Mistreatment on the part of beekeepers can lead to chilled brood. Chilled brood is not a disease it’s a problem regarding the young ones of the colony.
Pesticide can be the other culprit. It can also be caused by a sudden drop in temperature. The brood must be kept warm at all times. Nurse bees take care of these broods; they huddle over the brood to maintain it at the precise temperature.
When a beekeeper open the hive to inspect, to take honey, check the queen or just to look, he prevents the nurse bees to do their job thereby neglecting the brood. This causes the brood to become chilled, distorted or even killed.
The beekeeper should open the hive in warm days and at the hottest part of the day to prevent the risks of chilled brood. At this time, the majority of the bees are out foraging on flowers and the number of bees left in the hive is at its lowest. Practice inspecting your hive as swiftly as possible and place back the frames with brood back where the bees can cluster on it straight away.
Another reason for brood casualty is when they are underfed and when nurse bees cant keep up with their numbers. As a result, the broods turn grey and die. These dead broods will be removed as soon as the colony gets stronger and bee activity gets back to normal.