New Malaria Parasites

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QUESTION

I heard researchers have found a new malaria parasite—what is the name?

ANSWER

New malaria parasites are found quite regularly—the parasite that causes malaria, Plasmodium, actually infects birds, reptiles, rodents and non-human primates like monkeys and apes as well as humans. As such, non-human forms of malaria are discovered relatively frequently in other species. For example, a few years ago, some researchers looked at malaria in apes in Central Africa, and found a new species in gorillas, which is so new it has not even been fully described to science yet, and so remains unnamed! It is thought to be very closely related to Plasmodium falciparum, which is the most dangerous type of malaria in humans. Also recently, two new species were observed in chimpanzees, also in Central Africa, and names P. billcollinsi and P. billbrayi.

Even in humans, new infections are sometimes observed. One which has gained a lot of recent attention is not a new species, but what seems to be increasing numbers of cases of a monkey type of malaria (called P. knowlesi) in humans. It is unclear whether this is due to changes within the parasite, or changes to the landscape which might be creating more favorable conditions for the transmission of this malaria to humans. It is even possible that this malaria has always infected humans, and so this is not a new development, but due to diagnostic issues, it was mistaken for other, human malaria species, such as P. vivax and/or P. malariae.

Malaria Hosts

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QUESTION

Which are malaria hosts and how does malaria have an effect on one of them?

ANSWER

The parasite which causes malaria (called Plasmodium) requires two different hosts—a vertebrate intermediate host, such as a human, and an insect definitive host, also known as the vector. For the types of malaria which infect humans and other mammals, the vector is always a mosquito of the genus Anopheles.

However, there are other types of malaria which infect birds and reptiles, and these can use other genera of mosquito as their vectors, and some parasites closely related to Plasmodium can even use sandflies and other types of insects as their vectors too.

In humans, malaria usually causes disease, characterized by high fever, chills, aches and nausea. However, the presentation of symptoms and their severity depends on a number of factors, such as the type of Plasmodium (P. falciparum is the most dangerous to humans), the immune status of the host and the infective dose received from the vector.

Many mammals are also susceptible to malaria, such as macaque monkeys, and with them as well the effect of the parasite depends on a number of factors. For example, for macaques in south-east Asia, many will be co-infected with several different types of malaria simultaneously, though none appear to cause disease. However, if macaques from other parts of tje world, such as the Himalayas, are experimentally infected with these same types of malaria, they will get sick and possibly even die. As such, evolutionary history also plays a part in terms of how severe malaria will be in a particular host.

For the insect vector, infection with malaria parasites does not appear to have a strong deleterious effect, though some studies have shown reduced survival in mosquitoes infected with malaria. Also, changes in behavior have been observed. For example, some studies have shown that mosquitoes which are infected with malaria are more likely to continue seeking for food (i.e. through biting a host) even if they have recently fed than mosquitoes which are uninfected, or infected with non-transmissable life stages of malaria. This suggests that in some way the malaria parasite is manipulating the vector’s behavior in order to increase its own chances of being passed on to a new host.

Evolution of Malaria

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QUESTION

how did malaria evolve?

ANSWER

Malaria in humans probably evolved independently several times, and both times likely due to a cross-over event from a closely related primate malaria. For example, Plasmodium vivax is evolutionarily closely related to several species of malaria found in macaque monkeys in south-east Asia, and so a cross-over of one of those species into human, with subsequent adaptation and speciation, is one hypothesis as to the origin of P. vivax. Conversely, some people argue that P. vivax emerged in Africa, due to the high prevalence of certain genetic traits in African populations (such as the Duffy negative antigen), which protect against P. vivax.

In contrast, P. falciparum is agreed to have emerged in sub-Saharan Africa, and likely in the Congo basin, though the exact source of its origin has been under recent scientific dispute. Until 2010, it was thought that P. falciparum had crossed over to humans from chimpanzees, as chimps are known to be infected with P. reichenowi, a species very closely related to P. falciparum. However, a paper was published in 2010 which had sampled Plasmodium parasites of gorillas and revealed new species of Plasmodium which are even more closely related to P. falciparum, suggesting that the cross-over occurred from gorillas to humans.

As you can see, humans are not the only primates to get malaria; many species of monkey and ape are also susceptible to Plasmodium species, and even lemurs have their own suite of Plasmodium parasites. Among the mammals, rodents also can get malaria, and bats are infected with Hepatocystis, a malaria-like parasite which also infects hippos, primates and rodents. However, no other species of mammal appears to be susceptible to Plasmodium/Hepatocystis, and the reasons for this are not entirely clear.

Plasmodium probably crossed over to mammals from birds or lizards, both of which are infected with a vast number of species of Plasmodium. It is unclear in which of these groups Plasmodium first emerged, though it likely evolved originally from another type of blood-borne parasite called Leucocytozoon, which infects birds and uses blackflies (genus Simulium) as vectors.

A sister group to Plasmodium, called Haemoproteus, also evolved from Leucocytozoon but utilises a variety of different vectors, including mosquitoes, biting midges (Culicoides), louse flies (Hippoboscidae) and tabanids (Tabanidae). Plasmodium, by contrast, exclusively uses mosquitoes as its vectors (apart from one species of lizard Plasmodium, P. mexicanum, which uses sandflies), but while mammalian Plasmodium is only transmitted by Anopheles mosquitoes, bird and lizard Plasmodium can be transmitted by Culex, Aedes, Culiseta, Anopheles, Mansonia and Psorophora. As such, understanding the patterns of vector and host switches within Plasmodium and related taxa can actually provide interesting insights into the genus’ evolutionary history.