Plasmodium vivax malaria is an important threat to public health in many countries across the Asia-Pacific, Horn of Africa, and the Americas. It has been increasingly associated with severe and fatal outcomes, and repeated hypnozoite-induced relapses cause a cumulative risk of anaemia and malnutrition which results in substantial indirect morbidity and mortality.

When a mosquito infected with a Plasmodium species takes a blood meal from a human, it releases sporozoites into the human circulation, which infect liver cells, incubating for ~5 to 21 days before rupturing and releasing merozoites into the peripheral bloodstream. In the case of P. vivax or P. ovale, parasites can lie dormant in the liver (hypnozoites), “reactivating” weeks to months after the initial infection to cause recurrent episodes of malaria (relapses). Following rupture of a parasitised hepatocyte, approximately 104 merozoites are released into the peripheral circulation, subsequently undergoing asexual reproduction every 48 hours. In non-immune individuals, the peripheral parasite count rises exponentially, with symptoms associated with the innate immune response beginning to occur at about the level of microscopic detection (~50/µL); in an adult this equates to a total infection of approximately 108 parasites. However, repeated episodes of malaria can result in the host developing pathogen-specific immunity, allowing some individuals to harbor higher parasitaemias without manifesting symptoms.

The pyrogenic threshold of a Plasmodium infection is defined as the parasite density that is required to induce a fever. Pyrogenic thresholds vary according to parasite species and strain, and are dynamic within and between individuals according to age, immunity, and background levels of endemicity in a community.

In malaria-endemic countries, an understanding of the relationship between fever and P. vivax parasitaemia can inform both clinical management and epidemiological studies of disease burden. However, priorities differ in these two settings. In a clinical context, the priority is to identify as many patients as possible with malaria (by correctly attributing fever to parasitaemia). In an epidemiological context it is more helpful to understand the pyrogenic threshold and how this varies in different populations and locations.

We propose an individual patient data (IPD) meta-analysis of patients with P. vivax parasitaemia enroled into prospective clinical efficacy studies to explore both priorities.

1.            To inform the development and evaluation of diagnostic tools for vivax malaria by estimating the parasitaemia thresholds which capture certain percentages of febrile patients who present for treatment.

2.            To determine the pyrogenic density of P. vivax parasitaemia in patients with recurrent infection, and how this varies with age, location, and first versus second recurrence.  

•             All prospective clinical efficacy studies of P. vivax monoinfection held in the WWARN (WorldWide Anti-malarial Resistance Network) repository will be eligible for inclusion if they meet the essential inclusion criteria outlined below

•             A Data Access Request has been submitted to WWARN and granted pending investigator approval

•             Each identified study will be included if it is agreeable to the study’s primary investigator

•             Prospective clinical efficacy studies of uncomplicated P. vivax monoinfection with minimum 28 days follow up

•             Study meta-data including design, inclusion and exclusion criteria, and study location

•             Age and sex of the participant

•             Date of enrolment

•             Number of recurrent episodes of P. vivax parasitaemia per participant

•             For each episode of recurrent parasitaemia:

o             Date or timing of parasitaemic episode in relation to the date of enrolment

o             Temperature recording and/or the presence or absence of a recent history of fever (within the last 72 hours)

o             Presence and asexual density of P. vivax parasites based on blood film microscopy

Study site details

•             Microscopy quality assurance information

•             Residence in the study location during the last year

•             Malaria history of the participant in the last year

Weight and height of the participant

•             Nutritional status of the participant (using BMI (body mass index) or weight-for-age z score)

•             Haemoglobin at enrolment

•             Haemoglobin at the time of recurrent P. vivax parasitaemia

•             Schizontocidal treatment(s) tested

•             Administration of schizontocidal treatment for asymptomatic recurrences

•             8-aminoquinoline (8-AQ) treatment administered and timing of administration

•             Pregnancy

•             Severe malaria

All primary data sets will be pooled into a standardised format of quality-assured individual patient data (IPD) using a predefined Clinical Data Management and Statistical Analysis Plan to facilitate an IPD meta-analysis.

The distribution of P. vivax parasitaemia will be determined for all participants with a fever and/or a recent history of fever at presentation. In addition, subgroup analysis will be undertaken for each age group and transmission intensity setting.

The distribution of P. vivax parasitaemia will be determined for all participants with a fever and/or a recent history of fever at first recurrence. In addition, subgroup analysis will be undertaken for each age group and transmission intensity setting. Comparisons will also be made with the second recurrence for patients who experienced at least two recurrent episodes.

Univariable logistic regression analysis will be performed to define the relationship between the presence or absence of fever and/or a recent history of fever (dependent variable) and P. vivax parasitaemia at first recurrence (independent variable). A receiver operator curve (ROC) will be constructed and Youden’s index calculated to identify the parasite density value with optimal sensitivity and specificity for defining the pyrogenic threshold. Multivariable logistic regression analysis will be performed to investigate which variables of interest predict the presence or absence of fever or a recent history of fever during the first recurrent episode of peripheral P. vivax parasitaemia.

A detailed statistical analysis plan has been developed.

The P. vivax Fever Study Group comprises participating investigators who contribute relevant data sets to the pooled analysis. Data sets will remain the property of the investigator and will not be shared without their consent.

The Study Group will assign a Writing Committee to coordinate activities including data analysis and drafting of publications and reports for complete group review. The Writing Committee will comprise Emily Groves, Rob Commons, Nick Douglas, Benedikt Ley, André Daher, Ayodhia Pasaribu, Dhelio Pereira, Kavitha Saravu, Julie Simpson, Ric Price, and any other participating investigators interested in undertaking the data analysis and preparation of the manuscript.

Participating investigators will be recognised in publication as contributors under the banner of the P. vivax Fever Study Group unless they are a member of the Writing Committee, in which case they will be recognised in publication as named authors according to the ICMJE guidelines. The results of this study will be published in a peer-reviewed international journal and presented at international meetings with a focus on tropical medicine or malaria to inform other researchers and policy-makers.

For further information, please contact Emily Groves: esg.emily@googlemail.com