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Chronic respiratory disease (CRD)

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


Mycoplasma gallisepticum + chronic stress factor + secondary coliform organisms cause respiratory disease.

Mycoplasma gallisepticum is between a bacterium and a virus in size. The severity of infection is modified by such agents as respiratory viruses, secondary bacteria, E. coli, vaccinations and management factors.

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 tissues of healthy birds (carrier birds). Outbreaks occur most frequently when the flock's vitality is lowered at times of stress (for example, 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 - of major importance and the means by which the disease perpetuates itself.

Airborne (aerosol) - generally rapid, but does not travel distances.

Indirect or mechanical - the major means of entry of CRD is by the introduction of infected carrier birds or transport by persons such as service personnel, vaccinators and blood testers. The risk is great when people handle CRD-free birds on the same day after handling CRD-infected birds. Transfer can also occur on equipment (crates, vehicles, and vaccinating equipment).

Incubation period:

This is 5 to l0 days.


Sniffing, rattling, sneezing, coughing and other signs of respiratory distress. There may be wet noses, retarded growth in growing birds and a production loss (20 to 30%) in hens. The disease spreads slowly through the flock and there is a continual cycle of reinfection so that the disease never disappears by itself.


Deaths are few in uncomplicated cases. Financial losses are 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.

Cloudy appearance of the abdominal airsacs in this 7 week old chicken.

Pericarditis and perihepatitis in an eight week old chicken. E. coli was isolated from both lesions.


Isolation and identification of the causative agent. 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:

* Coryza

* Infectious bronchitis

* Infectious laryngotracheitis

* Fowl cholera


Medicinal - there are several antibiotics which, given in large enough doses, will help control the disease and minimize secondary bacterial complications, although they usually do not control the disease completely.

Management - correct faulty management practices that weaken flock vitality.


Control predisposing factors and attend to hygiene. Keep birds in older age groups separate from young birds. Isolate affected groups.


Medicinal :

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 more rapidly. Therefore treat the flock before the birds are stressed. Where CRD is a problem, birds should be treated with a suitable medication in the first 3 days of life, at four weeks of age and at maturity. These measures may not prevent the disease but will lessen the likelihood of an outbreak.

Vaccination :

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

Management :

House infected and uninfected flocks at least 50 to l00 m apart. Prevent the disease spreading by adopting basic principles of isolation and all-in, all-out management and buy vaccinated pullets.

Source : VetSoft Corporation