QUESTION
What is the life cycle of malaria?
ANSWER
The life cycle of malaria is complex, involving a definitive host, or vector, which for human malaria is a mosquito of the genus Anopheles, and also a human host. Inside both, the malaria parasite undergoes several different transformations and reproductive cycles, which are detailed in the schematic below.
You may also view a video of malaria life cycle.
Schematic of malaria life cycle, courtesy of CDC (www.cdc.gov)
QUESTION
What are the Host range?
ANSWER
Malaria is caused by a parasite, of the genus Plasmodium, which requires two organisms for the completion of its life cycle—an insect, which is usually referred to as the “vector,” in which sexual reproduction occurs, and a vertebrate “host,” in which asexual multiplication occurs.
Different species of Plasmodium infects a large range of vertebrate hosts, including many birds, reptiles, and dozens of different mammals, specifically primates (including humans) and rodents. These different types of Plasmodium are found all over the world; Plasmodium species that infect birds, for example, are found in much colder climates than human malaria.
The five species of Plasmodium that infect humans are found mainly in the tropics, and seasonally in the sub-tropics. Four of the species mainly infect humans only, but can also be found occasionally in other primates, such as chimpanzees and gorillas; the fifth species, P. knowlesi, is mainly considered a malaria of macaque monkeys in south-east Asia, but can also infect humans. As such, the host range of these malaria species extends wherever humans live throughout the world. However, in reality, the distribution of malaria is constrained to warmer, wetter climates based on the developmental needs of the parasite and also the vector.
For mammalian malaria, these vectors are all mosquitoes of the genus Anopheles, which breed in stagnant freshwater and also have temperature requirements, above or below which they will not develop. However, these mosquitoes are still found in many parts of the world, from far northern latitudes of Canada and Siberia in the summer to similar latitudes in the southern hemisphere and also throughout the tropics.
QUESTION
What is the life cycle of malaria?
ANSWER
Malaria is caused by a single celled organism in the genus Plasmodium. Five species of Plasmodium infect humans, but all follow a very similar life cycle, including two separate cycles of asexual reproduction in the human host (one in the liver, called the exo-erythrocytic cycle, and one in the blood, and specifically inside red blood cells, known as the erythrocytic cycle) and a sexual reproductive stage inside the mosquito definitive host (usually called the “vector”). A schematic of the full life cycle is below, courtesy of the CDC (www.cdc.gov).
Schematic of the malaria life cycle, courtesy of CDC (www.cdc.gov)
QUESTION
What are the effects of malaria on the body?
ANSWER
Malaria has a number of effects on the body. The parasite passes from the blood (where it enters via the bite of an infected mosquito) into the liver, where it reproduces and changes form. After a period of 1-4 weeks (usually – it can be longer) in the liver, the malaria parasite re-enters the blood and begins to infect red blood cells, undoing more reproduction inside the cells and then, in synchrony, bursting out once the cycle is complete. This process of reproduction and destroying red blood cells results in a build-up of toxins and debris in the blood; the resultant immune reaction produces side effects which are the common observable symptoms of malaria, such as fever, chills, nausea and aches.
One particular type of malaria, Plasmodium falciparum, is also able to modify the surface of red blood cells it infects. It causes these cells to become “sticky”, so they lodge in the small blood vessels leading up to major organs. This build-up is called sequestration, and results in reduced blood flow and oxygen deprivation in the organs. When sequestration occurs in the blood vessels in the brain, the patient may experience impaired consciousness, psychological disruption, coma and even death – this manifestation is called “cerebral malaria”.
If diagnosed and treated promptly, the malaria parasites in the blood can usually be killed rapidly and the patient will soon enjoy a complete recovery. With two forms of malaria, P. vivax and P. ovale, the parasite can remain dormant in the liver for months or even years, resulting in relapse of disease at a later date. To prevent this from occurring, patients with these types of malaria can sometimes take primaquine, a drug which kills the liver stages of the malaria parasite as well.
QUESTION
Please tell me the symptoms of malaria.
ANSWER
Symptoms of malaria include fever and flu-like illness, including shaking chills, headache, muscle aches, and tiredness. Due to the way in which the malaria parasite reproduces in the human body, fevers occurring in a cyclical fashion (i.e. high fever one day, no fever the next day, then recurrence of fever on the third or fourth day, depending on the type of malaria) are a characteristic symptom of malaria, but may not be present in all cases. Nausea, vomiting, and diarrhea may also occur. Malaria may cause anemia and jaundice (yellow coloring of the skin and eyes) because of the loss of red blood cells. Symptoms usually appear between 10 and 15 days after the mosquito bite. If not treated, malaria can quickly become life-threatening by disrupting the blood supply to vital organs. When blood supply to the brain is disrupted during malarial infection, the resulting condition, characterised by seizures, mental confusion, coma, and death, is known as “cerebral malaria”, and is associated with Plasmodium falciparum, the most deadly form of malaria.
Please check your article: Symptoms and Causes of Malaria
QUESTION
Why can’t mosquitoes become sick of malaria? What do they have in their system that prevent them from becoming sick yet they transmit the disease to humans?
ANSWER
First of all, mosquitoes do get “sick” when infected with malaria, though not in the same way as humans. This is due to the major differences between human and mosquito physiology and also differences in the life stage of the malaria parasite between the different hosts.
