Omphalitis, also known as yolk sac infection, is the main infectious cause of death in newly hatched chicks, during their first week of life. It is most commonly caused by infection with Escherichia coli
bacteria (approximately 70% of cases), however Staphylococcus aureus, Salmonella
spp, Enterobacter hafniae, Klebsiella
spp. , Pseudomonas
spp, and Aspergillus fumigatus
are also frequent causes. Chicks can become infected during incubation, while hatching, or when in the brooder.
- Incubation: For bacteria to enter incubating eggshells, it must be either already present in the incubator or introduced into it. This occurs as a result of improper or lack of disinfecting the incubator and/or associated equipment (egg turning device, thermometers), setting dirty eggs in the incubator, or by candling eggs with dirty, especially, moist or wet, hands. If infertile or dead embryos are left in the incubator these also have the potential to contaminate the incubator with bacteria, especially if they explode. This emphasizes the importance of candling to eliminate eggs that are not fertile or embryos that have died while in the incubator.
- Hatching: Chicks can become infected during the hatching phase if exposed to bacteria from old contaminated sponges used to help increase humidity levels, or if old shelf liners are used that are not clean.
- Post-hatch: Just before a chick hatches from an egg, it 'absorbs' the yolk sac through the navel. Once the yolk sac is fully absorbed, the navel immediately begins to heal and dries out. However, if chicks are unable to fully absorb the yolk sac prior to hatching (or are helped out of the egg before they were meant to hatch), they will have an attached mass apparent on the outside of their bodies, which is the unabsorbed yolk sac. Chicks with unabsorbed yolk sacs are at a great disadvantage. The yolk of the egg is a source of vital nutrients and protective antibodies for the chick. When chicks are unable to absorb all of the nutrients, they will not be as strong and have a weakened immune system. In addition, unabsorbed yolk sacs are essentially an unhealed naval or open lesion, which makes the chick more susceptible to bacteria infection. When yolk sacs become infected, they are often described as yellowish-brown to green to yellowish red in color.
Most chicks with a yolk sac infection die within 24 hours of hatching, peaking at 5 to 7 days. Chicks that make it to day 2 or 3 are often much smaller than the other birds. Any chicks that recover will have a poor immune system, making them more susceptible to infection with other opportunistic pathogens and developing chronic respiratory diseases.