Introduction
Bacterial contamination in newly hatched chicks is a major concern for hatcheries and poultry producers worldwide. It leads to poor chick quality, increased early mortality, reduced growth performance, and higher medication costs.
Contamination can occur at multiple points—during egg production, incubation, hatching, or post-hatch handling. Understanding the sources, types of bacteria, clinical impact, and preventive measures is critical for maintaining healthy flocks and ensuring high hatchability.
Sources of Bacterial Contamination
1-Breeder Flock Level
Vertical transmission
Pathogens are transmitted from hen to egg (Salmonella and Mycoplasma).
Dirty eggshells
Contaminated by faeces (Floor Egg), litter, or dust before collection
Poor nest hygiene
Nests with wet or dirty litter serve as a bacterial reservoir.Egg Handling & Storage
Improper egg
2-Egg Handling & Storage
Improper egg sanitation:
Delay in egg collection – improper handling with dirty eggs or lack of disinfection allows bacterial penetration through shell pores.
Contaminated trays and equipment:
Reused without proper cleaning.
Temperature fluctuations:
Sweating of eggs during transfer promotes bacterial growth.
3. Incubation & Hatching
Contaminated incubators: Inadequate cleaning between hatches allows pathogens to persist.
Exploder eggs: Bacterially infected eggs (Dirty Eggs) may burst, spreading contamination to healthy eggs.
High humidity and poor airflow: Favour bacterial survival and spread.
4-Post-Hatch Handling
Contaminated chick boxes, liners, and transport vehicles
Human handling: Poor biosecurity practices introduce bacteria during chick sexing,
vaccination, or packaging.
Contact with contaminated water or litter upon placement in the farm.
Common Bacteria Involved
Horizontally Transmitted Bacteria: Gram-negative bacteria as Escherichia coli (Most Common), Salmonella spp., Pseudomonas spp, Proteus spp., Klebsiella spp., Enterobacter spp. are the most common bacteria that cause contamination of newly hatched chicks.
• Vertically Transmitted Bacteria: as Mycoplasma spp. and Salmonella Spp.
Other Possible involved bacterial infections are Gram-positive bacteria, such as Staphylococcus and Streptococcus, be less commonly involved.
Clinical Impact on Chicks
I) Collibacillosis (E.Coli Infection)
B) Diarrheal Disease (Enterotyphlitis)
Birds may be clinically normal or have diarrhoea and be dehydrated (a chick with whitish grey pasting round the vent) (Pasty Vent)
The intestines are pale and distended with fluid, which may contain visible flecks of mucus and exudate.
Ceca are typically distended with pale brown fluid and gas• Vertically Transmitted Bacteria: as Mycoplasma spp. and Salmonella Spp.
Other Possible involved bacterial infections are Gram-positive bacteria, such as Staphylococcus and Streptococcus, be less commonly involved.
A) Omphalitis/Yolk Sac Infection (Mushy chick disease)
The affected chicks are listless and display a swollen, dirty stomach. They have inflamed, dirty navels, they stop eating and they often have dirty vents from loose droppings.
The yolk sac is typically distended and inflammatory products have been added
Yolk is abnormal in color, consistency, and smell, and may contain visible exudate & blood vessels of the yolk sac are often prominent.
II) Salmonellosis
Somnolescence, weakness, depressed appetite, poor growth & adherence of chalky white material to the vent (bacillary white diarrhea)
The liver is enlarged and often has pinpoint, pale to hemorrhagic foci of necrosis
White to yellow caseous exudate in the lumen of the ceca (Cheesy cecal cores)
Enteritis with mucosal necrosis is observed (possibly with tan pseudomembrane)
White nodules, resembling Marek’s disease tumors, may be present in the cardiac muscle and proventriculus and occationally in the pancreas and in the lung
Enteritis with mucosal necrosis is observed (possibly with tan pseudomembrane)
C) Neonatal Coliseptisemia
In Untreated survived chicks, E.coli spread systemically causing higher mortality rates
Fibrino-heterophilic polyserositis involving pericardial sac, pleura, airsacs & peritoneum
Proventriculus & lungs develop a dark color that can approach black as the interval between death and necropsy increases with splenomegaly
Salpingitis: immature oviduct is dilated, with thinned wall and filled with caseous exudate all along its length
III) Mycoplasmosis
Affected chicks show high depression and lethargy in addition to respiratory signs including gasping and rales.
Air sacs of infected chicks show yellow caseous material
Economic Consequences
• High first-week mortality: A critical KPI in hatchery performance.
• Lower hatchability: Contaminated eggs often die before hatching.
• Downgraded chick quality: Hatcheries face rejection from customers.
• Increased antibiotic use: Leading to higher costs and antimicrobial resistance risks.
• Reduced overall flock performance: Affecting uniformity, feed conversion, and final market weight.
