Although, according to the most recent World Malaria report, there were still an estimated 655,000 deaths from the disease in 2010, the majority occurring in African children, major investments in malaria control by the international community in the past decade have yielded excellent returns, with mortality rates falling by more than 25% globally. However as we approach this year’s World Malaria Day (25th April), it may be prudent to consider widening the focus of these international control efforts.
Of the five distinct species of Plasmodium causing human malaria, two species, namely P. malariae and P. ovale, have low prevalence and normally only cause mild disease. The thrust of global control efforts has been directed at P. falciparum, one of the two highly prevalent species, because it causes the highest mortality. However the former name of ‘benign tertian malaria’ given to P. vivax, the other highly prevalent species that is also the most widely distributed, is certainly a misnomer; infection with P. vivax is anything but mild. It has been estimated that this species, endemic in South and Central America, the Middle East, Africa and Asia, and found in temperate as well as tropical areas, causes over two hundred million cases of malaria per year. While fatal infections, usually resulting from a ruptured spleen, are infrequent, P. vivax is harder to diagnose than P. falciparum as it infects immature red blood cells and parasitaemias are thus lower. It is also harder to treat because the life cycle includes dormant liver stages (hypnozoites) that cause periodic relapse infections, accompanied by severe anaemia, respiratory distress and poor obstetric outcomes. And it affects all age groups rather than predominantly children, so as well as the human suffering endured, the economic impact is huge.
The fifth species of Plasmodium that can cause human malaria, P. knowlesi, was previously only thought to infect certain species of macaque monkeys, but has now been recognised as a clinically significant zoonosis. It has been reported from several South East Asian countries, including Thailand, Malaysia, Vietnam, Myanmar, Singapore, Indonesia and the Philippines, and causes up to 70% of the malaria cases in some of these areas. As with P. falciparum, infection with P. knowlesi is potentially fatal if it is not diagnosed and treated promptly; unfortunately microscopically it is very similar to the much less serious P. malariae and is frequently misdiagnosed. And a major concern is that deforestation and increasing human settlement in P. knowlesi endemic areas may result in humans, rather than macaques, becoming the preferred host, and thus the dissemination of P. knowlesi to neighbouring countries where there are no suitable simian hosts, but where the vector mosquitoes (predominantly Anopheles leucophyrus group) breed.
International investment and efforts to control malaria in the last decade have been truly laudable, but it is now time to look outside the P. falciparum box.