The largest-ever outbreak of Ebola could drain billions of dollars from economies in West Africa by the end of next year if the epidemic is not contained, the World Bank said in an analysis on Wednesday.

The global development lender predicted that slow containment of the deadly virus in Guinea, Liberia and Sierra Leone could lead to broader regional contagion, particularly through tourism and trade.

Under the worst-case scenario, Guinea's economic growth could be reduced by 2.3 percentage points next year while Sierra Leone's growth would be cut by 8.9 percentage points. Liberia would be hardest hit, with a reduction of 11.7 percentage points next year.

"We really need to scale up our response and what we have learned from this study is that time is of the essence," World Bank President Jim Yong Kim told reporters.

Even under the best-case scenario, countries would need a "massive" scaling up of their response to contain the disease in the next four to six months, the bank said.

The World Health Organization (WHO) said the outbreak requires a $1 billion expenditure to limit its spread. "The ($1 billion) is something we need right now, and it could go up rapidly if we do not respond," Kim said.

The United States announced on Tuesday it would send 3,000 troops to help tackle the Ebola outbreak.

The bank itself has pledged about $200 million in emergency assistance to Guinea, Liberia and Sierra Leone, the three countries most affected.

HARDEST-HIT ECONOMIES

The World Bank predicted the three West African countries so far affected by the virus would lose $359 million in economic output this year. All three also have significant funding gaps, totaling nearly $300 million.

Inflation and food prices were also starting to rise due to shortages, panic buying and speculation, the bank said.

Failure to contain the virus quickly could also affect business in neighboring countries, including Nigeria, Ghana and Senegal.

"The analysis finds that the largest economic effects of the crisis are not as a result of the direct costs ... but rather those resulting from aversion behavior driven by fear of contagion," the bank said in a statement.

The worst Ebola outbreak since the disease was identified in 1976 has already killed nearly 2,500 people, half of the number infected by the virus.

The bank said agreeing on a standardized Ebola treatment and prevention protocol, endorsed by the WHO, was critical and would significantly reduce the virus's death rate, now at 53 percent.

The outbreak of the highly contagious virus, which causes fever and uncontrolled bleeding, was first confirmed in the remote forests of southeastern Guinea in March.

(Reporting by Anna Yukhananov; Additional reporting by Stella Dawson, Thomson Reuters Foundation; Editing by Andrea Ricci and Jeffrey Benkoe)