Plasmodium knowlesi
Encyclopedia
Plasmodium knowlesi is a primate
malaria
parasite commonly found in Southeast Asia
. It causes malaria in long-tailed macaques (Macaca fascicularis), but it may also infect humans, either naturally or artificially.
Plasmodium knowlesi is the fifth major human malaria parasite. It may cause severe malaria as indicated by its asexual erythrocytic cycle of about 24 hours. The typical fever becomes quotidian. This is an emerging infection that was reported for the first time in humans in 1965. It accounts for up to 70% of malaria cases in certain areas in South East Asia where it is mostly found. This parasite is transmitted by the bite of an Anopheles
mosquito. Plasmodium knowlesi has health, social and economic consequences for the regions affected by it.
in 1927 when he was examining the blood of Macaca fasicularis and he noted that it differed from Plasmodium cynomogli and Plasmodium inui
. It was later seen by Campbell in 1931 in a long-tailed macaque imported from Singapore to the Calcutta School of Tropical Medicine and Hygiene in India. Campbell was interested in kala azar and was working under Napier. Napier inoculated
the strain into three monkeys, one of which was a rhesus macaque (Macaca mulatta), which developed a fulminating infection. Knowing that the Protozoological Department were looking for a monkey malaria strain, they handed the original infected monkey to Dr B. M. Das Gupta, who was Dr R Knowles' assistant. Dr Das Gupta maintained the species by serial passage in monkeys until Dr Knowlesi returned from leave. In 1932, Knowles and Das Gupta described the species in detail for the first time and showed that it could be transmitted to man by blood passage, but failed to name it. It was named by Sinton and Mulligan in 1932 after Dr Knowles. From early in the 1930s to 1955, P. knowlesi was used as a pyretic agent for the treatment of patients with neurosyphillis.
In 1957, it was suggested by Garnham et al. that P. knowelsi could be the fifth species capable of causing endemic malaria in humans.
In 1965, the first case of a naturally occurring infection of knowlesi malaria in humans was reported in an American man who had returned after working in the jungle in peninsular Malaysia. Although the infecting parasite was initially identified as P. falciparum, one day later it was then identified as P. malariae and it was only confirmed to be P. knowlesi after infected blood was used to inoculate Rhesus monkeys. A second report emerged in 1971 about the natural infection of a man in Malaysia with Plasmodium knowlesi followed by the description of a large focus of human infections in the Kapit Division of Sarawak, Malaysian Borneo. This was made possible due to the development of molecular detection assays which could differentiate between Plasmodium knowlesi and the morphologicall similar Plasmodium malariae. Since 2004, there has been an increasing number of reports of the incidence of P. knowlesi among humans in various countries in South East Asia, including Malaysia, Thailand
, Singapore, the Philippines
, Vietnam
, Myanmar
and Indonesia
.
Work with archival samples has shown that infection with this parasite has occurred in Malaysia at least since the 1990s and it is now known to cause 70% of the malaria cases in certain areas of Sarawak
.
Mosquito stages:
A mosquito ingests gametocytes, which have been formed in the mammalian host. These are either microgametocytes (which are male gametocytes) or macrogametocytes (which are female gametetocytes). These gametocytes mature into microgametes and macrogametes respectively, and then fertilize to form zygotes within the midgut of the mosquito. The zygotes mature into ookinetes, then into oocysts. Finally, the oocysts mature to release sporozoites which move to salivary gland of the mosquito.
Summary: gametocyte → (microgamete or macrogamete) → zygote → ookinete → oocyst → sporozoites.
In man: exoerythrocytic stage (in the liver): The sporozoites are injected into humans when the mosquito bites and they travel to the liver through blood stream and undergo asexual reproduction to become merozoites through schizonts in the liver cell. Hypnozoites in the liver has not yet been found.
Summary: sporozoites → schizonts → merozoites.
