European foulbrood (drawing: blochy and twisted EFB brood)
The most significant symptom of EFB is the non-uniform color change of the larvae. They change from a normal pearly-white to yellowish, then brown, and finally grayish-black but can be blotchy or mottled. Infected larvae lose their plump appearance and look under-nourished. Their breathing tubes or trachea are visible as distinct white lines. Larval remains often appear twisted or melted to the bottom side of the cell. Unlike larvae killed by AFB, recently killed larvae rarely pull out in a ropy string when tested with a toothpick. The dead larvae form a thin, brown or blackish-brown scale which can be easily removed. EFB usually does not kill colonies, but a heavy infection will seriously affect population growth.