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PostPosted: Sat Jan 05, 2013 5:04 pm 
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QLD DPI site information on Chronic Respiratory Disease:

Chronic respiratory disease

Chronic respiratory disease (CRD) is one of the most common respiratory diseases occurring in poultry in Australia. The disease occurs when birds infected with Mycoplasma gallisepticum are stressed. The subsequent invasion by secondary bacteria causes the major damage to the bird.

Scientific name
Mycoplasma gallisepticum, plus other organisms

Mycoplasma gallisepticum with a chronic stress factor and secondary coliform organisms cause this respiratory disease.

Species susceptible
Chickens and turkeys

Very widespread and present in most, if not all, commercial flocks. Each batch of new pullets will become infected.

CRD is the most prevalent of the so-called 'stress diseases'. Mycoplasma gallisepticum may be present in the tissues of healthy birds (carrier birds). Outbreaks occur most frequently when the flock's vitality is lowered either during times of stress - often caused by moving, chilling, vaccinating, beak trimming, worming, poor ventilation, damp litter and ammonia build-up - or in the presence of other diseases. Transmission may occur even in flocks that appear to be perfectly healthy.

Egg transmission is very important as it is the means by which the disease perpetuates itself. The disease is airborne (aerosol) and generally rapid, but it does not travel distances.

The disease is transferred most often when infected carrier birds are introduced to the flock or people such as service personnel, vaccinators and blood testers transport it. The risk is great when people handle CRD-free birds the day after handling CRD-infected birds. It can also be transferred on equipment, such as crates, vehicles and vaccinating equipment.

Incubation period
Incubation is 5 to 10 days.

Signs include sniffing, rattling, sneezing, coughing and other signs of respiratory distress. Birds may have wet noses, retarded growth (in growing birds) and a production loss of 20 to 30 per cent in hens. The disease spreads slowly through the flock with a continual cycle of reinfection, so the disease never disappears by itself. Deaths are few in uncomplicated cases. Financial losses occur due to poor feed conversion, retarded growth, drug costs, mortality, increased culling and poor production.

Mucus in the trachea, sinuses and bronchi. Cloudy, thickened air sacs, perihepatitis and pericarditis due to secondary bacteria.

Isolation and identification of the causative agent. The isolation of mycoplasma is difficult and an isolate must be confirmed to be Mycoplasma gallisepticum, as many serotypes of mycoplasma are present in the respiratory tract of birds. Blood tests vary in reliability.

Similar diseases
Infectious bronchitis
Infectious laryngotracheitis
Fowl cholera.


Regarding medicinal treatment, several antibiotics are available that, when administered in large enough doses, help control the disease and minimise secondary bacterial complications, though they do not usually control the disease completely. Correcting faulty management practices that weaken the flock's vitality will also assist in treating the disease.


Controlling the disease involves:

controlling predisposing factors and attending to hygiene
separating birds in older age groups from young birds
isolating affected groups.


The organism may be present in a flock without producing any signs of disease. It will spread slowly to other birds until the birds are 'stressed', when it will spread faster. Therefore, it is vital to treat the flock before the birds are stressed. If CRD is a problem, the birds should be treated with a suitable medication in the first three days of life, at four weeks of age and then at maturity. These measures may not prevent the disease but will reduce the likelihood of an outbreak.


Pullets reared in isolation can be vaccinated to prevent an infection of Mycoplasma gallisepticum. Suppliers of point-of-lay pullets can provide vaccinated pullets.


Infected and uninfected flocks should be housed at least 50 to 100 m apart. The disease may be prevented through the adoption of basic principles of isolation and 'all-in, all-out' management, and the purchase of vaccinated pullets.

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