Author
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Fadly, Aly |
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Submitted to: Meeting Proceedings
Publication Type: Proceedings Publication Acceptance Date: 4/22/2004 Publication Date: 4/22/2004 Citation: Fadly, A.M. 2004. Avian tumor viruses and their economic impact. Proceedings XII Moscow International Congress of Small Animal Diseases. p. 26. Interpretive Summary: Technical Abstract: Virus-induced neoplastic diseases of poultry are caused by either a herpesvirus (Marek's disease) or by a retrovirus (leukosis/sarcoma, reticuloendotheliosis and lymphoproliferative disease of turkeys). It is estimated that the annual cost to the poultry industry in the United States from avian tumor virus infections, primarily Marek's disease and avian leukosis is at least $150 million. Marek's disease (MD), a lymphoproliferative disease of primarily chickens, is caused by an alphaherpesvirus (MDV). Lesions of MD can be expressed in one or more of four forms, namely neural, visceral, ocular and cutaneous. MD is and has been controlled since early 1970s by use of conventional vaccines. During the last three decades, research on MD has resulted not only in improved conventional vaccines, but also in improved methods of vaccination (embryo vaccination). Good biosecurity practices and host genetic resistance are also recognized as important factors in implementing any strategy for control of MD. However, despite widespread use of vaccines and development of new methods of vaccination, economic losses from mortality of layers and breeders and condemnation of broilers continue to occur. Recent findings indicate that the host range of MDV has apparently expanded to include adult chickens and turkeys. Also, MDV continues to mutate to greater virulence, reducing the effectiveness of many existing vaccines. Such mutation of MDV to greater virulence leading to the emergence of more virulent pathotypes poses severe threats to the poultry industry. The major problem is the need for practical and cost-efficient technology that will better control losses, both now and in the future. Undoubtedly, the recent technological advancements in the field of molecular biology of MDV, particularly revealing DNA sequence of MDV genome and development of new technologies that allow manipulation of viral genes have enhanced our ability to identify and characterize functions of MDV genes. The knowledge generated from studying function of MDV genes will probably lead to development of more effective recombinant vaccines. The leukosis/sarcoma (L/S) viruses are avian retroviruses that infect chickens. Lymphoid leukosis (LL), a B-cell lymphoma of chickens affecting primarily the bursa of Fabricius and visceral organs is the most common form of leukosis that arise from infection with a member of L/S group of viruses known as avian leukosis virus (ALV). However, with the recognition of subgroup J ALV infection in the early 1990's, myelocytomatosis, has emerged as a neoplastic condition that is frequently detected in ALV-J- infected meat-type chickens. ALV is known to cause significant economic losses in commercial layers and breeder flocks due to mortality and lower productivity. As a potential contaminant of live-virus vaccines of poultry, ALV can also cause significant losses if contaminated vaccines were used in susceptible flocks. To date, no vaccines are available for control of ALV infection. Thus, eradication of virus infection at the primary breeder level is and has been the principal method for controlling ALV infection in chickens. Like other retroviruses, ALV mutates at a high rate and can recombine with endogenous (subgroup E ALV) elements resulting in new recombinant ALVs. Such endogenous subgroup E ALV elements not only contribute to recombination, but also can interfere with diagnosis and control of ALV infection. The new advancements in knowledge regarding molecular characteristics of ALV genome, development of highly specific reagents (monoclonal antibodies) and cloning technology have contributed significantly to: a) improved diagnostics (identification and characterization of new isolates including natural recombinant viruses), and 2) improved control methods. Reticuloendotheliosis (RE), a group of disease syndromes caused by an avian r |
