Marek’s Disease in Chickens: Paralysis, Tumours, Diagnosis and Prevention

A chicken dragging one leg, becoming paralysed or developing an abnormal eye may make a farmer suspect Marek’s disease. But paralysis alone does not diagnose Marek’s disease.

Marek’s disease is a highly contagious viral disease of chickens. It can damage nerves, weaken immunity and cause tumours in internal organs, skin or other tissues. There is no treatment that cures an infected bird, which makes hatchery vaccination, chick sourcing, sanitation and early disease investigation important.

This guide explains the signs farmers may notice, how the virus spreads, what can look similar, why laboratory and post-mortem findings matter, and what vaccination can and cannot do.

What to check before calling it Marek’s disease

When one chicken sits down or drags a leg, start with simple farm checks. Look for an injury, swollen joint, curled toes, a slipped tendon or signs that the bird was trapped under equipment. Confirm that feed is suitable for the bird’s age and ask whether other birds are showing weakness, twisted necks, breathing signs or unusual droppings. Then check the purchase record: source hatchery, delivery date, breed, vaccination information and the age when signs began.

This farmer-first check matters because “paralysis” describes what you see; it does not name the cause. If cases are increasing, do not keep testing home remedies while valuable time passes. Separate affected birds humanely, place feed and water within reach, and contact a poultry veterinarian or laboratory.

Why Marek’s disease can become a farm problem

Many Nigerian and African poultry farms keep different ages of chickens close together or reuse a brooder soon after an older flock leaves. Older birds may look healthy while shedding virus in feather dust. Young chicks entering that dusty environment can be exposed before vaccine protection has had enough time to develop.

Open-sided housing provides valuable airflow, but wind, sweeping and dry-season dust can move dander within and between pens. Avoid dry sweeping that sends dust into the air. Remove litter and dust carefully, clean from the roof and upper surfaces downward, and keep cleaned chick areas separate from older birds, used crates and unwashed equipment.

For a small farm, a notebook is enough to improve investigation. Record the date, bird age, leg or wing affected, deaths, hatchery source and vaccination evidence. A pattern across successive batches may reveal a sourcing or environmental problem that one isolated case would hide.

When buying chicks, ask for written vaccination details instead of relying only on a verbal promise. Keep that record with the batch file. It helps the veterinarian assess whether the problem involves vaccine handling, very early exposure or another disease entirely.

For a symptom-first overview of major flock problems and how to decide what needs urgent investigation, use the Poultry Diseases and Symptoms guide.

What is Marek’s disease?

Marek’s disease is caused by a herpesvirus of chickens. The MSD Veterinary Manual describes it as a highly contagious disease characterised by T-cell tumours and enlargement of peripheral nerves.

The disease is important because infected birds release virus from feather follicles. Virus-containing dust and dander can then contaminate the poultry house and expose other birds.

Signs farmers may notice

Marek’s disease does not look the same in every bird. Possible signs include:

• weakness or progressive paralysis;
• one leg held forward and the other backward;
• drooping wing or difficulty standing;
• weight loss despite feed being available;
• pale comb and poor condition;
• an irregular pupil or grey-looking iris;
• blindness;
• enlarged feather follicles or skin lesions;
• unexpected deaths or poor flock performance.

Internal tumours may affect the liver, spleen, kidneys, heart, lungs, reproductive organs, muscles or digestive tract. These cannot be identified reliably by looking at a live bird from outside.

Paralysis does not always mean Marek’s disease

Leg weakness or paralysis can also occur with injury, slipped tendon, nutritional problems, spinal disease, botulism, Newcastle disease and other nervous-system conditions. A bird lying down because it is extremely weak may also be mistaken for a paralysed bird.

Check the flock pattern:

• How old are the birds?
• Is one bird affected or are cases increasing?
• Are there respiratory signs, green droppings or twisted necks?
• Was the flock vaccinated at the hatchery?
• Are affected birds losing weight?
• Did the problem appear after birds were mixed or moved?

If several birds show nervous signs or mortality is rising, use the Signs of a Sick Chicken guide and obtain veterinary help. Newcastle disease can also cause nervous signs; compare the flock pattern with the Newcastle Disease guide.

How Marek’s disease spreads

Infected chickens shed virus from feather follicles. Dust and feather dander carry the virus through the house, on equipment, clothing and air movement. The MSD Veterinary Manual notes that the virus can survive for months in poultry-house litter or dust.

The infection is not considered vertically transmitted through the egg. Chicks are commonly exposed after hatching when they enter a contaminated environment or mix with shedding birds.

That is why cleaning between flocks matters, but cleaning alone cannot guarantee elimination once a poultry environment has been heavily contaminated.

How Marek’s disease is diagnosed

A presumptive diagnosis may combine flock history, age, clinical signs, enlarged nerves, tumours found during post-mortem examination and tissue changes under a microscope. PCR and other laboratory methods can support diagnosis.

A positive test for Marek’s virus does not automatically prove that the virus caused the bird’s signs because exposure can be common. The diagnostic question is whether the characteristic nerve damage or tumours are present and whether other tumour diseases or causes of paralysis have been excluded.

Keep freshly dead representative birds cool, not frozen, and contact a veterinarian or diagnostic laboratory promptly for submission instructions.

Can Marek’s disease be treated?

There is no curative treatment for Marek’s disease. Antibiotics do not treat the virus or remove tumours. Severely paralysed birds that cannot reach feed or water require prompt welfare assessment and humane action under veterinary guidance.

Do not treat the whole flock blindly for “paralysis.” Investigation matters because injury, nutritional imbalance and other infections need different responses.

Vaccination: important but not a universal schedule

Vaccination is the main prevention tool. It is usually planned for chicks before or around hatchery placement so protection can begin before heavy environmental exposure. Exact products and programmes vary by hatchery, vaccine label, disease pressure and local veterinary advice.

Marek’s vaccines greatly reduce disease and tumour development, but they do not always prevent infection or shedding. Vaccinated birds can still encounter field virus. Good protection also depends on correct vaccine storage, handling, administration, chick quality and the time between vaccination and exposure.

Ask the hatchery for written confirmation of the vaccine used and how it was administered. Do not copy a programme from another country or farm without checking that it is appropriate for your chicks and local conditions.

How farmers can reduce Marek’s disease risk

• Buy chicks from a reputable hatchery with a documented Marek’s vaccination programme.
• Prepare a clean brooding environment before chicks arrive.
• Remove old litter and dust thoroughly between batches.
• Avoid mixing young chicks with older birds.
• Use all-in/all-out management where practical.
• Control movement of people, crates and equipment between poultry houses.
• Follow the Poultry Biosecurity in Nigeria guide.
• Investigate paralysis and unexplained mortality rather than assuming a treatment.

When to seek veterinary or laboratory help

Seek help when paralysis affects more than one bird, cases continue to appear, birds lose weight, pupils or eye colour change, tumours are found, or the diagnosis remains uncertain. Rapid investigation protects later batches because the farm environment may remain contaminated.

Farmer-friendly disclaimer: This guide is for farmer education and early decision-making. Paralysis, tumours or eye changes can suggest Marek’s disease but cannot confirm it. Obtain veterinary or laboratory support for persistent, spreading or uncertain cases.