaria in human African trypanosomiasis by HAT type. ES effect size, CI confidence interval; black diamond symbol, point estimate, Dashed line: pooled prevalence of malaria in human African trypanosomiasis; I2, level of heterogeneity; p = 0.00 or less than 0.05, significant heterogeneity.

Full size image

Effect of Plasmodium spp. infection in HAT

Seven studies22,23,24,25,26,27,29 determined the effect of Plasmodium spp. infection in HAT. Blum et al.22 suggested that malaria might influence the risk of encephalopathy syndrome, as 14 of 16 patients with HAT and encephalopathy syndrome of the coma type were infected with malaria (87.5%) in Angola. Blum et al.29 demonstrated that patients with HAT who were co-infected with Plasmodium spp. had a significantly longer corrected QT time (QTc). Kagira et al.23 suggested that malaria might increase the risk of drug toxicity; however, the effect of Plasmodium spp. in HAT in Kenya was not investigated in their study. The study by Kato et al.24 conducted a retrospective study in Uganda and concluded that malaria did not significantly affect the clinical presentation or death rates of HAT. However, longer hospital stays and higher treatment costs were recorded among co-infected individuals. Kuepfer et al.25 conducted a clinical trial of patients with HAT in Tanzania and Uganda and also demonstrated that Plasmodium spp. did not influence the clinical presentation or treatment outcomes of HAT. Maina et al.27 conducted the cross-sectional study of HAT in Sudan demonstrating that Plasmodium spp. infection in HAT was more common in female patients. In addition, co-infection was observed in patients aged 10–19 and 20–39, whereas Trypanosoma spp. mono-infection was more common in patients aged 20–29. Meanwhile, Trypanosoma spp. mono-infection was associated with greater total leukocyte reduction (67%) than malaria co-infection (16.5%)27. Nsubuga et al.26 conducted a case–control study of HAT in Uganda that assessed TNF-α levels and found that Plasmodium spp. infection was linked to higher TNF-α levels in patients with HAT than observed in patients with HAT or malaria mono-infections.

Discussion

Testing for concomitant malaria infection in patients with HAT is not mandatory and is only conducted according to the decisions of the treating personnel6. In cases of malaria co-infection, antimalarial treatment is generally given as an ancillary drug before initiating HAT treatment to prevent encephalopathy30,31,32. The meta-analysis revealed that the high prevalence of malaria in HAT was high (50%). Nevertheless, the pooled prevalence appeared to be heterogeneous among the included studies. Subgroup analysis by HAT type indicated that the pooled prevalence of Plasmodium spp. infection was similar between patients with gambiense HAT and rhodesiense HAT (46% and 44%), and high heterogeneity was observed in the prevalence. Several reasons might explain the heterogeneity regarding prevalence among the included studies. The differences in the investigated participants might be the source of heterogeneity. For example, the study by Blum et al.22 examined Plasmodium spp. infection in patients with HAT and encephalopathy alone, whereas Kuepfer et al.25 examined Plasmodium spp. infection in patients with HAT and various types of complications. The participants in both studies had high rates of Plasmodium spp. infection. In addition to differences among the participants, the differences in the study design might be another cause of heterogeneity in the pooled prevalence. Two clinical trials22,25 included in the meta-analysis recorded a high prevalence of co-infection (83–88%). Blum et al.22 examined the blood slides of patients with HAT for malaria in the case of a reaction concurrent with fever, as malaria was suspected to increase the risk of encephalopathy in their participants; hence, the prevalence of malaria was high. Kuepfer et al.25 assessed malaria per a standard protocol to test co-infection with malaria at baseline and determine whether malaria influenced the clinical presentation or treatment outcomes in a Tanzanian study population; hence, the prevalence of malaria was high. The difference in the prevalence of co-infection might be explained by the difference in the study sites investigated. Blum et al.22 conducted their study in Angola, whereas the study by Maina et al.27 was performed in Sudan. The most recent WHO reports illustrated that the malaria burden share was higher in Angola (3%) than in Sudan (1%)9. Among studies conducted in East Africa, where T.b. rhodesiense is predominant, the study by Kagira et al.,23 which was conducted in Kenya, recorded a higher prevalence of co-infection (100%) than the studies by Kuepfer et al.,25 which was conducted in Tanzania and Uganda (83%), Kato et al.,24 which was conducted in Uganda (29%), and Nsubuga et al.,26 which was conducted Uganda (47%). A possible explanation of the high prevalence of Plasmodium spp. infection among patients with HAT (35–71%) was that it was mainly linked with the high prevalence of malaria in the general population of Africa, where malaria is highly endemic8.

