Nepal has culled more than 600,000 birds and destroyed around 1 million eggs as an H5N1 bird flu outbreak spreads across the Kathmandu Valley, shutting the capital’s only zoo and raising fears among scientists over the virus’ potential to mutate into a form transmissible between humans.The outbreak began in eastern Nepal in March before reaching the densely populated valley around mid-June – a development that health experts say raises the risk of human exposure. More than 60 poultry farms...