Marek’s disease is estimated as being responsible for the loss of 1% of all global poultry production.

And there is growing concern among some sectors of the UK poultry industry that changes in the herpes type viruses that cause Marek’s disease could lead to more virulent strains emerging in the UK similar to those already alarming poultry farmers in the US.

What is Marek’s disease?

It is a highly contagious herpes virus that can affect birds from a very young age, although older birds are also at risk. Birds have no natural immune system to combat Marek’s disease. It is found in the follicles of feathers and spread by “dander” – cells from the birds’ feathers and skin – and can survive for long periods in poultry houses.

Marek’s disease is particularly virulent and can be found in houses that have been free of birds for several months. The stability of the virus in the environment is one of its most serious characteristics. Birds with Marek’s disease will start to shed infectious dander into their environment within two to three weeks of being infected vaccinated birds can still contract the disease and shed infectious dander and will remain viraemic (infectious) for life.

How do I know my birds have Marek’s disease?

Vaccination against Marek’s disease is an essential mode of protection, but it doesn’t mean that vaccinated birds cannot become infected and act as a source of virus spread. Typical symptoms of the disease are paralysis of the legs, wings and neck, general morbidity and listlessness, weight loss, eye abnormalities and blindness.

Changes in the appearance of the skin close to the feather follicles also occurs. The disease occurs in several different types which can affect birds from as early as three weeks old. Acute Marek’s disease – usually hitting birds from 6-12 weeks old – causes serious tumours throughout the main organs. Unusual levels of mortality in a flock may be the first signs of an outbreak of Marek’s disease. If the earliest signs of the disease occur in the bird’s eyes these may be difficult to identify.

How do I protect my birds against Marek’s disease?

Birds should be vaccinated as day-old chicks but it’s essential that the vaccine is administered correctly. There is concern that a lack of adherence to high standards of vaccination procedures at some hatcheries is responsible for the spread of Marek’s among birds that have been vaccinated.

The vaccine contains live virus and must be stored in liquid nitrogen. It’s essential for the vaccine to be administered at the correct temperature it must be stored in ice during use to achieve that. It is feared that some hatcheries reduce the dosage by over 50% to cut costs – in effect these chicks are not being protected against Marek’s and have a high risk of contracting infection. Vaccinated chicks are not immediately protected and should be housed in a brooder building to allow sufficient immunity to develop.

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Some hatcheries and pullet rearers have employed double vaccination, believing the extra risk is worth it in reducing risk. This involves day-olds at the hatchery being given a second dose a couple of hours later. The thinking is that with a single vaccination cycle, there is always the possibility that a small percentage of chicks may not receive the full dosage. So a repeat dose protects these birds.

Although producers buy vaccinated birds, scientists involved in research into Marek’s say they must remain vigilant in checking for the disease once birds arrive on their premises. There is no room for complacency in UK flocks even though producers are dealing with birds that have been vaccinated. Producers are advised to maintain strict levels of hygiene at all stages of rearing and during the laying period. An “all-in all-out” system, which avoids mixing birds of different ages in the same house, is recommended as an important barrier to the spread of Marek’s.

There is shared responsibility for the control of Marek’s disease and hatcheries – rearers and producers all have a role to play. Correctly vaccinated birds and high standards of hygiene are the key elements of keeping Marek’s at bay but it’s vital to understand the impact of stress. Vaccinated birds that become stressed due to poor management are more at risk from Marek’s following the interaction the stress has with the birds’ immune system. All herpes-type viruses can be triggered by stress.

How do I know my vaccinated birds are protected?

Vaccinated birds can still contract Marek’s disease and although they will not display any clinical signs of infection they will act as a source of spread through their dander. Fort Dodge Animal Health has developed a method of assessing the level of immunity based on a feather test undertaken on vaccinated birds. The test indicates the bird’s health status and assesses how much vaccine is present and the level of protection it is achieving. This service is available to all users of the company’s vaccine.

Is Marek’s disease on the increase?

There are widely differing views on the subject of the increased spread and virulence of the disease. Concerns have been expressed in the UK that the increase in the number of free-range units is responsible for a higher number of outbreaks of Marek’s disease, particularly in the south west.

Marek’s disease is a world-wide problem but leading scientists working on research into the disease do not adhere to the fact that free-range units will inevitably lead to a faster spread of the disease. However there is unanimous agreement among experts that the standards of hygiene maintained in poultry accommodation plays a key role in combating the spread of Marek’s.

Fingers have been pointed at free-range units as being more susceptible to infection because of the “open environment” in which the birds live. But while producers are less able to manage this environment, compared with caged or housed birds, poultry health experts say that free-range birds that do contract Marek’s disease are likely to have been challenged at a younger age, well before they reach their free-range unit as point-of-lay pullets.

Is Marek’s disease becoming more virulent?

There are mixed messages coming from experts researching Marek’s on a global scale. There is no doubt that more virulent strains of Marek’s disease are being identified in the US and some say it’s a trend being reflected in parts of the EU. The Fort Dodge Marek’s test is intended to maintain a tight screening process to assess any changes in the disease and the effectiveness of the vaccine being used to combat it.

Some scientists working on Marek’s disease, while not admitting to an increase in the number of outbreaks in the UK, say the disease is demonstrating “ups and downs” in terms of incidence. If the disease is identified in the flock of a pullet rearer it’s inevitable that others further down the line will already have birds in their systems that are infected.

In the US poultry sector, an increasing number of new strains of Marek’s are being identified and these strains are becoming more virulent. It’s a situation that UK producers have so far been spared but scientists working on Marek’s say it is possible that the UK industry could face the same level of challenge in the future. New tests are being developed to keep pace with changes in Marek’s disease world-wide.

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Signs of the disease

  • Paralysis of legs, wings and neck
  • Weight loss
  • Grey iris or irregular pupil
  • Impaired vision
  • Skin around feather follicles raised and roughened