Page 354 - Avian Virology: Current Research and Future Trends

Marek’s Disease Virus

Blanca M. Lupiani , Yifei Liao , Di Jin , Yoshihiro Izumiya and Sanjay M. Reddy * 12
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1 College of Veterinary Medicine, Texas A&M University, College Station, TX, USA.
2 School of Medicine, University of California Davis, Sacramento, CA, USA.
*Correspondence: sreddy@cvm.tamu.edu
https://doi.org/10.21775/9781912530106.12

Abstract the disease as ‘fowl paralysis’ or ‘range paralysis’ since the disease
Marek’s disease virus (MDV) is a highly oncogenic alphaherpes- primarily affected peripheral nerves leading to paralysis.
virus that infects chickens, causing enormous economic losses Later studies showed that in addition to lesions in the nerves,
to the poultry industry. MDV belongs to the genus Mardivirus tumours were observed in various lymphoid organs (Pap-
and is classified into three serotypes. Only viruses belonging to penheimer, 1926, 1929). It was difficult to distinguish visceral
serotype 1 cause disease in chickens. In susceptible chickens, lymphomas caused by ‘range paralysis’ from neoplastic condi-
T-lymphocytes undergo neoplastic transformation. The manifes- tions of the haematopoietic system in chickens. These distinct
tation of the disease depends on the distribution of the neoplastic diseases were initially described as Avian Leukosis Complex, but
lesions. Typically, the disease is manifested by visible lymphomas later it was concluded that they were distinct diseases caused by
in various visceral organs and paralysis. Highly pathogenic MDV different viral aetiological agents.
also causes neurological disease and immunosuppression. MDV
is controlled by vaccination using live attenuated vaccines, but
vaccinated chickens do not provide sterilizing immunity and Infectious agent
cannot block virus transmission of field strains. Vaccination is
generally believed to have contributed to increase in virulence of Classification
the filed strains leading to vaccine breaks. Among several genes MD is a common, highly contagious, lymphoproliferative disease
encoded by MDV, meq, which encodes a bZIP protein, appears of chickens characterized by T-cell tumours, immunosuppres-
to play a critical role in the transformation of lymphocytes. With sion, partial or complete paralysis of legs and wings, skin leukosis,
the availability of genome manipulation tools and a natural host depression, and death (Marek, 1907; Pappenheimer et al., 1929;
system, MDV provides a relevant model to explore the molecular Calnek, 2001). The causative agent of MD is Marek’s disease virus
mechanisms of viral oncogenesis. In this chapter, we summarize (MDV serotype 1) or Gallid herpesvirus 2 (GaHV-2), and along
the current knowledge of MDV molecular biology, pathogenesis with two other non-oncogenic poultry viruses, Gallid herpesvirus
and control, as well as the role of individual viral genes in replica- 3 (GaHV-3 or MDV serotype 2) and Meleagrid herpesvirus 1
tion and transformation. [MeHV-1, MDV serotype 3 or turkey herpesvirus (HVT)], are
classified in the genus Mardivirus of the subfamily Alphaherpes-
virinae (Davison et al., 2009). MDV-1 includes oncogenic viruses
Introduction and history of variable virulence, MDV-2 includes non-oncogenic viruses
Marek’s disease (MD) is a lymphoproliferative disease of chick- from chickens, and MDV-3 includes non-oncogenic viruses from
ens caused by a herpesvirus. MD was first described by Dr Joseph turkeys (Table 12.1). The MDV-1 pathogenic strains vary greatly
Marek, a clinical pathologist at the Royal Hungarian Veterinary in their pathogenic and oncogenic potential and were originally
School in Budapest, in 1907. In his seminal report entitled ‘Mul- classified as either classical or acute forms of MD (Biggs et al.,
tiple Nervenentzündung (polyneuritis) bei Hühnern (chickens)’ 1965). The classical form referred to less pathogenic strains with
(Marek, 1907), he described the disease in four adult cockerels paralytic manifestation of the disease and the acute form referred
that were affected by paralysis of wings and legs. The affected to strains that caused lymphomas in visceral organs. With the
chickens showed thickening of the sacral plexus due to infiltration isolation of viruses with greater virulence than acute viruses this
of the mononuclear cells. He described the disease as a neurologi- classification became inadequate. An alternate nomenclature
cal disease of chickens due to polyneuritis of the affected nerves. was proposed in which the classical strains were considered mild
A similar condition was reported in the USA and in the Nether- MDV (mMDV) and acute strains were classified based on their
lands in the 1920s. These early descriptions led to the naming of ability to cause increasing disease in chickens vaccinated with