• Marek’s disease may not grab the headlines but it continues to negatively impact
on flock health and economics
• Marek’s disease is a very stubborn virus
• Marek’s disease is capable of causing clinical disease and mortality
• Marek’s disease can also damage the immune system leading to poor
performance, poor response to other diseases and their vaccinations
• The highly resistant virus is stable for 65 weeks in the un-cleaned environment
surviving in poultry feathers, dust, and dander
• Vaccination programmes are challenging to implement
• Vaccination alone will NOT control the effects of this virus
• Keeping the viral challenge low for the new crop is key
• Highly effective, four stage Terminal cleaning and disinfection programmes using
a proven virucidal disinfectant such as DuPont Virkon® S is recommended.
It is easy to get distracted by the headline grabbing poultry health issues of Avian
Influenza, Newcastle disease and Salmonella. Whilst these may well be the most
pressing disease problems for our industry, the danger is that concentrating all time and
resources on them tends to ignore those diseases that may be having a chronic and
significant impact on performance and profitability, day in and day out.
With outbreaks of Marek’s disease occurring in 2007 this article provides a timely
reminder to producers that Marek’s disease is a hidden menace. It is caused by a very
resistant and hence very persistent herpesvirus which causes infection in chickens and,
to a lesser extent, turkeys1.
Control of Marek’s disease infection depends on a combination of effective vaccination
and high quality cleansing and disinfection of poultry houses. Any deficiencies in either
area can tip the balance in favour of the virus rather than the bird.
Marek’s disease was first described over 80 years ago as a condition causing paralysis
of the legs and wings due to inflammation of the nerves. The condition was then known
as fowl paralysis. In the 1920’s and 1930’s, an association with internal tumours in
various organs such as liver, spleen, kidneys and lungs was made. The disease
became a significant limitation to the success of the developing layer and broiler
industries in the 1960’s due to some very high mortality rates of up to 80%!
The appearance of successful live vaccines in the late 1960’s was one of the most
significant factors in enabling the poultry industry to develop to the extent that it has
Disease can manifest itself in a number of ways related to the type of bird, when the
flock became infected, general hygiene and biosecurity on the rearing and laying farms,
the presence of other diseases and the vaccines used or the strain of virus involved.
Signs and Symptoms
In laying stock, producers may suspect Marek’s disease if they identify paralysed birds;
often with one leg trailing or wings dropped. In other cases, there may simply be higher
than expected mortality or a number of birds wasting away.
In broilers, losses are mostly due to high condemnation rates at slaughter predominantly
from the cutaneous (skin) form of Marek’s disease. In free range broilers or birds taken
to heavier weights, mortality may result from internal tumours, predominantly of liver and
In turkeys, acute mortality can be associated with internal tumours of liver and spleen
often where turkeys have been reared in the same accommodation as infected chickens.
Even where there is no acute mortality, the affects on internal organs can lead to
significant damage to the immune system. This can be associated with suboptimal
performance and clinical outbreaks of other diseases such as coccidiosis, heavy worm
infestations or viral diseases such as Gumboro disease. The impaired immune system
is also unable to respond to routine vaccinations and hence will leave flocks unable to
resist a whole range of disease challenges which affect performance.
All this means that if the background challenge from Marek’s disease virus is allowed to
build up, performance can suffer even without a specific diagnosis of clinical Marek’s
disease being made.
This vicious circle continues and “silent infection” build-up can put pressure on any
Marek’s disease virus vaccination programme. It is thought that this phenomenon has
led to the regular emergence of more and more virulent pathotypes of Marek’s disease
virus since vaccines first appeared in the 1970’s. Researchers then had to work hard to
develop increasingly effective vaccines to keep ahead of the virus and prevent clinical
Vaccination is far from simple. Due to the persistence of the virus in the environment,
vaccination is a race against time to get the bird to produce a protective response before
it meets field infection. These requirements and the nature of the virus mean that
protection can only be achieved by physically injecting chicks with live vaccine. This is
done either at one day old by intramuscular injection into the leg or neck, or increasingly
into the unhatched embryo by injecting hatching eggs at transfer.
The successful response to vaccination depends on choosing the correct vaccine strain,
correct preparation of this delicate vaccine and its accurate administration to each and
every chick. Even then, it is possible to overcome the protection offered by the vaccine
by exposing vaccinated birds to field virus too early or in such large amounts that the
vaccine response is overwhelmed.
To gain the benefit of all this hard work it is important to reduce the exposure of young
birds to virulent field virus. Newly hatched chicks should be placed in thoroughly cleaned
and disinfected houses that are well separated from houses with older chickens2. This
can be achieved by using a structured biosecurity programme concentrating on terminal
cleansing and disinfection using products of proven virucidal activity.
In infected birds, the virus is excreted in feather dander and dust and therefore
environmental contamination of all surfaces and litter can be very high from infected
flocks. Furthermore, since the virus survives at ambient temperature for a long time (65
weeks) when cell associated2 and is resistant to some disinfectants (quaternary
ammonium and phenol) the selection of an appropriately virucidal disinfectant is
It is therefore essential that first stage cleaning is capable of removing all organic
material from surfaces using good physical cleaning and degreasing agents to lift
material from perches, nests and other wooden surfaces.
