However, they may exert some adverse effects on egg production.

Live strains of Mycoplasma gallisepticum of lesser virulence as well as killed vaccines have little or no effect on egg production, but afford lower levels of protection.

This has led to research investigating their use in combination with a subsequent overlay vaccination of F-strain Mycoplasma gallisepticum given later in the production cycle.

Mycoplasma gallisepticum is considered to be the most problematic of the poultry mycoplasmal diseases.

It is a slow spreading infection and often infected birds remain healthy without showing any signs of the disease.

Once other complicating factors - such as environmental issues including a rise in temperature, ammonia levels or dust or cold drafts, problems with nutrition and other infections, such as infectious bronchitis or laryngotracheitis virus - Mycoplasma gallisepticum can flare up.

Mycoplasma gallisepticum adversely affects fertility, hatchability, and survival of baby chicks and it will spread easily to other flocks on the farm, to nearby farms and to other birds including wild birds.

Infection with Mycoplasma gallisepticum is associated with slow onset, chronic respiratory disease in chickens, turkeys, game birds, pigeons and other wild birds.

According to Paul McMullin in his Pocket Guide to Poultry Health and Disease, the condition occurs worldwide, though in some countries this infection is now rare in commercial poultry. In others it is actually increasing because there are more birds in extensive production systems that increase their exposure to wild birds.

In adult birds, though infection rates are high, morbidity may be minimal and mortality varies.

“The route of infection is via the conjunctiva or upper respiratory tract with an incubation period of six to 10 days.

“Transmission may be transovarian, or by direct contact with birds, exudates, aerosols, airborne dust and feathers, and to a lesser extent fomites. Spread is slow between houses and pens suggesting that aerosols are not normally a major route of transmission. Fomites appear to a significant factor in transmission between farms.

“Recovered birds remain infected for life; subsequent stress may cause recurrence of disease.

“The infectious agent survives for only a matter of days outwith birds although prolonged survival has been reported in egg yolk and allantoic fluid, and in lyophilised material. Survival seems to be improved on hair and feathers. Intercurrent infection with respiratory viruses (IB, ND, ART), virulent E. coli, Pasteurella spp. Haemophilus, and inadequate environmental conditions are predisposing factors for clinical disease,” said Mr McMullin.

Signs of the disease are:

• Coughing.
• Nasal and ocular discharge.
• Poor productivity.
• Slow growth.
• Leg problems.
• Stunting.
• Inappetance.
• Reduced hatchability and chick viability.
• Occasional encephalopathy and abnormal feathers.

Eradicating the infection has been the main goal for official poultry health programmes in most countries.

These programmes are based on buying of uninfected chicks, all-in/all-out production, biosecurity, and routine serological monitoring. In some circumstances preventative medication of known infected flocks may be of benefit.

Vaccination with live attenuated or naturally mild strains are used in some countries and according to Mr McMullin may be helpful in gradually displacing field strains on multi-age sites.

Productivity in challenged and vaccinated birds is not as good as in Mycoplasma gallisepticum-free stock.

In a study, on the effects of different vaccine combinations against Mycoplasma gallisepticum on the internal egg and eggshell characteristics of commercial layer chickens, published in Poultry Science, two trials were conducted to investigate the effects of pre-lay vaccinations of live and killed MG vaccines or their combination, in conjunction with an F-strain Mycoplasma gallisepticum vaccine overlay after peak production, on the egg characteristics of commercial layers.

The four treatments were administered at 10 weeks of age:

1. unvaccinated (Control),
2. MG–Bacterin (Mycoplasma Gallisepticum Bacterin) vaccine,
3. ts-11 strain Mycoplasma Gallisepticum (ts11 Mycoplasma Gallisepticum) vaccine, and
4. Mycoplasma Gallisepticum Bacterin and ts11 Mycoplasma Gallisepticum combination (Mycoplasma Gallisepticum Bacterin + ts11 Mycoplasma Gallisepticum).

At 45 weeks of age, half of the birds were overlaid with an F-strain Mycoplasma Gallisepticum vaccine.

The study’s authors, R. Jacob, S. L. Branton, J. D. Evans, S. A. Leigh and E. D. Peebles, from the Department of Poultry Science at Mississippi State University and the USDA's Agricultural Research Service, said that in each trial, internal egg and eggshell parameters including egg weight (EW), Haugh unit score (HU), eggshell breaking strength (EBS), percentage yolk weight (PYW), percentage albumen weight (PAW), percentage eggshell weight (PSW), eggshell weight per unit surface area (SWUSA), percentage yolk moisture (PYM), and per cent total lipids (PYL) were determined at various time periods between 21 and 52 weeks of age.

