الفهرس 
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Abstract
The study was carried out on a commercial broiler chickens, one hundred and fifty one day old chicks were divided into 6 separate groups.
The comparison between two drugs was depended upon:
I- Effect on clinical symptoms :
II- Effect on mortality rate.
III- Effect on lesion scores :
IV- Effect on the body weight and weight gain :
Doxycycline given to healthy chickens displayed the most significant increase in the body weight 604.77 compared with healthy, non-treated chickens 575.03 while healthy chickens treated with timaulin was 595.63gm.
V- Effect on feed consumption and conversion rate .
VI- Effect on serum biochemical parameter .
Effect on liver enzymes.
VII- Effect on kidney function tests .
VIII- Histopathological changes :
Tracheal section of chicken infected with Mycoplasma gallisepticum showed severe destruction of the epithelial layer of mucosa with oedema in submucosa.
Lung sections post infection with Mycoplasma showed emphysema, diffuse haemorrhage, diffuse mononuclear cell infiltration and congestion of pulmonary arteries.
Livers section of healthy chicken treated with doxycycline showed congestion of the blood vessels hyperplasia of the bile duct.
Kidney sections of healthy chickens treated with doxycycline showed slightly destruction of some renal tubules. While presence of Hyperplasia of the glomerul tufts with mild destruction of the renal tubules in case of kidney section of healthy chickens treated with Tiamulin.
Heart section of infected chickens with Mycoplasma showed severe oedema.