Marek's Disease in Chickens: Paralysis, Tumours and Prevention

A disease that waits for the flock to grow up

Marek's disease rarely kills a chick in its first week. It waits. The bill arrives when the birds should be laying or putting on the last of their weight, and by then the farmer has usually blamed the feed, the heat, the water or a bad batch of chicks.

We hear this story more than we would like. A farmer sends a picture of a pullet that cannot stand straight, or a list of birds that went quiet at four months. The chicks were healthy when they left us, which is true, and the virus was already in them by then, which is also true. Most chicks pick up Marek's early, from the dust of the house they grow up in, and the disease shows up much later.

That gap in time is why the disease is so easy to get wrong. A farmer who does not know about it looks for the cause in the wrong place, and the next flock gets the same treatment and the same surprise.

What the virus does inside the bird

Marek's disease is caused by a herpesvirus that infects the immune cells of the chicken. In most birds the immune system keeps the virus under control. In some, the infected cells multiply and form tumours. The virus also attacks the nerves, and the swollen nerves press on the muscles that let a bird stand, walk and balance.

Tumours can grow in the liver, the gonads, the heart, the skin, the feather follicles and the eyes. The nerve form is usually the first one farmers see, because a bird with a damaged nerve cannot balance well and soon cannot reach the feeder. A bird with tumours in its organs may lose weight, look pale and die suddenly.

Once a bird shows signs, there is no treatment that brings it back. The affected birds are a loss, and whether to cull them is a question for your vet. What matters most is what you do next to protect the birds that have not yet shown anything.

Four forms, four sets of signs

Vets usually sort Marek's into four forms. You will not diagnose the form yourself, but knowing the names helps when you describe the birds on the phone.

Ages are a guide, not a rule. A bird can show signs earlier if the virus pressure was heavy, or later if the house is clean and the birds are well vaccinated. If the timing does not fit the form, tell your vet, because it changes the list of likely causes.

How the virus travels: dander, dust and the house itself

The virus leaves the bird in the feather follicles, in the dander and skin flakes a flock sheds all day. Dander floats in the air, settles on the litter, lands in the feed and the drinkers, and sticks to walls and equipment. That is why the house becomes part of the problem. A house that held Marek's birds can keep the virus in its dust for a long time.

Infected birds shed virus for life, even when they look well. A flock can look fine and still be spreading the disease. Chicks are most at risk in their first weeks, because their immune systems are still learning to respond. A chick exposed in the first few days is more likely to develop tumours later than one exposed when it is older.

Harmattan dust makes things worse in the dry season. Fine dust travels, so dander from one house can settle in the next. Keep houses apart, and do not share equipment between them without cleaning it first. If you are sorting out the layout, a single-age house is the easiest to keep clean, and the day-old chicks hatchery page is a good place to start planning your intake.

The hatchery vaccine: what it does and what it does not do

Our chicks are vaccinated against Marek's at the hatchery, and the batch record shows it. The vaccine goes in on the first day of life, usually as an injection under the skin at the back of the neck. It is a live vaccine. It works by letting the chick's immune system meet a harmless form of the virus early, so it can handle the real one later.

The vaccine stops tumours. It does not stop infection. A vaccinated chick can still carry the virus and shed it into the house, which means it can still be a source for other birds. Farmers who treat the vaccine as a shield against every Marek's risk are surprised when the virus is still in their dust.

Handling matters a great deal. The vaccine is usually kept frozen in liquid nitrogen and has to be used quickly once it is mixed. The clock starts when it is thawed, and any hatchery should work to the time on the label. A vaccine that sits out in the heat loses its power, and nothing can fix that later on the farm.

When you buy chicks, ask for the batch record, look for Marek's on it and keep a copy in your farm file. If a supplier cannot show you the vaccination record, treat that as a warning. Your vet can then plan the house around what is actually known.

Why vaccinated birds still get tumours

When a vaccinated flock still shows Marek's, we look at the same few things. The vaccine itself may have been fine. The problem is usually timing, handling or the amount of virus already in the house.

