Good poultry practice is often a matter of common sense and none more so than health and hygiene measures required for high hatchability of healthy chicks
All else being equal, the cleaner the eggs—which are set for hatching in the incubator—the better are the results achieved as healthier and higher quality chicks. The science is simple. Pathogenic bacteria, which can never be totally eliminated from poultry practice, respond rapidly to warm and humid conditions in the incubator.
The bird’s egg is not only designed to exclude pathogenic microbes but also to allow free gaseous exchange as the chick embryo respires, grows and develops.This requires a porous calcium carbonate shell to allow oxygen to diffuse in and carbon dioxide to diffuse out. But egg shell pores are potential weak points through which bacteria can enter to disrupt and destroy developing chick embryos.
Microbial contamination of hen’s eggs can occur congenitally or extra-genitally. Congenital contamination invariably happens in the ovary and involves a range of well-known pathogenic microorganisms including salmonella bacteria, mycoplasmas and certain virus particles. These egg transmitted pathogens are controlled by vaccination, blood testing and subsequent removal of infected hens or in the case of mycoplasmas the use of antibiotic egg dipping.
Extra-genital contamination is something which happens after the egg is laid and typically causes rots, mould growth, bangers (eggs that explode once inside the incubator), early dead chick embryos and a higher first-week on-farm chick mortality, the latter often being the result of yolk sac infection. Heavily contaminated eggs, such as those laid-on and collected from the floor, will be the most heavily contaminated by potentially pathogenic microbes and therefore most at risk.
Evolution of the egg, complete with its membranes and hard and inert but porous calcium carbonate egg shell, has furnished the embryo with a comprehensive system of protection for unimpeded growth and development and successful hatching of the free living chick.
The eggshell has thousands of pores which are sufficiently large to permit bacterial cells and fungal spores to pass through and enter the egg. Passage through most of these pores is prevented by the presence of a blocking cuticle but sufficient numbers are open to allow the entry of microbes. Two inner shell membranes act as a further barrier and an antibacterial lysozyme, located in the egg albumen, is active against Staphylococcus bacteria.
Under normal circumstances, a healthy breeder hen lays eggs with an essentially sterile shell. At the point of lay (oviposition) the eggs are hot and moist but they cool down quickly which has the effect of drawing (pulling in) contaminating bacteria through the eggshell.
It follows that the environmental conditions into which the egg is oviposited will have a significant influence and effect on the concentration and range of pathogenic microorganisms that it carries and holds, and can transfer into the incubator. Motile bacteria in large numbers, especially in the presence of moisture (e.g. condensation) on the external surface of the egg, pose the biggest risk and likelihood of egg penetration and yolk colonisation and contamination.
Even eggs which are visibly clean can carry anything from 50,000 to l million bacteria. Soiled and dirty eggs may carry a bacterial load in excess of 10 million. For practical purposes the level of bacterial contamination will depend upon whether eggs are laid in the nest box or laid on the floor.
Condition of the floor litter further affects contamination. Damp litter sticks to the birds’ feet and feathers while exceptionally dry litter creates dust. Condition and depth of nest-box litter will affect contamination level of eggs laid there, while the manner in which eggs are handled and treated post collection will affect contamination level, irrespective of whether they were laid in a nest box or on the floor.
Clean nests
Key issues involved in the production of clean eggs for incubation and hatching are: design, nature and condition of the nests; separation of soiled eggs from clean ones; frequency of egg collection; degree of care exercised in egg handling; personal hygiene of those involved in the procedure; and the disinfection of eggs.
Nest boxes must be easily accessible (to hens), clean and hygienic, generously supplied with litter and sufficient in number with at least one nest allocated for every four hens. Overall they must be sufficiently attractive so that hens do not lay any eggs on the floor. Nest boxes must be in place and position prior to the onset of lay. They should be positioned for easy accessibility and comfort of hens, rather than convenience for the farmer, as the number one priority.
Nests should be started near the floor surface and elevated gradually up to 30-40cm from the litter. Softwood shavings are one of the best choices for litter and should be replenished (topped up) or replaced every seven days. Populations of bacteria in the litter can be reduced to safe levels by using paraformaldehyde prills deployed at the rate of 25g per box per month.
Automatic nests function best in slatted floor areas and should be closed up at night. This is because hens which sleep in nests, as well as hens with dirty feet resulting from wet floor litter, are significant factors which increase the contamination of nests.
Nest eggs and floor eggs must be collected separately while always making sure egg collectors wash their hands between collections to avoid cross contamination. Floor eggs should be packed separately with clear identification to avoid any mixing of clean and soiled eggs.
Some nests will invariably be more popular than others with the hens and if the eggs are not collected sufficiently frequently from these over-loaded nests then breakages will be unavoidable. Operators should collect eggs at least four times every day with a late afternoon collection to avoid eggs staying in the nest boxes overnight.
Automatic nests offer a range advantages including a reduction in the amount of egg handling and a corresponding reduction in the risk of shell breakage and egg contamination. Egg collection by hand allows eggs to be sorted and packed directly onto the incubator tray and subsequently conveyed on a monorail to the packing area.
Around four fifths of time spent on a breeder farm is concerned with egg collection and handling. Therefore detailed thought given to the most cost effective mode of harvesting of what is essentially a delicate living embryo in an equally delicate shell will surely pay dividends.
Disinfection and dry cleaning
There is general agreement amongst veterinarians that treatment of hatching eggs is the best way to control shell-borne infections such as salmonella. Disinfection of eggs can be carried out by dry cleaning followed up by fumigation with formaldehyde or alternatively by wet sanitising the eggs.
Dry cleaning using Scotchbrite pads or sandpaper is an ideal mechanical method for cleaning slightly soiled eggs and should be supported by follow-up fumigation using formaldehyde vapour as a chemical disinfectant. Fumigation with formaldehyde will usually destroy a high proportion of bacteria on the external surface of the shell but several important requirements must be satisfied for its success.
Eggs must be placed on plastic trays to allow adequate circulation of the formaldehyde vapour and the procedure should be conducted in a custom-designed chamber. This must be hermetically sealed (gas tight) to maintain a biocidal concentration of formaldehyde and to stop any vapour from escaping. Last but not least, operators must use the recommended dose and employ a gas neutraliser at the end of the procedure.
Fumigation will not be effective if eggs are very dirty or badly soiled because the active formaldehyde disinfectant will not be able to reach the pathogenic bacteria inside the particles of dirt and soil. That is why eggs must be pre-cleaned prior to fumigation. Use of formaldehyde vapour may be restricted or even banned in some countries due to current health and safety concerns.
Wet sanitising of eggs
Sanitising machines effectively and efficiently disinfect the eggshell and thus avoid practical problems associated with the ‘wet cleaning’ hatching eggs.Standard sanitising machines are designed to accept trays of eggs on a conveyor and to subsequently spray them with high-temperature disinfectant. When used correctly they are more cost-effective due to significant labour savings.
Temperature of the disinfectant should be higher than that of the egg to prevent bacteria from entering the egg by suction. Operators should only use the concentration recommended on the disinfectant product label which will be high enough to kill pathogenic bacteria without any risk of damage to the egg. Chlorine-generating products are popular but require constant monitoring and replacement with fresh product when the concentration falls below that required for effective disinfection.
Dr Terry Mabbett