In man: erythrocytic stage (in the blood):
Merozoites are unleashed into the blood stream to infect erythrocytes constituting one asexual cycle of infection of the erythrocytes. Within the red blood cells some merozoites develop into trophozoites, which in turn mature into schizonts that rupture to release merozoites, while others develop into microgametocytes or macrogametocytes. These gametocytes remain in the blood to be ingested by mosquitoes.
Summary: Merozoite → trophozoite → schizont → merozoites.
This parasite is mostly found in South East Asian countries particularly in Borneo
, Cambodia
, Malaysia, Myanmar
, Philippines
, Singapore
, Thailand
and neighboring countries and it appears to occur in regions that are reportedly free of the other four types of human malaria. Infective mosquitoes are restricted to the forest areas. Non-infective mosquitoes are typically found in the urban areas but transmission may occur due to the abundance of mosquitoes in this region. particularly Malaysia, but there are also reports on the Thai-Burmese border. One fifth of the cases of malaria diagnosed in Sarawak
, Malaysian Borneo are due to P. knowlesi.
Plasmodium knowlesi is absent in Africa
. This may be because there are no long-tailed and pig-tailed macaques (the reservoir hosts of P. knowlesi) in Africa and many West Africans lack the Duffy antigen
- a protein on the surface of the red blood cell that the parasite to uses to invade.
P. knowlesi is the most common cause of malaria in childhood in the Kudat district of Sabah
, Malaysia.
The known vectors belong to the genus Anopheles
, subgenus Cellia, series Neomyzomyia and group Leucosphyrus. Mosquitoes of this group are typically found in forest areas in South East Asia but with a greater clearing of forest areas for farmland, humans are increasingly becoming exposed to these vectors.
Within the monkey population in Peninsular Malaysia
, Anopheles hackeri is believed to be the main vector of P. knowlesi: although A. hackeri is capable of transmitting malaria to humans, it is not normally attracted to humans and seems unlikely to be an important vector for transmission to humans.
Anopheles latens
is attracted to both macaques and humans and has been shown to be the main vector transmitting P. knowlesi to humans in the Kapit Division of Sarawak, Malaysian Borneo.
Anopheles cracens has also been reported as a vector of P. knowlesi. Both species of mosquitoes have been shown to contain as many as 1,000 sporozoites suggesting that they may be efficient vectors.
Symptoms typically begin approximately 11 days after an infected mosquito has bitten a person and the parasites can be seen in the blood between 10 – 12 days after infection. The parasite may multiply rapidly resulting in very high parasite densities that may be fatal.
Although the current infection rate with Plasmodium knowlesi is relatively low, one risk it presents is misdiagnosis with other forms of malarial parasites such as P. malariae especially when microscopy is used. P. knowlesi can only be accurately distinguished from P. malariae using PCR assay and/or molecular characterization.
Symptoms of P. knowlesi in humans include headache, fever, chills and cold sweats. Singh et al. (2004) showed clinical symptoms in 94 patients with single species P. knowlesi infection at Kapit Hospital, Sarawak, Malaysian Borneo. Symptoms included fever, chills, and rigor in 100% of patients, headache in 32%, cough in 18%, vomiting in 16%, nausea in 6%, and diarrhea in 4%. Asexual cycle of the parasite in humans and its natural host macaque is about 24 hours. Hence the disease may be called quotidian malaria, in concert with designation of tertian malaria and quartan malaria. In addition to a lab diagnosis using PCR assay, knowlesi malaria may also present itself with elevated levels of C-reactive protein and thrombocytopenia.
This parasite causes non-relapsing malaria
due to lack of hypnozoites in its exoerythrocytic stage.
While infection with this organism is normally not serious, life threatening complications or even death may occur in a minority of cases. The most common complications are respiratory distress, abnormal liver function including jaundice and renal failure. Mortality in one series of cases was about 2%.
s. The appearance of P. knowlesi is similar to that of P. malariae
and is unlikely to be correctly diagnosed except by using molecular detection assays in a malaria reference laboratory.
