Bird flu raging uncontrolled in neighbouring Bangladesh was proving to be a continuing threat to Tripura, where culling was under way after the outbreak of the H5N1 virus in the State. "Even if we cull all our birds within a five km radius of the disease-hit areas, bird-flu affected areas in Bangladesh are very close to our border villages and there are places which fall within 100 yards of affected areas in the neighbouring country,' Animal Resource Department Director Asish Roy Burman said today.