Dakar - When a sick Muslim imam from Guinea entered Mali at the border town of Kouremale last month, he did not use the main tarmac road with its Ebola checkpoint but took a nearby dirt track.
When his green Mercedes was halted at a second checkpoint and he was asked to return to Kouremale for health checks, Oussa Koita, 70, managed to avoid them thanks to the intervention of a village chief, according to border officer Mamadou Diawara.
Koita is later believed to have died of Ebola - he was never tested - but not before he had visited Mali's capital Bamako. His journey highlights the porousness of national borders in a region struggling to stem the worst known outbreak of a disease which has now killed at least 5,420 people in eight countries.
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Authorities in Guinea and Mali are now scrambling to trace around 300 people on each side of the border who had contact with Koita, whose funeral in Kouremale drew thousands.
Health workers in Mali fear more Ebola cases will spill over the border from the Siguiri area of Guinea, where aid workers say resources to fight the disease are still meagre.
The World Health Organization (WHO) says there are five probable cases of Ebola in Siguiri as the contagion creeps northwards from its original forest epicentre of Gueckedou, deep in the south of Guinea.
Six people in Mali have died so far from Ebola. The WHO said in a report on Thursday that Mali would "remain at risk of further imported cases as long as transmission across the border is ongoing".
Since the Koita incident, Mali's President Ibrahim Boubacar Keita has stepped up border controls and this week he visited Kouremale to oversee their implementation. Medical charity MSF is also expanding its Mali team to help with the response.
But keeping cases out will not depend entirely on Mali.
"There is a problem with the Guinean response and if these people fall ill, the likelihood is that they come into Mali for treatment," said a senior source in Mali's Ebola response team.
Massama Keita, head of a local crisis committee fighting Ebola, said halting the disease at the border was impossible.
"Between Mali (and Guinea) it's not really a border. So many people reside together that you cannot control it," he said. (Additional reporting by Colin Baker in Bamako and Tom Miles in Geneva; Writing by Emma Farge; Editing by Gareth Jones)