Marek's Disease

Marek's Disease
Marek’s Disease
Marek's disease is a highly contagious
viral neoplastic disease in chickens.
Cause
it is caused by an alphaherpesvirus known
as 'Marek's disease virus' (MDV) or Gallid
herpesvirus 2 (GaHV-2).

General Discussion
A common lymphoproliferative disease of
chickens caused by a herpes virus that affects
peripheral nerves, brain, iris, skin, and other
tissues.
 Also causes atherosclerosis.
 Formerly a gamma herpesvirus; however genome
architecture most closely resembles alpha
herpesvirus.
 Model for herpesviral-induced lymphoma.
 Three serotypes that are designated 1,2,3; 2 and 3
are nononcogenic.
 Serotype 1 is further divided into pathotypes
classified as mild (mMDV), virulent (vMDV), or
very virulent (vvMDV), very virulent + (vv+MDV).
Pathogenesis
 Virus strain, dosage, route of infection, age,
genetic strain, and sex of the host influence the
incubation period and disease manifestation.
 Infected cells in the sloughed keratinized feather
follicle epithelium allow airborne, horizontal,
direct, or indirect transmission.
 The necrotizing effects of this infection provoke
an acute mixed inflammatory cellular infiltration.
A hyperplastic response in the spleen can
follow. Ultimately there may be atrophy of the
bursa and thymus.
Pathogenesis
 Susceptible birds develop a second wave of
cytolytic infection after 2 or 3 weeks involving
lymphoid organs, tissues of epithelial origin in
visceral organs, and feather follicle epithelium.
 The proliferative phase involving
nonproductively infected lymphoid cells may or
may not progress to the point of lymphoma
formation. Lesions may progress to tumor
development, but regression can and does
commonly occur either before or after frank
lymphoma develops.
 The virus transforms T cells. Virus has
incorporated onc genes that resemble those
found in avian retroviruses.
Clinical Findings
Four overlapping syndromes:
 Neurolymphomatosis - asymmetric paralysis of
wings or legs, incoordination, drooping wings,
head and neck.
 Acute Marek's disease - depression, ataxia,
paralysis and high mortality.
 Ocular lymphomatosis - irregular, eccentric
pupil with partial or total blindness.
 Cutaneous Marek's disease - usually not
recognized until after plucking.
paralysis (legs), dyspnea
Paralysis, paresis (legs) Paralysis (legs)
Torticollis
(head and neck)
opisthotonus
paralysis (neck)
paralysis
Paralysis
(legs, neck),
ataxia
depression,
diarrhea
nodules and
ulcers (skin)
swelling (wattles)
abnormal pupil
shape,
abnormal
iris color
A comparison between a normal chicken on the right and a

chicken infected with Marek's disease on the left. The eye
on the left shows how neoplastic cellular infiltration of the
iris can produce an abnormal iris shape and color.
abnormal pupil shape,
abnormal iris color
miosis (pupil)
Gross Findings
 Classically - enlargement of one or more
peripheral nerves, or spinal roots and ganglia,
often unilateral.
 Lymphoid tumors may occur in a variety of
organs with the gonads, especially the ovary,
most often affected; visceral tumors are
especially common in acute disease.
 Ocular lymphomatosis - graying of the iris of one
or both eyeballs (lymphoblastoid infiltration).
 Cutaneous Marek's disease - multiple, nodular
perifollicular proliferation.
Skin involvement in a chicken with Marek's disease.
normal irregular pupil
normal
lighter colored irises irregular pupil
irregular pupil lighter colored irises
left ischadic plexus normal
is asymmetrically enlarged
enlarged and yellow
edema, normal
loss of cross-striation,
normal
and a grey or yellow.
enlarged, yellowish, translucent and
loss of cross-striations.
enlarged sciatic plexus
dorsal root ganglia

multiple lymphomatous lesions in the
lungs.
Lymphomas on liver, lungs, and spleen tumors in the oropharynx.

the spleen, bursa, and thymus
the more virulent strain,, was associated with a greater
degree of bursal and thymic atrophy.
artherosclerosis of blood vessels on the surface of the gizzard.
tumors in the ovary. nodules on kidney and ovary
diffuse tumor involving several plicae,

