Changes

*High ambient environmental temperatures are associated with increased survival of infected animals.  Some authors consider the California strains of the virus (California rabbit fibroma virus) as distinct from the myxoma virus, while others feel that the antigenic differences do not justify the separate distinction.

*High ambient environmental temperatures are associated with increased survival of infected animals.  Some authors consider the California strains of the virus (California rabbit fibroma virus) as distinct from the myxoma virus, while others feel that the antigenic differences do not justify the separate distinction.

Pathogenesis

====Pathogenesis====

*MYX infects several cell types including mucosal cells, lymphocytes, and fibroblasts.  

*MYX infects several cell types including mucosal cells, lymphocytes, and fibroblasts.  

*In addition to primary and secondary tumor development, there is severe immunosuppression leading to overwhelming infections by opportunistic gram-negative bacteria.   

*In addition to primary and secondary tumor development, there is severe immunosuppression leading to overwhelming infections by opportunistic gram-negative bacteria.   

*Rabbit poxvirus gene sequences coding for products related to the C4 and C5 components of the complement cascade have been identified.  It is postulated that complement-like viral protein products may block the activation or action of complement.  The virulence factor T7 is an IFNγ receptor homologue.

*Rabbit poxvirus gene sequences coding for products related to the C4 and C5 components of the complement cascade have been identified.  It is postulated that complement-like viral protein products may block the activation or action of complement.  The virulence factor T7 is an IFNγ receptor homologue.

Clinical signs

====Clinical signs====

*Clinical disease varies with virus strain and host species.   

*Clinical disease varies with virus strain and host species.   

**Sylvilagus species are relatively resistant, and are  probably the natural host of the virus.  In Sylvilagus species, infection usually results only in the development of skin tumors at the site of inoculation.  The tumors appear 4-8 days after exposure and persist for up to 40 days.  Very young rabbits may succumb to generalized disease.

**Sylvilagus species are relatively resistant, and are  probably the natural host of the virus.  In Sylvilagus species, infection usually results only in the development of skin tumors at the site of inoculation.  The tumors appear 4-8 days after exposure and persist for up to 40 days.  Very young rabbits may succumb to generalized disease.

***Death (8-15 days post infection) is usually preceded by labored breathing and convulsions.

***Death (8-15 days post infection) is usually preceded by labored breathing and convulsions.

Pathology

====Pathology====

*The most prominent gross lesions in European rabbits with myxomatosis are the skin tumors and the pronounced cutaneous and subcutaneous edema,  particularly in the area of the face and around body orifices.   

*The most prominent gross lesions in European rabbits with myxomatosis are the skin tumors and the pronounced cutaneous and subcutaneous edema,  particularly in the area of the face and around body orifices.   

*Skin hemorrhages and subserosal petechiae and ecchymoses may be observed in the stomach and intestines.  Subepicardial and subendocardial hemorrhages may also occur.

*Skin hemorrhages and subserosal petechiae and ecchymoses may be observed in the stomach and intestines.  Subepicardial and subendocardial hemorrhages may also occur.

*Hares or young Sylvilagus rabbits may develop fibromatous to myxomatous nodules, however, lesions are usually mild and localized.

*Hares or young Sylvilagus rabbits may develop fibromatous to myxomatous nodules, however, lesions are usually mild and localized.

Histopathology

====Histopathology====

*The lesions are proliferative to degenerative, depending on the virus strain.   

*The lesions are proliferative to degenerative, depending on the virus strain.   

*The skin tumors result from proliferation of undifferentiated mesenchymal cells which become large stellate cells (myxoma cells).  These cells may contain intracytoplasmic eosinophilic inclusions. These cells lie in a homogeneous matrix of mucinous material interspersed with capillaries and inflammatory cells.  Endothelial proliferation with narrowing of the lumen and extrusion of stellate "myxoma" cells has been described.   

*The skin tumors result from proliferation of undifferentiated mesenchymal cells which become large stellate cells (myxoma cells).  These cells may contain intracytoplasmic eosinophilic inclusions. These cells lie in a homogeneous matrix of mucinous material interspersed with capillaries and inflammatory cells.  Endothelial proliferation with narrowing of the lumen and extrusion of stellate "myxoma" cells has been described.   

*Epithelial cells overlying the tumor may show hyperplasia or degeneration, or may appear normal.  Epithelial proliferation is characterized by ballooning of cells in the stratum granulosum and hyperkeratosis.  Epithelial vesiculation progresses to crust formation.  Large, eosinophilic, intracytoplasmic inclusions are especially prominent in the stratum spinosum.  Lesions in other organs include cellular proliferation of alveolar epithelium and the reticulum cells of lymph nodes and spleen.  Focal hemorrhage may be present in skin, kidneys, lymph nodes, testes, heart, stomach, and intestinal walls.  Myxoma cells may be present in lymph nodes, bone marrow, uterus, ovaries, testes, and lungs.

*Epithelial cells overlying the tumor may show hyperplasia or degeneration, or may appear normal.  Epithelial proliferation is characterized by ballooning of cells in the stratum granulosum and hyperkeratosis.  Epithelial vesiculation progresses to crust formation.  Large, eosinophilic, intracytoplasmic inclusions are especially prominent in the stratum spinosum.  Lesions in other organs include cellular proliferation of alveolar epithelium and the reticulum cells of lymph nodes and spleen.  Focal hemorrhage may be present in skin, kidneys, lymph nodes, testes, heart, stomach, and intestinal walls.  Myxoma cells may be present in lymph nodes, bone marrow, uterus, ovaries, testes, and lungs.

===Shope fibroma virus===

===Shope fibroma virus===