Marek's Disease.pptx

› Marek’s Disease History
› Marek’s Disease
– What is Marek’s disease?
– Etiology
– Economic importance
– Clinical signs
– Post mortem lesions
– Diagnosis and control
› Marek’sVaccine

› 1907
› MD was described by Josef Marek as a polyneuritis affecting mainly old chickens with
low morbidity and negligible mortality.
› 1921
› Van derWalle andWinkler-Junius in Europe and Kaupp inUSA consisted of neurological
clinical signs and was inflammatory in nature.
› 1922
› The disease was reported in most parts of USA.
› It appeared to be more severe with mortality of up to 20% and a significant proportion
of the affected chickens developed lymphoid tumors in visceral organs.
Marek’s Disease History

› 1950
› The disease became a serious problem at the end of 1950s when the nature of the
disease became predominantly neoplastic.
› This new form of MD was characterized by:
– The explosive nature of outbreaks.
– The young age at which the disease could first appear.
– The high mortality of 30% or more.
– The high incidence of visceral lymphoid tumors in addition to nerve lesions.
› 1960
› Acute MD became the predominant form in most countries that had a well-developed
poultry industry.

› 1965
› Biggs and coworkers proposed to call MD strains inducing this severe form of the
disease as “acute MD strains” to differentiate them from “classical MD strains” inducing
a much less severe form of the disease.
› This was the first classification of the etiological agent of MD based on virulence and
preceded the isolation and identification of the MDV by several years.
› Early 1970
› After the introduction and widespread use of HVT vaccines, the disease was at first well
controlled. However, those vaccines could not protect against super infection with other
MDVs.

Late 1970
› In theUSA, the next shift of virulence among MDV strains was recognized in association
with an unexplained increase in MD losses in vaccinated flocks.
› Witter and coworkers isolated and characterized several strains of MDV that caused
higher incidence of MD lesions in chickens vaccinated with HVT.
› Those isolates were originally considered as “variant” viruses.

› In order to control those viruses in the field, a change in vaccine strategy was made and
bivalent vaccines including HVT and naturally non-oncogenic serotype 2 MDV strains
were introduced.

› 1972
› In Europe and other parts of the world, the most widely used vaccine was an attenuated
serotype 1 MDV vaccine,CVI988.
› 1980s
› No evidence of MD inCVI988 vaccinated flocks was reported during 1980s.
› 1990s
› There was a significant incidence of MD in flocks vaccinated with the bivalent (USA) or
withCVI988 (Europe,Australia, SouthAmerica, andAsia).
› This suggested that a further change in virulence had occurred.
› In theUSA, the introduction ofCVI988 has been a solution to the problem since then.

› In countries that were already usingCVI988, the use of polyvalent vaccines (serotypes 1,
2, and 3), revaccination, improving vaccination techniques, and better control of other
immunosuppressive diseases have been the strategies taken to control MD.

Free range to intensive
production systems
Introduction of
HVT
Introduction of
bivalent vaccine
Introduction of
CVI988
Introduction of CVI 988 in Europe
and other countries
Introduction of revaccination and trivalent
vaccines in Europe and other countries
M V VV VV+
1970
1957 1981 1997 2007

Marek’s Disease
› Marek’s disease (MD) is a highly contagious viral infection that predominantly affects
poultry flocks worldwide.
› It can also affect pheasants, quail, gamefowl and turkeys.
› The disease is characterized by the presence ofT cell lymphoma as well as infiltration
of nerves and organs by lymphocytes.

Marek’s Disease
› Clinical disease is not always apparent in infected
flocks.
› It may appear as a decrease in:
– Growth rate.
– Egg production.
– Mortality rates can be very high in susceptible birds.

Etiology
› It is caused by an alpha herpes virus known as Marek's disease virus (MDV) orGallid
herpes virus 2 (GaHV-2).

Virus Classification
› An oncogenic (tumor-inducing) herpes virus
› Group:
› Group I (dsDNA)
› Family:
› Herpesviridae
› Genus:
› Mardivirus
› Species:
› Gallid herpesvirus 2 (GaHV-2)

Transmission
› Transmission
– MDV is horizontally transmitted from older
chickens by inhalation.
› Shedding
– Infected birds shed “dander” (feather dust)
contaminated with virus which can be
distributed by wind, equipment, and
personnel.

Virus Resistance
› The virus resists the environmental exposure and
can remain viable for long periods in houses
especially if units are not decontaminated
between cycles.

