Marek's diseaseFrom Wikipedia, the free encyclopedia

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Virus classification

Group: Group I (dsDNA)

Family: Herpesviridae

Genus: Mardivirus

Species: Gallid herpesvirus 2 (GaHV-2)

Marek's disease is a highly contagious viral neoplastic disease in chickens. Occasionally misdiagnosed as an abtissue pathology it is caused by an alphaherpesvirus known as Marek's disease virus (MDV) or gallid herpesvirus 2 (GaHV-2). The disease is characterized by presence of T cell lymphoma as well as infiltration of nerves and organs by lymphocytes.[1] Viruses related to MDV appear to be benign and can be used as vaccine strains to prevent Marek's disease. For example, the related Herpesvirus of Turkeys (HVT), causes no apparent disease in turkeys and continues to be used as a vaccine strain for prevention of Marek's disease (see below). Birds infected with GaHV-2 can be carriers and shedders of the virus for life. Newborn chicks are protected by maternal antibodies for a few weeks. After infection, microscopic lesions are present after one to two weeks, and gross lesions are present after three to four weeks. The virus is spread in dander from feather follicles and transmitted by inhalation.[2]

Contents

[hide] 1 Syndromes 2 Diagnosis 3 Prevention 4 Eponym

5 References

[edit] Syndromes

Left - normal chicken eye. Right - Eye of a chicken with Marek's disease

There are five syndromes known to occur after infection with Marek's disease. These syndromes may overlap.

Classical Marek's disease or neurolymphomatosis causes asymmetric paralysis of one or more limbs. With vagus nerve involvement, difficulty breathing or dilation of the crop may occur. Besides lesions in the peripheral nerves, there are frequently lymphomatous infiltration/tumours in the skin, skeletal muscle, visceral organs. Organs that are commonly affected include the ovary, spleen, liver, kidneys, lungs, heart, proventriculus and adrenals.

Acute Marek's disease is an epidemic in a previously uninfected or unvaccinated flock, causing depression, paralysis, and death in a large number of birds (up to 80 percent). The age of onset is much earlier than the classic form, birds are four to eight weeks old when affected. Infiltration into multiple organs/tissue is observed.

Ocular lymphomatosis causes lymphocyte infiltration of the iris (making the iris turn grey), anisocoria, and blindness.

Cutaneous Marek's disease causes round, firm lesions at the feather follicles.[2] Atherosclerosis is induced in experimentally infected chickens. [3] Immunosuppression Imparement of the T-lymphocytes prevent competent

immunological response against pathogenic challenge and the affected birds become more succeptible to disease conditions such as coccidiosis and "Escherichia coli" infection [4]. 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 immunocompromised.

[edit] Diagnosis

The demonstration of nerve enlargement, especially with the ischiatic nerve along with suggestive clinical signs in a bird that is around three to four months old is highly suggestive of Marek's Disease. The presence of nodules on the internal organs may also suggest Marek's disease but further testing is required for confirmation. This is done through histological demonstration of lymphomatous infiltration into the affected tissue. A range of leukocytes can be involved, including lyphocytic cell lines such as large lymphocyte, lymphoblast, primitive reticular cells and occasional plasma cells as well as macrophage and plasma cells. The T-cells are involved in the malignancy, showing neoplastic changes with evidence of mitosis.

The lymphomatous infiltrates need to be differentiated with another condition that affects poultry known as Lymphoid Leukosis as well as an inflammatory event associated with hyperplastic changes of the affected tissue.

[edit] Prevention

Vaccination is the only known method to prevent the development of tumors when chickens are infected with the virus. However, administration of vaccine does not prevent transmission of the virus; i.e., the vaccine is non-sterilizing.[1] However, it does reduce the amount of virus shed in the dander and hence reduce horizontal spread of the disease. Marek's Disease does not spread vertically. The vaccine was introduced in 1970. Before that, Marek's disease caused substantial revenue loss in the poultry industries of the United States and the United Kingdom. The vaccine can be administered to one day old chicks through sub-cutaneous inoculation or by in-ovo vaccination when the eggs are transferred from the incubator to the hatcher. In-ovo vaccination is the preferred method, as in does not require handling of the chicks and can be done rapidly by automated methods. Immunity develops within two weeks.[2]

The vaccine originally contained the antigenically similar turkey herpesvirus, which is serotype 3 of MDV.[5] However, because vaccination does not prevent infection with the virus, [6] the Marek's Disease virus has evolved increased virulence and resistance to this vaccine. As a result, current vaccines use a combination of vaccines consisting of HVT and gallid herpesvirus type 3 or an attenuated MDV strain, CVI988-Rispens. [7]

[edit] Eponym

The disease is named after Dr. Jozef Marek.

