Bangalore: The parasites that cause malignant malaria, a scourge that leads to 500 million clinical cases every year, may be single-celled protozoans, but there are multiple theories on their origin, which continues to remain a scientific mystery.
When they evolved and from which organism—human, avian or any other parasite—is something that is as difficult to decipher as the ability of the protozoan to dodge modern medicine and the human immune system.
New research presented in Monday’s issue of Proceedings of the National Academy of Sciences by an international team of researchers shows that malaria originated much the same way that other modern pandemics such as HIV, SARS (severe acute respiratory syndrome), or the swine flu did—by jumping from animals to humans. The malarial parasite came from chimpanzees to humans through mosquitoes and, contrary to popular scientific belief that malaria is just a few thousand years old, this study shows that the disease is a few million years old.
Root cause? A chimpanzee in the Mefou National Park, Cameroon. Global Viral Forecasting Initiative
“This is the best data so far and provides the most likely explanation of how malaria came into humans,” says Chetan Chitnis, a scientist at the International Centre for Genetic Engineering and Biotechnology in New Delhi. His team is awaiting government approval to start human trials of a new vaccine for Plasmodium vivax, one of the four species of plasmodium and the one that causes most of the malarial infections in India. According to provisional data from the National Vector Borne Disease Control Programme, India registered 1.52 million cases of malaria in 2008 and 271,037 cases in the first four months of 2009.
Chimpanzees have been known to harbour a parasite, Plasmodium reichenowi, which is closely related to Plasmodium falciparum, the dominant malaria parasite in humans, but it was widely assumed that these parasites had coexisted separately in chimpanzee and human ancestors for five million years.
Now, by studying wild and wild-born captive chimpanzees in three different wildlife sanctuaries in Cameroon and the Ivory Coast in Africa, researchers have identified many new parasites from the chimpanzees. They surmise that the agent for human malaria, Plasmodium falciparum, resulted from the introduction of a chimpanzee parasite into the human population.
“The fact that malaria is as widespread as it is today demonstrates the effectiveness with which this former chimpanzee parasite was able to establish itself in humans,” said co-author Brian L. Pike from the Global Viral Forecasting Initiative in San Francisco.
In a way, the study also suggests that modern pandemic-creating pathogens such as HIV or SARS may be around for very long.
Researchers say that the fact that a protozoan, consisting of a large cell, as complex as human cells, could be transferred from an animal to human means that other such transfers are possible and may have occurred in the past. “This should get the attention of the medical profession and public health officials,” said lead author Francisco J. Ayala from the University of California, Irvine.
Through future comparisons between the human malaria parasite and the chimpanzee parasite from which it is now known to have been derived, researchers may gain insight into how these parasites adapt to different hosts and cause disease. These insights could provide valuable clues into how to combat malaria in humans, said Pike.
The important message in this study is that we should be prepared for crossovers. “Who knows what kind of crossovers we’ll have; there’s already an epidemic of sorts in Indonesia from monkey malaria (caused by a monkey parasite called Plasmodium knowlesi),” said Chitnis.
Some researchers claim that knowing the broader range of relatives to the human parasite might provide key insights in drug development or even act as vaccines. But Chitnis isn’t quite convinced. “At least from the data presented here I am not clear how this could help in drug development.”
In May, the third phase of a malaria vaccine candidate from GlaxoSmithKline Biologicals was started in Tanzania. It’s the leading vaccine candidate by the PATH Malaria Vaccine Initiative and the only one with promising results so far.