1. Cell-associated virus, it replicates only inside the living cell.
2. Shed in dead skin cells and feather follicle.
3. Can remain infective for:
– 4 - 8 months at 22 ºC
– 10 years at 4 ºC
4. Has a varying pathotypes
Virus Characteristics

› Marek’s disease virus has 3 serotypes;
1. Serotype 1: field strains
2. Serotype 2: apathogenic
3. Serotype 3: herpesvirus of turkey HVT
Virus Serotypes

1. The virus enter through inhalation.
2. The virus is phagocytosed by the phagocytic cells in lungs.
3. Then, it is transported through blood and lymph (18hrs post infection) to lymphoid
tissues (spleen, 72hrs post infection)
4. The virus had an early cytolytic infection on B lymphocyte cells
5. At the same time, the virus activatesT lymphocyte cells to multiply to form mass.
6. Latent infection, the virus is stable insideT lymphocyte (7 days post infection), cell
mediated
7. Then, the virus is transport to feather follicles (virus insideT lymphocyte)
8. Productive infection, the virus multiply freely outsideT lymphocyte
Virus Life Cycle

Infection Early cytolytic
phase
Latent phase
Fully productive
infection FFE
Environment
B
B
T
T
T
Lung phagocytic
cell
B cell Cell death
Activated T cell
Cell death
Latently
infected T cell
MD tumours
cells
X
X

Viral Plaque
› Definition: viral plaque is a visible structure formed within a cell culture, as a result of
cell destruction.

What is Viral
Plaque?
A phage infects a single cell in a lawn of
sensitive, growing bacteria
A phage reproduces un the cell, typically
yielding about 50 progeny phage per infected
cell
Lysis of the cell releases phage into the
medium. The phage diffuses through the
medium and infects adjacent cells.

Phage reproduces in these cell, releasing 50
additional progeny phage per infected cell
These cell lyse, releasing more phage which
can then diffuse outward and infect
surrounding cells. Lysis of the cell results in a
circular clearing in the lawn of bacteria.
This region of visible lysis is called a plaque

Cont. …
› Counting the number of plaques can be used as a method of virus quantification.
› These plaques can be;
– Detected visually using colony counters, they are not always visible to the naked eye.
– Seen through a microscope.
– Seen using techniques such as staining or immunofluorescence.
– Counted by special computer systems.

Cont. …
Plaque appearance
› The appearance of the plaque depends on the virus strain and the conditions.
– Highly virulent or lytic virus strains give clear plaques.
– Less virulent strains that only kill a fraction of their hosts (due to partial resistance/lysogeny) or only
reduce the rate of cell growth give turbid plaques.
– Some partially lysogenic phages give bull's-eye plaques with spots or rings of growth in the middle of
clear regions of complete lysis.

Plaque Forming Unit
› Plaque FormingUnit is a measure of the quantity of individual infectious virus particles
based on the number of plaque formed per unit volume.
› Theoretically, the plaque forming unit includes only the infectious virus particles, since
a virus particle failing to infect a host cell will not be able to produce a plaque and will
not be counted.

Virus Plaque Size
› Increased virus plaque size indicates:
1. More virus adaptation to tissue culture cells.
2. More passages on the tissue culture.
3. More attenuation and less immnunogenicity.

Plaque size of SB-1 at various passage
levels in CEFs
0
0.1
0.2
0.3
0.4
0.5
0.6
SB-1 (11) SB-1 (23) SB-1 (64) Control (HVT)
0.03
0.095
0.32
0.59
Area
(square
mm
)
(Passage level)
Witter, Avian Diseases 34: 944-957

Protection of SB-1 at various passage
levels in CEFs
0
10
20
30
40
50
60
70
80
90
100
JM/102W MD5
92
57
69
0
38
0
SB-1 (23)
SB-1 (64)
SB-1 (98)
Witter, Avian Diseases 31: 752-765

Vaccine 2 SB-1
Vaccine 1 SB-1
Plaque Size Comparisons

IMMUNIZING DOSE
PLAQUESIZE
ADVANTAGETO
VACCINE PRODUCER
CONVENIENTTO
COUNT
MORECELL-FREE
VIRUS
General changes associated with virus passage as they relate to Marek's vaccines

ADVANTAGETO
CHICKEN PRODUCER
REPLICATION RATE
INCHICK
IMMUNOGENICITY
General changes associated with virus passage as they relate to Marek's vaccines

Summary of large plaque vs. small
plaque serotype 2 vaccines
› As the passage level increases, plaque size also increases.
› Larger plaque sizes indicate a higher level of attenuation.
› Increased adaptation toCEFs correlates to a slower onset of immunity and a lower
protective index.
› High passage vaccines may “get by” during low challenge.
› Low passage vaccines are best during high and low challenge.

Economic Importance
› Layer Flocks
– Increased mortality rates
– Drop in egg production.
› Broiler Flocks
– Increased mortality and condemnation rates at
processing due to neural and visceral tumors.

