Understanding the clinical spectrum of complicated Plasmodium vivax malaria: a systematic review on the contributions of the Brazilian literature

Plasmodium vivax is the most geographically widespread species of Plasmodium causing human disease, with most cases reported in Central and Southeast Asia, in the horn of Africa and in Latin America [1]. It is considered to be a potential cause of morbidity and mortality amongst the 2.85 billion people living at risk of infection, excluding the large African populations who are mostly Duffy negative and, therefore, naturally less susceptible to this infection. However recent data suggest that the parasite is evolving and may use alternative receptors other than Duffy (DARC) for erythrocyte invasion [2]. It is estimated that 5.5% of the population under risk live in the Americas [3]. The major biological characteristic of this parasite is the presence of liver hypnozoites responsible for the frequent relapses, which add a substantial number of cases to the general burden of the disease, what is being faced as one of the most challenging bottlenecks for vivax malaria eradication [4].

Although often regarded as causing a benign infection, there is recent increasing evidence that the overall burden, economic impact, and severity of P. vivax have been underestimated, in part due to a bias in the scientific literature which traditionally devoted most of its attention to the more lethal parasite Plasmodium falciparum, probably as a reflection of a more substantial funding [5]. Until 16 October 2011, the search in MEDLINE using P. vivax as keyword retrieved 5,026 indexed abstracts; using P. falciparum, on the other hand, retrieved almost five times more abstracts: 25,807. Even in places where P. vivax represents the major local problem to be tackled, clinical research is still focused on P. falciparum[6]. There is robust evidence in the past decade from hospital-based studies in India and Indonesia that P. vivax is able to cause severe disease [7, 8]. Some authors argue that this clinical severity may only now be properly recognized and announced by researchers in the field, but these complications apparently are not new from a historical perspective [9]. Actually, the case fatality rate (CFR) related to malarial infections in the English marshes during the 16th and 17th centuries, corresponding to the Little Ice Age, suggest that P. vivax (a parasite more prone to persist in vectors even under low temperatures) may have killed part of this population already victimized by famine [10]. During the first half of the 20th century, malariotherapy in patients with neurosyphilis, using essentially the 'non-severe' P. vivax parasite, led to diverse complications, CFR ranging from 3.3 to 30.3% [11]. The major related complications in these co-infected patients were liver damage, ruptured spleen, jaundice, delirium, uncontrolled vomiting and persistent headaches [11]. That reinforces the concept that P. vivax infection may synergize with other co-morbidities resulting in more complicated disease. Added to local geographical and social determinants, wide Annual Parasite Incidence (API) and CFR variations due to this species are seen around the world.

In summary, P. vivax, which has long been neglected and mistakenly considered 'benign' [12], is receiving an increasing amount of importance in the debates taking place on malaria epidemiology and control, drug resistance, pathogenesis and vaccines [13]. As reviewed elsewhere, the good clinical characterization of severe disease in vivax infection is the first step to understand how the inflammatory response to this parasite contributes to pathogenesis [14].

Traditionally, Brazil has been responsible for almost half of all cases of malaria in Latin America. In 2009, 308,498 cases of malaria were reported in this country (257,571 caused by P. vivax), representing 54.9% of all the malaria reported in the Americas [15]. Cases are virtually restricted to the Amazon Basin (constituted by the states of Amazonas, Acre, Roraima, Amapá, Pará, Tocantins, Rondônia, and parts of Mato Grosso and Maranhão). Amazonian urban agglomerations under continuous economical development trigger intense migration flows, such as in the city of Manaus (in the Western Brazilian Amazon), helping to maintain the disease under endemic levels [16, 17]. Malaria in Brazil is mostly related to P. vivax since the 1990s, when the available tools for control at the moment were put together and intensified, such as the fast diagnosis through thick blood smear (TBS) in all febrile patients, and free access to anti-malarials, integrated through a decentralized primary care-centred public health system [18]. Allied to that, an active community of local malariologists has been persistently identifying the profile of anti-malarial resistance with permanent counseling to the Brazilian Ministry of Health, which responds promptly to these evidences, changing the first line regimens [19]. As the sexual forms (which are infective for the vector) of P. falciparum generally appear later in the course of infection, opportune diagnosis and treatment tend to have a high impact on reducing the transmission intensity of this species but the same is not true for P. vivax, whose gametocytes are present in the very first days of the infection, before efficacious treatment is usually started. In 2008, 59% of all malaria cases registered in the Brazilian Amazon were treated in the first 48 h after appearance of symptoms (SIVEP-Malaria, 2009). These public health measures allied to a regularly updated online information system also impacted the number of deaths related to P. falciparum, which were not more than 58 in 2009 (Brazilian Ministry of Health, 2010). As a consequence, even in the non-indexed literature, severity due to P. falciparum is not frequently reported anymore in Brazil.

