What is malaria?

• Malaria is a life-threatening disease caused by a parasite that is transmitted through the bite of infected female Anopheles mosquitoes.
• The parasite that causes malaria is a microscopic, single-celled organism called Plasmodium.
• Malaria is predominantly found in the tropical and sub-tropical areas of Africa, South America and Asia.
• If not detected and treated promptly, malaria can be fatal. However, with the right treatment, started early enough, it can be cured.
• It is estimated that there were 198 million cases of malaria in 2013 and 584,000 deaths.
• Around 95% of deaths are in children under the age of five living in Sub-Saharan Africa.
• However, death rates have fallen globally by 47% since 2000 (WHO)
• There are more than 100 kinds of Plasmodium that can infect many animal species such as reptiles, birds and mammals.
• There are six different species of malaria parasite that cause malaria in humans: Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale curtisi, Plasmodium ovale wallikeri, Plasmodium malariae and the very rare Plasmodium knowlesi.
• Plasmodium falciparum and Plasmodium vivax are the most common types of malaria parasite that infect humans.
• Plasmodium falciparum causes the most serious, life-threatening infections in humans.

A map to show the distribution of malaria around the world
(Data source: WHO World Malaria Report, 2014)
Image credit: Genome Research Limited

How is malaria transmitted?

• Malaria is transmitted via the bite of the female Anopheles mosquito.
• These mosquitos most commonly bite between dusk and dawn.
• The Anopheles mosquito acts as a vector for the malaria parasite, carrying the parasite from host to host.
• If a mosquito bites a person already infected with the malaria parasite it can suck up the parasite in the blood and then spread the parasite on to the next person they bite.
• There are about 20 different Anopheles species around the world that are responsible for the spread of malaria between humans.
• There are five species of Anopheles in the UK that would be able to transmit malaria. Malaria was endemic in the UK until the 19th century but was eradicated and is no longer spread in the UK.
• Malaria is generally not spread directly from person to person. However, in some rare cases malaria has been spread through blood transfusions and the sharing of needles.
• Malaria is most severe in villages that are surrounded by forests where rainfall patterns and humidity levels suit the mosquito best. The longer the life span of the mosquito, the longer the malaria parasite has to complete its development inside the mosquito.
• Malaria transmission occurs mostly during the rainy season but rates of transmission can vary from one year to the next.

An Anopheles mosquito taking a blood meal from a human.

Image credit: Shutterstock

Malaria life cycle

• The malaria life cycle starts when a mosquito carrying the malaria parasite bites a human, injecting the parasite (in its sporozoite form) in its saliva into the human bloodstream.
• Once injected into the blood, the sporozoites head straight to the liver and within 30 minutes they have invaded the liver cells. Here they develop from sporozoites into merozoites and multiply rapidly to produce thousands of merozoites. They are usually in the liver cells for 10 days.
  • In some malaria species, such as Plasmodium vivax and Plasmodium ovale, the malaria parasites can lie dormant for months or years in the liver. This dormant form, the hypnozoite, can then become reactivated and continue its life cycle causing disease. This dormant stage does not happen in Plasmodium falciparum.
• The merozoites burst out of the liver and invade red blood cells in the bloodstream. Here, they multiply further.
• After 48 hours, the merozoites have multiplied so much that the red blood cells burst, releasing more merozoites into the bloodstream, which can then infect more red blood cells.
• Over 10 days some merozoites will develop into gametocytes. This is the sexual form of the parasite.
• When another mosquito sucks up blood from an infected human they take up the gametocytes.
• Once inside the mosquito gut, the gametocytes mature into gametes.
• Male and female gametes fuse together during sexual reproduction resulting in the formation of a mobile ookinete.
• The ookinete burrows through the mosquito’s stomach wall.
• The ookinete then forms an oocyst on the other side of the stomach wall. Within this oocyst a thousand new sporozoites form.
• After about 5-7 days the oocyst bursts, releasing the sporozoites. These then migrate up to the mosquito’s salivary gland, ready to be injected into the next individual it bites.

An illustration to show the life cycle of the malaria parasite.
Image credit: Genome Research Limited

What are the symptoms of malaria?

• Usually the incubation time for malaria is seven to 18 days.
• With some species of malaria, such asPlasmodium vivax and Plasmodium ovale, the parasite can lie dormant in the liver. It can therefore take up to a year for symptoms to develop (see life cycle above).
• Once inside the body, the malaria parasites grow and multiply inside the red blood cells. When the red blood cells burst releasing merozoites (usually every 48-72 hours), it results in an attack of flu-like symptoms such as fever, sweating, shivering and shaking, muscle aches, nausea and headache. These attacks last 6-10 hours.
• The type and severity of symptoms depends on which type of malaria parasite someone is infected with. For example, the most serious type of malaria is caused by the parasite Plasmodium falciparum.

Microscope image of a blood smear taken from someone with malaria:
1. Healthy red blood cell; 2. Malaria parasites developing within infected red blood cells;
3. Malaria parasites about to burst out of red blood cell.
Image credit: Will Hamilton

Complications associated with malaria

• All cases of Plasmodium falciparum malaria are potentially severe and life threatening.
• However, a major reason for complications is because of missed or delayed diagnosis.
• Plasmodium falciparum primarily destroys the red blood cells which can lead to severe anaemia.
• In severe Plasmodium falciparum infections, parasite-filled red blood cells may also block circulation to major organs, this can result in serious complications such as:
  • cerebral malaria – this is when malaria causes the brain to swell, potentially causing seizures, coma, permanent brain damage or death.
  • liver failure and jaundice – this is when the malaria prevents the liver from performing its normal functions leading to a build-up of toxins in the body. These toxins tend to cause the skin and whites of the eyes to go a yellow colour.
  • pulmonary oedema – this is a build-up of fluid in the lungs initially causing breathlessness but potentially leading to the lungs no longer functioning. This means that oxygen can’t be absorbed from the lungs into the blood, and carbon dioxide can’t be taken out of the blood and exhaled. As a result there are high levels of carbon dioxide and low levels of oxygen in the blood.
  • kidney failure – this occurs due to the build-up of waste products in the body causing weakness and shortness of breath.
  • acidosis – the pH of the blood becomes dangerously acidic due to the release of toxic compounds from bursting red blood cells.
  • tropical splenomegaly – chronic or repeated infection with malaria parasites can cause the spleen to enlarge. This is because the spleen is responsible for clearing infected red blood cells from the circulation and is overwhelmed during an infection. Eventually the spleen gets blocked up and no longer functions, leaving people vulnerable to other infections.

How is malaria diagnosed?

• Much of the time malaria is diagnosed on the basis of the symptoms of the individual. If a child living in a country with a high prevalence of malaria develops a high fever it is often assumed that they have malaria and they will be taken to a local hospital for treatment.
• The most accurate way to diagnose malaria is by taking a drop of blood, smearing it on a slide and then examining it under a microscope to look for malaria parasites inside the red blood cells (see microscope image above).
• This method of diagnosis relies on the availability of a clinic with a microscope, electricity and a trained technician to look at the slides. Unfortuantely in many countries with high levels of malaria these things are not always readily available.
• More recently, rapid diagnostic tests have been introduced. These involve dipping a test stick into a drop of blood to test for the presence of proteins from the parasite. If the malaria parasite is present in the blood sample bands appear on the test stick . Some of these tests are even able to identify which species of Plasmodium is present. These are convenient as they eliminate the need for high-tech equipment and a result can be generated within 15 minutes.

A smear of blood being taken on a slide for examination under the microscope.
Image credit: Shutterstock