17/04/2019
Malaria
Written by
Raphael Ajah I.
27 March 2019
Key facts
Malaria is a life-threatening disease caused by
parasites that are transmitted to people through the
bites of infected female Anopheles mosquitoes. It is
preventable and curable.
In 2017, there were an estimated 219 million cases of
malaria in 87 countries.
The estimated number of malaria deaths stood at 435
000 in 2017.
The WHO African Region carries a disproportionately
high share of the global malaria burden. In 2017, the
region was home to 92% of malaria cases and 93% of
malaria deaths.
Total funding for malaria control and elimination
reached an estimated US$ 3.1 billion in 2017.
Contributions from governments of endemic countries
amounted to US$ 900 million, representing 28% of total
funding.
Malaria is caused by Plasmodium parasites. The parasites
are spread to people through the bites of infected female
Anopheles mosquitoes, called "malaria vectors." There are 5
parasite species that cause malaria in humans, and 2 of
these species – P. falciparum and P. vivax – pose the
greatest threat.
In 2017, P. falciparum accounted for 99.7% of estimated
malaria cases in the WHO African Region, as well as in the
majority of cases in the WHO regions of South-East Asia
(62.8%), the Eastern Mediterranean (69%) and the Western
Pacific (71.9%).
P. vivax is the predominant parasite in the WHO Region of
the Americas, representing 74.1% of malaria cases.
Symptoms
Malaria is an acute febrile illness. In a non-immune
individual, symptoms usually appear 10–15 days after the
infective mosquito bite. The first symptoms – fever,
headache, and chills – may be mild and difficult to recognize
as malaria. If not treated within 24 hours, P. falciparum
malaria can progress to severe illness, often leading to
death.
Children with severe malaria frequently develop one or
more of the following symptoms: severe anaemia,
respiratory distress in relation to metabolic acidosis, or
cerebral malaria. In adults, multi-organ failure is also
frequent. In malaria endemic areas, people may develop
partial immunity, allowing asymptomatic infections to occur.
Who is at risk?
In 2017, nearly half of the world's population was at risk of
malaria. Most malaria cases and deaths occur in sub-
Saharan Africa. However, the WHO regions of South-East
Asia, Eastern Mediterranean, Western Pacific, and the
Americas are also at risk. In 2017, 87 countries and areas
had ongoing malaria transmission.
Some population groups are at considerably higher risk of
contracting malaria, and developing severe disease, than
others. These include infants, children under 5 years of age,
pregnant women and patients with HIV/AIDS, as well as non-
immune migrants, mobile populations and travellers.
National malaria control programmes need to take special
measures to protect these population groups from malaria
infection, taking into consideration their specific
circumstances.
Disease burden
According to the latest World malaria report, released in
November 2018, there were 219 million cases of malaria in
2017, up from 217 million cases in 2016. The estimated
number of malaria deaths stood at 435 000 in 2017, a
similar number to the previous year.
The WHO African Region continues to carry a
disproportionately high share of the global malaria burden.
In 2017, the region was home to 92% of malaria cases and
93% of malaria deaths.
In 2017, 5 countries accounted for nearly half of all malaria
cases worldwide: Nigeria (25%), the Democratic Republic of
the Congo (11%), Mozambique (5%), India (4%) and Uganda
(4%).
Children under 5 years of age are the most vulnerable group
affected by malaria; in 2017, they accounted for 61% (266
000) of all malaria deaths worldwide.
World malaria report 2018
Transmission
In most cases, malaria is transmitted through the bites of
female Anopheles mosquitoes. There are more than 400
different species of Anopheles mosquito; around 30 are
malaria vectors of major importance. All of the important
vector species bite between dusk and dawn. The intensity of
transmission depends on factors related to the parasite, the
vector, the human host, and the environment.
Anopheles mosquitoes lay their eggs in water, which hatch
into larvae, eventually emerging as adult mosquitoes. The
female mosquitoes seek a blood meal to nurture their eggs.
Each species of Anopheles mosquito has its own preferred
aquatic habitat; for example, some prefer small, shallow
collections of fresh water, such as puddles and hoof prints,
which are abundant during the rainy season in tropical
countries.
Transmission is more intense in places where the mosquito
lifespan is longer (so that the parasite has time to complete
its development inside the mosquito) and where it prefers to
bite humans rather than other animals. The long lifespan
and strong human-biting habit of the African vector species
is the main reason why approximately 90% of the world's
malaria cases are in Africa.