The morphology of Plasmodium knowlesi is similar to that of Plasmodium malariae
. P. malariae is characterized by a compact parasite (all stages) and does not alter the host erythrocyte's shape or size or cause enlargement. Elongated trophozoites stretching across the erythrocyte, called band forms, are sometimes observed. Schizonts will typically have 8-10 merozoites that are often arranged in a rosette pattern with a clump of pigment in the center.
Rapid diagnostic tests kits may or may not recognize P. knowlesi because of their specificity.
Currently PCR assay and molecular characterization are the most reliable methods for detecting and diagnosing P. knowlesi infection. PCR identifies the parasite DNA but this technique is not rapid and cannot be used for routine identification. PCR is also expensive and requires very specialized equipment.
and primaquine
. but if parasite counts are high and patients are severe, then infections should be treated as intensively as for severe falcipaum malaria.
In a clinical study of treatment where response was onserved after oral chloroquine was given for three days, and at 24 hours oral primaquine was administered for two consecutive days., it was found that this regime gave a rapid response with a median time to parasite clearance of three hours. This was more rapid that is found in Plasmodium vivax
malaria where the median time to clearance is between six and seven hours.
, potassium
, lactate dehydrogenase
and amino transferase values. Dengue fever was suspected but ruled out on investigation. Malarial parasites were seen on the blood film and later identified as Plasmodium knowlesi by PCR. At post mortum the liver
and spleen
were enlarged. The brain and endocardium
showed multiple petechia
l haemorrhages. The lungs had features consistent with acute respiratory distress syndrome
. Histological examination showed sequestration of pigmented parasitized red blood cells in the vessels of the cerebrum, cerebellum
, heart
and kidney
without evidence of chronic inflammatory reaction in the brain or any other organ examined. The spleen and liver had abundant pigment containing macrophages and parasitized red blood cells. The kidney had evidence of acute tubular necrosis. Endothelial cells in heart sections were prominent. Brain sections were negative for intracellular adhesion molecule-1.
The overall post mortem picture was very similar to that found in cases of Plasmodium falciparum
. There were important differences including the absence of coma despite petechial haemorrhages and parasite sequestration in the brain.
Primate
A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...
malaria
Malaria
Malaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...
parasite commonly found in Southeast Asia
Southeast Asia
Southeast Asia, South-East Asia, South East Asia or Southeastern Asia is a subregion of Asia, consisting of the countries that are geographically south of China, east of India, west of New Guinea and north of Australia. The region lies on the intersection of geological plates, with heavy seismic...
. It causes malaria in long-tailed macaques (Macaca fascicularis), but it may also infect humans, either naturally or artificially.
Plasmodium knowlesi is the fifth major human malaria parasite. It may cause severe malaria as indicated by its asexual erythrocytic cycle of about 24 hours. The typical fever becomes quotidian. This is an emerging infection that was reported for the first time in humans in 1965. It accounts for up to 70% of malaria cases in certain areas in South East Asia where it is mostly found. This parasite is transmitted by the bite of an Anopheles
Anopheles
Anopheles is a genus of mosquito. There are approximately 460 recognized species: while over 100 can transmit human malaria, only 30–40 commonly transmit parasites of the genus Plasmodium, which cause malaria in humans in endemic areas...
mosquito. Plasmodium knowlesi has health, social and economic consequences for the regions affected by it.
History of discovery
The first person to see P. knowlesi was probably the Italian Giuseppe FranchitiGiuseppe Franchiti
Giuseppe Franchiti was an Italian parasitologist who worked on malaria and leishmaniasis.He succeeded Alphonse Laveran as the head of the Laboratory of Institut Pasteur in Paris in 1922. He was appointed Professor...
in 1927 when he was examining the blood of Macaca fasicularis and he noted that it differed from Plasmodium cynomogli and Plasmodium inui
Plasmodium inui
Plasmodium inui is a protozoan parasite, one of the species of simian Plasmodium that cause malaria in Old World monkeys.-Hosts:* Assamese macques * long tailed macques -Life cycle:...
