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Plasmodium vivax Hospitalizations in a Monoendemic Malaria Region: Severe Vivax Malaria?

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  • Department of Parasitology, US Naval Medical Research Unit No. 6 (NAMRU-6), Lima and Iquitos, Peru; Sub-Regional Epidemiology Office, Peruvian Ministry of Health, Piura, Peru; Bellavista Health Center, Peruvian Ministry of Health, Piura, Peru; Universidad Peruana Cayetano Heredia, Lima, Peru

Severe malaria caused by Plasmodium vivax is no longer considered rare. To describe its clinical features, we performed a retrospective case control study in the subregion of Luciano Castillo Colonna, Piura, Peru, an area with nearly exclusive vivax malaria transmission. Severe cases and the subset of critically ill cases were compared with a random set of uncomplicated malaria cases (1:4). Between 2008 and 2009, 6,502 malaria cases were reported, including 106 hospitalized cases, 81 of which fit the World Health Organization definition for severe malaria. Of these 81 individuals, 28 individuals were critically ill (0.4%, 95% confidence interval = 0.2–0.6%) with severe anemia (57%), shock (25%), lung injury (21%), acute renal failure (14%), or cerebral malaria (11%). Two potentially malaria-related deaths occurred. Compared with uncomplicated cases, individuals critically ill were older (38 versus 26 years old, P < 0.001), but similar in other regards. Severe vivax malaria monoinfection with critical illness is more common than previously thought.

Introduction

Malaria continues to exert a significant disease burden across the world.1 Of five Plasmodium species that cause human disease, P. falciparum and P. vivax are responsible for most malaria cases worldwide. Research efforts have traditionally focused on P. falciparum because of the mortality that it causes in Africa.2 However, P. vivax is the most widely distributed malaria species, and more than 2.8 billion people are at risk of acquiring P. vivax. In the Americas, P. vivax is responsible for 70% of the reported malaria cases.3 Additionally, there is increasing concern regarding the global health impact of P. vivax infections because of our apparent underestimation of its potential disease burden and its extensive geographic distribution.4,5 Finally, even as global malaria elimination efforts focus on reducing the blood-stage P. falciparum cases, the latent hypnozoite liver stage confers a unique survival advantage to P. vivax.

Recent reports of severe and fatal malaria have challenged the dogma that P. vivax is a benign disease.6 A wide variety of severe complications have been described and associated with P. vivax monoinfection, ranging from single organ failure (brain, liver, lungs, kidneys, blood, spleen, etc.) to life-threatening multiple organ failure.712 Also, strong evidence from prospective studies with well-established examination of comorbidities, including P. falciparum, has been recently published.13,14 Based on this new evidence, more researchers now propose that severe malaria could be caused by P. vivax monoinfection,15 especially when comorbidities, such as dengue, leptospirosis, viral hepatitis, bacterial sepsis, and P. falciparum, have been ruled out by highly sensitive methods, such as polymerase chain reaction (PCR). Nevertheless, because most of the evidence originates from P. vivax/P. falciparum coendemic areas, recurrent or previous exposure to P. falciparum may prevent us from fully accounting for the role of P. vivax alone as the cause of severe disease.

In Peru, after the emergence of malaria in the 1990s,16 the number of cases of P. falciparum started to wane, particularly in the north coast of Peru. Tumbes, which borders directly with Ecuador, is the only area in this region that continues to report local transmission of P. falciparum.17 In the Luciano Castillo Colonna Health Region in the department of Piura, also located in the Peruvian north coast, P. vivax has been the single malaria species with autochthonous transmission for nearly 10 years. This finding provided a unique opportunity to retrospectively study the frequency, clinical presentation, and epidemiology of reported severe vivax malaria in a population with almost non-existent exposure to P. falciparum malaria.

Materials and Methods

Study design.