Marek's Disease

Clinical SignsDiagnosisSimilar DiseaseMethods of SpreadTreatmentPreventionFurther Information

Marek’s disease (MD or fowl paralysis) is a very common disease of chickens caused by a herpes virus. Marek’s disease affects both commercial and backyard poultry and may result in death or severe production loss. The disease causes changes in many of the nerves and may cause tumours in major internal organs.

Chickens are the main species affected. The disease occurs rarely in some other types of birds.

Clinical SignsYoung birds are most susceptible to infection. Most deaths from Marek’s disease occur between 8 and 20 weeks of age, although in some cases the disease may be seen in birds as young as 3-4 weeks of age or as old as one year of age.

Typically, Marek’s disease occurs as the nervous form, appearing as a progressive paralysis of one or more of the limbs or, less often, the neck or wings. The sciatic nerve (the main nerve to the leg) is commonly affected The birds are unable to stand, become paralysed, appear uncoordinated and slowly waste away from lack of food and water. In most cases the paralysis comes on quickly. In some cases the eyes may be affected, resulting in blindness.

In the visceral form, Marek’s disease occurs as tumours in internal organs, including the ovaries, liver, spleen, kidney and heart. Sometimes the liver and spleen are swollen without distinct tumours being present. Birds may show signs of depression, paralysis, loss of appetite, loss of weight, anaemia (pale combs), dehydration (shrunken combs), and sometimes diarrhoea. Some birds die without any clinical signs being noticed.

Most birds that develop Marek’s disease usually die.

DiagnosisVeterinary examination is necessary to diagnose Marek’s disease. The clinical signs, combined with post-mortem findings, will confirm the diagnosis in most cases, and, most importantly, rule-out other diseases. Enlargement of nerves such as the sciatic nerve are commonly seen at post-mortem. Changes in one or more internal organs may also be observed.

Similar DiseaseA different viral disease known as lymphoid leucosis also causes tumours in organs, but does not cause paralysis. It is usually seen in birds over 16 weeks of age, whereas Marek’s disease is commonly seen in younger chickens.

Methods of SpreadMarek’s disease virus occurs commonly wherever chickens are raised and most chickens probably become infected. The virus is highly infectious and once introduced into a flock, it spreads rapidly to unvaccinated birds.

Infected chickens carry the virus for life whether they develop the disease or not, and continue to shed the virus for long periods. The virus is shed from the feather follicles and spreads readily in fluff and dust, gaining entry when the bird breathes infected dust particles. This material can also be carried by people and equipment.

The virus can survive in the environment for as long as several months at room temperature. It is not spread from the hen to the chicken through the egg.

TreatmentThere is no treatment for Marek’s disease.

Diseased birds should be promptly removed from the flock and humanely destroyed. Other birds in the flock are likely to be infected at this stage also, so close monitoring of all birds is important.

Prevention

Although vaccines are commonly used in the commercial poultry industry, small numbers of doses cannot be purchased for use in backyard flocks.

For backyard flocks, the best protection against Marek’s disease is obtained by buying, from a commercial source, birds that have been correctly vaccinated.

Vaccination alone will not prevent Marek's disease. Particularly for commercial flocks, it is important to have good biosecurity to ensure that vaccinated chicks will develop immunity before they are subjected to a severe challenge of virus. For example, chicks need to be reared separately so that they are free from the infected fluff and dust of older birds. Standard hygiene measures are also important, including a thorough clean-out and disinfection of sheds and equipment between batches of chicks with a disinfectant effective against viruses. Good nutrition and maintenance of freedom from other diseases and parasites are also very important. These practices will help maintain the flock’s health and to ensure that the birds have optimum resistance against Marek’s disease infection.

For general advice on biosecurity, see the biosecurity checklist for bird keepers.

The breeding of genetically resistant strains of chickens, combined with the use of vaccination and good hygiene, can also be used to help control Marek’s disease.

For flocks with a serious Marek’s disease problem, the only solution is to depopulate, clean and disinfect all sheds and equipment, and spell for several months. Vaccinated chicks from a reputable hatchery would then need to be sought as replacements

Marek's disease

Introduction

Marek's disease is a Herpes virus infection of chickens, and rarely turkeys in close association with chickens, seen worldwide. From the 1980s and 1990s highly virulent strains have become a problem in North America and Europe.