. It was later seen by Campbell in 1931 in a long-tailed macaque imported from Singapore to the Calcutta School of Tropical Medicine and Hygiene in India. Campbell was interested in kala azar and was working under Napier. Napier inoculated
Inoculation
Inoculation is the placement of something that will grow or reproduce, and is most commonly used in respect of the introduction of a serum, vaccine, or antigenic substance into the body of a human or animal, especially to produce or boost immunity to a specific disease...
the strain into three monkeys, one of which was a rhesus macaque (Macaca mulatta), which developed a fulminating infection. Knowing that the Protozoological Department were looking for a monkey malaria strain, they handed the original infected monkey to Dr B. M. Das Gupta, who was Dr R Knowles' assistant. Dr Das Gupta maintained the species by serial passage in monkeys until Dr Knowlesi returned from leave. In 1932, Knowles and Das Gupta described the species in detail for the first time and showed that it could be transmitted to man by blood passage, but failed to name it. It was named by Sinton and Mulligan in 1932 after Dr Knowles. From early in the 1930s to 1955, P. knowlesi was used as a pyretic agent for the treatment of patients with neurosyphillis.
In 1957, it was suggested by Garnham et al. that P. knowelsi could be the fifth species capable of causing endemic malaria in humans.
In 1965, the first case of a naturally occurring infection of knowlesi malaria in humans was reported in an American man who had returned after working in the jungle in peninsular Malaysia. Although the infecting parasite was initially identified as P. falciparum, one day later it was then identified as P. malariae and it was only confirmed to be P. knowlesi after infected blood was used to inoculate Rhesus monkeys. A second report emerged in 1971 about the natural infection of a man in Malaysia with Plasmodium knowlesi followed by the description of a large focus of human infections in the Kapit Division of Sarawak, Malaysian Borneo. This was made possible due to the development of molecular detection assays which could differentiate between Plasmodium knowlesi and the morphologicall similar Plasmodium malariae. Since 2004, there has been an increasing number of reports of the incidence of P. knowlesi among humans in various countries in South East Asia, including Malaysia, Thailand
Thailand
Thailand , officially the Kingdom of Thailand , formerly known as Siam , is a country located at the centre of the Indochina peninsula and Southeast Asia. It is bordered to the north by Burma and Laos, to the east by Laos and Cambodia, to the south by the Gulf of Thailand and Malaysia, and to the...
, Singapore, the Philippines
Philippines
The Philippines , officially known as the Republic of the Philippines , is a country in Southeast Asia in the western Pacific Ocean. To its north across the Luzon Strait lies Taiwan. West across the South China Sea sits Vietnam...
, Vietnam
Vietnam
Vietnam – sometimes spelled Viet Nam , officially the Socialist Republic of Vietnam – is the easternmost country on the Indochina Peninsula in Southeast Asia. It is bordered by China to the north, Laos to the northwest, Cambodia to the southwest, and the South China Sea –...
, Myanmar
Myanmar
Burma , officially the Republic of the Union of Myanmar , is a country in Southeast Asia. Burma is bordered by China on the northeast, Laos on the east, Thailand on the southeast, Bangladesh on the west, India on the northwest, the Bay of Bengal to the southwest, and the Andaman Sea on the south....
and Indonesia
Indonesia
Indonesia , officially the Republic of Indonesia , is a country in Southeast Asia and Oceania. Indonesia is an archipelago comprising approximately 13,000 islands. It has 33 provinces with over 238 million people, and is the world's fourth most populous country. Indonesia is a republic, with an...
.
Work with archival samples has shown that infection with this parasite has occurred in Malaysia at least since the 1990s and it is now known to cause 70% of the malaria cases in certain areas of Sarawak
Sarawak
Sarawak is one of two Malaysian states on the island of Borneo. Known as Bumi Kenyalang , Sarawak is situated on the north-west of the island. It is the largest state in Malaysia followed by Sabah, the second largest state located to the North- East.The administrative capital is Kuching, which...