A retrospective case control study was conducted in the subregion of Luciano Castillo Colonna, province of Sullana (04°53′ S, 80°31′ W, population ∼250,000), department of Piura, in the northern coast region of Peru. We reviewed the clinical charts of malaria patients hospitalized at the Sullana Hospital, the single regional referral center for malaria hospitalization in the subregion. All malaria cases hospitalized from January of 2008 to December of 2009 were evaluated, and all cases that fulfilled the severe malaria criteria were enrolled. In addition, a randomly selected group of outpatient uncomplicated vivax malaria cases treated at other health facilities in the subregion (non-severe controls) was enrolled (1:4 case:control ratio).

From 2001 to 2010, Sullana has experienced exclusive transmission of P. vivax, with only eight possible P. falciparum malaria cases identified in 2005, 2006, and 2008, of which only four cases were officially reported.18,19 All these falciparum cases were investigated by E.P. and associated with a history of recent travel to the Amazon Basin, where P. falciparum is endemic. During the same period, over 10,000 cases of vivax malaria were reported, with a range of 588–4,037 cases per year. The highest incidence was reported during the years 2008 and 2009, with over 6,000 cases in total, representing 63% of total cases throughout the decade.19

Study population.

The Sullana Hospital is the only Ministry of Health hospital in the Luciano Castillo Colonna subregion and the only referral site to manage severe malaria cases. According to national guidelines, all malaria cases presenting with severe malaria complications must be hospitalized. In consequence, reviewing records of malaria hospitalizations in the Sullana Hospital was considered the best approach to identify cases of severe malaria, because the study area is urban with relatively good access to healthcare. Local regulations mandate that infectious diseases physicians in public hospitals evaluate all patients with suspicion of severe malaria using a standardized protocol, request additional tests to decide whether the patient fits the criteria for severe malaria, and prescribe the proper malaria treatment following World Health Organization (WHO) recommendations. From January of 2008 to December of 2009, in total, 16,874 patients were hospitalized at the Sullana Hospital for any cause, and those patients with blood smears positive for P. vivax were considered as potential cases. Uncomplicated vivax malaria control cases were selected by permuted block random sampling among outpatient cases registered in the subregion's system of febrile surveillance during the study period. Only the primary care health facilities that referred most of the hospitalized malaria cases during the study period were included in the selection of controls: Bellavista (40%), Sullana (16%), Santa Teresita (12%), and Querocotillo (12%).

Selection criteria for cases.

We included as potential cases all subjects hospitalized at the Sullana Hospital from 2008 to 2009 with a positive malaria thick smear for P. vivax. Among them, severe malaria was diagnosed if the medical chart of the patient had records of at least one of the criteria of the WHO for severe malaria.20 Severe hospitalized malaria cases were considered to be critically ill if they had any of the following malaria complications: (1) very severe anemia (hemoglobin < 5 g/dL), (2) lung injury, (3) shock (hypotension [systolic blood pressure < 80 mmHg] resistant to fluid), (4) renal failure, or (5) cerebral malaria. This subgroup of severe cases with life-threatening clinical features that demand intensive and immediate clinical care is important given the absence of a specific definition of severe vivax. Studying critically ill cases may help to elucidate the pathophysiology of severe vivax instead of evaluating clinical conditions that are life-threatening mostly when associated with P. falciparum rather than when associated with P. vivax. Multiorgan dysfunction was defined by the report of two or more of five malaria complications.14 Individuals who had a specific discharge diagnosis of coinfection or concurrent illness, such as typhoid fever, hepatitis B or C, leptospirosis, dengue, or human immunodeficiency virus (HIV), were excluded from the study. Therefore, morbidity in the study sample has a higher likelihood to be caused by vivax malaria.

Selection criteria for uncomplicated vivax controls.

We randomly selected four uncomplicated malaria controls for each case using the simple random sample with replacement methodology and the malaria registry program as the sample frame. The inclusion criteria for controls were (1) outpatient malaria cases were smear-positive for P. vivax monoinfection, (2) cases were registered in the subregion's system of febrile surveillance during years 2008–2009, (3) cases were fully treated by standard directly observed therapy at one of four aforementioned primary care health facilities, and (4) cases had a negative blood smear on the seventh day of treatment. Controls were selected using the proportional distribution of hospitalized patients per facility and the chronological order of the malaria registries. Controls who did not meet the inclusion criteria were systematically replaced during the selection process.