The disease has various manifestations: a) Neurological - Acute infiltration of the CNS and nerves resulting in 'floppy broiler syndrome' and transient paralysis, as well as more long-standing paralysis of legs or wings and eye lesions; b) Visceral - Tumours in heart, ovary, tests, muscles, lungs; c) Cutaneous - Tumours of feather follicles.

Morbidity is 10-50% and mortality up to 100%. Mortality in an affected flock typically continues at a moderate or high rate for quite a few weeks. In 'late' Marek's the mortality can extend to 40 weeks of age. Affected birds are more susceptible to other diseases, both parasitic and bacterial.

The route of infection is usually respiratory and the disease is highly contagious being spread by infective feather-follicle dander, fomites, etc. Infected birds remain viraemic for life. Vertical transmission is not considered to be important.

The virus survives at ambient temperature for a long time (65 weeks) when cell associated and is resistant to some disinfectants (quaternary ammonium and phenol). It is inactivated rapidly when frozen and thawed.

Signs Paralysis of legs, wings and neck. Loss of weight. Grey iris or irregular pupil. Vision impairment. Skin around feather follicles raised and roughened.

Post-mortem lesions Grey-white foci of neoplastic tissue in liver, spleen, kidney, lung, gonads,

heart, and skeletal muscle. Thickening of nerve trunks and loss of striation. Microscopically - lymphoid infiltration is polymorphic.

Diagnosis

History, clinical signs, distribution of lesions, age affected, histopathology. Differentiate from Lymphoid leukosis, botulism, deficiency of thiamine, deficiency of Ca/Phosphorus/Vitamin D, especially at the start of lay.

Treatment

None.

Prevention

Hygiene, all-in/all-out production, resistant strains, vaccination generally with 1500 PFU of HVT at day old (but increasingly by in-ovo application at transfer), association with other strains (SB1 Sero-type 2) and Rispen's.

It is common practice to use combinations of the different vaccine types in an effort to broaden the protection achieved. Genetics can help by increasing the frequency of the B21 gene that confers increased resistance to Marek's disease challenge.

Marek's Disease in Chickens

In an economic loss, Marek's disease is a most important malady of chickens. It often causes severe death loss in pullet flocks and has been a major cause of condemnations at the broiler processing plant. Generally, chickens under 16 weeks of age are most often affected.

Cause

Caused by a herpesvirus, the disease is often characterized by abnormal cell growth in the peripheral nerves and central nervous system. Hence, the common name for one form of Marek's: fowl paralysis. In addition to the nerves, however, the disease also may cause lesions on visceral organs and other tissues, including feather follicles of the skin. The most prominent lesions may be tumors on the liver, kidneys, testes, ova, spleen and lungs. In such cases, nerve swelling may not be involved.

How Marek's Disease is Spread

Chicken "dander" from feather follicles spreads the disease. The virus also is excreted in the saliva, and the virus probably enters the body through the respiratory system. Transmission via the egg is not significant.

Signs

Some chickens die without any clinical signs of Marek's disease. Most of the affected birds will have some degree of paralysis, although chickens with the acute form may not show this condition. Those with paralysis may die because they are unable to reach feed and water. The first indication of infection is a variation in the growth rate and degree of feathering.

Lesions

Swelling of the peripheral nerves, particularly of the nerves of the leg and wing, is often noticeable. The visceral organs may contain tumors ranging from microscopic size to fairly large. Such tumor lesions may be confused with those of lymphoid leukosis without a qualified laboratory diagnosis.

Prevention

Tumor formation from Marek's disease can be prevented through vaccination. Salsbury MD-Vac, a vaccine of chick-embryo tissue culture, is recommended. Vaccination at one day of age usually protects birds through their lifetime. There is no treatment for Marek's disease

Marek's disease

Description Marek´s disease (MD) is caused by a herpes virus that may result in death or severe production loss in both layer and meat chickens. Vaccination will reduce the losses. However, in recent years there has been an increase in Marek´s disease, due to new strains of virus and faster growing, more susceptible birds.

Marek´s disease is a problem in many household flocks, especially in those which have not been vaccinated. It causes changes in many of the nerves and may cause tumours in the major internal organs (ovary, liver, kidney, heart and spleen).