.
Evolution
Based on a Bayesian coalescent approach the most probable time of evolution of P. knowelsi is 257,000 years ago (95% range 98,000–478,000).Life cycle
Plasmodium knowlesi parasite replicates and completes its blood stage cycle in 24 hour cycles resulting in fairly high loads of parasite densities in a very short period of time. This makes it a potentially very severe disease if it remains untreated. Life cycle: merozoite → trophozoites → schizont → merozoite. These stages of Plasmodium knowlesi are microscopically indistinguishable from Plasmodium malariae and the early trophozoites are identical to those of Plasmodium falciparum .Mosquito stages:
A mosquito ingests gametocytes, which have been formed in the mammalian host. These are either microgametocytes (which are male gametocytes) or macrogametocytes (which are female gametetocytes). These gametocytes mature into microgametes and macrogametes respectively, and then fertilize to form zygotes within the midgut of the mosquito. The zygotes mature into ookinetes, then into oocysts. Finally, the oocysts mature to release sporozoites which move to salivary gland of the mosquito.
Summary: gametocyte → (microgamete or macrogamete) → zygote → ookinete → oocyst → sporozoites.
In man: exoerythrocytic stage (in the liver): The sporozoites are injected into humans when the mosquito bites and they travel to the liver through blood stream and undergo asexual reproduction to become merozoites through schizonts in the liver cell. Hypnozoites in the liver has not yet been found.
Summary: sporozoites → schizonts → merozoites.
In man: erythrocytic stage (in the blood):
Merozoites are unleashed into the blood stream to infect erythrocytes constituting one asexual cycle of infection of the erythrocytes. Within the red blood cells some merozoites develop into trophozoites, which in turn mature into schizonts that rupture to release merozoites, while others develop into microgametocytes or macrogametocytes. These gametocytes remain in the blood to be ingested by mosquitoes.
Summary: Merozoite → trophozoite → schizont → merozoites.
Epidemiology
P. knowlesi infection is normally considered a parasite of long-tailed (Macaca fascicularis) and pig-tailed (Macaca nemestrina) macaques but humans who work at the forest fringe or enter the rainforest to work are at risk of infection. With the increasing popularity of deforestation and development efforts in South East Asia, many macaques are now coming in close and direct contact with humans. Hence more and more people who live in the semi-urban areas are being found to be infected with knowlesi malaria.This parasite is mostly found in South East Asian countries particularly in Borneo
Borneo
Borneo is the third largest island in the world and is located north of Java Island, Indonesia, at the geographic centre of Maritime Southeast Asia....
, Cambodia
Cambodia
Cambodia , officially known as the Kingdom of Cambodia, is a country located in the southern portion of the Indochina Peninsula in Southeast Asia...
, Malaysia, Myanmar
Myanmar
Burma , officially the Republic of the Union of Myanmar , is a country in Southeast Asia. Burma is bordered by China on the northeast, Laos on the east, Thailand on the southeast, Bangladesh on the west, India on the northwest, the Bay of Bengal to the southwest, and the Andaman Sea on the south....
, Philippines
Philippines
The Philippines , officially known as the Republic of the Philippines , is a country in Southeast Asia in the western Pacific Ocean. To its north across the Luzon Strait lies Taiwan. West across the South China Sea sits Vietnam...
, Singapore
Singapore
Singapore , officially the Republic of Singapore, is a Southeast Asian city-state off the southern tip of the Malay Peninsula, north of the equator. An island country made up of 63 islands, it is separated from Malaysia by the Straits of Johor to its north and from Indonesia's Riau Islands by the...
