Malaria is a major public health problem in the developing world. Each year, more than 500 million cases of clinical malaria are reported worldwide, causing 3 million deaths. More than 90 percent of these deaths occur in sub-Saharan Africa. The disease is caused by a parasite that is transmitted to humans by a female Anopheline mosquito. Despite several decades of research, no antimalarial vaccine has been registered; the mosquito vector has developed widespread resistance to the leading insecticide (DDT) and is becoming increasingly resistant to newer insecticides. The parasite's resistance to cheap and affordable antimalarial drugs is also spreading.
Our primary research agenda is to understand how the deadliest species of the malaria parasite, Plasmodium falciparum, becomes resistant to malaria drugs and how that resistance spreads over time and space. We conduct field and laboratory-based studies to explore the relationship between genetic events in the parasite and the parasite's ability to survive in the presence of malaria drugs. Our research activities focus on five different, but convergent, directions.
Monitoring Malaria Parasite Resistance to Antimalarial Drugs in the Field
Until 2004, the National Malaria Control Program of Mali recommended chloroquine (CQ) as the first-line treatment for uncomplicated malaria followed by sulfadoxine pyrimethamine (SP) as the second line. Quinine was reserved for severe and complicated cases.
Drug resistance in Plasmodium falciparum has emerged as a critical public health problem in this region. One of the most effective and affordable antimalarials for the past 50 years, CQ has become useless in many parts of the world. Resistance to SP has also been widely reported and is spreading. Therefore, it is critical to regularly monitor the susceptibility of local malaria parasites to these drugs so as to devise the most up-to-date treatment policies. Since 1993, we have been following the efficacy of CQ and SP in five sentinel sites spread across Mali: Bandiagara, Kollé, Bancoumana, Bougoula Hameau, and Faladje. These sites were selected to include the various epidemiologic patterns of malaria found in Mali. We use protocols published in 1973, 1996, and 2003 by the World Health Organization. We demonstrated a steady increase in CQ resistance across the country, leading to a change in malaria treatment policy in Mali in July 2004. The new national policy recommends new and better drugs; artemisinine-based combination therapy is now the first-line therapy for uncomplicated malaria. On the other hand, we showed that SP is still highly efficacious, which justifies using it to prevent malaria during pregnancy.
Molecular Mechanisms of Antimalarial Drug Resistance
Understanding the molecular mechanism underlying drug resistance is crucial for designing strategies to overcome the resistance and to develop new drugs. Several in vitro laboratory-based studies showed that CQ resistance was primarily due to a genetic mutation on the pfcrt gene of the malaria parasite. Likewise, resistance to pyrimetamine and sulfadoxine are due to mutations in the dhfr and dhps genes, respectively. It was necessary to validate these observations in the field. In collaboration with scientists at the University of Maryland and the National Institutes of Health, we developed field-adapted nested PCR methods. With simple blood samples dried onto a piece of filter paper, these molecular biology methods readily detected the mutations. Using this simple and robust technique, we confirmed that the pfcrt K76T mutation was the best molecular marker of CQ resistance in the field. This approach is now being widely used by other scientists.
A recent publication suggests a role for pfNHE in P. falciparum resistance to quinine. Because quinine is the leading drug in the treatment of severe and complicated malaria in Mali and in most of Africa, we initiated studies aimed at establishing the role of this gene in quinine resistance in the field. We are conducting prospective in vivo quinine efficacy studies in two Mali villages. Parasites collected before and after quinine therapy are undergoing intensive laboratory investigations, including in vitro culture, in vitro quinine efficacy testing, PCR genotyping, and sequencing. We expect these studies to provide new data on quinine efficacy in Mali and to enhance our understanding of the molecular mechanisms involved in quinine resistance.
Novel Tools for the Surveillance of Malaria Drug Resistance in the Field
The level of in vivo drug resistance was consistently lower than the levels of molecular markers of drug resistance. Further investigations showed that host factors, including differences in age, ethnic group, and immunity, were responsible for these differences. We developed a simple mathematical model termed the "GRI genotype-resistance index model" that allows one to estimate in vivo resistance solely from molecular genotyping. We and other groups are currently validating the model; it has the potential to simplify the molecular surveillance of drug resistance in the field. Studies aimed at developing a similar model for SP resistance in western Africa are under way.
Interface Between Host Immunity and Drug Resistance
Our previous studies showed that host factors are involved in eliminating parasites even when the parasites carry mutations that would make them resistant to a drug. Epidemiologic investigations pointed out that age was the greatest confounding factor. Because age is also the best predictor of antimalarial immunity, we postulated that host immune factors may be responsible for clearing drug-resistant parasites; we proposed the ability to clear drug-resistant parasites as a model system for the investigation of antimalarial immunity. In this model, we compare the immune response and genetics of subjects capable of clearing drug-resistant parasites with those unable to do so. These studies of malaria patients include the genetic makeup of cytokine genes and measurements of cytokine production at the protein level and antibody production.
Impact of Drug Treatment on the Transmission and Spread of Drug-Resistant Parasites
Understanding the transmission of drug-resistant parasites to the mosquito will help in devising strategies to deter the spread of antimalarial drug resistance. To this end, we are measuring how various drugs affect the parasite's switching from the asexual form to the sexual form (gametocytes), which is transmitted to the mosquito. In addition, we are experimentally transmitting these gametocytes to laboratory-reared mosquitoes under various conditions. These studies will shed more light on the processes used in nature to transmit resistant parasites in time and space.
Our research is also supported by the U.S. National Institutes of Health, the Multilateral Initiative on Malaria, the European and Developing Countries Clinical Trial Partnership, and the International Atomic Energy Agency.
Last updated May 2007