Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: April 22, 2004
Publication Date: April 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. 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 retrovirus unrelated to the L/S group of viruses termed RE virus (REV). REV infects chickens, turkeys, ducks, geese, pheasants, quail, and probably many other avian species. The most common clinical diseases induced by REV are chronic lymphomas (reticular cell tumor, B- or T-cell lymphoma) and an immunosuppressive runting disease. Although REV is widespread, REV-induced clinical disease is infrequently diagnosed in commercial flocks. Although losses in REV affected flocks can be significant due to tumor mortality and or immunosuppression, the principal economic concerns of REV infection are as contaminants of live-virus vaccines of poultry or as a barrier to export of breeding stock to certain countries. To date, no vaccines are available for control of REV infection and no method has been routinely used by industry to control REV infection in commercial turkey and chicken flocks. However, some breeders have experimented with programs similar to that used in eradication of ALV. Recent findings on partial or complete REV genome insertion in large DNA avian viruses such as Marek's and fowlpox viruses have pointed to the need for further studies to determine the role of such insertion in the epidemiology of REV as well as the pathogenicity of these large DNA viruses. Lymphoproliferative disease (LPD), a neoplastic disease of turkeys caused by yet another retrovirus unrelated to L/S virus or REV had been reported in Europe and Israel. Because the incidence of LPD of turkeys has always been sporadic and the disease has not been reported during at least the last decade, further discussion of this rare neoplastic syndrome of turkeys is not included in this review.