Treatment of vivax malaria.

All patients received treatment uniformly for free according to the Peruvian malaria treatment guidelines.21 The treatment of choice for uncomplicated vivax malaria cases in adults and children is chloroquine (25 mg/kg, total dose) over 72 hours plus primaquine (0.5 mg/kg) taken orally daily for 7 days. Pregnant women receive chloroquine (25 mg/kg, total dose) over 72 hours and then a weekly dose of 5 mg/kg until delivery, after which time they receive primaquine phosphate (0.5 mg/kg) daily for 7 days. Infants under 6 months of age receive chloroquine only. Unconscious patients are treated with clindamycin (20 mg base/kg per day) and quinine sulfate (10 mg salt/kg per day) intravenously three times daily until they can start standard oral treatment.

Data collection.

We retrospectively reviewed the medical charts and registries at the Sullana Hospital corresponding to the study period and systematically recorded signs, symptoms, pre-conditions, comorbidities, and laboratory results of the first 72 hours using a structured form. The primary data source for hospitalized patients was the data recorded by the infectious diseases physician in the standardized admission sheet, which was complemented by available data from other sources. Baseline laboratory results and parasitemia were transcribed from clinical charts for cases and outpatient treatment registries for controls. In most severe malaria cases, we only had access to the laboratory results requested to support the patients' diagnoses and not the tests needed to rule out all of the malaria complications that might have affected them. To identify potential fatalities related to malaria, we cross-referenced the list of reported fatal malaria cases from the Epidemiology Directorate of the subregion and the list of all fatalities registered in the hospitalization database of the Sullana Hospital. Any patient who started malaria treatment within 1 week before hospital admission and died during hospitalization was considered a potential malaria-related fatality.

Data analysis.

Three main comparisons among malaria cases were conducted: (1) severe hospitalized versus uncomplicated hospitalized cases, (2) critically ill severe hospitalized versus non-critically ill severe hospitalized cases, and (3) critically ill severe hospitalized versus uncomplicated outpatient controls. Differences were tested using only available comparable data, which for comparison 3, included only age, sex, and parasitemia. Parasitemia was measured using the parasite counts as a surrogate (total number of asexual parasites detected on thick blood smears per 100 oil immersion fields [o.i.f.]) following the equivalencies and procedures used by the Ministry of Health.22 Parasite counts and sex were compared using the χ2 test, and age was compared using the Student's t test. All analyses were conducted with Stata 12.0 (Stata Corporation, College Station, TX) using a significance level of P < 0.05.

Ethical considerations.

The Institutional Review Board of the US Naval Medical Research Unit No. 6 (NAMRU-6) in Lima, Peru reviewed and approved the protocol of this study. Personal identifiers were coded immediately after collection to protect the confidentiality of the participants.

Results

Severe and non-severe malaria hospitalized cases.

The subregion reported 6,502 cases of malaria between 2008 and 2009, of which 106 cases (1.6%) were hospitalized at the Sullana Hospital. These cases included two hospitalized individuals who were identified as malaria cases only after matching the hospital mortality registries and the malaria records of the referral centers. An additional 10 hospitalized cases that had underlying or acute comorbidities (dengue = 6, HIV = 2, hepatitis B = 1, and hepatitis C = 1) were excluded from analysis. All clinical charts of 106 hospitalized patients who had malaria were found and reviewed, and 81 of these cases matched the WHO criteria for severe malaria (Figure 1). Twenty-five malaria cases that were hospitalized but did not fit the criteria for severe malaria had the following diagnoses other than malaria: hemoglobin between 7 and 9 g/dL (N = 8), jaundice (N = 6), dengue (N = 4), urinary tract infection (N = 2), decompensated diabetes (N = 2), hypoglycemia (N = 2), and decompensated hypertension (N = 1). Compared with severe cases, these 25 non-severe hospitalized malaria cases were slightly more often pregnant women (7/25 = 28% versus 10/81 = 12%, P = 0.062), but there were no differences in their age or sex.

