October 29, 2020

Malaria - Causes, Symptoms, Transmission, Treatment and Prevention

Malaria is a serious mosquito-borne disease caused by one of the Plasmodium parasites. People with malaria often experience fever, chills, and flu-like illness. Symptoms have been known to occur up to a year after travelling to an area in which malaria is present. Left untreated, individuals may develop severe complications and die. In 2018 an estimated 228 million cases of malaria occurred worldwide and 405,000 people died, mostly children aged under 5.

In this article:
What is malaria?
Symptoms of malaria
Complications of malaria
Who is most at risk from malaria?
Transmission of malaria
Diagnosis and treatment of malaria
How can malaria be prevented?
Vaccines against malaria

What is malaria?

Malaria is a life-threatening disease caused by parasites that are transmitted to people through the bites of infected female Anopheles mosquitoes, also called "malaria vectors". It is preventable and curable. It is common in tropical countries such as parts of Africa, Asia and South America. The parasites causing it are called plasmodium. A parasite is an organism that lives on an animal and feeds from it.

There are five main types of plasmodium that cause the disease. These are called Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale, Plasmodium malariae and Plasmodium knowlesi. 2 of these species – P. falciparum and P. vivax – pose the greatest threat, although Plasmodium falciparum is usually the most serious. Occasionally people have infection with more than one type.

P. falciparum is the most prevalent malaria parasite in Africa, South-East Asia, the Eastern Mediterranean, and the Western Pacific. It is responsible for most malaria-related deaths globally. P. vivax is the predominant malaria parasite in the Americas.

Symptoms of malaria

Symptoms may include: Symptoms usually occur between one to eight weeks (average of 10-15 days) after the initial mosquito bite. (This is the incubation period.) However, in some cases, depending on the type of plasmodium causing the infection, it can take up to a year before any symptoms start to show. The shorter incubation periods are observed most frequently with P. falciparum and the longer ones with P. malariae. The illness may start off with nonspecific flu-like symptoms. People who reside in areas of the world where malaria is normally prevalent (malaria endemic areas) may develop partial immunity to malaria (immune individuals), allowing malaria to occur with little of no symptoms (asymptomatic malaria infections). People who live in countries or regions where malaria does not normally exist do not have this kind of immunity to malaria (non-immune individuals).

In general, there are two types of malaria: uncomplicated and severe (otherwise called, complicated). Severe or complicated malaria is more serious and difficult to treat, and is more likely to be life-threatening.

Uncomplicated malaria

Any of the symptoms above may occur. The fever in most people has no specific pattern and may present 1-2 days after the symptoms start. If the malarial infection becomes established then symptoms can come in cycles, occurring every 2-3 days.

Severe (complicated) malaria

The more severe condition occurs when complications develop. It is most commonly caused by Plasmodium falciparum. It usually begins with similar symptoms to uncomplicated malaria, but serious problems in various body organs or systems develop, including:
  • Cerebral malaria: the brain or nervous system is affected. There may be lower levels of consciousness, coma or fits (seizures).
  • Kidney or liver problems.
  • Serious breathing problems (respiratory distress, usually in relation to metabolic acidosis). Also see Breathlessness and Difficulty Breathing (Dyspnoea).
  • Low sugar levels.
  • Very low blood pressure.
  • Sepsis.
  • Anaemia.
  • Abnormalities of blood clotting. Also see What Is Blood?
Note: pregnant women are at particular risk of severe malaria and should, ideally, not go to high risk areas (malaria endemic areas). This is because your immune system can be weaker during pregnancy. If you are pregnant and have malaria, you may pass the infection on to your baby.

Others who are more likely to get the severe form of the condition include elderly people, children, those with HIV/AIDS and others whose immune systems are not working normally.

Complications of malaria

Malaria relapses

In P. vivax and P. ovale infections, people having recovered from the first episode of illness may suffer several additional attacks (relapses) after months or even years without symptoms. Relapses occur because P. vivax and P. ovale have dormant liver stage parasites (hypnozoites) which can live in your liver for several months or years, and then may reactivate causing new disease. Treatment to reduce the chance of such relapses is available and should follow treatment of the first attack.

Other manifestations of malaria

  • Abnormalities of how the brain works (neurologic defects) may occasionally persist following cerebral malaria, especially in children. Such defects include trouble with movements (ataxia), palsies, speech difficulties, deafness, and blindness.
  • Recurrent infections with P. falciparum may result in severe anemia. This occurs especially in young children with frequent infections that are inadequately treated.
  • Malaria during pregnancy (especially P. falciparum) may cause severe disease in the mother, and may lead to premature delivery or delivery of a low-birth-weight baby.
  • On rare occasions, P. vivax malaria can cause rupture of the spleen.
  • Nephrotic syndrome (a chronic, severe kidney disease) can result from chronic or repeated infections with P. malariae.
  • Hyperreactive malarial splenomegaly (also called “tropical splenomegaly syndrome”) occurs infrequently and is attributed to an abnormal immune response to repeated malarial infections. The disease is marked by a very enlarged spleen and liver, abnormal immunologic findings, anemia, and a susceptibility to other infections (such as skin or respiratory infections).

Who is most at risk from malaria?

In 2018, nearly half of the world's population was at risk of malaria. Most malaria cases and deaths occur in sub-Saharan Africa. However, South-East Asia, Eastern Mediterranean, Western Pacific, and the Americas are also at risk.

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.

The risk of getting the disease is greatest if you are a non-immune individual (lacking any form of immunity to malaria) and do not take your antimalarial medication or do not take it properly when you visit a malaria endemic area or country. People who take last-minute holidays without good planning and also those visiting friends or relatives abroad have been shown to be the least likely to take their antimalarial medication.