Transmission also depends on climatic conditions that may
affect the number and survival of mosquitoes, such as
rainfall patterns, temperature and humidity. In many places,
transmission is seasonal, with the peak during and just after
the rainy season. Malaria epidemics can occur when climate
and other conditions suddenly favour transmission in areas
where people have little or no immunity to malaria. They
can also occur when people with low immunity move into
areas with intense malaria transmission, for instance to find
work, or as refugees.
Human immunity is another important factor, especially
among adults in areas of moderate or intense transmission
conditions. Partial immunity is developed over years of
exposure, and while it never provides complete protection,
it does reduce the risk that malaria infection will cause
severe disease. For this reason, most malaria deaths in
Africa occur in young children, whereas in areas with less
transmission and low immunity, all age groups are at risk.
Prevention
Vector control is the main way to prevent and reduce
malaria transmission. If coverage of vector control
interventions within a specific area is high enough, then a
measure of protection will be conferred across the
community.
WHO recommends protection for all people at risk of
malaria with effective malaria vector control. Two forms of
vector control – insecticide-treated mosquito nets and
indoor residual spraying – are effective in a wide range of
circumstances.
Insecticide-treated mosquito nets
Sleeping under an insecticide-treated net (ITN) can reduce
contact between mosquitoes and humans by providing both
a physical barrier and an insecticidal effect. Population-wide
protection can result from the killing of mosquitoes on a
large scale where there is high access and usage of such nets
within a community.
In 2017, about half of all people at risk of malaria in Africa
were protected by an insecticide-treated net, compared to
29% in 2010. However, ITN coverage increased only
marginally in the period 2015 to 2017.
Indoor spraying with residual insecticides
Indoor residual spraying (IRS) with insecticides is another
powerful way to rapidly reduce malaria transmission. It
involves spraying the inside of housing structures with an
insecticide, typically once or twice per year. To confer
significant community protection, IRS should be
implemented at a high level of coverage.
Globally, IRS protection declined from a peak of 5% in 2010
to 3% in 2017, with decreases seen across all WHO regions.
The declines in IRS coverage are occurring as countries
switch from pyrethroid insecticides to more expensive
alternatives to mitigate mosquito resistance to pyrethroids.
Antimalarial drugs
Antimalarial medicines can also be used to prevent malaria.
For travellers, malaria can be prevented through
chemoprophylaxis, which suppresses the blood stage of
malaria infections, thereby preventing malaria disease. For
pregnant women living in moderate-to-high transmission
areas, WHO recommends intermittent preventive treatment
with sulfadoxine-pyrimethamine, at each scheduled
antenatal visit after the first trimester. Similarly, for infants
living in high-transmission areas of Africa, 3 doses of
intermittent preventive treatment with sulfadoxine-
pyrimethamine are recommended, delivered alongside
routine vaccinations.
Since 2012, WHO has recommended seasonal malaria
chemoprevention as an additional malaria prevention
strategy for areas of the Sahel sub-region of Africa. The
strategy involves the administration of monthly courses of
amodiaquine plus sulfadoxine-pyrimethamine to all children
under 5 years of age during the high transmission season.
Insecticide resistance
Since 2000, progress in malaria control has resulted
primarily from expanded access to vector control
interventions, particularly in sub-Saharan Africa. However,
these gains are threatened by emerging resistance to
insecticides among Anopheles mosquitoes. According to the
latest World malaria report, 68 countries reported mosquito
resistance to at least 1 of the 5 commonly-used insecticide
classes in the period 2010-2017; among these countries, 57
reported resistance to 2 or more insecticide classes.
Despite the emergence and spread of mosquito resistance
to pyrethroids (the only insecticide class used in ITNs),
insecticide-treated nets continue to provide a substantial
level of protection in most settings. This was evidenced in a
large 5-country study coordinated by WHO between 2011
and 2016.
While the findings of this study are encouraging, WHO
continues to highlight the urgent need for new and
improved tools in the global response to malaria. To prevent
an erosion of the impact of core vector control tools, WHO
also underscores the critical need for all countries with
ongoing malaria transmission to develop and apply effective
insecticide resistance management strategies.
Diagnosis and treatment
Early diagnosis and treatment of malaria reduces disease
and prevents deaths. It also contributes to reducing malaria
transmission. The best available treatment, particularly for
P. falciparum malaria, is artemisinin-based combination
therapy (ACT).
WHO recommends that all cases of suspected malaria be
confirmed using parasite-based diagnostic testing (either
microscopy or rapid diagnostic test) before administering
treatment. Results of parasitological confirmation can be
available in 30 minutes or less. Treatment, solely on the
basis of symptoms should only be considered when a
parasitological diagnosis is not possible. More detailed
recommendations are available in the "WHO Guidelines for
the treatment of malaria", third edition, published in April
2015.
