In the global effort to eradicate malaria, the focus has often been on the number of lives saved—through insecticide-treated bed nets, artemisinin-based therapies, vector control and other strategies. Equally important in this fight is the concept of saving brains, particularly in young children.
Malaria is caused by an infection with the parasite Plasmodium falciparum and can manifest as either an uncomplicated or severe disease. The most severe neurological complication is cerebral malaria, a disease that disproportionally affects young children because they have not yet developed immunity against Plasmodium parasites. More than 785,000 children under the age of nine living in sub-Saharan Africa are affected by cerebral malaria each year. The idea of saving brains becomes especially relevant in this population because cerebral malaria can have long-lasting effects on the cognitive function of these children. An early study found that children who developed cerebral malaria were roughly three and a half times more likely to have a cognitive deficit than children who did not have malaria. Importantly, researchers observed this difference two years after the initial episode of cerebral malaria and long after the disease itself had been treated.
If exposure to malaria at a young age could have long-lasting effects on the cognitive abilities of children, what happened when that exposure happened much earlier? Like during pregnancy? In a new study published in PLoS Pathogens, a team of researchers led by Dr. Chloë McDonald and Dr. Kevin Kain showed that malaria in pregnancy leads to cognitive impairments in the offspring that persist into adulthood.
An estimated 125 million pregnancies worldwide are at risk of malaria infection, highlighting the need to understand the effects of malaria during pregnancy on offspring health and development. To study these effects, McDonald used an experimental mouse model where pregnant mice were infected with Plasmodium parasites mid-way through their pregnancy and allowed to progress naturally to a full-term delivery. “We’re basically modeling an infection in a first-time mother that happens mid-pregnancy and goes untreated,” says McDonald. “This model parallels a growing infection like what you would see in real life.
Malaria during pregnancy is known to be associated with an increased risk of premature, or preterm, birth and low birth weight, both of which are themselves risk factors for impaired neurodevelopment. To fully understand the effects of malaria in pregnancy on offspring cognitive functioning, the researchers needed to remove the confounding factors of preterm birth and low birth weight. They did this by infecting the pregnant mice with a lower dose of Plasmodium parasites. At this lower dose, the mothers still developed the same immunological response to the infection but there were no differences in the length of the pregnancies or the birth weight of the pups.
“The hypothesis was that if we eliminated the low birth weight and preterm birth [effects], we would still see impairments in learning and memory in the offspring,” says McDonald.
When the pups were four to seven weeks old, the researchers assessed their learning and memory using something called the Novel Object Recognition (NOR) test. In this test, a mouse is first exposed to a piece of LEGO and a Hot Wheels car and then exposed to one of the original objects plus a new object. Mice are naturally curious and will preferentially explore a new object over a familiar one. In the NOR test, mice born to infected mothers performed more poorly compared to mice that were born to uninfected mothers. That is, mice that had been exposed to malaria in the womb had trouble distinguishing the new object from the old one and spent equal amounts of time exploring both objects. Unexposed mice spent a larger proportion of time with the new object.
The researchers then used a test called the Tail Suspension Test (TST) to measure affective behaviour. For this test, a mouse was suspended by its tail for a short period of time and monitored for movement. Mice with more depressive-like behaviour spend less time moving and show greater immobilization. Using this test, the researchers found that the exposed mice spent significantly more time immobilized compared to the unexposed mice.
What was even more striking was that these deficits in learning, memory and affective behaviour were long-lasting. The researchers observed the same differences between exposed and unexposed mice in both the NOR test and TST at 20 weeks of age.
“I was surprised that the [effects] were as strong as they were,” says McDonald. “The majority of malaria-exposed pregnancies don’t have [observable characteristics] at birth. It’s not just the preterm births or low birth weight that are perhaps inducing a risk of neurodevelopmental deficits. It could also be the births that look completely healthy.”
Using different imaging techniques, the researchers next looked at changes in the brain that might be associated with the cognitive impairments. While they did not see a difference in brain size between exposed and unexposed mice, they did observe an increase in the number of small blood vessels in the brains of exposed pups. Further, these pups had lower levels of the neurotransmitters dopamine, norepinephrine and serotonin, which have previously been shown to be involved in memory, learning and affective behaviour.
To understand the underlying driver of these differences, the researchers honed in on the complement system, a part of the body’s immune system that plays an important role in helping to clear invading parasites like Plasmodium. In humans, malaria infection activates the complement system to fight pathogens. But in some cases, the complement system is turned on too much and this overzealous activation actually contributes to the development of severe disease like cerebral malaria.
Could the cognitive impairments seen in the malaria-exposed pups be caused by improper activation of the complement system in the pregnant moms?
To answer this question, the researchers repeated their earlier experiments with a strain of mice lacking a component of the complement system. They infected pregnant complement-deficient mice with Plasmodium parasites and tested their offspring using the same NOR test and TST. When the moms could not activate their complement system, there was no difference in performance between the pups of infected and uninfected mice. Loss of the complement system had reversed the learning and memory deficits and the depressive-like behaviour seen in the malaria-exposed pups. While levels of the neurotransmitters dopamine, norepinephrine and serotonin had been restored in these pups, they still had a higher number of small blood vessels in their brains than unexposed pups. These results suggest that the cognitive defects seen in the malaria-exposed mice were not related to the increase in small vessels.
This paper provides the first evidence that there is a causal link between malaria during pregnancy, activation of the complement system and cognitive impairments in the offspring. As a follow-up to this work, Kain’s team is currently conducting an observational study in Malawi to determine if the findings of their experimental mouse model will reproduce in a clinical human population. They are closely following a group of children born to women who developed malaria during pregnancy to see if the risk of cognitive deficiencies is higher in this group compared to children born to women who did not experience malaria during pregnancy. Based on their work, the researchers are also working on a new approach to treat the birth and cognitive effects of malaria in pregnancy that is safe, affordable and feasible in a low-resource setting.
McDonald is optimistic that their work will have a positive impact on efforts to ease the burden of malaria around the world. “The great thing about this story is that there are so many places to intervene,” she says. If the association between malaria in pregnancy and cognitive outcomes holds true in humans, it strengthens the case for protecting pregnant mothers from Plasmodium infections. It also provides opportunities for early intervention. A child born to a mother who experienced malaria during pregnancy can undergo treatments and therapies early on to improve his or her cognitive functioning—saving lives and brains.
Reference: McDonald CR, Cahill LS, Ho KT, Yang J, Kim H, Silver KL, Ward PA, Mount HT, Liles WC, Sled JG, & Kain KC (2015). Experimental Malaria in Pregnancy Induces Neurocognitive Injury in Uninfected Offspring via a C5a-C5a Receptor Dependent Pathway. PLoS pathogens, 11 (9) PMID: 26402732