Marek´s disease occurs in two main forms, depending on which parts of the body are affected by the tumours:

Nervous form

In this form the nerves, particulary the sciatic nerves (the main nerves to the legs), are affected. The birds are unable to stand, become paralysed and slowly waste away from lack of food and water. In most cases the paralysis comes on quickly. Sometimes the wings or neck are involved. In some cases the iris is involved and this can lead to blindness.

When the birds are autopsied the affected nerves can sometimes be seen to be swollen. However, in many cases changes can only be seen under the microscope.

Visceral form

In this form, greyish-white tumours are found in the ovaries, liver, spleen, kidney, heart and other organs. Sometimes the liver and spleen are swollen without

distinct tumours being present.

Birds may show signs of depression, paralysis, loss of appetite, loss of weight, anaemia (pale combs), dehydration (shrunken combs), and sometimes diarrhoea. Some birds die without any clinical signs being noticed.

Age of BirdsYoung birds are the most susceptible to infection Most deaths from Marek´s disease occur between 10 and 24 weeks of age, although in some cases the disease may not appear until later in life. Most birds that develop MD will die.

Similar Disease Lymphoid leucosis also causes tumours in organs, but

does not cause paralysis. It is usually seen in birds over 16 weeks of age, whereas MD is commonly seen under that age.

Methods of spread

The virus can remain alive in the environment for as long as eight months. It is shed from the feather follicles and spreads in fluff and dust, gaining entry when the bird breathes infected dust particles. It is not spread from the hen to the chicken through the egg.

The virus is highly infectious and, once it is present in a flock, it spreads rapidly to unvaccinated poultry. Healthy birds can be carriers and infect others.

TreatmentTreatment is not effective. Diseased birds should be removed from the flock and humanely destroyed. Other birds in the flock are likely to be infected at this stage - it is too late to protect them.

Prevention Vaccination

The introduction of more potent vaccines is giving better control. These are made either from Marek´s disease virus or from Herpes Virus of Turkeys (HVT), a closely related virus.  However, these new, cell-associated vaccines are only available in packs of 1000 doses and must be stored in liquid nitrogen, so

special equipment is needed.  This makes it costly to vaccinate small flocks.

A freeze-dried, cell-free, HVT vaccine is available which is stored under ordinary refrigeration and so is less costly.  This is also available in packs of 1000.

For backyard operations, protection is obtained by buying, from a commercial source, birds which have been correctly vaccinated either at day old or into 18-19 day old embryonated eggs (before hatching).

Breeders who wish to vaccinate their own day old chicks should follow the manufacturer´s directions for the storage and administration of the vaccine. The effectiveness of the vaccine is dependent upon its correct storage and administration.

Management

Vaccination alone will not prevent Marek's disease.  It is important to be attentive to the following management and hygiene procedures.

It is essential that vaccinated chicks be isolated during their first two weeks of life so that their immunity will develop before they are subjected to a severe challenge of virus. Chicks reared separately are free from the infected fluff and dust of older birds. This material can be carried by wind and by people and equipment.

Hygiene is important. There should be a thorough clean-out and disinfection of sheds and equipment between batches of chicks. To control MD, a disinfectant should be used which is effective against viruses, and not just against bacteria.  Care should be taken to protect oneself from spray by the wearing of suitable protective clothing and breathing apparatus.

Exposure of the sheds and runs to sunlight helps the disinfection process.

Good nutrition and freedom from other diseases and

parasites are necessary to maintain the flock´s health and to ensure that the birds have optimum resistance against MD infection.

Breeding

Breeding programs to select birds for resistance to Marek´s disease are important to give better control in the future.

In summary, Marek´s disease control requires attention to correct vaccination, isolation of young chicks, hygiene, good health and genetically resistant strains of birds.

For flocks with a serious MD problem, the only solution is to depopulate, clean and disinfect all sheds and equipment and spell for several months. Then replace with vaccinated chicks from a reputable hatchery.

The Condition

Marek’s disease is caused by a herpes virus and is one of the most widespread diseases afflicting chickens. The disease was first recognised by the Hungarian veterinarian Jozsef Marek in 1907 (Sluis, 1997), and was at one time the most common cause of losses in the poultry industry. It is now largely controlled by the use of vaccines.

Over the last ten years, the UK Veterinary Investigation Centres have diagnosed around 60 cases of Marek’s disease each year. Although there are no recent published data on incidence in the UK, losses from Marek’s disease in broilers in the UK are thought to be very low. It has been estimated that national mortality rates in layer flocks in 1977 were approximately 1.23% (Bennett et al, 1999).

