Contamination of eggs happens mainly directly after oviposition, when the eggs are cooling down from body temperature to environmental temperature. During this cooling process, a negative pressure in the egg develops as the egg content shrinks and the shell doesnt. This negative pressure results in an airflow through the pores into the egg, and when the egg is produced in a contaminated environment micro-organisms will be drawn into the pores.
When an egg is produced in a contaminated environment (a dirty nest but especially in the litter) there is an increased risk that it will be contaminated. As the contamination is located in the pores of the shell, it is a risk for the egg and it will be difficult to remove.
This means that the risk is during the cooling period, and that the after this cooling period not a lot can be done anymore. Washing the eggs will not remove the micro-organisms from the pores, but it will only make the surface of the eggs clean. Also disinfection, especially with a liquid, has very little effect. It will disinfect the surface of the shell but not the pores. A gas like formaldehyde will be more effective as it can enter the pores, but also a gas can not completely remove the contamination.
Eggs must be produced in a clean environment, and if they are not produced in a clean environment there is an increased risk of contamination, even if the surface of the egg by itself looks clean. This is well known, as we know that washing of dirty eggs doesn't really decrease the contamination level. Also clean eggs coming from the floor will still have an increased level of contamination, resulting in a lower hatchability and more exploding eggs.
When breeders are kept on the floor, a certain level of floor eggs will always be present, as usually floor eggs cannot be totally avoided. If we collect the floor eggs, a certain percentage of these eggs will be relatively clean, depending on the quality of the litter and the frequency of collection. But although these floor eggs look relatively clean, they are still contaminated, as can be seen when incubated separatedly.
Especially when we want to increase the quality of our day old chicks or limit or completely avoid anti-biotics in our production system, potential exploders and with it floor eggs are a risk, as they will contaminate the other chicks in the hatchers. It is therefore important that the floor eggs are delivered separatedly so the hatchery can treat them as contaminated risk eggs.
When we look at the floor eggs that are delivered separatedly to the hatchery, we often see that there are only dirty eggs on these trays. But when we look in the breeder house, we often see that a certain percentage of the floor eggs were actually clean, and could not be distinguised from clean nest eggs. If there are clean floor eggs in the house but there are no clean floor eggs in the floor eggs that are delivered to the hatchery, the conclusion must be that these clean floor eggs are delivered as clean nest eggs, and not as floor eggs, although they have the same contamination risk as floor eggs.
Most farmers will not recognize that clean floor eggs are potentially contaminated just as much as dirty floor eggs, for the simple reason that they look clean. They don't see any harm in delivering these eggs as nest eggs. Especially when they do not get the full price for floor eggs, they will feel even more or less forced to deliver them as nest eggs.
However, in a anti-biotic free production system these eggs will be quite dangerous. Checking the qualty of the floor eggs arriving in the hatchery is therefore important, and when there are no visually clean eggs in these floor eggs, there is a change that the clean floor eggs are delivered as clean eggs and form a potential risk.