الفهرس 
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Abstract
The present study was designed to investigate the molecular characterization of M. gallisepticum in order to better understand the epidemiology of M. gallisepticum outbreaks in chicken farms including broiler breeder, layer, and broiler flocks in Alexnadria, Behera, Kafr El-Sheikh, and Gharbia governorates during 2010/2011. Also to detect the role of vaccinal strain in these outbreaks or in the development of respiratory disease problems.
The study included the in-vitro testing the activity of various antimicrobials against MG isolates, then evaluation the virulence and pathogenicity, and transmissibility of these isolates and F vaccine through experimental infection.
The results were summarized as following:
1. The total number of 37 out of 49 poultry flocks were positive for Mycoplasma gallisepticum infection (75.5%) and distributed as follows: 66.6% in layer flocks, 100% in broiler breeder flocks and 75.75% in broiler flocks.
2. The C-terminus-encoding region of PVPA gene for 35 field isolates of MG out of 37 and the commercially used F strain vaccine were compared by PCR-RFLP technique.
a. At 38% of genetic similarity all isolates were divided into two main clusters.
b. Most of isolates that were sampled at the same month and in the same geographic area, and clinically caused similar pathogenic effect under field conditions, showed similar RFLP patterns and located very closely in the phylogenic tree with genetic similarity reached in some cases to more than 95%.
c. F vaccine located with 24 isolates in separate cluster with minimum 42% genetic similarity, but isolates 1,3,4,5,6, and 19 were more related to F vaccine with more than 92% genetic similarity.
3. The sensitivity test of 14 M. gallisepticum isolates to different antimicrobials revealed that all the tested isolates were sensitive to doxycycline, tiamulin, ciprofloxacin, enroflozacin, and tiamulin/chlorotetracycline, while they were resistant to tylosin and erythromycin.
a. Doxycycline showed the most activity on MG isolates where all isolates were highly sensitive with MICs ranged from 0.003-0.2g/ml.
b. Tiamulin was found to be active in inhibition of the growth of all MG isolates with MIC ranged from 0.003-0.8g/ml, and the same results with tiamulin/chlorotetracycline.
c. Ciprofloxacin were found to be active in inhibition the growth of all MG isolates with MIC 0.003-0.8g/ml. However, 8 isolates of 14 showed moderate sensitivity to enrofloxacin with MIC 0.4-0.8g/ml and 6 isolates were sensitive with MIC 0.002-0.8g/ml.
d. Tylosin and Erythromycin (two macrolides) failed to inhibit the growth of MG isolates until 6.4g/ml concentration. Ten isolates out of 14 were tylosin resistant, while one isolate was moderately sensitive and only 3 were sensitive. However, 13 isolates were resistant to Erythromycin and only one isolate were moderate in its sensitivity.
The pathogenicity and transmissibility of 3 MG isolates (no. 23, 36 and 37) and F vaccine were evaluated through experimental infection of one-day-old grandparent male chicks via intranasal and intraoccular inoculation of MG isolates and F vaccine one dose and double doses.
The infectivity and transmissibility of F strain vaccine and MG isolates were determined by PCR, serology and observation of clinical signs and PM lesions in different experimental period post infection.
The results appeared as follow:
Neither MG isolates nor F vaccine caused distinctive pathogenic effect in chicks, it just c aused conjuctivitis and turbidity in air sacs of both infected and contact birds.
At 14th days of age, all the infected birds with isolates 23 and 36 of group 1 & 2 respectively were positive for MG, and 75% of in-contact birds were positive. The number of positive in-contact birds decreased to 25% and 50% in group 1 & 2 at 28th days of age, respectively.
Birds infected by isolate 37 (group 3) were negative until age 14 day and became infected at 28 days PI and 50% of in-contact birds became infected at 28th days of age.
Both one dose or double doses vaccinated birds (group 4 and 5) tested positive by PCR at 14 and 28 days PV, and 50-75% of in-contact birds tested positive at 14th days PV which indicated the shedding and lateral transmission of vaccinal strain to in-contact birds.
The samples for ELISA collected at 5th wk PI or PV in all groups were revealed very slow serological response either to infection or vaccination. One dose- vaccinated group showed no response until the end of experiment (5 wks) except one bird which gave very low Abs titer, while in double dose-vaccinated group there was one bird out of 5 was positive and 2 birds gave very low Abs titer. The infected groups showed no response except one contact bird out of 5 in group 1 which was positive (20%).