Since the virus is not transmitted through the egg, chicks are born free of the disease. The infectious virus matures in the epithelium of feather follicles and infects other birds by inhalation of infected dust or dander.

Most flocks are infected, although clinical disease is not always seen (Biggs, 1997). It is a highly contagious disease that may survive for months or years in litter and poultry dust. Infection occurs through the respiratory tract and infected birds can remain carriers long after infection. Incubation periods range from 3 weeks to months. Chicks become infected at an early age, whilst the disease normally manifests itself at 8-24 weeks, although it may be observed, in some cases, much earlier or later.

There are three serotypes of the virus. Serotype 1 and 2 are found in chickens, while serotype 3 is related to herpesvirus in turkeys (HVT). Strains of serotype 1 can be divided into mildly virulent, virulent and very virulent. There is little information on the relative frequency of the serotypes, but mixed infections of serotype 1 and 2 are found in the same flock.

There are several forms of the disease. In the acute form the disease occurs rapidly and can result in high mortality rates. In a more classical latent form, Marek’s disease results in more persistent but much lower mortality rates. In the acute form, death frequently occurs within seven days and is often only preceded by a short period of depression. This acute or productive-restrictive infection occurs in lymphocytes, normally of B-cell origin, and results in antigen production leading to cell death. The classical form can lead to progressive paralysis of wings and legs, and in some cases respiratory signs may develop. In the classical form, the latent infection of T-cells is responsible for the long term carrier status of recovered birds. In some cases, latently infected lymphocytes undergo neoplastic development, and may develop into characteristic lymphoid neoplasms.

Classical Marek’s disease is characterised by enlargement of peripheral nerves up to three times the normal size. The most commonly affected nerves are the brachial and sciatic plexi, coeliac plexus, abdominal vagus and the intercostal. Nerve lesions are also found, and occasionally lymphomas (tumours). The acute form of the disease demonstrates lymphoma and enlargement of the liver, lungs, heart, gonads and kidney.

As the virus is widespread in poultry flocks, diagnosis of the disease cannot be made by detection using serological or virological tests. Marek’s disease is diagnosed by clinical signs and by the presence of gross and microscopic lesions. The presence of the virus, without clinical signs, can be demonstrated by isolation of the virus or by agar gel precipitation of the viral antigen in feather tips or serum antibody.

Marek’s condition can be confused with lymphoid leukosis. The two diseases are differentiated by the age at which birds are affected, the presence and focus of lesions, the presence and absence of paralysis and the category of neoplastic lymphoid cell affected

Recently, very virulent strains (vvMDV) (Witter, 1997; Venugopal, 1996) have emerged, which can induce a high incidence not only of lesions in the central nervous system, but also of visceral and nerve lesions in Marek's disease-resistant chickens (Cho et al, 1998). The impacts of more virulent strains are demonstrated and discussed by Hafez (1997).

The disease is immunosuppressive, and the degree of suppression is linked to the virulence of the virus strain (Calnek et al, 1998).

The most important route of infection is considered to be the airborne spread via the respiratory system. The darkling beetle (Alphitobius diaperinus) can carry the virus for several weeks, although this is not considered to be an important source of infection.

Methods of Control and Prevention

Management

Control of the disease is most effective through vaccination or the isolation of growing birds from sources of infection or the use of resistant breeds. The disease is not vertically transmitted and therefore all chicks hatched are virus free. Since the disease is highly infectious and the virus is present in most flocks, good management is required to delay infection and suppress the risk of serious disease. This should involve isolation of young chicks from older birds for the first 2-3 months. An all-in all-out housing policy, coupled with disinfection, is also recommended. Insects may act as virus reservoirs, and therefore creating conditions with low insect populations is desirable.

Vaccination

Vaccines are recommended as a supplementary control measure with good hygiene management. The normal vaccination practice is to use a live vaccine at one day. As all vaccines require 7-14 days to produce an effective immunity, it is critical to reduce exposure during the first week or so post-hatching.

Freeze-dried and ‘wet’ live vaccines are available. Both types require reconstitution with a diluent. The 'wet' vaccine is kept under liquid nitrogen and needs to be used within one hour and therefore is not really suited for average field use. Under severe conditions, the ‘wet’ vaccines are more effective than the freeze-dried (The Veterinary Formulary, 1998).

