Evaluation of the Protective of Morel’s Disease Vaccine in Sheep

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This study was carried out to assess and evaluate the effect of Morel’s disease vaccine for sheep. One hundred and seventy pus samples were obtained from sheep abscesses lymph node at meat inspection in three different abattoirs and subjected to bacteriological examination. Staphylococcus spp. were isolated from 68.8% of the pus samples, while Corynebacterium spp. were isolated from 26.5%. Mixtures of both organisms were isolated from the rest (4.7%) pus samples. Isolated staphylococci were subjected to further identification by biochemical tests and were found to be: 63.2% S. aureus subsp. anaerobius, 21.3% S. caseolyticus, 11.9% S. aureus, and 0.9% of each of S. simians, S. lugdunensis, S. warneri and S. epidermidis. In outbreak of the abscess disease of sheep in Alsamra village, Khartoum State, morbidity rate was 30%. Of the affected animals, 93.3% had Morel’s disease, as pus cultures of which yielded S. aureus subsp. anaerobius and the rest 6.7% had caseous lymphadentitis, as pus cultures of which yielded Corynebacterium spp. Comparison between the different isolates of S. aureus subsp. anaerobius using PCR based techniques (RAPD and polymorphism of coa and spa genes) showed that all Sudanese isolates were genetically identical. Complete sequence of the catalase gene of one outbreak isolate in addition to partial sequence of other two outbreak isolates, six from two different abattoirs and one reference strain was performed. Sequence results showed that all Sudanese isolates harbour a catalase gene which is distinct from the catalase gene of known reference strains suggesting that all Sudanese isolates originated from one clone. The deduce catalase-like protein encoded by the catalase gene of local Sudanese strains was found to be only 345 amino acids in length instead of 505 a.a. in S. aureus viii subsp. aureus (NCTC 8325 and Newman strains) and 445 a.a in S. aureus subsp. anaerobius strain MVF213. Ability of isolated staphylococci to induce abscess formation was tested. Except S. aureus and S. aureus subsp. anaerobius, none of the isolates was able to cause the clinical subcutaneous abscess of Morel’s disease and only S. caseolyticus formed caseated lymph abscess detected on post-mortem. Morel’s disease vaccine, made according to the method of Rodwan (1996), was injected into sheep at different doses. The animals were challenged with S. aureus subsp. anaerobius using three times the minimal abscess causing dose. The minimum protective dose of the vaccine was found to be 0.5 ml boostered by 0.25 ml after two weeks. This protocol minimized the known protective dose to the half. The protective ability of Morel’s disease vaccine against abscesses due to other two staphylococci was also tested. Lambs vaccinated with Morel’s disease vaccine were able withstand challenge by S. aureus or S. caseolyticus (no abscess formation in the sub cutis or in superficial lymph nodes). Assessment of the immunity of the sheep vaccinated with Morel’s disease vaccine was carried out using the Plaque Forming Cell Assay and by Opsonphagocytosis methods. Number of plaque antibody forming cells from vaccinated sheep was significantly higher (P<0.05) than from non vaccinated sheep. Bacteria opsonized for 2 hours by serum of vaccinated animals caused smaller subcutaneous abscesses in experimentally infected sheep when compared with that caused by non-opsonized bacteria. Also, a sharp decrease in the number of opsonized bacteria was observed indicating that the serum antibodies in response to vaccination with Morel’s disease vaccine had greatly increased.
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