Pathology in Chickens Experimentally Inoculated or Contact-Infected with Mycoplasma gallisepticum

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Abstract
Fifty-six 2-week-old chicks were inoculated in the foot pad with 10 CID50 [culture infective dose] doses of M. gallisepticum. Another 60 chicks were introduced for contact exposure, and 39 were maintained as isolated controls. Inoculated and contact-infected chickens were sacrified according to a schedule for an 8-month period. Clinical signs, hematology, and gross and microscopic pathology were studied. The clinical disease was characterized by a catarrhal exudate in the upper respiratory system of both inoculated and contact-infected birds, while a tendovaginitis and synovitis were observed in pelvic limbs of inoculated chickens only. A heterophilic leucocytosis occurred during the early phase of the disease in both groups. Gross pathology was manifested primarily as swelling in the foot pad, tarsometatarsal tissues, and tarsal joint area of the inoculated limb in the inoculated group, with a catarrhal exudate in respiratory passages and a splenomegaly in both inoculated and contact-infected chickens. Histopathologic lesions in contact-infected chickens were splenitis, myocarditis, hepatitis, sinusitis, tracheitis, and pneumonia. Inoculated birds had the additional findings of encephalitis, synovitis, tendovaginitis, and airsacculitis. Pathologic changes were generally more severe in inoculated chickens.