Figure 1.
Figure 1.

Flow diagram of patients' enrollment and inclusion criteria.

Citation: The American Society of Tropical Medicine and Hygiene 91, 1; 10.4269/ajtmh.12-0610

Critically ill and non-critically ill severe malaria hospitalized cases.

Twenty-eight (35%) of eight-one severe malaria hospitalized cases were identified as critically ill (Table 1). Therefore, we estimated that the proportion of critical illness among all 6,502 malaria patients was 0.43% (95% confidence interval [95% CI] = 0.28–0.62%). Of five malaria complications used to define critical illness, the most frequent was very severe anemia (57%). The remaining four complications were less common: shock (25%), lung injury (21%), acute renal failure (14%), and cerebral malaria (11%). The mean number of complications per patient was 1.3 ± 0.7 (range = 1–3, mode = 1). Multiorgan dysfunction (two or more malaria complications) was observed in 5 (18%) of 28 critically ill subjects. Compared with 53 non-critically ill cases, critically ill cases had hypoglycemia slightly less frequently (0% versus 11%, P = 0.064), but we failed to observe differences in the remaining malaria complications (i.e., thrombocytopenia and jaundice), which were possibly limited by the small sample size. There were no pregnant women among the critically ill compared with 10 (19%) of 53 pregnant women in the non-critically ill group (P = 0.010). Five of the pregnant women had miscarriages, and two pregnant women had threat of miscarriage without other severe complications. Four of five viable pregnancies had low birth weight neonates (one neonate was born pre-term), whereas the fifth neonate was normal. All other epidemiological characteristics of the critically ill and non-critically ill severe malaria hospitalized cases were similar (Table 2).

Table 1

Clinical presentation of critically ill P. vivax severe malaria cases

NumberAge (years)/sexParasites/100 o.i.f.Critical illness criteriaSevere but not critical illness criteriaCritical care*
12M1Hb < 5 g/dL, lung injury and shock ICU/transfusion
24M2–20Hb < 5 g/dL Transfusion
35M1Hb < 5 g/dL Transfusion
47M1Hb < 5 g/dLJaundiceTransfusion
520F1Cerebral malaria ICU
620M1Hb < 5 g/dLJaundiceTransfusion
722M2–20Hb < 5 g/dLJaundiceTransfusion
823M1Renal failure ICU
925M1Hb < 5 g/dL ICU/transfusion
1026M1Hb < 5 g/dLJaundiceTransfusion
1128F< 1Lung injury and shockHb < 7 g/dLFatal case
1228F< 1Hb < 5 g/dLJaundice
1333F< 1Shock ICU
1434F1Hb < 5 g/dL Transfusion
1537M1Lung injury ICU
1637F1Hb < 5 g/dLJaundiceICU/transfusion
1746M1Cerebral malaria ICU
1847M2–20Cerebral malaria ICU
1950F1Renal failureHb < 7 g/dLICU
2053M1Hb < 5 mg/dL Transfusion
2154M21–200Lung injury, shock, and renal failureHb < 7 g/dLICU/transfusion
2254F21–200Lung injuryJaundiceICU
2356F1Hb < 5 g/dL 
2457F1Hb < 5 g/dL Transfusion
2565F1Shock ICU
2668F2–20Shock and lung injuryHb < 7 g/dLICU/transfusion (fatal case)
2778M1Hb < 5 g/dL (shock) ICU/transfusion
2887F1Renal failure ICU

F = female; Hb = hemoglobin; M = male.

Transfusion implies red blood cell transfusion.

