Marek's Disease in Chickens: Why Vaccination Is Everything
TL;DR: Marek’s disease is a chicken herpesvirus that causes tumours and progressive paralysis, usually in young birds. There is no treatment. It spreads through feather dander and dust, and the virus persists in a contaminated environment for a long time. The vaccine is normally given at the hatchery on the day of hatch, and it protects against the disease rather than preventing infection. If you buy chicks, ask whether they were vaccinated — that question is the whole conversation.
Most poultry diseases give you something to do. This one mainly gives you a reason to have acted earlier.
What Is Marek’s Disease?
A viral disease of chickens caused by a herpesvirus. It produces tumours in nerves and internal organs, and the most recognisable presentation is progressive paralysis — a bird losing use of a leg or a wing, often with one leg stretched forward and one back.
Other things a keeper may see, depending on the form:
- Progressive weakness or lameness in a young bird that was previously fine.
- Wasting despite access to feed.
- Grey or irregular iris colour and pupil changes in one eye.
- Sudden deaths in growing birds with no other explanation.
There is no home test. A definitive answer requires post-mortem examination by someone qualified, and if you are losing growing birds to paralysis it is worth getting that answer rather than guessing.
Why There Is Nothing to Treat With
It is viral, and unlike a bacterial disease there is no antibiotic that touches it. Affected birds do not recover, and supportive care does not change the outcome.
That is the uncomfortable centre of this disease: by the time you see it, the decisions that mattered were made weeks or months earlier — at the hatchery, and at the moment new birds were brought onto the property.
How It Spreads, and Why It Stays
Through feather dander and dust, inhaled by other birds.
That transmission route explains everything difficult about Marek’s:
- You cannot see it moving. It is not in obvious discharge you could clean up — it is in the dust of the house.
- The virus is durable. It can persist in a contaminated environment for a long time, which is why a property that has had Marek’s tends to keep encountering it.
- Carrier birds look healthy. Older birds can shed virus without showing disease, so an apparently healthy adult can infect newly arrived chicks.
The practical consequence: never brood chicks in the same airspace as adult birds, and handle young stock before older stock in your daily routine, never the other way round.
Vaccination: What It Does and Does Not Do
The Marek’s vaccine is normally given at the hatchery on the day of hatch, before the chick can be exposed.
What it does: protects the bird against developing the disease — the tumours and paralysis.
What it does not do: prevent infection entirely. A vaccinated bird can still become infected and shed virus. That is why vaccination does not replace biosecurity.
Two practical points for a Philippine keeper:
- Ask your chick supplier what vaccinations were given. This should be a normal question and a good supplier will answer it — see where to buy day-old chicks.
- If you hatch your own chicks, discuss options with a veterinarian or agrivet. The vaccine has demanding storage and handling requirements and is normally administered at day old, which is a different situation from a backyard hatch. Our chicken vaccine guide covers handling and cold-chain generally, and chick vaccination schedule covers what else young birds usually receive.
What a Keeper Can Actually Control
| Action | Why it matters for Marek’s |
|---|---|
| Buy vaccinated chicks | The single most effective step available to you |
| Brood chicks away from adult birds | Adults shed dander; chicks are most vulnerable |
| Handle young stock first each day | Stops you carrying dander to them |
| Control dust | Damp-clean rather than dry-sweeping where practical |
| Quarantine new birds | Carriers look healthy |
| Clean out and rest a house after an outbreak | The virus persists in the environment |
| Full biosecurity routine | See backyard flock biosecurity |
If you have lost birds to paralysis and suspect Marek’s, be especially careful about selling or giving away stock from that flock. Moving carrier birds is how a local problem becomes a neighbourhood one.
What Else Causes Paralysis or Weak Legs?
Marek’s is not the only answer, and the others have very different implications:
- Newcastle disease — nervous signs, usually with respiratory signs and sudden deaths across the flock.
- Wry neck — a neck-twisting sign with several possible causes, some nutritional.
- Injury — a single bird, sudden onset, no flock pattern.
- Rapid-growth leg weakness in broilers — see Cornish Cross and broiler mortality causes.
- Crooked toes in chicks — a different problem entirely, present from very young.
A pattern across several growing birds is the signal to seek a proper diagnosis rather than to keep guessing.
The Honest Limits Here
Marek’s disease as a chicken herpesvirus, its tumour and paralytic presentations, transmission by feather dander, environmental persistence, and day-old hatchery vaccination that protects against disease rather than infection are all documented in veterinary literature. This guide names no medicine and gives no dosage, because there is no treatment for the disease. Definitive diagnosis requires post-mortem examination — if you are losing growing birds to paralysis, consult a veterinarian or your local agriculture office rather than treating on a guess.
Related: chick vaccination schedule, bakuna sa manok, quarantining new birds, chick mortality causes.
Frequently Asked Questions
What is Mareks disease?
A viral disease of chickens caused by a herpesvirus. It causes tumours in nerves and internal organs and commonly shows as progressive paralysis in young birds. It is one of the classic reasons chicks are vaccinated on the day they hatch.
Is there a treatment for Mareks?
No. There is no treatment and no cure. Affected birds do not recover. Vaccination before exposure is the only protection, which is why it is given at the hatchery at day old.
How does Mareks spread?
Mainly through feather dander and dust from infected birds, inhaled by others. The virus is durable in the environment and can persist in a contaminated house for a long time, which is why a farm that has had it tends to keep having it.
What age does it affect?
Typically younger birds, often becoming visible from a few weeks to several months of age. Older birds can carry and shed the virus without obvious illness.
Does the vaccine stop the bird getting infected?
It protects against the disease — the tumours and paralysis — rather than preventing infection outright. A vaccinated bird can still carry and shed virus, which is why biosecurity still matters.
Can I vaccinate my own chicks against Mareks at home?
It is usually given at the hatchery on the day of hatch, and the vaccine has demanding storage and handling requirements. Ask your chick supplier whether it was given, and talk to a veterinarian or agrivet about options if you hatch your own.
Can humans catch Mareks?
No. It is a chicken herpesvirus and does not infect people.