The more virulent Rispens strains of Marek’s virus are now being incorporated into vaccines (CyromarexTM, PoulvacTM Marek CVI and NobilisTM Rismavac). All are designed for day-old chicks and are administered by subcutaneous or intramuscular injection.

Although chicks are best vaccinated at one day, at the hatchery, chickens up to 3 weeks old can be vaccinated. Despite the advances in vaccine development, there remains the possibility that a virus might evolve that will overcome these control measures. This may result in a greater emphasis being put on breeding resistant poultry strains. The genetic variation in response to vaccines and the possibility of improving this response by selection are described by Bumstead (1998).

New, more virulent forms of the disease may require future vaccination programmes to rely on "designer vaccines" to combat this problem. Additionally, vaccines may fail to protect vaccinated birds if they are exposed to the virus before immunity develops or if exposure to other diseases (eg. infectious bursal disease) depresses their antibody response to the vaccine.

Breeding for resistance

Selection for resistance to Marek’s disease has been successfully achieved over relatively short periods, and resistant genes have been identified (Hutt, 1958; Cole and Hutt, 1973; Cole, 1972; Gavora and Spencer, 1979; Briles et al, 1977). Selection procedures have been based upon exposing birds to the disease based on the incidence of tumours. Cole (1972) demonstrated a selected resistant strain of White Leghorns with a susceptibility of 4% compared to an unselected strain with a 91% susceptibility. This type of selection is expensive and more recently breeding for resistance has been replaced by vaccination.

Gavora and Spencer (1979) indicate that even under vaccination, genetic resistance within a flock can significantly reduce mortality rates from Marek’s. Yonash et al (1998) in studies on resistant and susceptible strains indicate that susceptible strains provide a valuable resource to study the complex nature of Marek’s disease resistance by simplifying a multigenic trait to a series of single gene traits. Heier et al (1999) showed significant differences in mortality between strains of White Leghorn and concluded that Norwegian farmers should be able to reduce the losses from Marek’s disease by replacing the Norwegian strain of White Leghorn with imported strains, such as the Lohmann White and Shaver White.

Nutrition

Deaths due to Marek’s disease in layers may be influenced by the method of feeding and calcium provision. Taylor et al (1999) showed excessive losses of choice-fed birds given ground calcium included with a protein concentrate. They suggest that lower calcium intake may have affected the immune responsiveness of the birds. Restriction of total feed intake accompanied by retardation of body growth rate has been shown to delay the time of development significantly and to reduce the overall incidence of Marek's disease (Han and Smyth, 1972).

Good Practice based on Current Knowledge

Vaccination is advised. This is normally done at the hatchery, aged one day.

Recommended management practices include:

- All-in, all-out policy, coupled with disinfection between batches.

- Rear young birds away from older birds for the first 2-3 months.

- Reduce insect populations in houses.

Marek's disease(thing)by Rai Tai Sat Nov 18 2006 at 8:01:21

So, you entered your chicken house today to find that, unfortunately, your poultry is dying at a

relatively unhealthy rate. Birds are lying all over the floor, some looking at you pitifully through misshapen eyes. Some look like they're preparing for a track competition. A necropsy reveals a body full of lumps, bumps, and assorted nastiness. What are you to do? What has gone wrong here? I'm sorry to tell you, friend-- It's beginning to look a lot like Marek's. What is Marek's, you might ask? I'm glad you did. I'm here to tell you.

What IS this foul monster?

Marek's disease, discovered by Dr. Josef Marek, is a neoplastic disease caused by a type B herpes virus . There are three different serotypes of the virus, though it seems that only the first serotype actually causes disease. This bugger has an incubation period of approximately 4-12 weeks, with manifestations normally seen around 5-6 weeks of age or after the 12 week period. The mortality curve often follows production curves in layers and broiler-breeders-- that is to say, as birds become more stressed out, the disease peaks. It can form latent infections that reoccur during times of stress, causing problems that can last a lifetime. This is a host-specific pathogen, meaning that humans are not at risk of infection.

But where does it come from?

