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Lumpy skin disease (LSD) is a viral infectious disease of cattle that has been reported in Iran since 2014 and has spread to different parts of the country. This disease causes significant losses to the livestock industry due to reduced milk production, abortion, infertility, decreased skin quality and skin damage. Therefore, recognizing the characteristics of the virus to produce vaccines and control and eradication programs is an important step in reducing the damage caused by it.In order to produce a vaccine, virulence factors of virus must be reduced or eliminated, which used to be done by repeated passages in non-target animal or cell culture, but now, using biotechnology techniques, the genes that cause the virus can be removed that accelerated producing attenuated virus. Therfore, the virulence factors must first be well known. The LSD142 gene is similar to the N1L gene in vacinia virus that the deletion of it in vacinia virus has been shown to reduce the virulence of the virus in mice. But in LSD virus, its effect on the virulence of the virus is still unknown. Therefore, this study was designed to investigate the effect of LSD142 gene deletion on reducing the virulence of LSD virus.At first, the sequence of LSD142 gene with 500 to 600 bp sequences of its left and right genes in 334 isolates of Kermanshah was determined and its identity with other capripoxes was determined and it was shown that this virus is in LSD group and subgroup of acute strains. The right and left parts of the gene were amplified and inserted into the vector. By subcloning the vectors prepared with vectors containing gpt and EGFP cassette, donor vectors containing right and left parts of the gene and gpt and EGFP genes were prepared. By using this vector and homologous recombination method during virus replication in the cell culture, LSD142 gene was removed from LSD virus. By replicating and observing the virus on a fluorescent microscope, the recombinant virus was identified. Then the recombinant virus amplified in the culture medium containing mycophenolic acid, and by PFU, clones containing the recombinant virus were isolated and purified. LSD142 gene deletion was confirmed by PCR.Six three or four-month-old male calves were used to assess the virulance of the recombinant virus. The recombinant virus was injected into three cases and the wild-type viruses were injected other three calves. Ten folds dilutions of the viruses were injected intradermally into the flank at four points for each dilution. All animals had lesions at the injection sites and the disease became general in all animals. At necropsy, lesions caused by the virus were observed in various organs, especially the mesentery. In result, it seems that gene deletion did not reduce the virulence of the virus in cattle.The LSD virus has multiple virulence factors and many genes are involved in the virulence of the virus, therefor deletion of one gene alone may not attenuated the virus. Thus, in order to produce the attenuated virus, while examining separately the effect of hipotetical virulence factors of virus, the effect of their combined deletion should also be investigated.Keywords: Lumpy skin disease, gene deletion, virus, homologous recombination
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