Researchers Identify New Malaria Species

Satisfied: Female Anopheles after a blood-meal.

Researchers have identified a ‘new’ and distinct form of the malaria parasite which naturally infects humans. This finding adds another human malaria parasite to the previously identified 5 species of Plasmodium already known to infect humans. After analysing the DNA sequences of Plasmodium parasites from the ovale species, the investigators determined that the Plasmodium ovale parasite, which has until now been considered to be a single species, is actually 2 different species. It seems that the 2 variant species have historically been misclassified as a single species of Plasmodium ovale. As described by the authors “it is unclear which of these should be considered the agent of “true” ovale malaria” which was first identified in 1922.

Malaria symptoms are caused by Plasmodium parasite infections which are transmitted by anopheles mosquitoes. P. ovale infections can cause long persisting parasites in the liver and disease relapses; infections cause illness but are rarely fatal. As such, to date there has been limited research investigating P. ovale malaria infections; especially so in comparison to P. falciparum which causes 90% of the estimated 1 million annual malaria-related deaths.

The authors provide compelling evidence that all P. ovale malaria infections, which had previously been defined as originating from a single Plasmodium species, may actually result from one of 2 distinct species; the findings change the future characterisation of all P. ovale infections. The investigators “propose to name these species Plasmodium ovale curtisi (classic type) and Plasmodium ovale wallikeri (variant type), in honor of [malaria researchers] Christopher F. Curtis (1939-2008) and David Walliker (1940-2007)”.

The authors initially anticipated that different P. ovale parasites would be found in different geographical areas. However, the study results show that both parasites exist in the same regions and can both be found in Africa and Asia. The independent teams of investigators who collaborated to conduct the study and publish these findings have now identified the 2 differing types of P. ovale species in DNA samples from Ghana, Nigeria, Sa˜o Tome´, Sierra Leone, Uganda and Burma (Myanmar).

Lead author Colin Sutherland of the London School of Hygiene and Tropical Medicine commented that “we estimate there could be 5 million cases of symptomatic ovale malaria each year in Nigeria alone. The abundance of P. ovale curtisi and P. o. wallikeri in Asia is less clear, but it is certain both species are present in the region.” In considering other limitations for these parasites, the authors, commented Sutherland “hypothesize that the two species are restricted to human hosts with different blood groups.”

Furthermore, added Sutherland, “elucidation of such a mechanism [of parasite restriction] would provide new understanding of how malaria parasites may be prevented from invading red blood cells of the wrong type. This could lead to new drug or vaccine strategies for all malaria species.”

25^th April 2010 is World Malaria Day.