A useful, newly-published article by UK vet David Burch entitled "Egg Drops - Not So Much a Syndrome, More a Production Failure" details the range of Viruses, Bacteria, Mycoplasmas, Parasites and Management factors that can be involved in "Egg-Drop" problems.

He comments:

"The term "Egg Drop Syndrome" was first used in the 1970s to describe a virus infection of poultry flocks. Falls in egg production could be as much as 40% but were more commonly 10-15%. In present day layer production 'egg drops' or reduced egg production are frequently an indication that something in the system has gone wrong and the flock is showing the farmer that he needs to pay attention to their management, health and welfare to solve the problem and restore production. Maintaining good farm biosecurity, avoiding multi-age sites where possible, good vaccination programs for the pullets and prompt attention to egg drops all seem to be important parts in minimising resulting losses."

The article is online here:
www.octagon-services.co.uk/articles/poultry/Egg_Drops.htm

David Burch has now published an illustrated follow-up article on the symptoms,treatment and control of mycoplasma infections in poultry...

 www.octagon-services.co.uk/articles/poultry/antibiotics.htm

Production losses associated with M. gallisepticum

Mycoplasma synoviae has a much greater variability in pathogenicity and effects on production. In the past, some strains were as pathogenic as MG and could induce severe respiratory signs and mortality in broilers, associated with complicated chronic respiratory disease (CCRD) with mortalities approaching 20% (see Figure 1).

An increasingly common cause of egg-drop in free-range flocks is Brachyspira infection [Avian Intestinal Spirochaetosis (AIS)] 

In a survey in the UK of 222 commercial layer, pullet and breeder farms 74% of the flocks were positive and 25% were associated with the pathogenic strains. When this was broken down by type of farm a different picture arose Breeder layer flocks could be infected, but it was not found in in-rear pullet flocks up to 15 weeks of age. The organism is spread by the faecal-oral route and there is a natural break as it dies out on the egg surface during hatching.

Free-range and free-range organic flocks were heavily infected (approx. 90%) and less so in caged flocks (76%). Barn flocks were all infected. Brachyspira infection began at a much earlier stage in free-range flocks, by 22 weeks of age, soon after the birds were allowed to go outside. By comparison, caged flocks took longer for the infection to spread, but the majority of flocks were positive by 36 weeks of age. This explains the high level of infection in the US survey in 2009 with 86% of flocks infected as they were over 40 weeks of age. There were not many barn flocks tested, but the younger ones of 31-34 weeks of age were all positive.

Interestingly, there did not appear to be any difference in age of infection between caged flocks using deep pits for waste disposal or belt cleaning. This was considered surprising as the number of flies in deep-pit systems is usually much higher. It is also thought that flies are important, mechanical carriers of Brachyspira from faeces to feed and thereby complete the infection cycle.

A new [August 2010] comprehensive review article and photos can be found here:

www.octagon-services.co.uk/poultry.htm