Antimalarial drug resistance
Resistance to antimalarial medicines is a recurring problem.
Resistance of P. falciparum malaria parasites to previous
generations of medicines, such as chloroquine and
sulfadoxine-pyrimethamine (SP), became widespread in the
1950s and 1960s, undermining malaria control efforts and
reversing gains in child survival.
Protecting the efficacy of antimalarial medicines is critical to
malaria control and elimination. Regular monitoring of drug
efficacy is needed to inform treatment policies in malaria-
endemic countries, and to ensure early detection of, and
response to, drug resistance.
In 2013, WHO launched the Emergency response to
artemisinin resistance (ERAR) in the Greater Mekong
Subregion (GMS), a high-level plan of attack to contain the
spread of drug-resistant parasites and to provide life-saving
tools for all populations at risk of malaria. But even as this
work was under way, additional pockets of resistance
emerged independently in new geographic areas of the
subregion. In parallel, there were reports of increased
resistance to ACT partner drugs in some settings. A new
approach was needed to keep pace with the changing
malaria landscape.
At the World Health Assembly in May 2015, WHO launched
the Strategy for malaria elimination in the greater mekong
subregion (2015–2030) , which was endorsed by all the
countries in the subregion. Urging immediate action, the
strategy calls for the elimination of all species of human
malaria across the region by 2030, with priority action
targeted to areas where multidrug resistant malaria has
taken root.
With technical guidance from WHO, all countries in the
region have developed national malaria elimination plans.
Together with partners, WHO is providing ongoing support
for country elimination efforts through the Mekong Malaria
Elimination programme, an initiative that evolved from the
ERAR.
Latest bulletin of the Mekong Malaria Elimination
programme
Surveillance
Surveillance entails tracking of the disease and
programmatic responses, and taking action based on the
data received. Currently, many countries with a high burden
of malaria have weak surveillance systems and are not in a
position to assess disease distribution and trends, making it
difficult to optimize responses and respond to outbreaks.
Effective surveillance is required at all points on the path to
malaria elimination. Stronger malaria surveillance systems
are urgently needed to enable a timely and effective malaria
response in endemic regions, to prevent outbreaks and
resurgences, to track progress, and to hold governments
and the global malaria community accountable.
In March 2018, WHO released a reference manual on
malaria surveillance, monitoring and evaluation. The
manual provides information on global surveillance
standards and guides countries in their efforts to strengthen
surveillance systems.
Elimination
Malaria elimination is defined as the interruption of local
transmission of a specified malaria parasite species in a
defined geographical area as a result of deliberate activities.
Continued measures are required to prevent re-
establishment of transmission.
Malaria eradication is defined as the permanent reduction
to zero of the worldwide incidence of malaria infection
caused by human malaria parasites as a result of deliberate
activities. Interventions are no longer required once
eradication has been achieved.
Countries that have achieved at least 3 consecutive years of
0 local cases of malaria are eligible to apply for the WHO
certification of malaria elimination. In recent years, 9
countries have been certified by the WHO Director-General
as having eliminated malaria: United Arab Emirates (2007),
Morocco (2010), Turkmenistan (2010), Armenia (2011),
Maldives (2015), Sri Lanka (2016), Kyrgyzstan (2016),
Paraguay (2018) and Uzbekistan (2018). The WHO
Framework for Malaria Elimination (2017) provides a
detailed set of tools and strategies for achieving and
maintaining elimination.
WHO certification of malaria elimination
Vaccines against malaria
RTS,S/AS01 (RTS,S) is the first and, to date, the only vaccine
to show partial protection against malaria in young children.
It acts against P. falciparum, the most deadly malaria
parasite globally and the most prevalent in Africa. Among
children who received 4 doses in large-scale clinical trials,
the vaccine prevented approximately 4 in 10 cases of
malaria over a 4-year period.
In view of its public health potential, WHO’s top advisory
bodies for malaria and immunization have jointly
recommended phased introduction of the vaccine in
selected areas of sub-Saharan Africa. The vaccine will be
introduced in 3 pilot countries – Ghana, Kenya and Malawi –
in 2019.
The pilot programme will address several outstanding
questions related to the use of the vaccine in real-life
settings. It will be critical for understanding how best to
deliver the required four doses of RTS,S; the vaccine’s
potential role in reducing childhood deaths; and its safety in
the context of routine use.
This WHO-coordinated programme is a collaborative effort
with ministries of health in Ghana, Kenya and Malawi and a
range of in-country and international partners, including
PATH, a non-profit organization, and GSK, the vaccine
developer and manufacturer.