Terminal Biosecurity Programme
An effective Marek’s disease terminal biosecurity programme should comprise the
following 4 specific stages, the objective of which is simple – to prevent the “carry-over”
of pathogenic organisms, thus ensuring that each new crop gets a completely clean
Stage 1: Removal of Equipment and Dry Cleaning
• Remove any residual food from the feeder system and silo.
• Remove all movable equipment.
• Blow or wash down all surfaces to remove dust from ceilings, ledges, water
pipes, fan boxes and inlets.
• Remove all litter from the house using scrapers and blow or brush loose debris
from the ceilings and floor.
• Load litter onto lorry ensuring that all outside areas such as concrete pads by
doors and silos are cleared of old litter, dust etc.
• Make sure that the load is covered before transporting from site.
• Blow or wash down bulk feed bins before disinfecting.
Stage 2: Water System Clean and Disinfect
• Isolate header tank at mains inlet point, check that it is free from debris and then
drain entire system from the drinker points farthest away. Clean as required.
• Fill the tank with the necessary volume of water to fill the entire drinking water
system adding the appropriate amount of disinfectant concentrate to achieve the
• Allow disinfectant solution to fill the system to the drinker points farthest away
from the header tank. Leave for one hour.
• Thoroughly flush lines through with clean water. Repeat process if scale and
• Refill the header tank with clean fresh water.
Stage 3: Cleaning Buildings and Equipment
Thorough washing of all surfaces and equipment is essential to soak and dislodge as
much organic material as possible and achieve the best results from the subsequent
Using a pressure washer apply an appropriate detergent solution (such as DuPont
Biosolve® Plus) to all surfaces. Ensure inclusion of air inlets, fan boxes, partitions,
feeders, drinkers and other equipment, including any removed from the house. Include
any service/utility rooms in this cleaning procedure before rinsing all surfaces and
equipment with clean water at high pressure.
• Externally, spray air inlets and built-up deposits from around fan boxes and
loading areas with detergent solution. Ensure that all dirty areas such as
concrete aprons around the houses and bulk bin pads are washed clean.
• Internally, soak all surfaces thoroughly with detergent solution applied at low
• Leave for 20-30 minutes, and then rinse at high pressure using clean water.
Ensure all surfaces and equipment are visibly clean before moving on to the
Stage 4: Disinfection
The level of disease-causing organisms present after the cleaning stage is high enough
to present a serious disease challenge to a new crop of birds. Broad spectrum
disinfectants such as DuPont Virkon® S are recommended. Tested at the Institute for
Animal Health, Compton, UK on Marek's disease virus strain HPRS16, using the DEFRA
testing method at 4°C in a 5% yeast solution organic challenge, Virkon® S passed at all
dilutions tested including the lowest test dilution of 1:200.
• Use either a knapsack sprayer or pressure washer on a low pressure setting
(300psi) to disinfect all equipment removed from the building.
• Place the equipment back in the cleaned building prior to disinfecting the house
• Using a pressure washer on a low pressure setting (300psi) or an orchard
sprayer for larger premises apply the disinfectant solution evenly to all washed
internal surfaces to achieve thorough wetting.
• Pay particular attention to corners, cracks and seams ensuring that all sides of
supporting posts are covered.
• Spray into the apex of the roof and work down the walls to the floors.
• Working from one end of the building to the other, apply disinfectant solution to
the floor of the building.
• Upon completion of the disinfectant process, close all doors and place
disinfectant footdips at all entrances.
• To control organisms introduced to the house during the setting up procedure
and to disinfect the air and other inaccessible areas of the building use either a
fine mist sprayer or thermal fogging machine to apply a solution of Virkon® S
Marek’s is a virus disease that can cause significant economic damage to poultry
production through a variety of direct and indirect effects. Vaccination alone will NOT
control all these effects. This stubborn virus can survive in the environment for 65
weeks. Feather follicles in the skin are the site of viral replication resulting in feather,
dander, dust and litter containing the virus which is then spread by the airborne route.
Good hygiene practices which prevent “carry-over” of Marek’s organisms, thus ensuring
that each new crop gets a completely clean fresh start in combination with effective
vaccination are key to controlling the disease.
1. The Economics of Marek’s Disease, University of Reading publication
2. Control Strategies for Marek’s Disease: A Perspective for the Future. Author: R.
L. WITTER 1998, Poultry Science 77: 1197-1203. USDA-Agricultural Research
Service, Avian Disease and Oncology Laboratory, East Lansing, Michigan 48823
3. Immunosuppressive effects of Marek’s disease virus (MDV) and herpesvirus of
turkeys (HVT) in broiler chickens and the protective effect of HVT vaccination
against MDV challenge. Author: A. F. M. F. Islam et al 2002, Avian Pathology 31, 449-461
Stephen Lister BSc BVetMed CertPMP MRCVS
Crowshall Veterinary Services
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