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.