That incident underlines the problems in tracking the virus as a WHO expert panel on Friday is set to discuss whether to declare a global health emergency
As a second person infected with Ebola died in Uganda, WHO emergencies chief Dr Mike Ryan told The Associated Press he did not believe the man had been on any list of high-risk Ebola contacts that health workers use to track infections and contain the outbreak.
Three of the pastor's family members were infected with Ebola when they travelled from Uganda to DR Congo to visit him.
His five-year-old grandson became the first Ebola case in Uganda and the first death after several family members quietly returned home on footpaths that bypassed border screening for Ebola.
The boy's grandmother also died after their return, Ugandan officials said on Thursday.
"It's an unfortunate occurrence that a pastor who's taking care of people and providing care to people is himself infected in the line of his own work and then ultimately goes on to infect others," Dr Ryan said.
The pastor's positive test for Ebola came back only after he was buried, Dr Dominique Kabongo, coordinator of Ebola response teams in Kasindi, said.
Family members traditionally wash and dress the corpse.
More than 80 people attended the burial, Dr Kabongo said.
The first cross-border cases in this Ebola outbreak - the second-deadliest in history - have prompted the WHO expert committee to again consider declaring a global health emergency after twice deciding against it.
Such declarations almost always boost attention and donor funding.
Dr Ryan said there is no evidence Ebola is spreading within Uganda and that authorities have contained the virus to the family.
He said 27 people who may have been exposed are being followed. Ugandan authorities said three suspected Ebola cases not related to the family were awaiting test results.
More than 1,400 people have died in this outbreak declared in August in eastern DR Congo, one of the world's most turbulent regions, where rebel attacks and resistance by community members wary of authorities have badly hampered Ebola containment work.
For months, officials have feared the virus would cross the heavily travelled border where health workers have been screening millions of people for fever and other symptoms.
Uganda, Rwanda and South Sudan have been preparing for possible cases.
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