Table 2

Critical illness among severe cases of P. vivax malaria monoinfection

CharacteristicsCritically ill (N = 28)Non-critically ill (N = 53)P value
Age (years)38 ± 2233 ± 200.313
 < 184 (14%)8 (15%)0.928
 18–6420 (72%)39 (74%) 
 ≥ 654 (14%)6 (11%) 
Female sex13 (46%)29 (55%)0.478
Pregnant0 (0%)10 (19%)0.014
Parasite counts*
 < 13 (11%)4 (8%)0.946
 119 (68%)37 (70%) 
 2–204 (14%)9 (17%) 
 21–2002 (7%)3 (6%) 
Number of symptoms/patient6.8 ± 2.35.1 ± 2.60.006
 Fever at hospitalization26 (93%)45 (85%)0.301
 Malaise18 (64%)23 (43%)0.074
 Headache18 (64%)27 (51%)0.250
 Myalgia17 (61%)15 (28%)0.005
 Chills14 (50%)27 (51%)0.936
 Appetite loss14 (50%)23 (43%)0.570
 Joint pain11 (39%)12 (23%)0.114
 Ocular pain10 (36%)8 (15%)0.034
 Nausea10 (36%)19 (36%)0.990
 Vomiting9 (32%)22 (42%)0.409
 Abdominal pain9 (32%)16 (30%)0.856
 Bone pain8 (29%)6 (11%)0.051
 Lumbar pain6 (21%)5 (8%)0.071
 Cough2 (7%)3 (6%)0.792

Total number of asexual parasites detected on thick blood smears per 100 o.i.f.

Clinically, critically ill patients reported more symptoms, on average, than non-critically ill severe malaria patients (6.8 ± 2.3 versus 5.1 ± 2.6, P = 0.006) and also, more myalgia (61% versus 28%, P = 0.005) and ocular pain (36% versus 15%, P = 0.034). Additionally, marginal differences were found in bone pain (29% versus 11%, P = 0.051), lumbar pain (21% versus 8%, P = 0.071), and malaise (64% versus 43%, P = 0.074). The most frequent symptoms reported by the critically ill severe malaria cases were fever at hospitalization (93%), malaise (64%), headache (64%), myalgia (61%), chills (50%), and appetite loss (50%). In addition, both critically ill and non-critically ill severe malaria cases presented with relatively low parasite densities; the most frequent parasite density detected was 1 parasite/100 o.i.f. (68% and 70%, respectively).

Clinical outcomes of critically ill malaria cases.

During 2008 and 2009, no fatal malaria cases were reported in the subregion, but two individuals registered in the hospital mortality records had been diagnosed with malaria at one of the referral centers in the previous 7 days. Therefore, they were included in our study among 28 critically ill severe malaria hospitalized cases and considered critically ill severe malaria cases. These two individuals died during hospitalization without a malaria diagnosis made at the hospital and as such, had not been officially recorded as deaths secondary to P. vivax infection. The first patient, a 28-year-old non-pregnant female, was diagnosed at a health center with a parasite density of 1 parasite/100 o.i.f. and began receiving antimalarial treatment as an outpatient 1 day before hospitalization. She was admitted to the Sullana Hospital with a diagnosis of acute respiratory distress syndrome (ARDS) and died 2 days later. The second possible malaria death, the death of a 68-year-old male, was also diagnosed with P. vivax malaria at a health center. He presented with a parasite density of 2–20 parasites/100 o.i.f. and began antimalarial treatment as an outpatient 2 days before admission to the Sullana Hospital. This patient died 2 days after hospital admission and 1 day after being admitted to the intensive care unit (ICU) with a diagnosis of septic shock and severe anemia. Based on the clinical presentation of these two cases, we highly suspect that they died because of the severity of their malaria infection. However, we cannot be entirely sure that other underlying medical conditions may better explain their deaths. No autopsy findings or materials were available. The remaining 26 critically ill malaria hospitalized cases (93%) showed signs of favorable response to treatment with chloroquine and primaquine. Before the fourth day of treatment, ICU patients were transferred to the regular hospitalization ward, and non-ICU patients were medically discharged.

Case control analysis.

Compared with uncomplicated outpatient vivax cases, critically ill severe hospitalized cases were older (38 versus 26 years old, P = 0.001) and showed a different distribution of parasite densities, with 1 parasite/100 o.i.f. being the most common (68% versus 31%, P = 0.004) (Table 3). Missing data did not allow other categories to be strictly compared. Four pregnant women were randomly sampled in the outpatient non-severe malaria controls.