Humans get sick from malaria because the malaria parasite, when inside the human host, sequentially infects red blood cells, multiplying inside them and then destroying them. This process of destruction and the resultant debris in the blood results fever and other symptoms (some of which are exacerbated by the body’s immune response).
Mosquitoes are insects, and do not have blood in the same way as humans. Instead, in the mosquito, the malaria parasites differentiate into male and female life stages (called gametocytes) which fuse into the mosquitoes gut. After further reproduction, they eventually produce sporozoites that rupture out of the mosquito gut wall and migrate to the salivary glands, where they can be infected into a new human host.
Mosquitoes also have a very different immune system to humans, and so the concept of “sick” is not the same for them as for humans. However, there is a lot of evidence that infection with malaria results in reduced reproductive success for the mosquito, as well as lower survival. This could be due to protective responses on the part of the mosquito, but also could be mediated by the malaria parasite, by diverted resources towards its own development that could have been used for mosquito reproduction or survival.
QUESTION
What are the three stages of malaria?
ANSWER
The three parts of the malaria life cycle are known as the exo-erythrocytic cycle, the erythrocytic cycle and the sporogonic cycle. The first two of these take place in the human (or other mammal) host, while the third occurs in the mosquito vector. The below diagram, courtesy of the CDC, shows the complete life cycle of malaria in more detail.
Schematic of the malaria life cycle, courtesy of the CDC (www.cdc.gov)
QUESTION
What countries have malaria?
ANSWER
Prior to the advent of modern methods for controlling malaria, it was present on every continent in the whole except Antarctica. The transmission of malaria depends on an appropriate climate, both for the development of the parasite and the mosquitoes that it requires as a vector. This limits malaria to areas that are sufficiently warm and with sufficient rainfall to provide pools of stagnant water for the development of mosquito larvae.
In practice, this means that malaria can be transmitted year round in the tropics (apart from areas of high altitude and deserts), most of the year in the sub-tropics (predominantly during rainy periods) and even seasonally in temperate latitudes (during the warmer months). As a result, malaria has historically been present in the United States and even in England, at a latitude of over 50 degrees north.
However, modern control measures, such as insecticide spraying and epidemiological surveillance, has greatly reduced transmission of malaria in many parts of the world, and especially in temperate regions where the force of infection was already lower than elsewhere in the world. As such, nowadays malaria is confined to tropical and sub-tropical Central and South America, certain Caribbean islands (such as Haiti), sub-Saharan Africa (apart from most of Namibia and South Africa), parts of the Middle East, the Indian sub-continent, south-east Asia (excluding major cities such as Singapore) and many of the Indo-Pacific islands (including Papua New Guinea). Of these, by far the largest number of deaths from malaria occur in sub-Saharan Africa.
See the CDC’s interactive map of malaria distribution for more information.
QUESTION
Why can the malaria parasite be transmitted from mosquito to human, but not from human to human (via blood)?
ANSWER
This is a very good question, and actually, malaria can be transmitted via human blood directly to another person, but this occurs relatively rarely!
It has to do with the life cycle of the malaria parasite. When a mosquito bites a human host, it injects sporozoites from its salivary glands into the blood. This life stage first migrates to the liver, where it undergoes a cycle of multiplication, before entering the blood stream. Here, in the so-called “erythrocytic” portion of the life cycle, the parasite reproduces a series of times in red blood blood, before finally forming gametocytes, which are required to be ingested by a mosquito vector during another blood meal for the life cycle to be continued. As such, when blood is passed between people, they would have to pass infected red blood cells, and not gametocytes (which are not infective to humans, only to mosquitoes) in order for the other person to become infected.
As such, when a person who is infected with malaria donates blood, there is a chance that they might pass on some red blood cells which are infected with mature trophozoites or schizonts; these could then go on to infect more red blood cells in the person who received the blood. However, in most countries, blood is screened for malaria, and in fact, in many places, people who might have been exposed to malaria are not allowed to donate whole blood, only plasma (in which the red blood cells have been removed, and therefore there is no risk of transmission). Given the high levels of malaria prevalence in some endemic countries, however, preventing people from donating blood who are positive for malaria may result in too little blood being collected; in these cases, other strategies, such as treating donors or closely monitoring patients post-transfusion, may prove to be a better strategy.
Schematic of the malaria life cycle, courtesy of CDC (www.cdc.gov)
QUESTION
Where does malaria normally come from?
ANSWER
Malaria is caused by single-celled parasites of the genus Plasmodium; the parasites enter the human body via the bite of an infected mosquito. However, the mosquito has to pick up the infection in the first place from another infected human; so in a sense, you could say that malaria both comes from mosquitoes, as well as from other people, although only indirectly!
More generally, your question could be read as asking where does malaria come from geographically—malaria is actually distributed over most of the world, and at some point or another has been found on all continents except Antarctica. In modern times, it tends to be restricted to the tropics and sub-tropical regions of the world, since temperature is a key factor pertaining to its survival. However, seasonal transmission (especially in the summer, hotter months) still persists even in some temperate regions. Moreover, with global climate change, it is feared that the distribution of malaria will continue to spread, and even more people will be affected.