Mycoplasma gallisepticum is associated with chronic respiratory disease (CRD)/air sac syndrome in chickens and turkeys CRD was first recognised as a chronic but mild respiratory disease of adult chickens. It reduced egg production but caused little or no mortality. Afterward, a condition known as "air sac disease" became a problem in young birds. It caused high mortality in some flocks. Many birds became stunted, feed efficiency was reduced, and many fowl were rejected as unfit for human consumption when processed.

CRD is caused by a pure MG infection while the air sac syndrome is caused by an infection of MG in combination with E. coli. These conditions are triggered by acute respiratory infections such as Newcastle Disease or Infectious Bronchitis.

Mycoplasma gallisepticum is widespread and affects many species of birds. Eradication programmes have reduced the incidence in recent years. It is primarily spread through the egg. Infected hens transmit organisms and the chick is infected when it hatches. Organisms may also be transmitted by direct contact with infected or carrier birds.

The true CRD produces slight respiratory symptoms such as coughing, sneezing and a nasal discharge. In the air sac syndrome there is an extensive involvement of the entire respiratory system. The air sacs are often cloudy and contain large amounts of exudate. Affected birds become droopy, feed consumption decreases and there is a rapid loss of body weight.

Treatment of MG-infected chickens or turkeys with suitable antibiotics or chemotherapeutics has been found to be of economic value. However, control by medication or vaccination and eradication of Mg infections has been by far the most effective method of combating the disease. Blood serum testing of breeder chickens for MG antibodies has become a routine to test flocks for a MG infection.