Table 3

Critically ill hospitalized versus uncomplicated outpatient malaria cases caused by P. vivax monoinfection

CharacteristicsCritically ill hospitalized malaria cases (N = 28)Uncomplicated outpatient malaria cases (N = 112)P value
Female sex13 (46%)38 (34%)0.219
Age (years)38 ± 2226 ± 15< 0.001
Parasite counts*
 < 13 (11%)38 (34%)0.004
 119 (68%)35 (31%) 
 2–204 (14%)30 (27%) 
 21–2002 (7%)9 (8%) 

Total number of asexual parasites detected on thick blood smears per 100 o.i.f.

Discussion

We conducted a comprehensive characterization of severe vivax malaria in an endemic area with an extended absence of P. falciparum and found that approximately 0.4% of all vivax cases presented with critical clinical conditions, including two potentially malaria-related fatalities. Over one-half of these cases had severe anemia, and nearly 20% presented multiorgan dysfunction. These estimates correspond to a coastal urban area with lower malaria endemicity, greater access to healthcare, and better overall health status than the Amazon region,23 where 85% of malaria cases reported in Peru occur. Additionally, tests assessing malaria severity were only conducted in subjects with clinical suspicion, and it is possible that our results underestimate the incidence of this poorly understood facet of vivax malaria in other regions. However, the retrospective assessment of coinfections and comorbidities may have prevented assessment of all causes of severe disease, potentially overestimating the true burden of severe vivax malaria. In general, this study highlights our limited knowledge regarding the morbidity and mortality of vivax malaria, particularly regarding the frequency and etiology of its severe and critical presentations.

Our results concur with the growing evidence supporting the hypotheses that vivax malaria patients can experience severe and life-threatening illness and that P. vivax is associated with severe illness similar to P. falciparum.79,13,2434 Furthermore, our study in a vivax monoendemic setting adds to the only two case series that strictly ruled out P. falciparum coinfection by highly specific techniques (i.e., PCR) and shows that P. vivax alone may, indeed, present with severe manifestations.13,14 In agreement with these two studies conducted in Brazil and India, respectively, we found that four criteria (severe anemia, shock, lung injury, and acute renal failure) (Table 4) are commonly present in critical illness with P. vivax, despite our use of few criteria to identify critically ill severe vivax cases. However, our fifth criterion for critical illness, cerebral malaria, was infrequent in both Peru and India and not observed in Brazil, supporting the hypothesis that cerebral malaria caused by P. vivax is probably a rare event. Also, Kochar and others14 observed that acute renal failure was very common, possibly a unique feature of severe vivax malaria in India. Severe anemia (hemoglobin < 5 g/dL) was consistently the most frequent finding in all three studies, consistent with observation that anemia is a frequent and often severe result of vivax malaria.12,3538 However, the anemia of P. vivax is a very complex phenomenon, and important questions remain unanswered, such as the roles of acute, chronic, and relapsing infections and the interaction of vivax-related anemia with iron deficiency resulting from helminthiasis and malnutrition.39 Proper understanding of vivax pathogenesis and identification of clinical manifestations that are directly life-threatening and demand critical care are needed. Additional prospective studies may lead to the establishment of a much-needed globally accepted definition of severe vivax malaria.

Table 4

Clinical presentation of critically ill severe vivax malaria cases

Severe malaria criteriaBrazil13 (N = 17) WHO criteria (severe)India14 (N = 40) WHO criteria (severe)Peru* (N = 28) modified WHO criteria (critically ill)
Critical illness criteria
 Severe anemia (Hb < 5 g/dL)5 (29%)13 (33%)16 (57%)
 Shock (systolic blood pressure < 80 mmHg) resistant to fluid1 (6%)3 (8%)7 (25%)
 Lung injury (ARDS or pulmonary edema)2 (12%)4 (10%)6 (21%)
 Acute renal failure (creatinine > 3 mg/dL)2 (12%)18 (45%)4 (14%)
 Cerebral malaria (Glasgow < 9/14)0 (0%)5 (13%)3 (11%)
Non-critical illness criteria
 Jaundice (bilirubin > 3 mg/dL)10 (59%)23 (58%)7 (25%)
 Hypoglycemia (glucose < 40 mg/dL)0 (0%)1 (3%)0 (0%)

More strict criteria to identify critical hospitalized cases of severe vivax malaria, including only five WHO criteria, with a more demanding cutoff to select only life-threatening disease.

