* Governments, philanthropists pledge most of needed funds
* So far in 2013, only 19 polio cases seen worldwide
* Dramatic progress "puts us within sight of the endgame"
By Kate Kelland, Health and Science Correspondent
LONDON, April 25 (Reuters) - Health groups said on Thursday they could rid the world of polio by 2018 with a $5.5 billion vaccination and monitoring plan to stop the disease taking hold once more now there are only a handful of cases worldwide.
Experts say the plan offers the best chance yet to eradicate a disease that until the 1950s crippled many thousands of people every year but has been brought almost to extinction though effective vaccine campaigns.
In 1988, more than 350,000 children were paralysed by polio and the disease was endemic in more than 125 countries. Last year, worldwide polio cases plunged from 650 in 2011 to 223, the largest drop in a decade.
So far in 2013, 19 cases have been reported and polio remains endemic in just three countries - Pakistan, Afghanistan and Nigeria - after India celebrated its second polio-free year.
"Today we have the fewest cases in the fewest places ever, making it critical to use the best opportunity the world has ever had to put an end to this terrible, preventable disease," Anne Schuchat, a global health specialist at the U.S. Centers for Disease Control and Prevention (CDC), said in a statement.
The virus attacks the nervous system and can cause irreversible paralysis within hours of infection. It often spreads in areas with poor sanitation and children under five are the most vulnerable, but it can be halted, as it was in many developed countries, with comprehensive vaccination programmes.
The polio plan's $5.5 billion budget includes the costs of reaching and vaccinating more than 250 million children multiple times every year, monitoring and surveillance in more than 70 countries, and securing the infrastructure that health campaigners hope will go on to help other health programs.
In a statement issued by the World Health Organisation, world leaders and individual philanthropists backed the plan by the Global Polio Eradication Initiative (GPEI) by pledging almost three-quarters of the funds up front.
"After millennia battling polio, this plan puts us within sight of the endgame," said the WHO's director-general Margaret Chan. "We have new knowledge about the polio viruses, new technologies and new tactics to reach the most vulnerable communities."
The GPEI, launched in 1988, is a grouping of governments, the WHO, Rotary International, the CDC and the United Nations children's fund UNICEF, supported by philanthropic groups such as the Gates Foundation.
Speaking at a summit on vaccines in Abu Dhabi, Bill Gates said his foundation would stump up $1.8 billion - a third of the total cost of the GPEI's six-year budget.
Another $335 million was promised by a seven-strong group of other philanthropists, including the Tahir Foundation, Bloomberg Philanthropies and the Carlos Slim Foundation.
Multiple government donors - among them Britain, Germany, Norway, Pakistan and Nigeria - also made pledges, bringing the total promised so far for the plan to just over $4 billion.
Public health experts say if the polio eradication campaign succeeds, the world would not only declare its second eradicated disease - smallpox was wiped out in 1979 - it would also be billions of dollars richer.
A 2010 analysis found that if polio transmission were to be stopped by 2015 the net benefit from reduced treatment costs and productivity gains would be $40 billion to $50 billion by 2035.
(Editing by Alison Williams)