Basically, all chickens produced in the United States (and probably most other intensive production nations) have Marek's disease. This means that the disease is ubiquitous. Because of the devastation that the disease caused in the industry before it began to be controlled, vaccination is now the absolute norm. Vaccination utilizes an attenuated strain, but still results in infection. We already established that infected birds form latent infections-- they become carriers for the vaccine in the environment. They can shed the disease at intermittent times throughout their life and production cycles. Shedding occurs in the form of sloughed-off epithelial cells from feather follicles. These, along with respiratory secretions and other excretions can cause lateral transmission from the vaccine-infected birds to those that were not previously infected. This can lead to a phenomenon called a rolling infection- if any birds are immunosuppressed, have vaccine failures or other confounding factors, the increased viral load produced by vaccinated birds will laterally infect the unprotected birds in the house. This results in a delayed peak of more severe disease. Bad times. Further, these danderbits can contaminate the environment long after the birds have come and gone, meaning that litter used between bird cycles can contain infected fomites from earlier flocks. Obviously, the older the litter, the stronger the build-up of junk in the dust and dirt, and the stronger the challenge to the baby chickies placed on said litter at day of age. Darkling beetles, a little litter beast, can also be a vector of the disease because they feed on chicken dander.

What did chicken #44369 ever do to deserve this?

Unvaccinated birds are very susceptible to this disease due to its field prevalence. This infection hits all types of chickens, from broilers to layers to broiler breeders, but

mostly occurs in older birds. Turkey can be affected to some extent, but the disease is much more prevalent in

chicken. Birds in heavy production systems are at a greater risk than those who are in a lower

intensity rearing system. Birds in very unhygienic houses are at risk. Birds in a multi-age flock are particularly at risk because of the rolling infections-

imagine putting babies under the challenge of not only the stress of the house and movement, vaccine reactions, and contaminated environment, but also the established hot strain of the virus that has come rolling off of birds who have been in the system for several months. Recipe = disaster.

Other predispositions include damage to the respiratory tract through wet litter, disease, or vaccines, temperature abuse, and immunosuppression from other diseases.

What should I be looking for?

Marek's has many manifestations, including neural, visceral, and cutaneous. Any or all of these signs can help point you to a guess of Marek's for your sick birds. Here are some general signs to be on the lookout for.

General: o Severe depression-- caused by generally feeling like all hell o Emaciation-- caused by a refusal to eat, due to feeling like all hell

Neurological: o Paralysis-- a characteristic "hurdling" position- one leg forward and one leg

back. Paralysis is a result of the neurological component of this disease, and is one of the more characteristic symptoms and is very important for differential diagnosis. The idea behind the paralysis is that tumor cells invade the nerves and brain tissue, resulting in neurological symptoms. Paralysis is also seen in some of the viscera- some birds will have a distended, over-filled crop due to nerve inhibition. The nerves controlling the tone and stretch of the bird's crop are out of commission, and so the little storage baggy overfills and looks pretty darn grotesque.

o Swollen nerves-- especially the sciatic/isciatic nerve. This can be one of the most acute symptoms indicating the disease and is another important differential

Visceral: o Diarrhea -- either a green or white diarrhea. Because normal bird droppings are

fairly dry, diarrhea can be said to be anything that is particularly wet, and especially anything that pastes up the vent. Diarrhea can be a complicating factor to many other diseases because wet litter is generally just not good to have around.

o Eye lesions-- grey iris and misshapen pupils, caused by the infiltration of tumor cells into the eyes.

o Visceral tumors-- Arguably the most important manifestation of this disease. Tumors will appear on/in/around nearly every organ in the body. This is not necessarily a diagnostic lesion, but it IS a very significant one. Tumors can cause the kidneys to be several times larger than normal, the liver to appear necrotic, and the nerves to lose striation. Tumors may also occur on the thymus and bursa of fabricius, resulting in immunosuppresion. Tumors are nasty business.

Cutaneous: o Shriveled, pale comb o Lesions on shanks and/or comb-- red shanks are indicative of cutaneous

infiltration of tumor cells. o Follicular tumors-- These are relatively unpronounced infiltrates of tumors on

the skin. Basically, they will result in what appears to be exaggerated goosebumps on the skin. Goosebumps are normal; very swollen bumps accompanied by redness and congestion are not.

Sweet Jesus on a stick! Are you SURE it's Marek's? Maybe it's something else!