› Impairment of theT-lymphocytes prevents competent immunological response against
pathogenic challenges.
› Affected birds become more susceptible to disease conditions such as coccidiosis and
"Escherichia coli" infection.
› Furthermore, without stimulation by cell-mediated immunity, the humoral immunity
conferred by the B-cell lines from the Bursa of Fabricius also shuts down, thus resulting
in birds that are totally immune-compromised.
Marek’s & Immunosuppression

› Involvement of the peripheral nerves results in paresis
(weakness) of the legs or wings which progresses to
paralysis.
› Death occurs in both caged and floor-housed birds as a
result of dehydration and persecution.
Clinical signs

Clinical Signs
› There are 5 syndromes known to occur after infection
with Marek's disease and these syndromes may
overlap:

Cont. …
1. Classical Marek's disease or neurolymphomatosis
2. A symmetric paralysis of one or more limbs.
3. Difficulty breathing or dilation of the crop may occur due to vagus nerve involvement.
4. Frequently lymphomatous infiltration (tumors) in the skin, skeletal muscle, and visceral
organs.
5. Organs that are commonly affected include the ovary, spleen, liver, kidneys, lungs,
heart, proventriculus and adrenals.

Acute Marek's disease
1. It is epidemic in a previously uninfected or unvaccinated flock
a) Depression
b) Paralysis
c) Death in a large number of birds (up to 80 %).
2. The age of onset is much earlier than the classic form; birds are 4-8
weeks old when affected.
3. Infiltration into multiple organs/tissue is observed.

4. Ocular lymphomatosis
› Causes lymphocyte infiltration of the iris
(making the iris turn grey), anisocoria, and
blindness.
Cont. …

5. Cutaneous Marek's disease
– Causes round, firm lesions at the feather follicles.
Cont. …

6. Cutaneous Marek's disease
– Causes round, firm lesions at the feather follicles.
7. Atherosclerosis
– Is induced in experimentally infected chickens.
Cont. …

Kidney Tumors
Sciatic Nerve Tumor

Post Mortem
› Enlargement of the feather follicles is observed on the skin of de-feathered broilers.
› The characteristic MD lesion comprises enlargement of the peripheral nerves of the
sciatic or brachial plexus.
› Occasionally visceral lesions are observed and the kidney, eye, proventriculus, ovary or
other organs may be affected.

Tumors in Proventriculus & Spleen

Diagnosis and Confirmation
› The gross appearance of neural lesions is generally diagnostic.
› Histological examination of nerve and visceral lesions will show characteristic
lymphocytic proliferation.
› The causal virus may be isolated and identified by submitting tissues to a suitably
equipped laboratory using specific tissue culture techniques.

› Vaccination is the only known method to prevent the
development of tumors when chickens are infected
with the virus.
› However, administration of vaccines does not prevent
transmission of the virus, i.e., the vaccine is non-
sterilizing.
› However, it reduces the amount of virus shed in the
dander and hence reduce horizontal spread of the
disease.
Control

1. Stimulates full immune response by 7 - 14 days of age.
– Maternal antibodies declines rapidly and depleted at this age.
2. Establishes life time immunity.
– No secondary vaccination is necessary.
3. Reduces or prevents:
– Virus shedding
– Mortality.
– Tumor development.
– Clinical signs.
– Immune depression.
Ideal Marek’s Vaccine

4. Highly protective vaccines
– Replicates better in vivo
– Early stimulationT-cell lineage
› Higher response of totalT cells andCTL’s
› Early NK activity
Ideal Marek’s Vaccine

1. Decreases field challenge viremia
– Greatly reduces early cytolytic infection (B-cell destruction)
– Minimizes inflammatory reaction (activation ofT-cells)
2. Fewer activatedT-cells, so immune system can better control:
– Second cytolytic phase (causes permanent immune suppression)
– Transformation phase (causes tumor production and mortality)
3. Prevents shedding of field virus.
Effect of vaccine on field challenge

1. Type 1: attenuated chicken strain (e.g. Rispen’s orCVI)
– Cell associated, frozen
2. Type 2: apathogenic chicken strain (e.g.SB1)
3. Type 3: apathogenic turkey strain (e.g. HVT)
– Cell free, freeze dried
4. Bivalent vaccines (CVI/HVT – SB1/HVT)
5. Recombinant vaccines
Types of vaccines

› This includes all the pathogenic strains of the virus, ranging from strains that are:
1. Very virulent plus (e.g. 648A)
2. Very virulent (e.g. Md/5, Md/11,Ala-8, RB-1B)
3. Virulent (e.g. HPRS-16, JMGA)
4. Mildly virulent (e.g. HPRS-B14,ConnA)
5. Weakly virulent (e.g.CU-2,CVI-988).
Serotype 1