with focal areas of necrosis the bursa on the left is from an MD
Microscopic Findings
 Classification of lymphoproliferation in nerves:
 Type A Neoplastic - Pleomorphic lymphoblastic
cells.
 Type B Inflammatory - Small lymphocytes and
plasma cells.
 Type C - light infiltration of small lymphocytes
and plasma cells.
 In the brain - focal perivascular cuffs, or nodules
of lymphocytes, vasculitis and endothelial
hypertrophy, edema prominent in the white
matter, Purkinje cell layer, and granular cell layer
of the cerebellum
 Spinal cord - focal accumulations and regional
infiltrates of lymphocytes with root ganglia
intensely infiltrated, producing a disease
primarily of peripheral nerves
 Eye - mononuclear infiltration of the iris is the
most constant ocular change.
 Viscera - lymphomatous visceral lesions have a
cellular composition like type A nerve lesions.
 Cutaneous - lesions in the skin are largely
inflammatory but may be lymphomatous.
Massive accumulations of mononuclear cells
are usually localized around infected feather
follicles, but often perivascular aggregates of
lymphocytes with a few plasma cells and
monocytes are present in the dermis.
 In the bursa and thymus, productive replication
results in degeneration and atrophy.
.Typical presentation of leg paresis in a chicken with Marek's disease
Unilateral enlargement of sciatic nerve and sacral plexus in a chicken with
.Marek's disease
Enlarged and mottled liver and spleen in a chicken with Marek's disease.
Skin involvement in a chicken with Marek's disease.
Eye involvement with neoplastic cell infiltration into the iris,
resulting in an irregular pupil, in a chicken with Marek's
Marek's disease - peripheral nerves -
chicken
Peripheral nerves : Multifocally separating and

surrounding individual nerve fibers and small caliber
vessels within the nerve are moderate numbers of
lymphocytes with fewer plasma cells and macrophages.
There are scattered aggregates of lymphocytes and mild
hemorrhage within the surrounding adipose tissue.
Neuritis : Inflammation is composed of atypical lymphocytes
with fewer plasma cells and macrophages.
Marek's disease - feathered skin - chicken
Feathered skin: Multifocally within the dermis, subcutis,
surrounding follicles and vessels, and extending into the
adjacent tissue, are low to moderate numbers of
pleomorphic lymphocytes and few scattered macrophages.
The pleomorphic lymphocytes range in appearance from
small lymphocytes with scant to indiscernible amounts of
eosinophilic cytoplasm and round, small hyperchromatic
nuclei to large lymphoblastic cells with small amounts of
eosinophilic cytoplasm and large, irregularly round nuclei
with finely stippled chromatin and variably distinct
nucleoli. Dermal and subcuticular vessels are occasionally
lined by hypertrophic endothelial cells. Multifocally, dermal
collagen is separated by a small to moderate amount of
clear space (edema), and diffusely, feather follicles are
devoid of feathers and follicular epithelium is disrupted or
absent.
Pleomorphic lymphocytes surround a
vessel lined by reactive endothelial cells.
Leukosis/sarcoma
group
It is a neoplastic disease of 4 or more
month old chickens. It start in the
bursa of fabricius and metastesize to
internal organs
Cause
it is caused by an Alpharetrovirus genus of
.the family Retroviridae
RNA viruses known as 'avian leukosis virus
(ALV)'
1-Lymphoid leukosis
It is a neoplastic disease of 4
or more month old chickens. It
start in the bursa of fabricius
and metastesize to internal
organs
Signs
Gross appearance
Nodular or diffuse tumors are seen on
the liver, spleen and bursa of fabricius.
Moreover, lungs, kidneys, gonads and
heart may be involved. Tumors are
soft, smooth and glistening with
grayish white in cut surface
Microscopic

appearance
The tumors are focal and multicentric
in origin. The tumor cells displace and
compress cells of organs rather than
infiltrate between them.
 Tumors consist of aggregates of large
lymphoid cells (lymphoblasts) of the
same primitive development stage.
They have basophilic cytoplasm and
vesicular nucleus.
Microscopic
 The other
appearance
diseases of
leukosis/sarcoma include
erythroblastosis, myeloblastosis and
myelocytomatosis,
reticuloendotheliosis, hemangioma,
osteopetrosis nephroma and
nephroplastoma
Comparison between Marek's disease and Lymphoid leukosis
Lymphoid leukosis Marek's disease Criterion
Not less than 16 weeks 6 weeks or old Age
Non specific Frequently paralysis signs
Rarely above 5% Frequent above 5% Incidence
Macroscopic lesions
Absent Frequent 1-Neural degeneration
Nodular tumors Diffuse enlargement or 2-Bursa of fabricius
atrophy
Usually absent May be present 3-Tumor in skin muscles
and proventriculus
Microscopic
No yes 1-Neural involvement
Focal or diffuse Often perivascular 2-Liver tumor
Focal Diffuse 3-spleen
interafollicular Interfollicular and/or 4- Bursa of fabricius
atrophy of follicles
No Yes Central nervous system

No Yes Skin and feather
involvement
Lymphoblasts Lymphoblasts, small, Cytology of the tumors
medium and large
lymphocytes
B-lymphocytes T-lymphocytes Type of lymphocytes
Large tumor of bursa of Fabricius (B) and kidneys (arrow in
the adult hen.
Liver tumor from a 20-week-old chicken. Note
displacement and compression of hepatic parenchyma

Marek's Disease

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Marek's Disease

A comparison between a normal chicken on the right and a

enlarged and yellow

enlarged sciatic plexus

dorsal root ganglia

Lymphomas on liver, lungs, and spleen tumors in the oropharynx.

diffuse tumor involving several plicae,

Peripheral nerves : Multifocally separating and

No Yes Central nervous system

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