One of the easiest methods of differentiating one disease from others is to pick out important-looking lesions/signs/symptoms/gross things and see if they can be related to any other disease.

visceral tumors = lymphoid leukosis, and that's about it. Swollen nerves = possible riboflavin deficiency Hepatic lesions = Fowl Cholera, bacterial hepatitis, aspergillosis Skin lesions- trauma, bacterial infection

Neurological symptoms = Newcastle Disease, Avian Influenza, encephalomyelitis, botulism, 3-Nitro toxicity, Monensin toxicity, methionine toxicity

Because this disease occurs in older birds and has tumors and swollen nerves and neurological symptoms and skin lesions, it is most likely Marek's. This is the art of differential diagnosis. Excluding other possibilities makes it possible to take a stab at what on earth is causing your problem. You can learn a lot by examining the epidemiology. Further tests for serotyping can be done by submitting pathology samples to the lab. Submitting samples of brain tissue, nerves, and fresh viscera and tumor tissues will provide the best results.

Determining the source of infection is much more difficult. Because birds are supposed to be vaccinated, it's important to look at the hatchery. They will tell you it is not their fault. Growers are supposed to keep their houses hygienic and keep up with vaccines. They will tell you it is not their fault. Finding the real culprit is going to be a party. Have fun.

Well... Hell. Maybe this isn't so bad. What's the worst that could happen?

Unvaccinated birds can reach high levels of mortality, either directly or due to the repercussions of their horrific pathologies. Many birds will be culled due to paralysis and stunted growth. Those that do make it to the processing plant are likely to be condemned due to the tumors and skin lesions. In a business where the loss of pennies is a catastrophic economic blow, this is not an acceptable scenario. So what are you going to do about it?

What CAN I do about it?

There is no real treatment for this disease other than the preventative treatment. Vaccine is the best means to this end. Vaccination is, in this day and age, normally performed in ovo at day 18 of incubation. This is simple, because at this age, the eggs are being moved from the setter to the hatcher. While they are suctioned up, a needle pops a tiny hole in the egg and injects vaccine into the fluids surrounding the embryo. When the yolk is taken up, the vaccine will be as well. Combined with maternal antibodies formed from vaccination of parent flocks, the chickies will be protected for some time. The other option is for hand-vaccination of day-old chicks. Chicks are pressed against an injection machine that pops the vaccine subcutaneously into their fuzzy yellow little chicken necks.

Unfortunately, the Marek's vaccine is fairly easy to screw up. Maternal immunity may be screwed up by only using a weak strain vaccine and never graduating to a more stringent vaccine that will induce high immunity. The vaccine must be stored in liquid nitrogen until it's used, and reconstituting can then be easily seen to get screwed up, rendering the vaccine ineffective. Some people are just bad at hand-vaccinating. They get tired when they get to the end of the 6 hour vaccination shift, and those little birdies don't get the protection they need. The diluent or vaccine itself may become contaminated and cause disease or simply fail to work.

Hygiene also becomes extremely important in prevention. Allowing litter to accumulate without reducing the disease load is a bad idea. Increasing air flow through the houses can help to prevent dander from building up and recirculating into the respiratory tracts of the birds. Adjusting temperature and humidity can help prevent respiratory distress that would aid Marek's ability to enter the body. Killing off little beetly-bugs can also help prevent the spread of disease. Biosecurity is a huge issue on most poultry farms, and practising good sanitation and biosecure practices can decrease risk between houses and between farms.

There are also attempts to genetically select birds that are resistant to the disease. This can help ameliorate some of the problems with vaccine failure and hygiene, but it should instead be viewed as a way to aid those other preventative practices, not supplant them.

Marek's DiseaseMarek's disease is a herpes virus-induced neoplastic disease of chickens characterized by tumor formations in nerve, organ, muscle and epithelial tissue with pleomorphic lymphoid cells. Affects chickens 2 to 16 weeks of age, stresses from other disease increase severity of MD. Herpes virus is cell associated and shed in skin scales and feather dander.

Birds remain viremic for life but infected carriers may or may not be clinically ill. vaccination protects against tumor formation but not against MD infection. Occurrence is world wide wherever poultry is produced. Transmission is primarily by air within the poultry house, in feather dander, chicken house dust, feces and saliva. Infected birds carry virus in blood for life and are a source of infection to susceptible birds. Transmission by egg is of no significance.

Clinical signs "gray eye" caused by tumors in the pupils and blindness, tumors of the liver, kidneys spleen, gonads, pancreas, lungs, muscles and skin. Birds develop tumors, emaciation and death. Diagnosis is based on history of no vaccination, presence of typical tumor pattern and affected birds.

There is no treatment. Prevention is by vaccination at the hatchery, but vaccination only prevents tumor formation, not MD infection. All Marek's Disease vaccine must include HVT.