Page 243 - Avian Virology: Current Research and Future Trends

236 | Yao and Nair

tolerance to the virus and after hatching make up the V+A– class, myeloblasts comprising up to 75% of peripheral blood cells and
with high levels of virus in the blood and tissues and an absence of forming a thick buffy coat and usually an anaemia and thrombo-
antibodies. By 22 weeks of age, up to 25% of meat-type chickens cytopenia.
exposed to ALV-J at hatch were found to be V+A–, although this Tumours of myelocytomatosis (myelocytic myeloid leuko-
could be affected by a number of factors including the virus strain sis) are distinctive and can be recognized on gross examination
(Pandiri et al., 2007). with some degree of certainty. Characteristically, they occur on
the surface of bones in association with the periosteum and near
cartilage, although any tissue or organ can be affected. Myelocy-
Spread of infection tomas often develop at the costochondral junctions of the ribs, on
Birds from infected flocks, and products, such as eggs and meat, the inner sternum, pelvis, and on the cartilaginous bones of the
could spread infection to other flocks and locations. Because ALV mandible and nares. Flat bones of the skull are also commonly
is vertically transmitted, infection can spread transnationally in affected. Tumours may also be seen in the oral cavity, trachea,
hatching eggs and day-old chicks, and potentially through the and in and around the eye (Pope et al., 1999). The tumours are
semen. Importers need to guard against the introduction of ALV usually nodular and multiple, with a soft, friable consistency and
by requiring health certificates from the exporter relating to the of creamy colour. In the disease caused by subgroup J ALV, myelo-
specific disease and infection status of source flocks. Similarly, cytomatous infiltration often causes enlargement of the liver and
there has to be a guard against the introduction of ALV through spleen and other organs, in addition to skeletal tumours (Wil-
live cell culture vaccines. Introduction of a new ALV into sus- liams et al., 2004) and myelocytic leukaemia (Payne et al., 1991).
ceptible poultry populations could have serious consequences Haemangioma is found in the skin or in visceral organs in
because of absence of immunity and of effective control measures chickens of various ages. They appear as blood-filled cystic masses
in breeding and production populations other than at the primary (blood blisters) or more solid tumours and consist of distended
breeding level. blood-filled spaces lined by endothelium or as more cellular,
proliferative, lesions (Campbell, 1969). They are often multiple
and may rupture, causing fatal haemorrhage (Soffer et al., 1990).
Clinical features and pathogenesis More recently, many workers have reported the incidence hae-
Outward signs of the leukotic diseases are mostly non-specific. mangiomas in layer chickens infected with ALV-J in China (Lai
They include loss of appetite, weakness, diarrhoea, dehydration, et al., 2011; Pan et al., 2011; Zhang et al., 2011). Renal tumours
and emaciation. In LL especially, there may be abdominal enlarge- may cause paralysis due to pressure on the sciatic nerve. Sarco-
ment. The comb may be pale, shrivelled, or occasionally cyanotic. mas and other connective tissue tumours may be seen in the skin
In erythroblastosis and myeloblastosis, haemorrhage from feather and musculature. When advanced, these various other tumours
follicles also may occur. After clinical signs develop, the course is may be accompanied by the non-specific signs given previously.
usually rapid, and birds die within a few weeks. Other affected Benign tumours may follow a long course, malignant tumours a
birds may die without showing obvious signs. rapid one.
Fully developed LL occurs in chickens of about 4 months of In osteopetrosis, the long bones of the limbs are commonly
age and older. Grossly visible tumours almost invariably involve affected. Uniform or irregular thickening of the diaphyseal or
organs such as the liver, spleen, and bursa of Fabricius. Other metaphyseal regions can be detected by inspection or palpation.
organs including kidney, lung, gonad, heart, bone marrow, and The affected areas are often unusually warm. Birds with advanced
mesentery also may show lesions. The tumour cells are B-cells, lesions have characteristic ‘bootlike’ shanks. Affected birds usu-
expressing immunoglobulin M (IgM) and other B-cell mark- ally are pale, stunted and walk with a stilted gait or limp. In recent
ers, which originate in the bursa of Fabricius but subsequently years, ALV has also been shown to be associated with the ‘so
develop metastasis to other visceral organs. called fowl glioma’ (Ochiai et al., 1999), associated with cerebel-
Natural cases of erythroblastosis (erythroid leukosis) usually lar hypoplasia and myocarditis (Iwata et al., 2002; Hatai et al.,
occur in birds between 3 and 6 months of age. The liver and kidney 2005, 2008; Toyoda et al., 2006; Nakamura et al., 2011).
are moderately swollen, and the spleen often is greatly enlarged.
The enlarged organs are usually cherry red to dark mahogany and
are soft and friable. The bone marrow is bright red and liquid. Immune responses
Affected birds are often anaemic, with muscle haemorrhages and
occasionally abdominal haemorrhage from a ruptured liver. The Active immunity
disease is an intravascular erythroblastic leukaemia. Immune responses to oncogenic viruses including ALV have
Myeloblastosis (myeloblastic myeloid leukosis) is predomi- been reviewed (Nair, 2013). Under natural conditions, most
nantly a sporadic disease of adult chickens. The liver is greatly chicks become infected by exogenous ALV from penmates or
enlarged and firm with diffuse greyish tumour infiltrates, which their surroundings and, after a transient viraemia, develop virus-
give a mottled or granular (‘Morocco leather’) appearance. The neutralizing antibodies directed against virus envelope antigens
spleen and kidneys are also diffusely infiltrated and moderately that rise to a high titre and persist throughout the life of the bird.
enlarged. The bone marrow is replaced by a solid, yellowish- The virus-neutralizing antibodies serve to restrict the amount
grey tumour cell infiltration. A severe leukaemia exists, with of virus in the bird, which in turn, will limit neoplasia, but they