• The chicks met heavy virus before the vaccine had time to build protection, which takes about a week or two.
• The vaccine was not stored or used properly on the farm, or the chicks received it late.
• The house still held old dander and dust from a previous flock that was never properly cleaned.
• Older birds were added to a young house, bringing a fresh load of virus with them.
• The flock had a second infection or a stress event that weakened the immune response.

The fix is partly the vet's job and partly yours. Your vet can decide whether a booster, a change of timing or a better clean-out is needed. Your job is to keep the records clear enough that the vet can see the whole picture.

Mixed ages and village birds: how Ghanaian flocks catch it

Many Ghanaian farms keep more than one age group in the same yard. A grower pen beside an old layer house, a few cockerels from the village and a neighbour's free-range birds wandering through the compound are all ways virus moves from older birds to young ones.

Keep the age groups apart as far as you can. Use separate houses, separate equipment and, where possible, separate people. If you must add birds from outside, buy them from a known source with a vaccination record, quarantine them, and watch them for a few weeks before they mix with the flock. Our infectious bronchitis article covers the same quarantine logic for breathing diseases.

Village birds are often unvaccinated, and they are the birds that rarely see a vet. That is not a criticism of village farmers. A virus that has a yard full of unvaccinated birds to move through will stay there.

Telling Marek's from its look-alikes

Paralysis has many causes. A bird that cannot stand may have Marek's, but it may also have Newcastle disease (see our Newcastle guide), a Gumboro infection (see our Gumboro guide), a leg injury, a vitamin shortage or a bacterial joint infection. Lymphoid leukosis is a different disease that also causes tumours, and it is a common mix-up in older birds. The only reliable way to separate them is a post-mortem with laboratory tests, usually arranged through your vet.

Look for the details that matter. The eye is one of the most useful clues. In a classic Marek's eye the iris loses its normal colour and goes grey, and the pupil can look small, misshapen or irregular. Take the bird into good light and compare both eyes. A single grey eye is worth a call to the vet.

Look at the rest of the bird too. Weight loss, a swollen nerve in the leg, a bird that lies flat and keeps trying to stand, or lumps felt in the abdomen all add to the picture. Write them down before you call.

What to do if Marek's is on your farm

1. Call your vet and describe the age, the number and the signs of the affected birds.
2. Separate birds that are paralysed or losing weight, and keep them away from the feed and water used by the rest.
3. Follow your vet's advice on culling. In most cases, birds with visible tumours or paralysis are removed rather than kept as a source.
4. Do not put new chicks into a house that still holds old dander until it has been cleaned and rested.
5. Clean the house thoroughly, including walls, ceiling, nest boxes, perches and equipment, before the next flock.
6. Ask your vet whether the next flock should start with a different vaccine or a booster.

A clean-out is not a sweep. Remove the litter, scrape the dust off the rafters, wash the equipment and let the house dry before the next batch arrives. That takes days, not hours, and most people try to cut it short.

A practical checklist for the chick house and the grower pen

• Get the batch record, with Marek's listed, before the chicks leave the supplier.
• Keep the vaccination date and batch number in your farm file.
• Run a single-age house where you can, and clear it fully before the next flock.
• Keep feather dust down with clean litter and good airflow rather than with extra water.
• Check the eyes and walk of your birds every week from about eight weeks on.
• Note any bird that stands strangely, and report it to your vet within the day.
• Ask your vet to review the vaccination plan at each stage of the flock.

What our batch record gives you

Every batch we send out comes with a record of the vaccines it received, including Marek's, and the record is yours to keep. It cannot tell you the future. What it does is give your vet a clear starting point, so the plan for the next flock rests on facts rather than memory.

If you are planning a layer flock and want to talk through the house, the layer day-old chicks page is the place to start. Our chicks go to all 16 regions, and delivery costs are confirmed before you pay. WhatsApp +233 24 915 1503 with your numbers and your region, and we will talk through the record, the timing and what your vet is likely to ask. When you are ready, you can place your order here. Common questions about the process are on our FAQ page.

Questions farmers ask