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Marek’s Disease Virus | 357

induce the transformation of T lymphocytes, which provide a severe and acute TP. However, some infected chickens can fully
novel insight for better understanding of MDV pathogenesis. recover or may transit to persistent neurological disease (PND),
which is characterized by persistent non-paralytic but neuro-
logical related signs, including ataxia, torticollis, and nervous
Pathogenesis tics (Gimeno et al., 2001). PND is observed in chickens 12–15
In MDV infected chickens, lymphoproliferative lesions appear, days post infection, and chickens showing signs of PND usually
as early as two weeks post infection, in various organs including die within a week (Fig. 12.6A). Not all chickens infected with
spleen, kidney, liver, skin, eye, nerve, and heart (Payne, 2004). MDV develop neurological CNS lesions (like TP and PND),
Sometimes, lymphoma lesions may regress (Sharma et al., 1973), however most chickens develop lymphoproliferative lesions in
but often progress to massive size compromising the function of various tissues.
the organ. Clinical manifestation of the disease depends on the
organ being affected. MDV infection can result in paralysis when
peripheral nerves are involved, blindness if the ciliary body of Diagnosis
the eye is infiltrated, or death due to visceral organ involvement Outbreaks of MD usually occurs due to vaccine failure, which
(Fig. 12.6). Lymphoma development is generally coupled with a is primarily due to improper vaccination or infection with
permanent immunosuppression which affects both humoral and highly virulent field viruses. Manifestation of MD is typically
cell-mediated immunity and may result in chickens succumbing seen by the presence of gross lymphomas in various visceral
to lethal infection with other pathogens. It has also been reported organs and skin (Zelnik, 2004). Lymphoproliferative lesions in
that early exposure of chickens to very virulent plus MDV strains peripheral nerves results in paralysis and sometimes blindness
results in inadequate immune response to other vaccines resulting is observed due to accumulation of tumour cells in the cili-
in vaccine failures (Faiz et al., 2016). ary body of the eye. In addition to clinical signs, other factors,
Infection of chickens with very virulent plus MDV strains such as vaccination program, history of disease, and the age of
can result in the development of additional lesions in the cen- affected chickens, should also be considered for accurate diag-
tral nervous system (CNS) at 9–10 days post infection, called nosis (Zelnik, 2004). Virulence of MDV and co-infection with
transient paralysis (TP), which is characterized by sudden onset other immunosuppressive viral pathogens aids in diagnosis as
of flaccid paralysis in neck and wings (Gimeno et al., 2001). well. Infection with avian leukosis virus (ALV) or reticuloen-
Infected chickens usually die within 1 to 3 days after onset of a dotheliosis virus (REV) will also cause lymphomas in poultry,

A B
(B)
(A)

D
C (D)
(C)

Figure 12.6 Marek’s disease pathogenesis. Chickens infected with vv+ strain of MDV showing neurological signs (A), paralysis (B), gross
MD lymphomas in liver (C) and lymphoproliferative lesions in peripheral nerves (D).
Figure 6