Brazil has reported 85% of its cases related to P. vivax in 2010, which puts this country in a peculiar epidemiological situation, as one of the few countries around the world with P. vivax predominance. The impact of P. vivax/P. falciparum co-infections or simultaneous circulation of both species with similar frequencies in a given population, upon the immunological status and clinical presentation of malaria is still unclear [20, 21], but most probably clinical data from population from certain areas should not be extrapolated to other areas in distinct epidemiological conditions. Actually, the lack of data on clinical presentation of P. vivax infection allied to the several particularities of this region, including the diverse genetic background of its population, implicate that the generalization of the findings from Southeast Asia may be inappropriate.

In Brazil, in 1903, the young physician Carlos Chagas (most known for the discovery of American trypanosomiasis afterwards) wrote his MD thesis on the haematological complications of malaria, which, at that moment, also occurred in the non-Amazon area. His major findings in studying P. vivax patients were severe anaemia, splenomegaly, leukopenia, cachexia and jaundice associated to concomitant staphylococcal disease [22]. Bone marrows were also analysed in these patients with no conclusive findings. Later on, during the 1940s, Djalma Batista in Manaus described a series of malarial cases from his outpatient clinics in whom large splenomegaly, cachexia and minor bleeding were frequent among those with the 'benign' tertian malaria [23]. More recently, from 1998 to 2008, 234 deaths related to vivax disease were officially reported to the Brazilian Ministry of Health [18], and an increase in the hospitalization trends for vivax patients was published in a tertiary care hospital from Manaus [24]. To complicate matters, these facts parallel a lack of robust biomarkers and specific criteria for severe disease for this species in the literature. A sine qua non requisite in the analysis of clinical severity related to P. vivax infection is the exclusion of mixed infection with P. falciparum through a more sensitive technique such as PCR and the exclusion of other co-morbidities which may be responsible for the clinical presentation per se. In the literature, in general, reports of 'complicated/severe' cases lack more precise and uniform definition criteria, in part due to the rare application of more robust endpoints such as death and admission to the intensive care unit (ICU), and therefore end up suffering bias through individual judgment of authors, editors and reviewers. As in most of the data published there were no systematic exclusion of co-morbidities and/or mono-infection confirmation using PCR, performing a meta-analysis of severe manifestations of P. vivax becomes virtually impossible. The other bias in the case of Brazil is that many relevant data are confined in abstracts from national scientific meetings and graduate students' dissertations and theses. The systematic review of these unpublished data therefore could contribute to the understanding of the clinical spectrum of vivax infection in this country and ultimately as a representative sample from Latin American vivax malaria.

In the present systematic review, the major limitation was the fact that most of the information was retrieved from non-peer reviewed sources. However, it seems clear that vivax patients in Brazil are calling the attention of their physicians only recently. Like other infectious diseases, defining severity criteria is a major challenge. As an example, dengue fever specialists have defined 'warning signs' for dengue haemorrhagic fever, the most lethal complication of the infection due to dengue virus, which are early signs that should raise the suspicion of severe dengue but are not applied themselves to the final classification [178] as proper intervention can avoid the patient evolving to more severe stages. Sometimes in the literature potential 'warning signs' for severe vivax malaria are mistaken for severity criteria, which are those ultimately related to increased fatality. WHO severity criteria formerly developed for falciparum disease seem to apply reasonably to vivax disease as well, but there are clearly 'warning signs' that should motivate clinicians from the tropics to observe patients more closely, such as isolated thrombocytopaenia, isolated jaundice or the presence of chronic or acute co-morbidities. For example, during influenza outbreaks, the virus does not necessarily kill per se, but compromises the most vulnerable population and facilitates fatal secondary bacterial infections. The most common complications observed in the field are not necessarily the most frequently reported in the literature, sometimes biased by the uniqueness or exoticness of the cases reported. It is only after 1987 that these cases started to be reported in Brazil in indexed and non-indexed publications, which may simply parallel the increase in the absolute numbers of vivax cases in Brazil, culminating in the more frequent observation of rare clinical events triggered by this parasite. Publication bias may also impact the chronology of these complicated case reports, especially when research group leaderships based in the endemic areas start to look for clinical aspects more closely. It is noteworthy however that studies on pathogenesis must be careful when dealing with severe vivax disease as a single entity. The best approach is to study groups of patients with specific complications (e.g., severe anaemia or ARDS) in order to minimize the risk of heterogeneous groups with probable multifactor causality, including the diversity of host genetics. The amount of complications related to anti-malarial drug use is not negligible, especially primaquine. Multicentric studies using standard protocols, with the proper care of confirming mono-infection by more specific tools (e.g. PCR) and ruling out co-morbidities, are urgently needed to characterize the real spectrum of vivax disease worldwide. Tissues from deceased patients are also waited, in order to support more robust analyses of the mechanisms of death. Without that information, vaccine clinical trials against P. vivax will not be able to include among their endpoints the protection against the severe disease (essentially severe anaemia), which parallels the frequency of severe falciparum anaemia in some endemic areas. The recent discussion on malaria eradication will only succeed if the two parasites which most affect humans begin to be treated as distinct and not causing a single disease. Clinical characterization is the first step to estimate its burden and ultimately to plan any control strategy in the near future.