* Nigeria one of three nations where polio endemic
* Gates' sees fight against malaria making headway
* Billionaire backs efforts to boost Africa agriculture
By Edmund Blair
NAIROBI, July 24 (Reuters) - Nigeria could cut the number of polio cases to zero next year and be declared free of the disease in 2018 even though a national eradication campaign has had to contend with an insurgency in the north, Bill Gates told Reuters.
The Bill and Melinda Gates Foundation supports the global initiative to wipe out polio, which includes a campaign in Nigeria, one of three nations where the crippling virus is still endemic. The other two are Pakistan and Afghanistan.
"We have got all the challenges up in northern Nigeria, the violence from Boko Haram, and the distraction of an upcoming election," Gates said in a telephone interview, referring to an Islamist rebel group that has in the past targeted vaccination workers, and to Nigeria's national vote next year.
"Despite all that, we've got by far the lowest numbers of cases ever," he said. "We hope by the end of next year we'd be at zero." He added that if there were no more cases for three years after that, Nigeria could be certified clear in 2018.
The technology billionaire-turned-global philanthropist was speaking last week before a speech on Thursday at Addis Ababa University on development in Africa, mainly in health and agriculture.
"We've got a pretty optimistic view of what can happen in Africa in those two areas," he said before his trip to Ethiopia, a nation stricken by famine 30 years ago but which has doubled farm output in the last eight years.
In health work, one of his most high-profile programmes is the fight against malaria, the mosquito-borne disease that infects more than 200 million people a year and kills more than 600,000 people. Nine out of 10 deaths are in Africa.
The campaign includes promoting bed nets, protecting homes with insecticide sprays and using the artemisinin drug in treatment to bring down fatalities, he said.
"On the negative side, we have artemisinin resistance emerging in southeast Asia, and that can take away the very best drug tool we have right now," he said, adding that there were efforts to contain that problem from spreading.
British drugmaker GlaxoSmithKline is seeking regulatory approval for the world's first vaccine, known as RTS,S, which has been supported by Gates and which he said would help in the battle although it was only "partially effective."
"We are investing in other malarial vaccines but just given the complexity and time for doing trials we won't have a second-generation vaccine for like four or five years," he said.
In agriculture, his foundation is the primary backer of the eight-year-old Alliance for a Green Revolution in Africa, which supports smallholder farmers with projects ranging from improving seed varieties to increasing soil fertility.
Productivity in Africa per hectare where most farmers till small plots is only about a fifth that of the United States and Europe, where farms are far bigger and better equipped.
Global research has often focused on improving rice, wheat and maize, while popular African staples like sorghum, cassava and millet have received less attention, Gates said, adding that his foundation was seeking to redress some of that imbalance.
Bringing down fertiliser prices, better understanding of soils and improving credit systems were vital to boosting output in Africa, while making sure knowledge and expertise reached smallholders in far-flung places was also valuable, he said.
The Digital Green project, which is supported by the foundation, aimed to help exchange knowledge between smallholder farmers via video clips that could be watched on smartphones. In Africa, the programme runs in Ethiopia, Ghana and Tanzania.
Gates said this encouraged better adoption of new practices because farmers were listening "to somebody speaking local dialect and working on the same problem they are."
The man behind Microsoft said he was also backing "old school" projects using radio to spread better techniques. That project operates in Mali, Burkina Faso, Ghana, Ethiopia, Tanzania, Uganda and Malawi.
While expertise extension programmes were proving effective in countries like Ethiopia, he said: "There are lot of countries in Africa where the extension system is hardly working at all, and there we work with the government to improve that." (Editing by Hugh Lawson)