, Thailand
Thailand
Thailand , officially the Kingdom of Thailand , formerly known as Siam , is a country located at the centre of the Indochina peninsula and Southeast Asia. It is bordered to the north by Burma and Laos, to the east by Laos and Cambodia, to the south by the Gulf of Thailand and Malaysia, and to the...
and neighboring countries and it appears to occur in regions that are reportedly free of the other four types of human malaria. Infective mosquitoes are restricted to the forest areas. Non-infective mosquitoes are typically found in the urban areas but transmission may occur due to the abundance of mosquitoes in this region. particularly Malaysia, but there are also reports on the Thai-Burmese border. One fifth of the cases of malaria diagnosed in Sarawak
Sarawak
Sarawak is one of two Malaysian states on the island of Borneo. Known as Bumi Kenyalang , Sarawak is situated on the north-west of the island. It is the largest state in Malaysia followed by Sabah, the second largest state located to the North- East.The administrative capital is Kuching, which...
, Malaysian Borneo are due to P. knowlesi.
Plasmodium knowlesi is absent in Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
. This may be because there are no long-tailed and pig-tailed macaques (the reservoir hosts of P. knowlesi) in Africa and many West Africans lack the Duffy antigen
Duffy antigen
Duffy antigen/chemokine receptor also known as Fy glycoprotein or CD234 is a protein that in humans is encoded by the DARC gene....
- a protein on the surface of the red blood cell that the parasite to uses to invade.
P. knowlesi is the most common cause of malaria in childhood in the Kudat district of Sabah
Sabah
Sabah is one of 13 member states of Malaysia. It is located on the northern portion of the island of Borneo. It is the second largest state in the country after Sarawak, which it borders on its southwest. It also shares a border with the province of East Kalimantan of Indonesia in the south...
, Malaysia.
Vectors
Theoretically there are four modes of transmission: from an infected monkey to another monkey, from an infected monkey to a human, from an infected human to another human and from an infected human back to a monkey. In practice human malaria appears to be almost entirely due to monkey to human transmission.The known vectors belong to the genus Anopheles
Anopheles
Anopheles is a genus of mosquito. There are approximately 460 recognized species: while over 100 can transmit human malaria, only 30–40 commonly transmit parasites of the genus Plasmodium, which cause malaria in humans in endemic areas...
, subgenus Cellia, series Neomyzomyia and group Leucosphyrus. Mosquitoes of this group are typically found in forest areas in South East Asia but with a greater clearing of forest areas for farmland, humans are increasingly becoming exposed to these vectors.
Within the monkey population in Peninsular Malaysia
Peninsular Malaysia
Peninsular Malaysia , also known as West Malaysia , is the part of Malaysia which lies on the Malay Peninsula. Its area is . It shares a land border with Thailand in the north. To the south is the island of Singapore. Across the Strait of Malacca to the west lies the island of Sumatra...
, Anopheles hackeri is believed to be the main vector of P. knowlesi: although A. hackeri is capable of transmitting malaria to humans, it is not normally attracted to humans and seems unlikely to be an important vector for transmission to humans.
Anopheles latens
Anopheles latens
The Anopheles latens mosquito is an important vector for the transmission of malaria in humans and monkeys in Southeast Asia...
is attracted to both macaques and humans and has been shown to be the main vector transmitting P. knowlesi to humans in the Kapit Division of Sarawak, Malaysian Borneo.
Anopheles cracens has also been reported as a vector of P. knowlesi. Both species of mosquitoes have been shown to contain as many as 1,000 sporozoites suggesting that they may be efficient vectors.
Clinical
Two possible modes of transmission to humans have been proposed: either from an infected monkey to a human or from an infected human to another human.Symptoms typically begin approximately 11 days after an infected mosquito has bitten a person and the parasites can be seen in the blood between 10 – 12 days after infection. The parasite may multiply rapidly resulting in very high parasite densities that may be fatal.
Although the current infection rate with Plasmodium knowlesi is relatively low, one risk it presents is misdiagnosis with other forms of malarial parasites such as P. malariae especially when microscopy is used. P. knowlesi can only be accurately distinguished from P. malariae using PCR assay and/or molecular characterization.