At 28 weeks of age, SWUSA was lower in the ts11 Mycoplasma gallisepticum and Mycoplasma gallisepticum bacterin + ts11 Mycoplasma gallisepticum groups compared to the Control group.

Conversely, at 43 weeks of age, SWUSA was higher in the ts11MG than in the Mycoplasma gallisepticum bacterin group.

Between 23 and 43 weeks of age, PYL was higher in the Mycoplasma gallisepticum bacterin and ts11 Mycoplasma gallisepticum groups in comparison to the Control group.

In conclusion, the researchers said that vaccination with Mycoplasma gallisepticum bacterin alone or in combination with ts11 Mycoplasma gallisepticum at 10 weeks of age with or without a Live F-strain Mycoplasma gallisepticum vaccine overlay at 45 weeks of age does not adversely affect the internal egg or eggshell quality of commercial layers throughout lay.

According to Dr Gary Butcher from the University of Florida in his paper, 'Mycoplasma gallisepticum - A Continuing Problem in Commercial Poultry' the organism is similar to bacteria, but lacks a cell wall. This characteristic makes Mycoplasma gallisepticum extremely fragile and they are easily killed by disinfectants, heat, sunlight, and other factors.

They only remain viable in the environment, outside the chicken, for typically up to three days.

For this reason, Dr Butcher said Mycoplasma gallisepticum is fairly easy to eliminate on single-age, all-in all-out poultry farms. If a laying flock is infected, complete depopulation of the farm at the end of the laying cycle and providing down-time prior to reintroducing chickens will be successful in eliminating Mycoplasma gallisepticum.

However, complete depopulation must be performed to break the cycle and prevent re-infection in subsequent flocks on the premises.

When a chicken is infected with Mycoplasma gallisepticum, 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 and eggs.

After several weeks, the level of infection and shed 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 Mycoplasma gallisepticum extremely difficult in multi-age breeder and laying complexes.

As Mycoplasma gallisepticum-clean pullets, raised in single-age farms and in isolation, are brought onto the complex, they are often exposed to the organism at probably the worst possible time - at the onset of production. This cycle of spread continues in a complex with new flock introductions, said Dr Butcher.

Efforts to reduce the adverse effects of the disease on breeders and egg-type layers in complexes have included 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, reducing sensitivity to air quality, limiting shed level and duration into the poultry house environment, and reducing egg transmission to broiler progeny.

These efforts have not been successful, however, in eliminating infection and shed.

Dr Butcher said that live vaccines have become commercially available that do not spread from bird to bird, do not cause disease in turkeys, and cause a very mild and predictable reaction in pullets.

These, he said, offer many advantages over the live vaccines used in the past.

Most Mycoplasma gallisepticum-positive breeder and egg-type layer complex managers administer these products to pullets prior to moving the Mycoplasma gallisepticum-clean pullets onto infected complexes.

Use of killed vaccines is common in some farms, especially broiler breeder complexes.

While live vaccines are more commonly used in egg-type commercial layers.

However, combinations of live and killed vaccines and antibiotics are used depending on local conditions.

Use of antibiotics is most practical in broilers for controlling respiratory reaction, he said.

The decision to vaccinate or simply accept performance losses in commercial layers will depend on several factors, he said.

The strain of Mycoplasma gallisepticum 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.

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 performance losses.

He said that vaccination programmes for Mycoplasma gallisepticum must take into account the air quality where layers will be housed.

Concurrent diseases such as coryza and infectious laryngotracheitis and the intensity of the live virus vaccination programme, especially against IBV, NDV, ILT, are also variables to take into consideration.

Mycoplasma gallisepticum infection in heavy breeders, almost without exception, requires intervention with vaccines and antibiotics. These breeders suffer significant loses and shed the organism to the progeny.

Mycoplasma gallisepticum vaccination has been shown to reduce shed level and duration. If the aim is to eradicate Mycoplasma gallisepticum on a commercial layer or breeder farm or reduce potential spread to neighbouring non-infected farms, vaccination is suggested.

Affected broiler breeder flocks should be vaccinated prior to onset of infection and broilers managed and treated to reduce adverse of effects of Mycoplasma gallisepticum Dr Butcher said.

“It is unlikely Mycoplasma gallisepticum will be eradicated from the commercial poultry industry in the coming years. However, through biosecurity programmes and effective use of vaccines, losses can be reduced,” he said.

July 2015