Malaria eradication calls for detection and treatment of asymptomatic cases

Asymptomatic malaria infections don’t reduce the risk of serious illness, as is commonly believed, but suppress the immune system and prevent it from getting rid of parasites that persist in the bloodstream, research suggests.

According to the World Health Organization, there were an estimated 241 million cases of malaria worldwide in 2020, of which 627,000 were fatal. The disease is caused by Plasmodium parasites transmitted by the female’s bite Anopheles mosquito.

Some people develop immunity to the malaria parasite Plasmodium falciparum after many years of repeated infections, but they harbor a small number of parasites that continue to live in the bloodstream without causing the typical feverish symptoms, says Diana Hansen, an associate professor at the Walter and Eliza Hall Institute of Medical Research, in Melbourne, Australia.

“These infections have traditionally been considered beneficial because they were thought to protect against symptomatic diseases,” said Hansen, who was part of a team that published the results of their research in Molecular systems biology.

“Based on this assumption, asymptomatic malaria is often left untreated in countries where malaria is endemic, despite our poor understanding of the real impact these persistent infections have on humans.”

Hansen emphasizes that asymptomatic malaria infections are common in endemic areas, “with recent studies estimating their prevalence to be up to five times higher than symptomatic infections”.

“A central item on the malaria eradication agenda is the challenge that asymptomatic infection poses to elimination efforts, as this clinically silent reservoir of parasites has been shown to help maintain transmission,” says Hansen. “Progress towards the WHO-set target of eliminating malaria by 2030 has stalled in many endemic countries since 2015.”

The researchers analyzed white blood cells from patients with asymptomatic and symptomatic infections in an endemic area of ​​Indonesia. They found that patients with chronic, asymptomatic malaria infections suppressed the immune system, producing more proteins to help the parasites survive.

Hansen says that because the immune system is suppressed and can’t work at full capacity, the body can’t control the parasites and remove them from the bloodstream. because the immune system doesn’t have the capacity to be trained properly,” she explains.

“If we treated individuals with asymptomatic malaria infections, we would also reduce the invisible reservoir of parasites that sustain transmission and deter the efforts of malaria elimination campaigns,” she says, adding that the study provides a framework for the consider new policies to support screening and treatment of asymptomatic malaria in endemic areas around the world.

“Individuals living in malaria-endemic areas only develop what we call clinical immunity after many years of repeated infections. Clinical immunity does not completely cure the infection, but prevents symptomatic episodes by significantly reducing the number of parasites in the blood, with adults often non-fever asymptomatic experienced malaria.”

She adds that since non-fever-infected individuals do not actively seek treatment, asymptomatic infections last longer than symptomatic episodes and become chronic in nature. “The full impact of persistent asymptomatic malaria on the host was unclear, so we decided to embark on this study.”

Hansen said the complete lack of information about the true impact of chronic infections discouraged strategies such as mass drug administration. “Our study provides evidence that asymptomatic malaria is not benign or harmless. This information is critical in providing a framework for new policies to support screening and treatment of non-febrile malaria in pre-elimination areas of Southeast Asia.”

Epidemiologists have recently used molecular (PCR-based) methods to determine that asymptomatic malaria is more common than symptomatic cases, as transmission decreases and can account for as much as 80 percent of all malaria cases, said Alyssa Barry, professor of systems epidemiology at the University of Groningen. infection at Deakin University and principal investigator at the Burnet Institute, Australia, says: SciDev.Net

“As malaria transmission has declined dramatically over the past two decades, these hidden cases are increasingly recognized for their key role in perpetuating malaria transmission and preventing the disease from being eliminated. has not been studied is the impact of asymptomatic malaria on human health,” says Barry.

The researchers hypothesize that the suppressed immune response of asymptomatic cases may also prevent appropriate immunity from developing after vaccination, and may partly explain why so many malaria vaccines that were successful in clinical trials in unexposed volunteers failed when tested in malaria-endemic diseases. areas,” she says.

Barry thinks one solution could be to give malaria treatment before vaccination to clear the infection.

“These results are extremely important for malaria immunity and represent a major paradigm shift from the widely held belief that asymptomatic cases are benign. They also provide new opportunities to further investigate the biological mechanisms involved,” said Barry.

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