Printer friendly version
Malaria: how did it reach the Americas?
28 March 2012
Institut de Recherche pour le Développement (IRD)
The malaria parasite, Plasmodium falciparum, originates from Africa, and is found on every continent. Over 200 million people are infected every year from Africa to Asia, as well as in America and the Middle East. How did it spread to the entire planet? It is not entirely clear how it conquered the New World. Scientists from the UMR Migevec (1) and their partners (2) have recently shown in the journal PNAS that the pathogen was introduced by ship during the slave trade.
The research team collected samples of infected blood taken across the whole distribution area of the disease. Analysis of genetic material extracted showed that the American P. falciparum is a close cousin of its African counterpart. In addition, two separate genetic groups exist in Latin America, as a result of two distinct slave routes, one towards the Spanish empire in the North – West Indies and present-day Mexico and Colombia – and the other towards the Portuguese empire – today’s Brazil. Nearly half of the 2.7 million annual cases of malaria in America are now occurring in Brazil.
This recent expansion of the disease shows the parasite’s ability to spread.
The most virulent malaria parasite, Plasmodium falciparum , is currently found in all the continents of the intertropical zone. It kills more than 650,000 people every year, mostly in Africa, but also in Latin America, in the Middle East, in Asia and in Oceania. Recent studies have shown that it is present in African monkeys, and demonstrate that monkeys were the source of the disease. But how it spread across the whole planet, in particular the New World, is still not entirely clear. Did it come from Asia, following human settlement across the Bering Strait, 15,000 years or so ago? Or did it arrive more recently, straight from Africa?
The pathogen crossed the oceans
A large international study recently published in the PNAS by scientists from the UMR Migevec ( 1) and their partners ( 2) has shown that P. falciparum crossed the ocean on slave ships which crossed the Atlantic between the 16th and the 19th century, some 500 to 200 years ago. The research team has indeed just demonstrated that the parasite which is now found in America has African origins.
Through a global international scientific collaboration, biologists have collected several hundred samples of infected human blood from 17 countries representing the parasite’s entire distribution area. It is one of the largest sets of P. falcifarum genetic data ever collected. The analysis of genetic material extracted from those samples has taught the scientists several things. First of all, the American pathogen is genetically distant from its Asian cousin, thus precluding an Asian origin. It is, however, close to the African parasite.
Two slave routes
This study has also shown that P. falciparum colonized America via two separate introduction routes. The biologists have identified two separate genetic groups in Latin America, one in the North and the other further South. Each of these entities is genetically closer to its African source than they are to each other. By being geographically separated by the Andes, they have evolved separately from each other over time.
The two genetic groups stem from separate slave trade routes. During European colonization, the New World was divided between the colonial powers, namely the Spanish and the Portuguese empires. Both imported their own slaves from Western Africa. The Spanish transported their slaves to the West Indies, Central America or what is now Colombia, whereas the Portuguese took them further south, to present-day Brazil. Around 40% of Africans deported across the Atlantic were taken to Brazil, totalling nearly 5 million individuals over three hundred years, mainly in the two main slave trade ports of Rio de Janeiro and Bahia.
In America, the countryside remains affected
Although 80% of malaria cases occur in Africa, Latin America is also affected. The disease is also endemic in the intertropical zone, predominantly in the Amazon Basin, where 90% of infection cases are found, as well as Central America and southern Mexico. Nowadays, the risk of infection in cities is now low; but the risk is very real in the rural areas of Bolivia, Colombia, Ecuador, Peru and Venezuela, and very serious in the entire Amazon region. Each year 2.7 million people are affected in Latin America, half of those cases occurring in Brazil. Yet the proportion of infections caused by P. falciparum does not exceed 50% in the Guyana region. Another species, Plasmodium vivax , which is not deadly but can be associated with recurrences of the disease, is responsible for 75% of Latin American cases.
This work shows that the spread of malaria in the New World is a recent event and has occurred extremely fast. This conclusion underlines the highly invasive ability of P. falciparum , particularly through the explosive reproductive ability of the Anopheles mosquito vector, a fact which must be taken into account by anti-malaria programmes.
(1) UMR Maladies infectieuses et vecteurs : écologie, génétique, évolution et contrôle - MIVEGEC (IRD / CNRS / Université Montpellier 1)
(2) The investigations were conducted jointly with the CNRS, the universities of Toulouse, Montpellier I and II, the National Laboratory of Public Health of the Republic of Congo, the Texas Biomedical Research Institute and the University of California in the United States, Imperial College, London, UK, the Angolan Society of Metallurgy, the French Armed Forces Biomedical Research Institute and the Urmite UMR in Marseille, the Institute of Tropical Medicine in Belgium, the Universidad Peruana Cayetano Heredia, the University of the Mediterranean, the CIRMF in Gabon, the University of Antioquia in Colombia, the Mahosot Hospital in Ventiane, Laos, the Pasteur Institutes of Cambodia, French Guiana and Iran, the Central University of Venezuela, and the Cayenne General Hospital in French Guiana.