Research Explained

Can you have malaria and not know it?

Researchers at KNUST tested 113 schoolchildren in western Ghana. Eighty-six were carrying malaria parasites but not one of them felt ill.

Amma Aboagyewa Larbi and her colleagues at Kwame Nkrumah University of Science and Technology went to Ahanta West, a coastal district in Ghana's Western Region, and screened schoolchildren aged six to 15 for malaria. Of 113 children, 86 tested positive. None had a fever, none had been to a clinic, and most were carrying two species of the parasite at once.

Their parents did not know. The children did not know. Because none of them ever presented at a health facility, none appear in Ghana's malaria case figures either.

Ahanta West is not unusual. Pooling 24 studies that covered 19,169 children across ten sub-Saharan countries, Daniel Asmelash and colleagues at Mizan-Tepi University in Ethiopia put the average at around one in four.

Why don't they feel ill?

Malaria hurts most the first time. With each infection the body learns a little more, and after enough of them it can hold the parasite below the level that triggers fever without ever getting rid of it. Immunologists call this partial or semi-immunity. It is never complete, and it is the reason silent carriage clusters in older children and adults in places where transmission never stops.

So the honest answer to whether a visitor could be carrying malaria unknowingly is: usually not, and that is not good news. A traveller with no exposure history has no such tolerance, which is why they get sick, and quickly. The fever is the immune system's inexperience.

There are exceptions, and one of them is unsettling. A Dutch blood donor who had travelled in East Africa carried Plasmodium malariae for years with no symptoms at all, according to a case report in the Malaria Journal. It surfaced only when a transfusion recipient developed malaria and investigators traced the blood back to him. A low platelet count was the sole clue anything was wrong.

People who grew up in an endemic country and then moved abroad occupy the ground in between. Immunity fades with time away, which is why returning to visit family carries a risk that childhood exposure no longer covers.

Why does it matter if nobody feels ill?

Larbi's team point to what happens when species mix. Most of the children they tested carried P. falciparum alongside P. malariae, and that combination raises the odds of the infection turning severe. Non-falciparum species have been linked to severe anaemia and kidney disease in adolescents.

Silent infection also shows up in growth. Researchers analysing blood spots from 840 infants in rural Malawi found that those carrying parasites weighed less for their age than uninfected children of the same age.

Then there is everyone else. Someone who never feels ill never seeks treatment, so the parasite stays in circulation, available to the next mosquito.

Why doesn't a test find it?

At Obom, near Accra, a University of Ghana team led by Mawusi Adepa Mawuli, Linda Eva Amoah and Yaw Asare Afrane screened 98 children three ways. Microscopy caught 13.7%. PCR caught 78.08%. Nearly two thirds of the carriers had parasite loads too low for a microscope to see.

The parasite has also started evading the tests directly. Most rapid tests hunt for a protein called HRP2, and in places where the parasite has lost the genes that make it, an infected person tests clean. Work in Malawi last year found this in 12.3% of samples. Ghana, so far, appears clear of it.

Do silent carriers spread malaria?

Here the science is genuinely unsettled, and it matters, because the argument for hunting carriers down rests on the answer.

The Obom team fed blood from 73 symptomless children to mosquitoes. Four samples produced infections. In Ouidah, southern Benin, the same experiment found every single gametocyte carrier infectious. Earlier work in Burkina Faso put 24.2% of one community's entire transmission down to carriers invisible under a microscope.

The Ghanaian researchers are careful about their own limits, noting they used a laboratory mosquito strain bred to be highly susceptible and did not measure gametocyte density.

What is being tried

Four approaches are under test, and none of them is settled.

The bluntest is mass testing and treatment: screen a whole community, treat everyone positive, symptoms or not. Researchers have run it in Ghana's Pakro sub-district and measured what happened to prevalence afterwards.

The second targets the sexual-stage parasite directly. A trial in The Gambia is testing whether adding primaquine to standard treatment clears the gametocytes that carriers pass to mosquitoes.

The third is a transmission-blocking vaccine. Two parasite proteins, Pfs230 and Pfs48/45, are the leading candidates, and the Obom team measured natural antibodies to both. Their finding was awkward: two of the four children whose blood infected mosquitoes had antibodies to both proteins.

The fourth is the simplest. Ghana gives seasonal preventive medicine to children under five. Ten years of surveillance in Bongo District, in the Upper East Region, found prevalence near zero in that group and stubbornly high in everyone older.