* Guinea worm cases fell 50% last year despite pandemic * Campaigners credit community-led efforts with success * Access to clean water is key, say campaigners By Emeline Wuilbercq ADDIS ABABA, April 29 (Thomson Reuters Foundation) - Okello Aballa Ognum regularly has to walk deep into the jungles of south-west Ethiopia to treat the water ponds that harbour a debilitating parasitic disease. Painstakingly, he measures the water volume to determine how much chemical treatment to use against copepods, the tiny water fleas that carry the Guinea worm larvae. If ingested by humans, the larvae can grow up to a meter long before emerging through the skin, leading to serious disability and amputation in the worst cases.