Note: malaria can kill people very quickly if it is not diagnosed promptly, this is especially true for non-immune people. If you are non-immune and feel unwell after you have recently visited an area known to be affected with malaria, you should seek prompt medical advice, even if you have taken your antimalarial medication correctly.

Transmission of malaria

In most cases, malaria is transmitted through the bites of infected 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. If a female Anopheles mosquito bites a person who is infected with malaria, the now infected mosquito can then carry the plasmodium parasite and spread it to other people when it bites and feeds from their blood.

Anopheles mosquito taking a human blood meal
Anopheles mosquito taking a human blood meal. Image credit: Mary Soan

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 (non-immune). 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.

When the plasmodium parasite enters your blood from the bite of an infected female Anopheles mosquito, it travels to your liver and then re-enters the bloodstream where it can invade your red blood cells to multiply. Eventually, these infected red blood cells burst which leads to them releasing even more of the tiny parasites into your blood. These infected red blood cells tend to burst every 48-72 hours. Each time they burst, you may experience an episode of chills, high temperature (fever) and sweating.

Diagnosis and treatment of malaria

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).

How is malaria diagnosed?

If you think you may have malaria you need to seek medical advice without delay. If you have symptoms and live in a malaria-endemic area, or you have travelled to a malaria-endemic area, you may be at risk of this disease, even if you took antimalarial medication when you travelled. Usually your doctor will immediately want you to undergo rapid testing if this is the suspected diagnosis.

Your doctor will perform a blood test. The blood sample will be sent to the laboratory and will be examined for the presence of the plasmodium parasite. The type of plasmodium causing the infection will also be determined. If the first blood test is negative but your doctor suspects you have malaria then you may be asked to have another blood test taken a couple of days later.

The World Health Organization (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.

Malaria treatment

If malaria is promptly diagnosed and treated, most people make a full recovery. Treatment is with antimalarial medicines. A number of these are available including:
  • Artemisinin combination therapy (ACT) This is usually artemether-lumefantrine (sold as Coartem® or Riamet® among others) or dihydroartemisinin-piperaquine (sold as Eurartesim® among others).
  • Quinine.
  • Quinine with doxycycline.
  • Atovaquone-proguanil (sold as Malarone® among others).
  • Chloroquine.
  • Artesunate.
  • Primaquine.
The type of medicine prescribed and the duration of treatment can vary from person to person. It depends on various factors such as:
  • The type of plasmodium infection that you have.
  • If you have taken any antimalarial medication (for example, when you were travelling).
  • The severity of your symptoms.
If your symptoms are mild then you will be treated at home. However, if you have the Plasmodium falciparum type or are pregnant then it is very likely you will be treated and monitored in hospital. This would also be needed if there are complications, or for children or elderly people.

Some people are given more than one type of medication or an alternative medication if they develop side-effects to a medication. Resistance to antimalarial medicines has become a problem. This means that newer medicines or a combination of medicines may be given. Also see What You Need To Know About Antimicrobial Resistance.

If the first antimalarial medicine that you are prescribed fails to improve your symptoms, you may have to try a variety of other medicines as part of your treatment. You may find that your treatment for malaria leaves you feeling very weak and tired for several weeks afterwards.

Other things you should do

Other advice after a diagnosis of malaria:
  • You should warn non-immune individuals who were with you (if you caught malaria whilst travelling) to urgently report any symptoms of illness to a health care professional.
  • After treatment, it is possible to have a relapse. If this occurs you may require further treatment.
  • Having had malaria does not make you immune from getting it again so you would still need to take precautions if you are travelling to areas where malaria is known to be found.
  • You cannot donate blood for some time after having this disease. If you donate blood regularly, inform the blood donation service of your diagnosis.

How can malaria be prevented?

Vector control is the main way to prevent and reduce malaria transmission. Female Anopheles mosquito is the malaria vector. 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 (insect death) 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 2018, 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 has been at a standstill since 2016.

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 2% in 2018, with decreases seen worldwide, apart from the Eastern Mediterranean countries. 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 (taking drugs to prevent disease), 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 (sold as Fansidar® among others), 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 (taking drugs to prevent disease) 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.

Despite the emergence and spread of mosquito resistance to pyrethroids, 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.

Vaccines against malaria

RTS,S/AS01 (RTS,S) is the first and, to date, the only vaccine to show that it can significantly reduce malaria, and life-threatening severe malaria, in young African children. It acts against P. falciparum, the most deadly malaria parasite globally and the most prevalent in Africa.

Reference(s)
1). World Health Organization: Malaria. WHO Fact Sheets 2020.
2). World Health Organization: World Malaria Report 2019. WHO Publications.
3). Centers for Disease Control: Malaria.
4). Lalloo DG, Shingadia D, Bell DJ, et al; UK malaria treatment guidelines 2016. J Infect. 2016 Jun72(6):635-649. doi: 10.1016/j.jinf.2016.02.001. Epub 2016 Feb 12.
5). Brigadier General James Stevens Simmons, M.C., A.U.S. Global Malaria. N Engl J Med 14 Oct 1943; 229:605-610.

4 comments:

  1. Any new updates on malarial vaccines?

    ReplyDelete
    Replies
    1. The only known malarial vaccines at this time is RTS,S/AS01 (RTS,S). It is now being introduced in selected areas of sub-Saharan Africa – Ghana, Kenya and Malawi – began introducing the vaccine in selected areas of moderate and high malaria transmission in 2019.

      Delete
  2. I'm sort of wondering what was the results of the RTS, S/AS01 falciparum vaccine deployment tested in some African countries in 2015? Any updates on this malaria vaccine?

    ReplyDelete
    Replies
    1. Apologies for late response, but see answer above.

      Delete

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