Richard Turner MA VetMB MRCVS
As vets, one of the most common scenarios that we are presented with is an egg drop in apparently normal birds. Sometimes we see this in its early stages, but often it is after the drop has been ongoing for maybe two weeks and various tests have been carried out. So, where do we start in looking for a cause?
Egg drops can be due to infectious and non-infectious causes. Too often the focus is with infectious agents but in many cases, there are other reasons. It is important as a start to look at the eggs themselves. Firstly, is there not only a drop in total egg numbers but also a change in the egg appearance? Pale egg colour might suggest a mycoplasma infection, but equally a virus such as Avian Pneumovirus (ART/TRT) could be involved.
Egg shell colour change will also occur due to feed issues and in the summer, due to long periods of bright sunshine. A change is therefore not indicative of one cause. Feed testing, ensuring the correct sample size and relevance is not easy. In general, proving a feed issue can be difficult and therefore we would not look at feed as a first option, but rule out other causes first.
High worm burdens or specific types of worms might also affect gut health and lead to changes in egg shell strength and colour. When taking faeces samples for worm egg counts its essential to sample enough individual faeces, and to also be sure that the lab carrying out the test can differentiate the common worm species. In many cases, routine worming of a free-range flock every 6- 8 weeks is the best policy, but of course not all farms want to commit to that extra expense.
Egg shell wrinkles and roughness might point towards a coronavirus IB challenge, but this is usually short term as most flocks are well vaccinated. There are of course new variants (as we hear all too much in the moment in the press!), and if this is suspected then there are a range of PCR tests which will make a diagnosis easier.
Blood testing at the time of an egg drop has its own problems. In most cases, pullets are vaccinated against a wide range of viruses that will lead to eggshell changes and loss of production, so looking at antibody levels at a fixed time might not help. In general, antibody levels rise until around peak production then generally fall unless there is challenge on the farm. We need a standard to compare with and that is often not present.
Antibody rises do not necessarily indicate that the clinical signs (egg changes) are due to the pathogen being tested. Birds with good vaccine protection will respond to a challenge by an increase in antibodies and the bird will not develop disease. The simplest thing to do is to take around 20 bloods per house at about 28-30 weeks of age and store these bloods in an approved laboratory so that in the event of a possible infectious disease challenge it is possible to look at the titres at peak production and then also when there is a problem. This is not an expensive option and should be regarded as a routine procedure like worming a flock.
When testing a flock, it is essential to collect a statistically significant sample number. For example, testing just a few birds with tracheal PCR is not likely to find the infectious agent where the egg drop is 5%. You would need to bulk together maybe 60 samples to get a significant result.
One of the more common causes of egg drop is poor digestion due to some form of gut damage. Where mortality is also present it is worth looking at the crop and gizzard for signs of ulcers and bleeding which will lead to anaemia, production drops and death. Equally red mite infection if high enough will of course affect both eggshell quality and production but diagnosing this is a lot easier!
I have already mentioned the effect of worms, but any other disturbances to the intestinal microbiome will lead to failures in digestion. Studying the consistency, colour and amount of gas in droppings will give a good indication of gut heath. We used to see a lot of cases of frothy droppings which were really the fermentation in the caeca. This is brought about by failure of digestion in the upper intestine leading to an overload of undigested nutrient in the caeca and a bacterial blooming and fermentation. The caeca is also important in the body’s water regulation, and when there is a bacterial overload and disturbance there is also the likelihood of failure of water control. This leads to increase in water intake, wet litter and other challenges to the bird’s environment.
Frothy droppings associated with specific Brachyspira bacterial infections seems less common, at least in our client base, but a general disruption of the microbiome is still common. We use either antibiotics, or preferably mixtures of essential oils, to try to rebalance the microbiome. However, as with all egg drops it can take as long to get the production back to normal as it did to find the cause.
Regular weighing of birds, especially as they come into peak production, will give a very good indication of the level of nutrition and the ability of the bird to digest it. The most common egg drop is probably the post peak drop, which is often due to a mismatch between the bird’s nutritional needs and its genetic drive to produce eggs. On multi age sites of course there is always the possibility that as peak production is reached the endemic variants of various viruses have been able to establish themselves in the new flock.
Water deprivation is not common, but the quality of the water and especially the mineral levels can be an issue. We see some farms where bore holes are so heavily loaded with iron that filters are partially blocked making a physical lack of water, through to other minerals such as manganese, that lead to intestinal damage and poor digestion.
In summary, there are many causes of a drop in production and frustratingly, often by the time tests have been carried out the birds have returned to normal production anyway. Even after looking at all these possible causes there are a few farms where we still cannot find the cause of an egg drop. It is important to note that there are a wide range of viruses that have no vaccines and no routinely available tests. A bird with a low-level fever will have reduced feed intake and water intake leading in a few days to a drop in production. Where mortality is normal, and the flock appears healthy it can be very difficult for the vet and farmer to select the birds to be tested and testing normal production birds just leads to more confusion and false results.