Plasmodium knowlesi
The 4 Plasmodium species known to cause malaria include Plasmodium falciparum,
Plasmodium vivax, Plasmodium ovale, and Plasmodium malariae. A fifth species,
Plasmodium knowlesi, is typically found in nature as macaques (monkeys) and has
recently been identified as a clinically significant pathogen in humans. This is an
emerging infection that was reported for the first time in humans in 1965[1] . It accounts
for up to 70% [2] of malaria cases in South East Asia particularly the forested areas where
it is mostly found. This parasite is transmitted by the bite of an 'Anopheles leucosphyrus'
mosquito.
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 the
infected blood was used to inoculate Rhesus monkeys[3]
Transmission
Transmission is constant due to abundance of the Anopheles l. Plasmodium knowlesi
appears to occur in regions that are reportedly free of the other four types of human
malaria. There are two possible modes of transmission to humans which are either from
an infected monkey to a human or from an infected human to another human. The
gametocytes of the parasites can be seen in the blood between 10 – 12 days of infection.
Plasmodium knowlesi is less prevalent in Africa because it appears that West African
blacks are les likely to be infected by plasmodium knowlesi. This seems to be the case
because many West African blacks have Duffy-negative antigen which may be a factor
that decreases the ability of the parasite to bind to red blood cells.
Pathogenesis/Incubation Period
Incubation period for P. knowlesi is about 12 days - the shortest known malaria of all the
known malarias that infect humans and primates.[4] P knowlesi has a 24 hour asexual
cycle.
Clinical Presentation in Humans
Symptoms of P. knowlesi in humans include headache, fever, chills and cold sweats[5] . In
addition to a lab diagnosis using PCR array, Malaria knowlesi could also present itself
with elevated levels of C-reactive protein and thrombocytopenia in the patient [6].
Morphology
The morphology of Plasmodium knowlesi is similar to that of P. malariae. P, malariae is
characterized by a compact parasite (all stages) and does not alter the host erythrocyte or
cause enlargment. 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[7].
Diagnosis and Diagnostic Tests
P. knowlesi infections is diagnosed by examining thick and thin blood films in the same
way as other malaria. The appearance of P. knowlesi is similar to that of P. malaria and is
unlikely to be correctly diagnosed except by using molecular detection assays [8]in a
malaria reference laboratory.
There are several methods for detecting and diagnosing P. knowlesi [9]. At the moment,
PCR assay and molecular characterization are the most reliable methods for detecting P.
knowlesi . PCR identifies the parasite protein but this technique is not rapid and cannot
be used for routine identification. Rapid diagnostic tests kits may or may not recognize P.
knowlesi because of its specificity. Microscopy techniques can also be used to detect the
presence of the P. knowlesi parasite in erythrocytes however this technique is not reliable
because P. knowlesi can also be confused with P. malariae because of their
morphological similarities.
Treatment
Plasmodium knowlesi is most effectively treated if the infection is detected early. P.
knowlesi takes only 24 hours to complete its life cycle, which means it can result in very
high parasite density in a short amount of time, which can be fatal in humans . However,
the disease can be treated using already existing anti-malaria therapy such as mefloquine
and chloroquine. Chloroquine is specifically preferred for non-complicated infections
References
1. Chin W, Contacos PG, Coatney RG, Kimbal HR, 1965. Naturally acquired
quotidian-type malaria in man transferable to monkeys. Science 149:865.
2. McCutchan TF, Piper RC, Makler MT, 2008. Use of Malaria Rapid Diagnostic
Test to Identify Plasmodium knowlesi Infection. Emerging Infectious Diseases
14:11.
3. Coggeshall L.T. (1940) The occurrence of malaria antibodies in human serum
following induced infection with Plasmodium knowlesi. The Laboratories of the
International Health Division of the Rockefeller Foundation, New York
4. Bronner U, Divis PCS, Färnert A, Singh B, 2009. Swedish traveller with
Plasmodium knowlesi malaria after visiting Malaysian Borneo Malaria Journal
8:15.
5. Bronner U, Divis PCS, Färnert A, Singh B, 2009. Swedish traveller with
Plasmodium knowlesi malaria after visiting Malaysian Borneo Malaria Journal
2009, 8:15.
6. Bronner U, Divis PCS, Färnert A, Singh B, 2009. Swedish traveller with
Plasmodium knowlesi malaria after visiting Malaysian Borneo Malaria Journal
2009; 8:15
7. ^ http://www.tulane.edu/~wiser/protozoology/notes/pl_sp.html#morph
8. Singh B, Lee KS, Matusop A, Radhakrishnan A, Shamsul SSG, Cox-Singh J,
Thomas A, Conway DJ (2004). "A large focus of naturally acquired Plasmodium
knowlesi infections in human beings". Lancet 363: 1017–24. doi:10.1016/S01406736(04)15836-4
9. Bronner U, Divis PCS, Färnert A, Singh B, 2009. Swedish traveller with
Plasmodium knowlesi malaria after visiting Malaysian Borneo Malaria Journal
2009; 8:15.
Case report of Swedish Traveller with Plasmodium knowlesi malaria after visiting
Malaysian Borneo, published on 16th January 2009 can be traced through the following
link
http://www.malariajournal.com/content/8/1/15