This is caused by various bacteria during incubation which has gotten into the egg yolk such as E. coli. Staphylococci, Proteus, Clostridia, Facali and Pseudomonas.
This will occur in chicks in their first week of life and is usually terminal. Their life expectancy is usually around 48 hours.
Prevention is the best method.
- Place only clean dry eggs into the incubator.
- Clean your hands thoroughly before turning eggs manually, or during candling. Use an alcohol based hand cleaning gel, allow for air drying time before touching eggs.
- Alternatively, use disposable gloves when handling the eggs.
- Clean the incubator equipment thoroughly after every hatch before storage. Dirty incubators harbour all sorts of bacteria. Do not submerge the motor. Please refer to the manufacturer's instructions for best advice.
For more pictures and information on this condition, please check out this amazing article on backyardchickens.com.