Duration of Malaria Symptoms

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QUESTION

If untreated how long will a person suffer symptoms caused by Malaria?

ANSWER

The answer to that depends a lot on what type of malaria they have as well as their own immune status, and particularly if they have been infected with malaria before. So, for example, P. falciparum is the most severe and deadly kind of malaria, and without treatment, many people who do not have acquired immunity (from previous infections) are likely to die within just a few days. This is the type of malaria that causes the most deaths, and explains why most of the fatalities occur in young children, who have not had the chance to acquire immunity.

In contrast, other kinds of malaria are less severe, and so symptoms can persist before the malaria parasite is naturally cleared by the parasite, usually within 1-2 weeks. In extreme cases, this can last much longer; Plasmodium malariae is the slowest replicating form of malaria, and so frequently causes mild infections which can last weeks, if not months. In some cases, people are infected with low levels of P. malariae for years without even experiencing symptoms, since their own immune system is able to keep levels of the parasite low enough so that they don’t cause noticeable disease.

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.

Persistent Antibodies to Malaria?

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QUESTION

I had malaria as child, more than 30 years ago. It was successfully treated with no relapse. I have since travelled to malaria- endemic countries, but the last time was over 3 years ago. No symptoms. I recently donated blood and routine screening has detected malaria antibodies. How long do the antibodies persist?

ANSWER

Based on your experience, I would say at least three years! While I doubt your antibodies would persist since your infection as a child, it is more likely that in your more recent trips to malarial areas you have been re-exposed to the parasite, but for whatever reason, the infection didn’t progress into a full-blown episode of malaria. This could well be due to some residual immunity from childhood, or you just received a light enough infection that your general immune system was able to fight off. Either way, this would have produced new antibodies against malaria, which were picked up by the blood screen.

The length of time antibodies persist is important information in the control of malaria, since serological tests (which detect antibodies) can be used for screening of populations in low-transmission environments, but their efficacy is reduced in locations where people have been treated for malaria but their antibodies persist. Also, understanding how antibodies are created and maintained in the body is necessary for gaining an appreciation of how preventive measures, such as bednets, might potentially leave populations more vulnerable to malaria later on, through lack of acquired immunity.

Pregnant Women and Malaria

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QUESTION

Why are pregnant women and children at the highest risk of getting malaria?

ANSWER

The reason why pregnant women and children are at greater risk of contracting malaria is due to their reduced levels of immunity. Children living in malarial areas have less immunity than adults because they have had less exposure to malaria, and so their immune systems have not had a chance to develop antibodies to protect against the disease. Pregnant women, although they may normally have good immunity, witness changes in their immune systems during pregnancy in order for the woman to carry the child.

Given that a baby is genetically distinct from its mother, the mother’s immune system has to be modulated in order to ensure that the body’s defenses do not act against the baby, as they would some other foreign object such as an infection. While this immune modulation allows the mother to carry her foetus, it does leave her vulnerable to other infections, such as malaria. In areas of high malaria transmission, the mother may still retain enough immunity to not present with severe symptoms of malaria. In these cases, the dangers of malaria in pregnancy come from resulting anemia in the mother, as well as passage of malaria antigens or the parasite itself through the placenta and to the foetus, which can result in problems for the baby.

Interestingly, a study came out last year which showed evidence that using long-lasting insecticide treated bednets increased malaria prevalence in older children and adults, probably because the nets protected against even sub-clinical exposure to malaria and therefore the levels of natural immunity in people using the nets declined over time, leaving them more susceptible to infection. However, this result has been controversial, as in other studies community-wide coverage of bednets has reduced overall malaria incidence due to reduced transmission.