Page 238 - Avian Virology: Current Research and Future Trends

Avian Leukosis Virus

Yongxiu Yao and Venugopal Nair* 8

Viral Oncogenesis Group, Pirbright Institute, Surrey, UK.

*Correspondence: venugopal.nair@pirbright.ac.uk
https://doi.org/10.21775/9781912530106.08

Abstract the erythropoietic (red cell) system, and the myelopoietic
The leukosis/sarcoma (L/S) group of diseases comprise of a (myelocytic) system. Presenting signs of the leukosis are mostly
variety of transmissible benign and malignant neoplasms, such as non-specific (loss of appetite, weakness, emaciation, diarrhoea,
lymphoid, myeloid and erythroid leukosis, caused by avian leuko- pale wattles). Lymphoid leukosis has been the most common
sis viruses (ALV), belonging to the family Retroviridae. Extensive form of L/S group of diseases seen in field flocks, although more
research in the last several decades have uncovered interesting recently, myeloid leukosis had become more prevalent.
biology of these viruses and their pathogenic mechanisms. ALV Infection of chickens with ALV is the most common L/S
are characterized by the unique possession of a reverse tran- virus infection encountered in field flocks and is known to be of
scriptase enzyme that drives the generation of the DNA provirus, significant economic importance. Economic losses from ALV-
which is integrated into the host genome during viral replication, induced diseases are attributed to two sources. First, the tumour
and induction of diseases is from insertional activation as well as mortality that usually accounts for 1–2%, with occasional losses
transduction of oncogenes such as c-myc. ALV-associated dis- as high as 20% in some flocks. Second, subclinical infection by
eases are widespread with significant economic losses resulting ALV, to which most flocks are subject, produces a depressive
from tumours and subclinical infections and loss of productivity. effect on a number of important performance traits, including egg
ALVs infecting chickens belong to six envelope subgroups A, B, C, production and quality (Gavora et al., 1980, 1982; Gavora, 1987).
D, E (endogenous retroviruses) and J, with A, B and J subgroups Economic losses due to ALV tumour mortality and reduced pro-
the most important ones in terms of distribution and induction of ductivity were estimated to be in millions of US dollars each year
diseases. While A and B subgroups are primarily associated with (Nair and Fadly, 2013). Currently the virus is causing huge eco-
lymphoid leukosis, ALV-J primarily induces myeloid leukosis, nomic losses to the poultry industry in countries such as China
currently a major problem in countries such as China. With the (Gao et al., 2010, 2016; Payne and Nair, 2012).
virus transmitted both vertically through the eggs from infected The earliest reports of leukotic diseases in fowl are those of
hens and horizontally, prevention of the disease is mainly by Roloff (1868), who described a case of ‘lymphosarcomatosis’ in
preventing the introduction of viruses and by eradication, taking 1868, and of Caparini (1896), who in 1896 described cases of
advantage of the diagnostic tests to identify and disrupt the infec- ‘fowl leukaemia’. In 1905, Butterfield diagnosed ‘aleukemic lym-
tion cycle. Genetic resistance to ALV infection has also been phadenosis’ in the USA (Butterfield, 1905). In 1908, Ellermann
observed primarily based on the presence or absence of specific and Bang, working in Copenhagen, founded the discipline of viral
receptor sequences. Recent advances in genome editing offers oncology when they transmitted erythroleukemia and myelog-
the possibility of exploiting genetic resistance in the fight against enous leukaemia by inoculation of chickens with cell-free filtrates
ALV. (Ellermann and Bang, 1908).
Ellermann (1921) also provided further classification of the
pathological forms of avian leukosis that essentially still holds
Introduction and history today. He described in his monograph ‘The Leucosis of Fowls
The leukosis/sarcoma (L/S) group of diseases comprise a variety and Leucemia Problems’ (Ellermann, 1921) (1) ‘lymphatic
of transmissible benign and malignant neoplasms of chickens leucosis’ with lymphoblastic hyperplasia; (2) ‘myeloic leucosis’
caused by members of the family Retroviridae (Fauquet, 2005). with leukaemia and general hyperplasia of ‘myeloic’ cells (mye-
The term ‘leukosis’ embraces several different leukaemia-like pro- locytes, ‘large mononuclear cells’ and ‘poikilonuclear cells’); and
liferative diseases of the different cell types of the hemopoietic (3) ‘intravascular lymphoid leucosis’ involving ‘lymphoidocytes’
system caused by avian leukosis virus (ALV); the term leukosis which he concluded were erythrocytic cells and that this intravas-
is used because a leukaemic blood picture is not always present. cular form was erythroleukosis.
The components of the hemopoietic system that undergo neo- Also, in the early decades of the twentieth century, Peyton
plastic change include the lymphopoietic (lymphocytic) system, Rous, working in New York, was undertaking studies on the