Asymptomatic malaria ‘super-spreaders’ regularly infect mosquitoes

Researchers found that a small number of infected, asymptomatic school-aged children in an area of eastern Ugandan are responsible for the majority of remaining malaria circulating in local mosquitoes.

The so-called “super spreaders” complicate long-term efforts to eliminate malaria, according to a press release about the findings, which were presented at the American Society of Tropical Medicine and Hygiene Annual Meeting.

Chiara Andolina, MSC, a PhD student at Radboud University Medical Center in the Netherlands, said the study amounts to the first direct evidence that even in areas with intensive malaria control, a small number of asymptomatic people can “quietly sustain transmission, and finding and treating them could prove very challenging.”

“The existence of asymptomatic malaria infections is well known, but it was surprising to see just how much they can contribute to infecting mosquitoes,” Andolina said in the press release.

According to Andolina, school-aged children are an “accessible reservoir” of infection because most of them are in school, meaning they can be more easily targeted for interventions, like malaria prophylactic medicines.

The study showed that asymptomatic infections were mostly found in children aged 5 to 15 years and were sufficient to sustain the circulation of malaria parasites in local mosquitoes, Teun Bousema, PhD, a malaria epidemiologist at Radboud and co-author of the study, said.

The 2-year study involved testing more than 500 people regularly for evidence of malaria parasites. The authors reported that only four children were linked to 60% of the infected mosquitoes studied. One of the four children showed no symptoms but concealed seven different variations of the Plasmodium falciparum malaria parasite and was responsible for almost 25% of the infected mosquitoes. Two asymptomatic youth were infectious for more than 6 months.

“Our study adds another challenge because it shows that making and maintaining progress against malaria cannot rely on simply treating people who are sick and thus eliminating a source of parasites that can infect local mosquitoes,” Bousema said in the release. “Some have argued that when transmission is very low and treatment is available, you can relax somewhat — because most new infections will produce symptoms and patients will come to clinics for medicines. But what if the biggest source of parasites is a group of people who never show any signs of getting sick?”

Efforts to prevent COVID-19 also could complicate malaria control efforts.

Andolina and Bousema said a troubling aspect to the study was learning how asymptomatic spreaders developed a level of immunity, even though they grew up in an area with decreasing levels of malaria infections.

“It could be evidence that you don’t need a lot of previous exposure to acquire a level of immunity that allows you to get infected with malaria parasites without getting sick,” Andolina said. “This supports previous work showing that children can acquire immunity very efficiently and, at the same time, remain very important as a source for spreading infections.”