MDV and HVT are typical herpesviruses, in the family Herpesviridae, subfamily Alphaherpesvirinae, genus Marek's disease-like viruses. The nucleocapsid measures 85-100 nm and has 162 hollow capsomeres. Enveloped particles measure up to 400 nm. The nucleoid has a spherical to toroidal structure. Although originally placed with the gammaherpes-viruses because of their lymphotropism, molecular studies show that MDV and HVT are more closely related to the alphaherpesviruses, such as herpes simplex virus.
Three serotypes of the virus are recognized: serotype 1, which comprises a variety of strains of MDV which vary from being highly to weakly oncogenic; serotype 2, which contains nononcogenic strains of MDV, although a virulent strain has been reported, and serotype 3, represented by HVT, which is also nononcogenic. These serotypes are distin-quished in polyclonal or monoclonal antibody tests, polypeptide pattern and DNA analysis.
Serotype 1 isolates can also be classified patho-typically as mild (mMDV), virulent (vMDV), very virulent (vvMDV) and very virulent plus (vv + MDV).
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