Symptoms of P. knowlesi in humans include headache, fever, chills and cold sweats. Singh et al. (2004) showed clinical symptoms in 94 patients with single species P. knowlesi infection at Kapit Hospital, Sarawak, Malaysian Borneo. Symptoms included fever, chills, and rigor in 100% of patients, headache in 32%, cough in 18%, vomiting in 16%, nausea in 6%, and diarrhea in 4%. Asexual cycle of the parasite in humans and its natural host macaque is about 24 hours. Hence the disease may be called quotidian malaria, in concert with designation of tertian malaria and quartan malaria. In addition to a lab diagnosis using PCR assay, knowlesi malaria may also present itself with elevated levels of C-reactive protein and thrombocytopenia.
This parasite causes non-relapsing malaria
Malaria
Malaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...
due to lack of hypnozoites in its exoerythrocytic stage.
While infection with this organism is normally not serious, life threatening complications or even death may occur in a minority of cases. The most common complications are respiratory distress, abnormal liver function including jaundice and renal failure. Mortality in one series of cases was about 2%.
Diagnosis
P. knowlesi infection is diagnosed by examining thick and thin blood films in the same way as other malariaMalaria
Malaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...
s. The appearance of P. knowlesi is similar to that of P. malariae
Plasmodium malariae
Plasmodium malariae is a parasitic protozoa that causes malaria in humans. It is closely related to Plasmodium falciparum and Plasmodium vivax which are responsible for most malarial infection. While found worldwide, it is a so-called "benign malaria" and is not nearly as dangerous as that...
and is unlikely to be correctly diagnosed except by using molecular detection assays in a malaria reference laboratory.
The morphology of Plasmodium knowlesi is similar to that of Plasmodium malariae
Plasmodium malariae
Plasmodium malariae is a parasitic protozoa that causes malaria in humans. It is closely related to Plasmodium falciparum and Plasmodium vivax which are responsible for most malarial infection. While found worldwide, it is a so-called "benign malaria" and is not nearly as dangerous as that...
. P. malariae is characterized by a compact parasite (all stages) and does not alter the host erythrocyte's shape or size or cause enlargement. Elongated trophozoites stretching across the erythrocyte, called band forms, are sometimes observed. Schizonts will typically have 8-10 merozoites that are often arranged in a rosette pattern with a clump of pigment in the center.
Rapid diagnostic tests kits may or may not recognize P. knowlesi because of their specificity.
Currently PCR assay and molecular characterization are the most reliable methods for detecting and diagnosing P. knowlesi infection. PCR identifies the parasite DNA but this technique is not rapid and cannot be used for routine identification. PCR is also expensive and requires very specialized equipment.
Treatment
Because P. knowlesi takes only 24 hours to complete its erythrocytic cycle, it can result in very high parasite density quickly and may be fatal in humans. For this reason early treatment is advised. It responds well to treatment with chloroquineChloroquine
Chloroquine is a 4-aminoquinoline drug used in the treatment or prevention of malaria.-History:Chloroquine , N'--N,N-diethyl-pentane-1,4-diamine, was discovered in 1934 by Hans Andersag and co-workers at the Bayer laboratories who named it "Resochin". It was ignored for a decade because it was...
and primaquine
Primaquine
Primaquine is a medication used in the treatment of malaria and Pneumocystis pneumonia. It is a member of the 8-aminoquinoline group of drugs that includes tafenoquine and pamaquine.-Radical cure:...
. but if parasite counts are high and patients are severe, then infections should be treated as intensively as for severe falcipaum malaria.
In a clinical study of treatment where response was onserved after oral chloroquine was given for three days, and at 24 hours oral primaquine was administered for two consecutive days., it was found that this regime gave a rapid response with a median time to parasite clearance of three hours. This was more rapid that is found in Plasmodium vivax
Plasmodium vivax
Plasmodium vivax is a protozoal parasite and a human pathogen. The most frequent and widely distributed cause of recurring malaria, P. vivax is one of the four species of malarial parasite that commonly infect humans. It is less virulent than Plasmodium falciparum, which is the deadliest of the...
malaria where the median time to clearance is between six and seven hours.
