Hello!
For this week’s NTD of the week we’re going to give a little introduction to malaria, a deadly disease caused by parasites that are transmitted to people by mosquitoes [1].
The disease earned its name from an early theory that the disease was caused by bad air (Mal-aria).
Malaria has been around for much longer than you may think. Ancient writings show the presence of malaria a very long time ago, with documents depicting key symptoms of malaria. For example, in 400 BC, Hippocrates describes the ‘unhealthiness of air’ and other symptoms of malaria [2].
Transmission
Malaria is caused by a parasite that is a protist (single-celled organism) called Plasmodium, of which there are five species known to infect humans: P. falciparum, P. vivax, P. ovale, P. malariae and P. knowlesi. The disease pathology and hence treatment can differ across the Plasmodium species. Plasmodium falciparum is the most prevalent species worldwide, owing to the majority of annual malaria infections.
An infected female Anopheles mosquito takes a blood meal from a human and transmits the parasite to the human host
The parasites travel in the bloodstream and infect the liver cells, where they develop and rupture the cells, releasing more parasites
The parasites then enter red blood cells (RBC) and multiply, causing the RBC to rupture
It is the rupturing of RBC that causes the clinical manifestations of the disease
The parasites in the blood are then taken up by the next mosquito when they come to take a blood meal
The parasites multiply inside the mosquito and eventually make their way to the salivary gland of the mosquito - ready to transmit to the next human host
[3]
Some species of malaria (P. vivax and P. ovale) can cause a relapse if left untreated by remaining dormant in the liver cells and invading the bloodstream weeks, months or years later!
Here is a microscope image of Plasmodium multiplying inside a red blood cell!
Image: Aimee Whalley
Common symptoms
A febrile illness develops approximately 10 days after the bite from the infected mosquito. Symptoms can initially present themselves as chills, sweating and aching. It is important these signs are recognised and treated early to prevent severe complications and fatality.
Severe complications
Anaemia: when red blood cells cannot carry sufficient oxygen, leading to tiredness.
Cerebral malaria: blood vessels in the brain become blocked, and this can lead to brain damage and coma.
Treatment
A prompt diagnosis of malaria in the blood is required using a rapid diagnostic test (RDT) or microscopy. The Plasmodium species must be identified to ensure the correct drug is given. The drug administered will depend on the species of parasite, your symptoms, your age and whether you are pregnant. The most widely administered drugs are chloroquine and artemisinin-based combination therapies (ACTs). [1]
Burden
As of November 2020 there were approximately 229 million cases of malaria globally, an increase from 2018 which had 228 million cases [1]. In 2019, there were an estimated 409 000 deaths due to malaria, of which the majority were young children [1]. The disease poses a particularly huge burden in the WHO African region, with 94% of global malaria cases found in these regions in 2019 [4]. Malaria poses the highest risk to young children and pregnant women, who have lower immunities to the disease.
Control and Prevention
Vector control
The main way to prevent malaria transmission is through control of the vector by controlling adult mosquitoes and preventing breeding. There are several methods, but the most effective are Long Lasting Insecticidal Nets (LLINs) and Indoor Residual Spraying (IRS), which
together prevented 663 million malaria cases worldwide between the years 2000-2015 [5].
LLINs: bed nets impregnated with insecticide that is lethal to mosquitoes. These nets repel mosquitoes and prevent them biting human hosts when they sleep. They are the most widely used control tool as they have a mass community effect.
IRS: insecticide residue is sprayed on indoor surfaces of dwellings and kills mosquitoes when they come to rest.
Antimalarial drugs
Intermittent preventive treatment can be given to high risk groups (pregnant women and infants below the age of five) in high transmission areas during transmission seasons.
Travellers should take a course of antimalarials when travelling to high risk areas, to prevent malaria, as they have a low immunity to the disease.
New challenges
Insecticide resistance
Many mosquito species have developed resistance to the insecticides used in vector control and this resistance is spreading across endemic areas, compromising the effectiveness of LLINs and IRS. In 2019, 73 endemic countries reported that there were mosquitoes resistant to at least one type of insecticide [6].
