Guest commentary: Plasmodium knowlesi-need to diagnose in India.

Plasmodium knowlesi was first described in the long tail macaque (Macaca fascicularis) in 1931.[1] It also causes naturally acquired malaria in pigtail macaques (Macaca nemestrina), the mitred leaf monkey (Presbytis melalophos) that live in the broad-leaf rain forests, Macaca inus and Saimiri scirea.[2] The most effective mosquito vector is Anopheles lateens that bites both humans and monkeys. In the year 1932, scientists succeeded in transmitting P. knowlesi to a human through blood inoculation experimentally,[1] while attempting to induce monkey malaria to humans as an alternate pyretic agent for neurosyphilis (general paralysis of insane). However, the practice was discontinued due to life-threatening nature of the infection. The first natural infection of P. knowlesi in humans was, however, reported in 1965 in an American traveller returning from Malaysia[3] and the first large focus of human infection was reported in Malaysian Borneo in 2004.[4] Subsequently, the parasite was recognized as the fifth human malaria parasite,[5] with the major infective foci described in Malaysia in the states of Sarawak and Sabah (Malaysian Borneo) and in the Pahang region (peninsular Malaysia), though not confining only to Malaysia. Cases have been increasingly detected in other South-East Asian countries such as Thailand, Vietnam and Cambodia, as well as from regions other than South-East Asian countries, mainly in travelers from several countries such as Japan, Spain, Germany, France, Italy, New Zealand and Netherlands.[678910]

P. knowlesi has all the characteristics of an emerging pathogen due to changes in the ecosystem, international travel and cross border migration. This parasite has added a new dimension to our existing knowledge of malaria and posed several challenges which indicate the need to revamp our malaria control programs. Over the next few years, it may become mandatory to mount an adequate surveillance network to keep a watch on this parasite. An accurate appraisal of the epidemiology, vector-host dynamics, pathophysiology, immunobiology, clinical features, complications, diagnostic tools and treatment options for P. knowlesi infections will be strongly needed over the time. This will be important not only in terms of administration of adequate therapy but also to bolster the preventive measures for re-introduction of endemic malaria in countries with low endemicity.

A recent analysis reporting increasing incidence of P. knowlesi malaria in Sabah, Malaysia threatens the prospects of malaria elimination in spite of control of Plasmodium falciparum and Plasmodium vivax in the period 1992-2011. P. falciparum notifications peaked in 1994 and decreased 55-fold in 2011 while P. vivax peaked in 1995 and decreased 25-fold in 2011. Plasmodium malariae and P. knowlesi (considered as a single group due to microscopic near-identity) notifications, however, demonstrated a >10-fold increase between 2004 and 2011, after having decreased to 100/year in the late 1990s. In 1992, P. malariae/P. knowlesi infections accounted for 1% of malaria notifications compared to 35% in 2011.[11] Determination of transmission dynamics and risk factors for knowlesi malaria is required to guide measures to control this rising incidence. It is suggested that P. knowlesi malaria in Sarawak, Malaysian Borneo remains zoonotic, therefore assuming adequate response to available anti-malarial treatments. However, when ex-vivo sensitivity of human P. knowlesi isolates in Malaysian Borneo was studied, it was found that though the parasite is highly sensitive to artemisinins, there is variable and moderate sensitivity to chloroquine and it is less sensitive to mefloquine. This data, taken together with reports of clinical failures with mefloquine suggest that caution is required if using mefloquine in the prevention or treatment of P. knowlesi.[12]

P. knowlesi malaria causes severe disease in up to 10% of cases with a case fatality rate of 3% in the Malaysian regions. Respiratory distress, acute renal failure, shock and hyperbilirubinemia are the most frequently observed complications of severe P. knowlesi malaria.[13] Cytoadherence of infected red blood cells (RBCs) to brain endothelium mediated by endothelial receptors ICAM-1 and VCAM has been shown for P. knowlesi, similar to comatose patients with falciparum malaria.[14] Thus, it is important to come up with laboratory markers with the ability to identify patients at risk of developing complications. A case control study in Malaysian Borneo has suggested a parasite count ≥35,000/μl or ≥1% or a platelet count ≤45,000/μl as markers indicative of developing complications.[15] More such studies are required to understand the severity and complications of P. knowlesi malaria. As proven in in-vitro culture systems, the parasite can adapt to invade a wider age range of RBCs resulting in proliferation in normal human blood, contrary to the previous knowledge that P. knowlesi parasite proliferates poorly in human blood due to a strong preference for young RBCs.[16] Such cellular niche expansion may increase pathogenesis in humans and will be a key feature to monitor as P. knowlesi emerges in human populations. Parasite adaptation for invasion of older RBCs as a likely mechanism leading to high parasite densities in clinical infections has been reported in cases with hyperparasitemic human P. knowlesi infection with atypical morphology from peninsular Malaysia.[17]

The clinical course of P. knowlesi malaria is indistinguishable from that of P. falciparum, highlighting the importance of taking detailed travel history, careful examination of malaria blood films and judicious use of molecular techniques. Antigen tests alone may miss a malaria diagnosis altogether, whereas blood film examination may wrongly identify the species. The emergence of P. knowlesi as a fatal pathogen in humans poses several challenges for the development of useful diagnostic tools for endemic areas, the study of the vectors involved and the therapeutic efficacy of various drug regimens. Misdiagnosis of P. knowlesi as both P. vivax and P. falciparum and vice versa, is common, potentially leading to inappropriate treatment, including chloroquine therapy for P. falciparum and a lack of anti-relapse therapy for P. vivax.[18] This supports the application of unified blood-stage treatment strategies for all Plasmodium species. Molecular methods have showed the existence of human infections by P. knowlesi, otherwise not detectable by microscopy. Therefore, the need for development of such diagnostic tests which are suitable for all species and confirmation by nucleic acid amplification assays is crucial for appropriate treatment and surveillance.

Until now, P. knowlesi malaria was not reported from India, however, in a recent report from Andaman and Nicobar Islands, among 445 samples analyzed, 53 had P. knowlesi-specific gene sequences, of which 33 had monoinfection, while the rest were coinfections with P. falciparum or P. vivax.[19] Thus, a larger human population in South-East Asia may be at risk of P. knowlesi infection than reported so far. The risk of this parasite entering India and spreading across the country cannot be ignored and is a matter of serious concern. The North-Eastern states of India share borders with P. knowlesi malaria endemic countries and travelers from countries in South-East Asia visit India frequently and vice versa. Furthermore, since India has both the suitable vectors like Anopheles dirus and the hosts like pigtailed macaques in the North-Eastern part of India, it supplies a fertile ground for entry and proliferation of this parasite.[20] Moreover, since P. knowlesi can be easily misdiagnosed as P. malariae or P. falciparum by routine microscopy, cases of P. knowlesi might already be present but missed in this country. Therefore, it is necessary to supplement light microscopy with other advanced molecular diagnostic tests which will help us to detect cases of P. knowlesi. It should be considered in the differential diagnosis of any febrile illness in travelers coming back from endemic regions of South-East Asia. Apart from improved diagnostic facilities, constant surveillance of transmission and host-switch events along with an increased awareness and a high index of suspicion among physicians and microbiologists about this emerging pathogen are vital to detect P. knowlesi in India, which is essential for appropriate control of this severe form of malaria.