Public health, Prevention strategies and Vaccines
- Mosquito bed nets
- Medication – MefloquineMefloquineMefloquine hydrochloride is an orally administered medication used in the prevention and treatment of malaria. Mefloquine was developed in the 1970s at the United States Department of Defense's Walter Reed Army Institute of Research as a synthetic analogue of quinine...
, ChloroquineChloroquineChloroquine is a 4-aminoquinoline drug used in the treatment or prevention of malaria.-History:Chloroquine , N'--N,N-diethyl-pentane-1,4-diamine, was discovered in 1934 by Hans Andersag and co-workers at the Bayer laboratories who named it "Resochin". It was ignored for a decade because it was... - Vector control
- Residual spraying using insecticides
Pathology
A single post mortem case has been described to date The patient was a male who became unwell 10 days after exposure. After four days he presented acutely unwell to a hospital. He was found to have a raised eosinophil count, to be thrombocytopaenic, hyponatraemic with an elevated blood ureaUrea
Urea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....
, potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
, lactate dehydrogenase
Lactate dehydrogenase
Lactate dehydrogenase is an enzyme present in a wide variety of organisms, including plants and animals.Lactate dehydrogenases exist in four distinct enzyme classes. Two of them are cytochrome c-dependent enzymes, each acting on either D-lactate or L-lactate...
and amino transferase values. Dengue fever was suspected but ruled out on investigation. Malarial parasites were seen on the blood film and later identified as Plasmodium knowlesi by PCR. At post mortum the liver
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
and spleen
Spleen
The spleen is an organ found in virtually all vertebrate animals with important roles in regard to red blood cells and the immune system. In humans, it is located in the left upper quadrant of the abdomen. It removes old red blood cells and holds a reserve of blood in case of hemorrhagic shock...
were enlarged. The brain and endocardium
Endocardium
The endocardium is the innermost layer of tissue that lines the chambers of the heart. Its cells are embryologically and biologically similar to the endothelial cells that line blood vessels....
showed multiple petechia
Petechia
A petechia is a small red or purple spot on the body, caused by a minor hemorrhage ."Petechiae" refers to one of the three major classes of purpuric skin conditions. Purpuric eruptions are classified by size into three broad categories...
l haemorrhages. The lungs had features consistent with acute respiratory distress syndrome
Acute respiratory distress syndrome
Acute respiratory distress syndrome , also known as respiratory distress syndrome or adult respiratory distress syndrome is a serious reaction to various forms of injuries to the lung....
. Histological examination showed sequestration of pigmented parasitized red blood cells in the vessels of the cerebrum, cerebellum
Cerebellum
The cerebellum is a region of the brain that plays an important role in motor control. It may also be involved in some cognitive functions such as attention and language, and in regulating fear and pleasure responses, but its movement-related functions are the most solidly established...
, heart
Heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...
and kidney
Kidney
The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...
without evidence of chronic inflammatory reaction in the brain or any other organ examined. The spleen and liver had abundant pigment containing macrophages and parasitized red blood cells. The kidney had evidence of acute tubular necrosis. Endothelial cells in heart sections were prominent. Brain sections were negative for intracellular adhesion molecule-1.
The overall post mortem picture was very similar to that found in cases of Plasmodium falciparum
Plasmodium falciparum
Plasmodium falciparum is a protozoan parasite, one of the species of Plasmodium that cause malaria in humans. It is transmitted by the female Anopheles mosquito. Malaria caused by this species is the most dangerous form of malaria, with the highest rates of complications and mortality...
. There were important differences including the absence of coma despite petechial haemorrhages and parasite sequestration in the brain.
Useful Web Links
- 1. CDC: cdc.gov/malaria, http://www.cdc.gov/EID/content/14/11/1750.htm
- 2. WHO: http://www.who.int/topics/malaria/en/
- 3. Short film on the discovery of the role of P. knowlesi in human malaria: http://www.abc.net.au/catalyst/stories/2533454.htm