› These strains may be attenuated by passage in tissue culture, with loss of pathogenicity
but retention of immunogenicity, to provide strains that have been used as vaccines.
› The number of passages necessary to attenuate serotype 1 MDV strains depends on:
1. The initial virulence of the strains.
2. The days between passages.
3. The type of cell culture.
Cont. …

› HPRS-16
– The first commercially effective vaccine, it required 33 passages in chicken kidney cell cultures.
› CVI988
– It has a low oncogenic potential upon initial isolation, it was attenuated by 26-35 serial passages in
duck embryo fibroblast culture.
– Serotype 1 vaccines are prepared in a cell-associated form that must be stored in liquid nitrogen.
Cont. …

› They are apathogenic strains isolated from clinically normal chickens.
› This includes naturally avirulent strains of MDV:
1. SB-1
2. HPRS-24
3. 301B/1
4. HN-1
› Several serotypes of these have been shown to provide protection against virulent
strains.
Serotype 2

› The level of protection alone is low, but when combined with serotype 3 they exhibit
protective synergism as seen in bivalent vaccines.
› TheSB-1 and 301B/1 strains have been developed commercially and used, particularly
with HVT, in bivalent vaccines for protection against the very virulent strains.
› Serotype 2 vaccines exist only in the cell-associated form.
Cont. …

› This contains the strains of naturally avirulent non-oncogenic viruses of turkeys (HVT):
1. FC126
2. PB1
› HVT has been the most widely used poultry vaccine in the world.
› But, due to the increased virulence of MDV, the use of HVT alone is now restricted to
the vaccination of broilers.
Serotype 3

› It is still used as component of polyvalent vaccines for the vaccination of breeders and
layers in combination with serotype 1 and 2 strains in bivalent or trivalent vaccines
against the very virulent strains of MDV.
› A unique feature of HVT is that it can be obtained as cell-free virus (sonication of
infected cell cultures, lyophilized HVT vaccines).
› The efficacy of HVT cell-free vaccines is not as high as cell-associated vaccines due to
interaction with maternal antibodies.
Serotype 3

› Synergism
› Combinations of theSB-1 strains (serotype 2) and the FC126 strain (serotype 3, HVT)
provide better protection than either vaccine alone.
› This phenomenon, termed protective synergism, is the basis for the bivalent vaccines
developed in the 1980s.
› Synergism is a serotype specific and is especially evident between combinations of
serotypes 2 and 3.
Bivalent Vaccine

› No protective synergism has been reported between combinations of serotype 1 with
serotypes 2 or 3.
› However, augmentation of protection for MD vaccines has been observed with two
additional systems.
1. Combinations of HVT and glutaraldehyde-inactivated serotype 1 MDV obtained from feather
follicles.
2. Combinations of HVT and a recombinant fowl pox virus expressing gB and other glycoproteins of
serotype 1
– The mechanisms involved are poorly understood and represent an overlooked but important area for
future research.
Cont. …

› Several attempts have been made to develop MD vaccines based on recombinant
techniques.
› Strategies for recombinant vaccines:
1. Live virus vectors
– based on HVT and serotype 1 MDV which express inserted genes obtained from other MDV
serotypes.
2. Gene deletion within serotype 1 MDVs.
– The very virulent strain Md5 lacking the oncogene meq appears to be the most promising candidate
at the moment.
3. Modification of domains within the oncogene meq in the very virulent strain RB1B.
Recombinant Vaccines

› None of the recombinant DNA (rDNA) vaccines have yet been licensed for commercial
use in theUnitedStates, as none of them exceed the efficacy ofCVI988 strain.
› It is very likely that recombinant DNA vaccines will be the basis for control of MDV in the
future, but there is limited knowledge about which viral genes are involved with
immunity or virulence and what combination of genes must be expressed (or deleted) to
produce an effective vaccine.
Cont. …

› MD vaccines are administered to the 1 day old chicks or in ovo at 18 days of
embryonation.
› Early immunity is essential since most chicks face challenge from MDV within a few
days of being introduced into their brooding pens.
› Serotype 3 MDV strains replicate better in ovo than viruses of other serotypes.
› StrainCVI988 has been shown to induce higher protection against challenge during the
first 3 days of life when administered in ovo than when administered at hatch.
In Ovo Vaccination

1. Bivalent vaccines (CVI + HVT)
2. CVI-988 Rispens - low passage
3. CVI-988 Rispens - higher passage
4. Bivalent vaccines (serotypes 2&3)
5. HVT monovalent or Serotype 2 monovalent--similar protection
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