In our retrospective study, we could not conclusively rule out the role of other coinfections, such as dengue, leptospirosis, bacterial sepsis, and other conditions, that also might explain the severe illness observed because of lack of laboratory data. However, one important selection criterion in our study is that we only included those subjects who received the standard directly observed treatment of non-complicated vivax malaria and except for the two deaths, had a negative blood smear on the seventh day of treatment and an otherwise favorable outcome. Indeed, before it disappeared, P. falciparum in this area was largely resistant to chloroquine,16 arguing against successful outcomes if, indeed, P. falciparum alone or in a mixed infection was the culprit. Additionally, all cases had a relatively short hospital course after starting treatment (3–5 days). Nevertheless, the possibility that the critical illness observed in these cases could be caused by comorbidities cannot be completely rejected, and the role of coinfections should be evaluated in future studies.

Critically ill cases were primarily adults, with equal sex distribution. The older age of critical compared with outpatient cases may be related to their higher rates of comorbidities and greater resulting chances of hospitalization and decompensation, which was observed by Lacerda and others.40 Additionally, critically ill patients commonly showed low parasite counts, even lower than the parasitemia of uncomplicated outpatient cases. Relatively low parasitemia of severe vivax infections was observed by Kochar and others14 (19,665 ± 9,876 parasites/μL) and Alexandre and others13 (8,077 ± 8,685 parasites/μL). Nearly all the severe P. vivax malaria cases had parasite densities below 30,000 parasites/μL, consistent with the notion that high parasitemia is not a typical feature of severe vivax.41 P. vivax can cause fever at a lower parasitemia compared with P. falciparum, and it is also associated with increased cytokine production at similar parasitemias.41 However, the mechanisms leading to that increased inflammatory response at similar parasitemia have not been elucidated.

Another important finding of our study was the observation that vivax malaria may lead to severe complications during pregnancy. These complications, largely described as almost exclusive complications of P. falciparum infection, also have been reported as P. vivax complications.42 We know that, in Africa, falciparum malaria infections have been associated with severe maternal anemia, low birth weight, and perinatal mortality, whereas outside Africa, vivax malaria has been associated with severe malaria, pre-term births, and fetal loss.42 These findings are particularly true in Latin America, where the main pregnancy complications of P. vivax are pre-term births and miscarriages43 regardless of whether associated with critical illness. The burden of vivax malaria on pregnancy fetal outcomes still needs to be better estimated using controlled studies.

Our data add to the evidence that indicates that vivax malaria can be a debilitating disease. Critically ill vivax malaria cases had significantly more symptoms than non-critically ill severe malaria cases; additionally, critically ill vivax malaria patients reported more pain-related symptoms than non-critically ill severe malaria patients. Malaise was also common, although symptoms should not be equated to severity and the limitations of retrospective data should be considered when interpreting these data. Considering the overall frequency of P. vivax worldwide, it can be a meaningful figure when translated into quality of life and disease burden. Chloroquine resistance could add to the severity of vivax malaria but probably not in our study, given that only two cases have been reported in Peru.44,45 Also, none of them occurred in the north coast where our study took place.

