Your Chicken Has Marek’s Disease Virus.

Marek’s disease is one of the most important viral diseases of chickens and occurs worldwide. It is caused by a highly contagious herpesvirus that can produce nerve damage, paralysis, immune suppression and tumours.

One of the most important things to understand about Marek’s disease is that infection is lifelong. Once a chicken becomes infected, it can remain infected and shed virus for the rest of its life.

Vaccination is highly effective at reducing the risk of disease, but it does not completely prevent infection or shedding of the virus.

What Does Marek’s Disease Look Like?

Marek’s disease can be frustrating to recognise because it does not have one single presentation. The virus can affect the nerves, eyes, skin and internal organs.

Neurological Marek’s Disease

This is probably the presentation most backyard chicken owners associate with Marek’s.

The virus can cause lymphoid infiltration and enlargement of peripheral nerves, producing signs such as:

• Lameness
• Weakness or paralysis of one or both legs
• Wing paralysis
• Difficulty standing or walking
• Abnormal posture
• Neck weakness or paralysis
• Crop dysfunction

The classic image of Marek’s is a chicken with one leg positioned forwards and the other backwards, but not every bird with Marek’s will show this posture, and not every paralysed chicken has Marek’s disease.

Transient Paralysis

Some chickens develop a temporary neurological syndrome characterised by weakness, incoordination or paralysis.

Birds may recover, but supportive care can be required while they are unable to move, eat or drink normally.

Ocular Marek’s Disease

Marek’s can infiltrate the tissues of the eye.

Signs may include:

• Changes in iris colour
• An irregularly shaped pupil
• Changes in pupil size
• Reduced response to light
• Partial or complete blindness

This form is sometimes referred to as grey eye.

Cutaneous Marek’s Disease

Marek’s can also affect feather follicles and skin.

Affected follicles may become enlarged or raised, sometimes occurring in clusters. This form may be much less obvious in a living bird than neurological or visceral disease.

Visceral Marek’s Disease

Marek’s can cause lymphoid tumours within internal organs.

These may involve organs such as the liver, spleen, kidneys, gonads, heart and other tissues.

Clinical signs depend on which organs are affected and may simply include:

• Weight loss
• Reduced appetite
• Lethargy
• Poor condition
• Reduced egg production
• Progressive deterioration

In some birds, internal tumours may be the first significant evidence of disease.

How Does Marek’s Disease Spread?

Marek’s disease virus is shed primarily from infected feather follicles.

Virus-containing feather dander and dust accumulate within the environment and can then be inhaled by other chickens.

This makes Marek’s particularly difficult to control once it becomes established in a poultry environment.

The virus can persist in contaminated dust and housing for extended periods, so simply removing an affected bird does not necessarily remove the infection risk.

When Do Chickens Develop Marek’s Disease?

Chickens are commonly infected when young, but clinical disease does not necessarily develop immediately.

Disease is frequently seen in younger chickens, although birds of any age can potentially develop clinical Marek’s disease.

Whether an infected bird actually becomes sick is influenced by several factors, including:

• Vaccination status
• Age at exposure
• Viral strain and exposure level
• Genetics
• Immune status
• Concurrent disease
• Environmental and physiological stress

This helps explain why several chickens can be exposed to the same environment while only some develop obvious disease.

Vaccination Against Marek’s Disease

Vaccination is one of the most effective tools available for controlling Marek’s disease.

Ideally, chicks are vaccinated before they are exposed to the virus, commonly at hatch in commercial production systems.

Vaccination significantly reduces the development of clinical disease and tumours. However, Marek’s vaccines are unusual because vaccinated chickens can still become infected with field virus and potentially shed it.

This means vaccination protects the individual bird primarily against disease, rather than creating a completely infection-free bird.

It also means biosecurity remains important even in vaccinated flocks.

Does Vaccination Make a Chicken Dangerous to Unvaccinated Birds?

This is an area where Marek’s disease is often misunderstood.

A vaccinated chicken does not automatically become a source of virulent Marek’s disease simply because it has been vaccinated.

The concern is that a vaccinated bird can still become infected with field Marek’s virus and shed that virus without becoming clinically sick.

So rather than assuming vaccinated and unvaccinated birds can never be mixed, the more important issue is understanding the disease status and exposure history of the flock.

Can Marek’s Disease Be Treated?

There is currently no treatment that eliminates Marek’s disease virus from an infected chicken.

Management therefore focuses on supportive care and the welfare of the individual bird.

Birds with mild neurological impairment may sometimes stabilise or improve, but progressive paralysis, severe internal disease or an inability to independently eat and drink can carry a poor prognosis.

Diagnosing Marek’s Disease

Paralysis does not automatically mean Marek’s disease.

Other diseases, injuries, nutritional disorders, toxins and neurological conditions can produce similar signs.

Diagnosis may involve:

• Clinical examination
• Flock and vaccination history
• PCR testing
• Post-mortem examination
• Histopathology of affected nerves, organs or other tissues

Post-mortem examination and histopathology can be particularly valuable when investigating unexplained paralysis, tumours or repeated losses within a flock.

Managing Marek’s Disease in a Backyard Flock

If Marek’s disease is suspected or confirmed, management should focus on reducing exposure and protecting future birds.

Good practices include:

• Vaccinating replacement chicks where appropriate
• Protecting newly vaccinated chicks from early exposure
• Maintaining good hygiene and reducing accumulated feather dust
• Avoiding unnecessary movement of birds between flocks
• Quarantining new birds
• Investigating unexplained paralysis rather than assuming it is Marek’s
• Maintaining good overall flock health and minimising unnecessary stress

A closed flock can significantly reduce the number of new pathogens being introduced, although it cannot remove Marek’s virus once it is already established in the environment.

The Takeaway

Marek’s disease is common, highly contagious and capable of producing everything from subtle weakness to paralysis, blindness and internal tumours.

One of the biggest misconceptions is that vaccination completely prevents infection. It doesn't. Vaccination primarily protects chickens from developing Marek’s disease after exposure, while infected vaccinated birds may still carry and shed field virus.

For backyard flock owners, early vaccination, sensible biosecurity and accurate diagnosis are the most useful tools we have for reducing its impact.

Written by Dr Sam Hurley, BSc, BVSc, MAHM