DAKAR, Senegal —The number of Ebola cases reported in Guinea and Sierra Leone last week dropped to its lowest total this year, the World Health Organization announced Thursday, just days before Liberia is due to be declared Ebola free.

Health experts acknowledge they are still unable to track exactly where the virus is still spreading in West Africa, more than 16 months after the first deaths in the remote forests of Guinea. The outbreak has killed more than 11,000 people.

In Guinea, five of the country’s nine new cases were detected only after the patients died; none had sought treatment at an Ebola clinic. And only two of the nine cases in Sierra Leone were identified as contacts of previous Ebola patients. “It will still take some time before we are going to be able to actually celebrate an Ebola-free West Africa,” said Peter Jan Graaff, the U.N. secretary-general’s acting special representative and head of the United Nations Mission for Ebola Emergency Response.

Graaff emphasized the importance of knowing where the virus is moving within communities. In Guinea, where Ebola first emerged, health workers are still struggling to trace cases. “As long as we have this, it’s going to be very difficult to get to zero. Not impossible – but very difficult,” Graaff said.

In another development, for the first time, Ebola has been discovered inside the eyes of a patient months after the virus was gone from his blood.

Ebola has infected more than 26,000 people since December 2013 in West Africa. Some survivors have reported eye problems but how often they occur isn’t known.

The new report concerns Dr. Ian Crozier, a 43-year-old American physician diagnosed with Ebola in September while working in Sierra Leone.

He was treated at Emory University Hospital’s special Ebola unit in Atlanta and released in October when Ebola was no longer detected in his blood. Two months later, he developed an inflammation and very high blood pressure in one eye.