We observed two likely but unconfirmed fatal cases of severe vivax malaria (2.4% of all severe malaria cases and 7.1% of all critically ill cases). Similar rates were reported by Kochar and others14 (5%) and Alexandre and others13 (5.6%), showing that the case fatality rate among severe vivax malaria is not negligible. All deaths in the three studies had ARDS or lung injury, identical to many deaths in the literature. Severe anemia alone could have triggered respiratory decompensation, because all lung injury cases were anemic (two patients with hemoglobin < 5 g/dL and two patients with hemoglobin < 7 g/dL. This finding suggests that prompt treatment of subjects with two or three of these conditions is crucial, and because it has been shown in a recent post-mortem study, it is particularly true among cases with ARDS or pulmonary edema.40 Additionally, although only one of our six lung-injured critically ill patients was an infant, lung ARDS is a relatively frequent event and particularly threatening, especially among children.46 Therefore, we feel that the survival rate of patients, particular those patients presenting vivax-related lung injury, requires additional study as well.

It is important to further elucidate the mechanisms by which P. vivax can cause severe illness, and an appropriate definition for severe malaria related to P. vivax monoinfection will be vital to this effort. We propose that the five common life-threatening malaria complications that we used (very severe anemia, shock, lung injury, acute renal failure, and cerebral malaria) could be specific enough to guide future studies while still being reasonably sensitive. The value of other criteria, such as thrombocytopenia, is not necessarily practical,47 particularly in the absence of coagulation complications. Along the same lines, hypoglycemia and jaundice do not seem to influence the outcome or appearance of severe or critical illness, because these complications have been more collateral than life-threatening in most cases in Peru. Jaundice, hypoglycemia, and thrombocytopenia most likely are not fatal unless occurring simultaneously with another organ failure. In general, we strongly support the hypothesis that P. vivax may present with life-threatening features requiring ICU support or a blood transfusion. Additional studies should help to delineate the proper criteria to identify such critically ill cases, which we try to propose from our results.

In summary, we observed that cases of vivax malaria monoinfection could become clinically severe, which is in agreement with observations made in other regions. Despite the recent literature on the subject, there is need for a consensus definition for severe vivax malaria as well as a clear understanding of the mechanism and global burden of this condition. A great need remains for increased surveillance and prospective, comprehensive research to better understand this understudied condition.

ACKNOWLEDGMENTS

The authors thank Drs. Pilar Campos, Lourdes Jimenez, and Ana M. Palacios for their valuable support to study activities and Ms. Elena Sarango for her devoted work in the malaria program of Bellavista Health Center.

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Author Notes

* Address correspondence to Andres G. Lescano, Department of Parasitology and Public Health Training Program, US Naval Medical Research Unit No. 6 (NAMRU-6), Av. Venezuela Cdra. 36 S/N, Centro Médico Naval, Lima Callao 02, Peru. E-mail: willy.lescano@med.navy.mil

Financial support: Funding for this study was received through Grants CO348_10_L1 and CO466_11_L1 from the Armed Forces Health Surveillance Center/Global Emerging Infections Surveillance and Response System of the US Department of Defense. Additionally, this work was supported by Training Grant 2D43 TW007393 awarded to the US Naval Medical Research Unit No. 6 by the National Institutes of Health/Fogarty International Center.

Authors' addresses: Antonio M. Quispe, Salomón Durand, G. Christian Baldeviano, Kimberly A. Edgel, Department of Parasitology, US Naval Medical Research Unit No. 6 (NAMRU-6), Lima and Iquitos, Peru, E-mails: aquispe@jhu.edu, salomondurand@gmail.com, christian.baldeviano@med.navy.mil, and kimberly.edgel@med.navy.mil. Paul C. F. Graf, US Naval Medical Center San Diego, San Diego, CA (NMCSD), E-mail: paul.graf@med.navy.mil. Edwar Pozo, Sub-Regional Epidemiology Office, Peruvian Ministry of Health, Piura, Peru, E-mail: epozo@dge.gob.pe. Edith Guerrero, Bellavista Health Center, Peruvian Ministry of Health, Piura, Peru, E-mail: edithguerrerov@hotmail.com. Andres G. Lescano, Department of Parasitology, US Naval Medical Research Unit No. 6 (NAMRU-6), Lima, Peru, and Universidad Peruana Cayetano Heredia, Lima, Peru, E-mails: wlescano@hotmail.com and willy.lescano@med.navy.mil.

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