WHO says cash crunch, rains could thwart Ebola efforts

* Need for progress in stemming disease by April-May

* WHO will run out of cash in mid-February, needs funds

* WHO Executive Board to hold special session on Sunday (Adds quotes, figures)

By Stephanie Nebehay

GENEVA, Jan 23 (Reuters) - Halting the spread of Ebola in West Africa will depend on mobilising funds and aid workers before the rainy season hits in April-May, otherwise it could up to take a year, the World Health Organization (WHO) warned on Friday.

But the WHO is set to run out of cash in mid-February, a key period as it tries to halt the deadly disease, a senior WHO official said.

"It is a programme that can stop transmission if we have the money and the people, and we don't have either," Dr. Bruce Aylward, WHO assistant director-general in charge of the Ebola response, told a news briefing before a special session of WHO's Executive Board on Sunday.

The number of Ebola cases week-on-week has declined for each of the past four weeks in hard-hit Guinea, Liberia and Sierra Leone, which is promising, he said after a tour of the region.

There has been a "real substantive reduction" in cases in the past 21 days, corresponding to the incubation period for the haemorrhagic fever, a crucial barometer for tracking its spread.

"We run out of cash in mid-February, that is four or five months before that virus is going to stop in a best-case scenario," Aylward said. "So it is a bit of a race against time right now."

The U.N. health agency still needs $260 million for its $350 million budget for Ebola for the next six months, Aylward said. It is seeking to raise the money from donor countries.

The key target was getting down to zero new infections.

"You're looking at months...it really depends on the progress they can make between now and the wet season. Because if you go into a real wet season with this disease you're looking at another hard year of work or plus."

The rains could wash away roads, complicating logistics for aid delivery and movement of health care workers, the WHO says.

In all, 21,724 cases of Ebola have been reported in nine countries in the past year since the epidemic began in Guinea, including 8,641 deaths, the WHO said on Thursday.

It currently deploys 700 experts in Ebola zones, but needs another 300 to help with identifying cases and tracing contacts of those who are infected, Aylward said.

"There tends to be a false sense of security that this is somehow a controllable disease. There is no such thing as Ebola control, it has got to drive to zero," he said.

"It's still an incredibly dangerous situation."

The first batch of GlaxoSmithKline (Other OTC: GLAXF - news) 's experimental Ebola vaccine has been dispatched to West Africa and is expected to arrive in Liberia later on Friday, the British drugmaker said. (Reporting by Stephanie Nebehay; Editing by Mark Trevelyan)