TECHNICAL ARTICLES Broilers/Layers/Breeders

Mg in Commercial Layers Still a World-Wide Problem

Mycoplasma gallisepticum (Mg) infection in the commercial layer Industry is common in many parts of the world. The disease persists at a high incidence in commercial layers, despite success in eliminating the disease in layer parent stock, turkeys, broiler breeders, and broilers in most parts of the world.

The continued presence of Mycoplasma gallisepticum in commercial layers suggests extreme efforts at eradication were not implemented as they were in other segments of the poultry industry. Losses in the commercial layer sectors have not been catastrophic, as was the case in the broiler and turkey sectors. However Mg infection in the commercial layer industry is likely to continue, and limiting losses will be the primary objective.

Clinical symptoms of Mg vary slightly between species. Infected adult chickens may show no outward signs if the infection is uncomplicated. However, sticky, serious discharge from nostrils, foamy discharge in eyes, and swollen sinuses can occur, especially in broilers. The air sacs may become infected, infected birds can develop respiratory rales and sneeze, and affected birds are often stunted and unthrifty.

Mg infection is caused by an organism classified as a mycoplasma. This organism is similar to bacteria, but lacks a cell wall, which makes Mg extremely fragile.

They are easily killed by disinfectants, heat, sunlight, and other factors. They remain viable in the environment, outside the chicken, typically for up to three days.

For this reason, Mg is fairly easy to eliminate on single age, all-in all-out layer farms. If a flock is infected, complete depopulation of the farm at the end of the laying cycle and providing downtime prior to reintroducing chickens will successfully eliminate the presence of Mg. However, complete depopulation must be performed to break the cycle and prevent re-infection in subsequent layer flocks on the premises.

Multi-age farms

When a chicken is infected with Mg, the infection is of long duration. In the period after infection, the organism is present in the respiratory tissues in high levels and is shed into the environment. After several – weeks, the level of infection in the flock and shedding of the organism decreases. However, the infection persists in the flock indefinitely and the chickens may shed the organism intermittently, especially following a period of stress. This characteristic makes elimination of Mg extremely difficult in multi-age egg laying complexes. As Mg clean pullets, raised in single-age farms and in isolation, are brought onto the complex, they are exposed to the organism at probably the worst possible time – the onset of egg production. This cycle of spread continues in a complex with new flock introductions.


Efforts to reduce the adverse effects of the disease on infected layers in complexes have included the use of antibiotics, killed vaccines, and live vaccines. These efforts have been successful in reducing drops in egg production following infection, maintaining levels of egg production throughout the cycle. Reducing severity of concurrent respiratory diseases, controlling excess vaccine reactions following live-type respiratory vaccines, reducing sensitivity to

Poor air quality. And limiting shed level and duration of Mg in the poultry house environment. These efforts have not been successful in eliminating infection and shed.

More recently, live vaccines have become commercially available that cause only a very mild and predictable reaction in pullets, that do not spread from bird to bird, and do not cause disease in turkeys. These new live Mg vaccines offer many advantages over the live vaccines used in the past. Most managers of Mg positive complexes administer these products to pullets prior to moving the Mg clean pullets into infected laying complexes.

Mild or virulent strains

The decision to vaccinate or simply accept performance losses in commercial layers will depend on several factors. The strain of Mg in a farm must be considered, as some strains of Mg are mild while others are highly virulent. House construction is a major factor in determining the severity of clinical disease. Birds in open-sided houses and closed houses with excellent ventilation do not experience recognisable losses in performance, while the same layers in a closed-type house with poor ventilation will experience considerable perform losses. Thus, vaccination programs must take into account the air quality where layers will be housed. Concurrently diseases such as Coryza and Infectious Laryngotracheitis and the intensity 0 live-virus vaccination program (especially against IB, ND, ILT).

Mg vaccination has been shown to J shed level and duration. Thus, if effort being made to eradicate Mg on a commercial layer farm or reduce potential spread to neighbouring non-infected farms, is recommended.

Importance of biosecurity

Air-borne Mg can only be spread over short distances. Where excellent biosecurity is practised, there have been many instances where infection has not spread to adjacent houses within a complex. The disease spread from farm to farm predominantly by movement of contaminated people equipment and vehicles. Thus, biosecurity is the best means of prevention introduction of Mg into a layer complex Data have demonstrated that wild birds may become infected with Mg.


Gary D. Butcher, Richard D. Miles, and Jacqueline P. Jacob, University of Florida, Gainesville, USA

Date published: 2003-10-20

G. Butcher, R. Miles
and J. Jacob