Antimalarial drug resistance
Resistance to antimalarial drugs has been seen in both P. falciparum and P. vivax species. Resistance to the drug chloroquine is most prevalent globally, though less widespread resistance to other antimalarials has been documented in P. falciparum.
COVID-19
The COVID-19 pandemic has disrupted malaria services including large IRS campaigns, the distribution of LLINs and antimalarial treatment as they must be conducted in adherence to COVID-19 restrictions. The true impact COVID-19 has had on the transmission of malaria will be clearer over time, but an increase in cases and deaths over the last year is likely.
Future
Despite great collective efforts worldwide to prevent malaria transmission, there is still a long way to go to achieve the 2030 and 2040 elimination and eradication goals. Malaria is a completely preventable disease and it is vital we work together to stop further suffering.
Elimination = interruption of disease transmission in a given area due to the input of control tools
Eradication = permanent zero incidence of disease transmission worldwide, with no need to continue with control efforts
2030 Elimination Goal = no reported malaria cases in at least 33 endemic countries
2040 Eradication Goal = elimination of malaria worldwide
In order to eliminate malaria:
Governments across the world must invest financial and human resources into control and prevention campaigns, as currently the funding is not enough to overcome the sheer scope of the problem.
Surveillance systems put in place in every endemic country to improve readiness to respond to outbreaks. This will include monitoring trends in disease transmission and resistance to antimalarials and insecticides amongst mosquito populations.
Tackle the problem of insecticide resistance via discovering novel insecticides and insecticide resistance management plans put in place.
The development of innovative new control tools to create a toolbox of control methods that can be used if current tools begin to lose their efficacy.
There is currently one malaria vaccine, RTS,S, that works against P. falciparum. It has been used in pilot immunisation programmes against young infants, and has shown an approximate 40% malaria case reduction in Kenya [7]. This vaccine is a beacon of hope for the future of malaria prevention and hopefully will be administered in other endemic countries in the future.
There is lots more to discuss about malaria, including the effects of climate change, the latest control tools being developed, the biology of mosquitoes, social and economical impact and much more! We hope this short introduction has made you curious for more malaria information.
- A and L
FACT CHECK
Malaria is caused by a parasite of the Plasmodium species
The parasite is transmitted to a human host via a bite from an infected female Anopheles mosquito
Children under the age of 5 are the most at risk group, due to the lack of immunity to the disease
The highest burden falls to WHO Africa regions, where 94% of malaria cases and deaths are found
Malaria transmission is largely prevented via two vector control methods: LLINs and IRS
Abbreviations
NTD = Neglected Tropical Disease
VBD = Vector Borne Disease
LLIN = Long-Lasting Insecticidal Net
IRS = Indoor Residual Spraying
RBC = Red Blood Cell
WHO = World Health Organisation
References
World Health Organisation, (2020). Malaria https://www.who.int/news-room/fact-sheets/detail/malaria
Hempelmann, E., & Krafts, K. (2013). Bad air, amulets and mosquitoes: 2,000 years of changing perspectives on malaria. Malaria journal, 12, 232. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3723432/
Centers for Disease Control and Prevention,(2020) Malaria https://www.cdc.gov/parasites/malaria/index.html
World Health Organisation, (2020). World malaria report 2020 https://www.who.int/publications/i/item/9789240015791
Bhatt, S., Weiss, D. J., Cameron, E., Bisanzio, D., Mappin, B., Dalrymple, U., Battle, K., Moyes, C. L., Henry, A., Eckhoff, P. A., Wenger, E. A., Briët, O., Penny, M. A., Smith, T. A., Bennett, A., Yukich, J., Eisele, T. P., Griffin, J. T., Fergus, C. A., Lynch, M., … Gething, P. W. (2015). The effect of malaria control on Plasmodium falciparum in Africa between 2000 and 2015. Nature, 526(7572), 207–211. https://doi.org/10.1038/nature15535
World Health Organisation, (2019). World malaria report 2019 https://www.who.int/publications/i/item/world-malaria-report-2019
World Health Organisation, (2020). MVIP update – 1 million doses administered, Kenya 1st anniversary, cooperation for vaccine access. https://www.who.int/news/item/15-11-2020-mvip-update-1-million-doses-administered-kenya-1st-anniversary-cooperation-for-vaccine-access