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Malaria is a serious mosquito borne, febrile illness due to infection of red blood cells with a Plasmodium parasite. Five species of Plasmodium regularly infect humans:
Of the five species, Plasmodium falciparum is the most severe and is responsible for most deaths. However, in recent years Plasmodium knowlesi is increasingly giving cause for concern because it too can cause severe illness.
P. knowlesi (hereafter termed as knowlesi) is a natural parasite of macaque monkeys found in South East Asia. So far there have been no recorded cases of knowlesi anywhere that macaques are not found (2); human – human transmission has not been established and the disease therefore remains classed as a zoonotic infection i.e. a disease transmitted (by mosquitoes) from animals to humans. Since 2004 there have been increasing reports of knowlesi in Malaysia, Thailand, Philippines, Vietnam, Indonesia and Myanmar; it is now the most common type of malaria found in Malaysia and Western Indonesia – countries that are nearing the elimination of malaria due to P. falciparum and P. vivax. (3)
It is thought that knowlesi is under-diagnosed; diagnosis by microscopy is difficult and it may be mistaken as P.malariae – a much less severe form of disease.
Non specific symptoms of knowlesi infection are the same as for other malarias but because the knowlesi parasites can grow and divide quickly once in the red blood cells (over 24 hours), infection can progress rapidly into severe disease. Of note, severe illness is more likely in older people (>45 years). (4)
Knowlesi drug resistance is not reported. Standard antimalarial treatments are effective. Full information can be found in UK Malaria Treatment Guidelines. (5)
Where knowlesi is reported, TRAVAX will indicate this on the country specific advice pages under malaria advice. This includes countries that are considered *malaria free, but where knowlesi is reported such as Brunei and Singapore. (6)
[* The ‘malaria free’ status of a country refers to the lack of cases of malaria transmitted human to human. Knowlesi is transmitted animal to human and is thus not considered in the determination of malaria-free status of a country.]
The mainstay of prevention of knowlesi is the same as for all other forms of malaria and is based around:
All travellers visiting areas where malaria is transmitted should be given the A, B, C, D of malaria prevention advice. For travellers going to countries classed as “malaria free” but where knowlesi is reported (Brunei and Singapore), antimalarials are not currently advised, but awareness of risk, bite avoidance and prompt investigation of fever are important.
The Travel and International Health Team would like to thank Dr Lisa Ranford-Cartwright (University of Glasgow: Institute of Biodiversity, Animal Health & Comparative Medicine) for providing the presentation: “Plasmodium Knowlesi: Should we be thinking about it for travellers?” Glasgow, August 2019.
References
1. Chiodini P, Patel D, Whitty C. Guidelines for Malaria Prevention in Travellers from the United Kingdom. Public Health England. August 2019. https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/833506/ACMP_Guidelines.pdf [ Accessed November 2019].
2. Singh B & Daneshvar C (2013). Human infections and detection of Plasmodium Knowlesi. Clin Microbiol Rev, 26(2), 165-184
3. WHO. World Malaria Report 2018. https://www.who.int/malaria/media/world-malaria-report-2018/en/ [Accessed November 2019]
4. Grigg M.J, William T, Barber B.E. et al (2018). Age-related Clinical Spectrum of Plasmodium knowlesi Malaria and Predictors of Severity. Clin Infec Dis,67(3), 350-359.
5. Lalloo D, Shingadia D, Bell D et al (2016). UK Malaria Treatment Guidelines 2016. Journal of Infection, 72 (6), 635-649
6. Koh G.J, Ismail P.K, Koh D (2019). Occupationally Acquired Plasmodium knowlesi Malaria in Brunei Darussalem. Saf Health Work, 10(1), 122-124.
