Immunity and climate in African malaria

A new study published this week in the Proceedings of the National Academy of Sciences of the United States of America by scientists from CONICET Bariloche, Institut Pasteur both at Paris and Dakar, IRD-Dakar and the Institut Català de Ciències del Clima (IC3) at Barcelona, shows unequivocally how the incidence of clinical malaria is both influenced by climatic factors and immunity. Disentangling the relative contributions of these factors is a major challenge. The study benefits from a unique long-term follow-up of 2 adjacent populations spanning two decadesin Senegal, that have very different levels of malaria endemicity. ‘By using recently developed mathematical modelling techniques and extremely detailed records from both malaria incidence, parasite inoculation variability within hosts by mosquito population, and local climate data, the study was able to demonstrate how the influence of immunity and climatic factors varies according to the force of infection’ highlights Karina Laneri, main author of this study and scientist at both CONICET in Bariloche, Argentina and IC3, Barcelona Spain. Moreover, a common modelling framework elucidates important information on how immunity influences transmission of the parasite from men to mosquitoes. ‘Notably, the models show how individuals, who are infected without symptoms or infected with parasites at an undetectable level by classical diagnostic methods, contribute significantly to the infectious population. Undetectable but infectious individuals are likely to present a problem for elimination of the parasite reservoir in humans, which is the final step towards elimination of the malaria in a population’, remarks Richard Paul, research scientist at the Institut Pasteur in Paris, coauthor of the study.

Malaria is responsible for an estimated 700,000 deaths per year caused by the protozoan parasite Plasmodium falciparum and transmitted by Anopheline mosquitoes. The force of infection is determined by mosquito density, longevity, speed at which the parasite develops within the mosquito and biting frequency, all of which are influenced by climatic factors, namely temperature and rainfall. Although complete sterilising immunity to the parasite is never achieved by humans, a clinical immunity develops after repeated infection, whereby the individual can be infected without showing clinical symptoms. Furthermore, individuals may be infected with parasites at undetectable levels using conventional diagnostic methods (thick blood smear or Rapid Diagnostic tests).

The study presents one general mathematical model that is fitted exactly in the same way to the two close locations. This unique setting, with both villages in Senegal separated only 5km apart, allowed the researchers to test hypothesis in the way it is customarily done in the laboratory, something absolutely unique and unprecedented in climate change research.

‘This study’, according to Xavier Rodó, senior author of the study and ICREA Professor at IC3, ‘clearly demonstrates how climate is a key driver of malaria variability in epidemic fringes, ending this way the long disputed debate on whether climate affects deadly diseases such as malaria’. ‘At the same time’, highlights Rodó, ‘the study conversely serves also to unequivocally show how climate effects can be nearly totally buffered by levels of acquired herd immunity, highlighting the complex interaction existing instead in the settings where the disease has been present since long’.

Press release for the article entitled ‘Dynamical malaria models reveal how immunity buffers effect of climate variability’, published in the Proceedings of the National Academy of Sciences of the USA the week of June 29 th , 2015.

Authors of the study are Karina Laneri, scientist of the Grupo de Física Estadística Interdisciplinaria at CONICET, Bariloche, Argentina and IC3 associated researcher, Richard Paul, from the ‘Unité de Génétique Fonctionnelle des Maladies Infectieuses at the Institut Pasteur, Paris, Adama Tall, Joseph Faye, Fatoumata Diene-Sarr and Cheikh Skhna, from the Institut Pasteur in Dakar, Jean-François Trape from the Institut de Recherche pour le Dévelopement in Dakar and Xavier Rodó, ICREA research professor at the Institut Català de Ciències del Clima (IC3) in Barcelona.

MEDIA CONTACT: *Karina Laneri, Grupo de Física Estadística e Interdisciplinaria, Centro Atomico Bariloche (8400) San Carlos de Bariloche, Rio Negro, Argentina

Phone/fax: +54-294-445100 int 5345 OPCION 30. e-mail: karinalaneri@gmail.com; *Pilar Racero, Institut Català de Ciències del Clima, c/ Dr. Trueta, 203, 08005 Barcelona; tel: +34 935679977; email: pilar.racero@ic3,cat; *Xavier Rodó, Climate Dynamics and Impacts Unit, Institut Català de Ciències del Clima, c/Dr. Trueta, 203, 08005 Barcelona; e-mail: xavier.rodo@ic3.cat and *Richard Paul, Unité de Génétique Fonctionnelle des Maladies Infectieuses at the Institut Pasteur, Rue du Dr. Roux 75724, Paris cedex 15, Phone: +33(0)140613626; e-mail: rpaul@pasteur.fr.