The occurrence of Plasmodium spp. infection among patients with HAT suggested that these patients were susceptible to other infections, which might influence the pathogenesis and prognosis of the disease. As HAT is a chronic disease that leads to long-term immunosuppression27,33, which could increase the possibility of Plasmodium spp. infection in patients with HAT. Nevertheless, HAT is usually a chronic disease, but rhodesiense HAT often causes a more acute and rapidly progressive disease than gambiense HAT34. The role of immune responses in co-infected patients was demonstrated by Nsubuga et al.26, who revealed that co-infection by P. falciparum increased TNF-α levels in patients with HAT. Previous studies revealed associations of TNF-α with the progression and severity of HAT35,36. The synergistic effect of TNF-α on disease progression was related to increased IFN-γ levels37,38. The possibility of Plasmodium spp. infection among patients with HAT was probably high in previous studies6,22,25,28 that enrolled only patients with late-stage HAT. Although the study by Blum et al.6 that enrolled patients with late-stage HAT did not report the number of cases of Plasmodium spp. infection among their participants, a malaria prevalence of 50% among patients with HAT was recorded in several endemic countries including Angola, Central African Republic, Côte d’Ivoire, Democratic Republic of Congo, Equatorial Guinea, Republic of Congo, and Southern Sudan.

Testing for concomitant malaria infection in patients with HAT was not mandatory, and it was only performed according to the discretion of the treating personnel6. In cases of malaria co-infection, antimalarial treatment is generally given as an ancillary drug before initiating HAT treatment to prevent encephalopathy30,31,32. Because the examination of malaria parasites among patients with HAT was not mandatory, the possibility of Plasmodium spp. infection in HAT could not be discounted. The presence of fever in patients with HAT might support the presence of malaria parasites, and further examinations of malaria parasites are required. Fever was reported in half of the patients with HAT, resulting in a febrile reaction during HAT treatment6. If malaria parasites are present, then antimalarial treatment is generally given before initiating HAT treatment31,32. Antimalarial treatment was described by Blum et al.22 in which a full course of 1500 mg of chloroquine was given before the start of HAT treatment with melarsoprol.

Concerning the effect of Plasmodium spp. infection in HAT, a previous study found that the co-infected patients required multiple tests and experienced higher treatment costs because of the use of multiple drugs23. In addition, co-infected patients might have a poor prognosis due to prolonged hospital stay, receiving more drugs, and experiencing increased drug toxicity from melarsoprol treatment23,24. Kagira et al.23 observed that malaria could increase the toxicity of drug therapy. Suramin was injected in patients with early-stage rhodesiense HAT, whereas melarsoprol was used to treat patients with late-stage HAT23. Previous research demonstrated that the most serious side effect of melarsoprol treatment is post-treatment reactive encephalopathy characterized by increased mental deterioration, coma, and convulsions or death in some cases after treatment39. Blum et al.29 reported increased QTc in patients with HAT and concomitant malaria, suggesting the probability of melarsoprol-induced ventricular dysrhythmia in late-stage HAT. However, one study suggested that malaria and other co-infections did not significantly affect the clinical presentation and case fatality rates of HAT24. A similar observation was reported by Kuepfer et al.,25 who conducted a study in Tanzania and Uganda. They revealed that Plasmodium spp. infection did not influence the clinical presentation or treatment outcomes of HAT25. However, the outcome of this co-infection might be influenced by several factors such as the stage of infection and age and sex of the patient40.

The present study had several limitations. First, the number of studies that examined malaria and trypanosome co-infection was limited. Therefore, the differences in clinical characteristics, laboratory parameters, and treatment outcomes between co-infected patients and patients with malaria or HAT mono-infection could not be investigated. Second, there was high heterogeneity regarding the pooled prevalence of co-infection in the meta-analysis; therefore, the pooled prevalence should be carefully interpreted.

Conclusion

Because of the high prevalence of malaria among patients with HAT and the endemic nature of malaria in Africa, malaria was present in some asymptomatic patients. Therefore, it is suggested to test every patient with HAT for malaria before initiating HAT treatment. If malaria is present, patients should receive antimalarial treatment before HAT treatment is initiated. Antimalarial treatment in patients with HAT might decrease the probability of poor clinical outcomes and case fatality rates.