9-week-old, female, Leghorn chicken, avianIn March 2003, extensive swelling or bulbous protrusions of the integument were noticed in the layer chickens of approximately 11 weeks of age of a commercial flock. The affected chickens had almost no clinical signs other than the tumor and ate, drank, and walked normally. Approximately 80 of 27,000 (0.3%) chickens in the affected flock were slaughtered because of the tumor. Similar tumors had also occurred in the chickens of another farm originating from the same lot of breeder hens.15 Both affected flocks had been vaccinated by the wing-web method for fowl pox and Marek's diseases and were sprayed with the infectious bronchitis vaccine at hatching

Gross Description:  

The tumors were primarily observed in the head or wing, had a maximum diameter of 10 cm, were usually solitary, soft to firm but not hard, and creamy white to a dull red in color and grew slowly. The overlying skin was wounded in some cases, resulting in ulceration.

Histopathologic Description:

The tumor was unencapsulated and composed of loose areas with abundant mucinous matrix. Stellate or spindleshaped cells were predominant, whereas mitotic figures were uncommon. Short collagen fibers, which are stained blue by the azan stain and Masson trichrome stain, were observed in the matrix. The mucinous matrix was stained positively with alcian blue pH 2.5 and colloidal iron and showed metachromasia by toluidine blue staining: it was negative with alcian blue pH 1.0, high iron diamine, and periodic acid-Schiff reaction. The positive reactions disappeared after digestion with hyaluronidase. These results demonstrate that the matrix contained hyaluronic acid.

Tumors were specifically stained by a rabbit anti-ALSV serum and two mouse monoclonal antibodies against subgroup A ALSV, and the inner bulb of Herbst corpuscles was strongly stained. Normal cutaneous and subcutaneous tissues of the affected chickens were negative. 

Electron microscopy revealed viral particles in the tumor that were 74-97 nm in diameter, had a core (34-46 nm in diameter) and envelopes, and sometimes showed the budding process. Their ultrastructural characteristics were identical with those of type C retroviruses.8

Morphologic Diagnosis:  

Skin: myxoma, Leghorn, chicken, avian

Lab Results:  

RT-PCR analyses were performed on the tumor tissues of two affected chickens with a primer pair targeted the gp85 region of the env gene of subgroups A to E avian leukosis/sarcoma virus (ALSV). Tumor samples were PCR positive. Nucleotide sequence analyses indicated that these viruses belonged to subgroup A of ALSV and had 97.5% nucleotide sequence homology with myeloblastosis-associated virus type 1-like strain 1 of Canadian isolate.17 Fifteen of 20 serum samples obtained from 19 weeks of age clinically normal chickens from the affected layer farm were strongly reactive against subgroup A of ALSV, and eight of them were also strongly reactive against subgroup B by an enzymelinked immunosorbent assay (ELISA).19



Contributor Comment:  

Generally, neoplastic diseases other than Marek's disease or lymphoid leukosis are sporadic; however, in the present case, subcutaneous tumors were observed in many layer chickens in a flock. A congenital or genetic disease was suspected at first because a similar disease had occurred in the chickens of another farm originating from the same lot of breeder hens. The results of immunohistochemistry, electron microscopy, RT-PCR analysis, and ELISA indicate that the neoplastic diseases in the present case are associated with subgroup A of ALSV infection. It seems that the virus is exogenous because the ALSV antigen positive area is localized and, in the tumor, it corresponds with the C-type viral particles observed area. Eight of 20 chickens from the affected farm had an antibody to subgroup B; however, it might be a cross-reaction caused by group-specific antigens of ALSV. We conclude that 17 samples had antibodies against subgroup A ALSV. The existence of matrix-inclusion bodies containing ribonucleic acid in the myocardium is further evidence in favor of this viral infection.3 It is possible that the virus is transmitted vertically from hen to progeny through the egg or horizontally from bird to bird at the breeding site. Chickens inoculated in the wing web with avian sarcoma virus develop sarcomas at the site of inoculation5. Epizootic outbreaks of solid tumors have been reported, and the physical transmission of ALSV among newly hatched chickens during the vaccination procedure has been suggested7. However, in our case, tumors were observed not only at the site of inoculation, but also in other areas. Further studies are required to elucidate the epizootiology of our case.

The tumor contained large amounts of mucin and involved histologic lesions similar to those previously reported in cases of myxoma in chickens2, 16 and was therefore diagnosed as myxoma. Myxoma is composed of embryonal connective tissues.9 Myxoma in the subcutis, spleen, kidney, ovary, and mesentery in chickens has been previously reported.1, 16, 20 Replication-defective avian retroviruses and Rous sarcoma virus affect mesenchymal cells and cause sarcomas.12 Various tumors have been reported in chickens inoculated with specific strains of ALSV; for example, strain F-1A of subgroup A has been associated with lymphoid leukosis, erythroblastosis, fibrosarcoma, and hemangioma in inoculated chickens.7

Today, it seems that leukosis-free flocks have been established, most commercial flocks consist of genetically resistant lines, the eradication of horizontally transmitted viruses has been accomplished,12 and, accordingly, there has been a sharp reduction in the incidence of diseases associated with ALSV infection with the exception of subgroup J ALSV.14,18 However, in our unpublished data, layer flocks frequently have ALSV antigens in Japan, and we are concerned about an outbreak of diseases associated with ALSV. The present epizootic outbreak of neoplastic disease is therefore unusual and worthy of study.

JPC Diagnosis:  

Feathered skin: Myxoma, leghorn chicken (Gallus domesticus), avian.

Conference Comment:  

The contributor gives a good overview of the ALSV-induced myxoma in this flock of commercial chickens. 

Viruses of the avian leukosis/sarcoma virus (ALSV) group are members of the Alpharetrovirus genus of the family Retroviridae. Other species within this genus include the Rous sarcoma virus and other replication defective viruses that carry various oncogenes. ALSVs are divided into 6 subgroups, A-E and J, based on differences in their viral envelope. Viral replication requires a reverse transcriptase that synthesizes a DNA provirus of the RNA virus. This DNA provirus is then integrated into the host cell genome where viral RNAs are transcribed, which are then translated into precursor and mature viral proteins.

In addition to subgroups, strains of ALSV are generally classified according to the predominant neoplasm they produce, such as lymphoid leukosis virus (LLV), avian erythroblastosis virus (AEV), avian myeloblastosis virus (AMV), and avian sarcoma virus (ASV). Although the oncogenic spectrum of the strains are usually characteristic, they can overlap and are affected by viral origin, dose, and route of inoculation, as well as by host age, genotype, and sex.

Gross and histologic differential diagnoses for neoplasms and lesions that may be caused by ALSV strains include:

Retrovirus of Animals12
AlpharetrovirusAvian leukosis viruses, avian carcinoma viruses, avian sarcoma viruses, Rous sarcoma virus, duck spleen necrosis virus
BetaretrovirusMouse mammary tumor virus, Jaagsiekte
GammaretrovirusFeline leukemia virus, feline sarcoma virus, porcine type C virus, many murine leukemia viruses, many murine sarcoma viruses
DeltaretrovirusBovine leukemia virus, human and simian T lymphotropic viruses
EpsilonretrovirusWalleye dermal sarcoma virus, walleye epidermal hyperplasia viruses
LentivirusHuman immunodeficiency virus, simian immunodeficiency viruses, maedi/visna virus, caprine arthritis-encephalitis virus, feline immunodeficiency virus, equine infectious anemia virus, bovine immunodeficiency virus
SpumavirusBovine, feline, simian, and human foamy viruses


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