The outbreak's spread into Uganda prompted the World Health Organization to revisit whether the second-largest Ebola epidemic in history should be declared a global health emergency. A WHO expert committee meets on Friday. Such declarations almost always boost attention and donor funding.
The boy's mother had taken him and his brother from Uganda into Congo, where her father was ill. WHO said he died of Ebola, and officials believe those who mourned him became infected, too.
The family then crossed back into Uganda via an unguarded foot path, bypassing official border crossings where health workers have been screening millions of travelers since the outbreak was declared in August.
Authorities in both countries now vow to step up border security.
Experts have long feared Ebola could spread to neighboring countries because of rebel attacks and community resistance hampering containment work in eastern Congo, one of the world's most turbulent regions. The virus can spread quickly via close contact with bodily fluids of those infected and can be fatal in up to 90% of cases.
The 5-year-old boy's mother and grandmother, along with several other children, were stopped at a border post before crossing into Uganda. A dozen of them already showed symptoms of Ebola.
Congo's health ministry said those 12 were put in an isolation center, but in fact they were told to remain where they were staying until transport was found to an Ebola treatment unit, Dr. Dominique Kabongo, a local coordinator of response teams, told The Associated Press.
Instead, six family members quietly crossed into Uganda.
"Many people are evading (border) customs and using small footpaths and it is difficult for us to follow the contacts," Kabongo said.
On arrival in Uganda, where authorities had been alerted by Congolese colleagues, the boy received treatment while relatives were isolated and tested. The boy's uncle is among seven suspected cases now identified in Uganda.
On the Congo side, five family members who did not cross into Uganda have tested positive for Ebola, the health ministry said.
Health teams in Uganda "are not panicking," Henry Mwebesa, the national director of health services, told the AP. He cited the East African nation's experience battling previous outbreaks of Ebola and other hemorrhagic fevers.
This outbreak "is not going to go beyond" the boy's family in Uganda, he added.
While officials vowed to close unauthorized crossings, an AP reporter in the border area where the family crossed saw surveillance teams patrolling the Ugandan side. Some footpaths, however, remained unguarded. Some people wade across the shallow Lubiriha River.
The "stubbornness of Congolese" is a challenge in screening, a Ugandan Red Cross official, Francis Tumwine, told the AP at one border crossing last week. "They have failed to understand that Ebola is there, they think that it is witchcraft which is killing them."
A Congolese trader, Muhindo Kaongezekela, added: "We are not sure if there's Ebola in Congo. In Congo, if they find you with a headache, they take you to the hospital and later say they died of Ebola."
This is the first time this restive part of vast Congo, veteran of several Ebola outbreaks, has experienced the virus.
Resistance by residents wary of authorities has hurt containment efforts in an outbreak where an experimental but effective Ebola vaccine is being widely used for the first time. More than 130,000 people have received the vaccine.
Uganda is more stable than eastern Congo, and it has vaccinated nearly 4,700 health workers. WHO is shipping another 3,500 doses this week for health workers and contacts of those infected.
The WHO expert committee has twice decided that this outbreak, while of "deep concern," is not yet a global health emergency . But international spread is one of the major criteria the United Nations agency considers before making a declaration. WHO has advised against travel restrictions.
The first cross-border case is "tragic but unfortunately not surprising," said Dr. Jeremy Farrar with the Wellcome Trust, which funds Ebola vaccine research.
While Uganda is well-prepared, he added, "we can expect and should plan for more cases in (Congo) and neighboring countries. This epidemic is in a truly frightening phase and shows no sign of stopping anytime soon."
Muhumuza reported from Kampala, Uganda. Associated Press writers Maria Cheng in London; Saleh Mwanamilongo in Kinshasa, Congo; Krista Larson in Dakar, Senegal and video journalist Tina Smole in Mpondwe, Uganda contributed.
Follow Africa news at https://twitter.com/AP_Africa
Copyright 2019 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.