Last month's Ranger Magazine (www.theranger.co.uk; June 2008) took up the contentious issue of Marek's disease. One of the reasons why the effects can be so shocking for producers is that whilst the virus is latent there are no obvious signs of disease. An infection in a flock may escape detection until suddenly, without warning, mortality commences. Since by that time there is no cure, prevention is doubly important.

For the purposes of this follow-up article, Farming UK spoke to two sectors of the industry that play a very important role in minimising the risk of the disease being present in the first place: firstly, the pullet rearers whom producers rely on to supply them with problem-free pullets, and second, the animal health companies who produce the vaccines used throughout the industry.

Important Lessons Learnt

During the last few years, Marek's disease has impacted badly on the free-range sector, and lessons have been learnt the hard way. Many pullet rearers were unfortunate enough to experience Marek's, and responded by looking very carefully at every aspect of what they do. As a result, the pullet rearing industry today is probably operating to higher standards than at any time in the past. Having spoken to a number of rearers, it seems that the important lessons can be summarised as follows:

• There is a very strong case to be made for operating a rearing farm as all in, all out, i.e. no chicks and pullets at same time.
• Double vaccination by the hatchery is strongly advisable. Double vaccination means that day-olds at the hatchery are given a first vaccination in the normal way and a couple of hours later, they are all vaccinated again.
• The storage of the vaccine and how it is administered is highly important. Hatchery audits should be carried out to monitor and observe. Like any process done by humans, there is the potential for error and variability between personnel.
• A healthy chick is imperative if it is to respond to the Marek's vaccine optimally. Day-old health issues, such as pseudomonads, can cause high mortality. If treated, the mortality is short-lived but in the meantime, the chick's immune system has been suppressed, so that uptake of the vaccine may be compromised.
• Likewise, it is important that immunosuppressive diseases like infectious bursal disease or chicken anaemia are kept under control to avoid immunosuppression that may interfere with Marek's vaccination.
• If there is a level of Marek's challenge on a rearing farm, this does not mean that all chicks will go on to develop Marek's in lay. Cases have been recorded where flocks of birds of the same breed have been sent out of the same house, and one flock then experienced issues while the other did not. This is strong evidence of a link to stressors in the laying environment triggering the underlying disease. There have been cases, too, where some breeds go on to have issues and others do not, and this can often be traced back to day-old chick quality/rearing issues.
• Once an all-in, all-out status has been achieved, thorough cleaning is essential to reduce the Marek's challenge. An effective method is for the house to be washed with detergent and disinfectant twice, followed by an ammonia product such as Bio-Oocyst, then gassed with formulin and subsequently locked up for 48 hours. It is important to keep up with a thorough turnaround programme thereafter.
• Once the chicks are housed, all visitors should be kept to a minimum for the first six weeks, and a good bio-security programme should be adhered to. Overalls, boot covers and the use of foot dips (regularly changed) should be enforced.
• Finally, it is also important to bear in mind that if a producer has birds sent for post mortem examination and Marek's is found, this does not necessarily mean that high mortality will continue. Marek's is no different from any other disease in that there is always a possibility of it being found in one or two birds in a flock, normally in birds that are sick or underweight.

A Three-Way Partnership

On a positive note, all the evidence indicates the Marek's threat has now receded.

Looking back on her own company's experience, Olivia Potter highlights the need for co-operation between all those involved. "What is most important to remember is that defeating Marek's requires a three-way partnership. The hatchery has a very important role to play in administering the vaccine effectively; the rearer has an equally important role in keeping a 'clean' farm and observing chick quality; and thirdly, the egg producer also has a role to play in housing the pullets into a clean environment and keeping stress to a minimum."

Double Vaccination

One of the recommendations listed above was double vaccination, and this is a practice that is gaining strong support within the industry. Olivia Potter describes it as a 'must'.

"For the sake of the small extra cost, it is not worth the rearer or egg producer taking the risk," she says.

Steve Carlyle, of Country Fresh Pullets, is another firm advocate of double vaccination.

Country Fresh began looking for new answers when, after many trouble-free years, they started seeing Marek's. Breaks occurred at free-range sites around three or four years ago: at first, these occurred only at sites where a high challenge was to be expected, for instance where the young birds were in close proximity to layers.

Tightening up on biosecurity seemed the appropriate solution. This was duly done and initially, it appeared to have worked. But then, even with all the correct biosecurity in place, another break happened at an isolated site where there was no reason to suspect any significant level of risk.

It was at this point that Country Fresh decided to switch to double vaccination. Since then, they have experienced no breaks whatsoever.

"Double vaccination seems to have been the solution for us. There is no question in my mind that it does bring a benefit. The one major change we made on the rearing farm is that we began double vaccinating all our litter chicks; that was two years ago, and since then we've never had a problem," said Mr Carlyle.

"It puts £0.03 onto the price of a pullet and in my opinion this is a very small premium to pay for a very valid extra insurance.

"I realise that some rearers who have not yet had breaks are still sceptical but I believe that more of them will recognise the need to adopt this practice in the future."

There are many in the industry who agree with Ms Potter and Mr Carlyle. John Bowler, for instance, is already committed to the double vaccination route. Although there is as yet no clear-cut explanation as to why double vaccination works, experience indicates quite clearly that it does, and work is being carried out on this subject.

Sue Baigent, a BBSRC research scientist who specialises in Marek's disease, has been involved in a recent study carried out by Fort Dodge. She points out that when evaluating the results of any such study, it must be borne in mind that with a single vaccination cycle, there is always the possibility that a small percentage of chicks may not receive the full dosage – for instance, if the needle is not positioned 100% accurately.

Double vaccination would eliminate this risk as it is extremely unlikely that this would happen twice to the same chick. Her view is that in-ovo vaccination is a more accurate technique, as the machine uniformly vaccinates a tray of eggs at once. Nonetheless, she does not discount the possibility that administering a second vaccination may provide an extra boost to the chicks.

Recent Studies Carried out by Fort Dodge

Herve Le Galludec, Poultry Business Unit Manager at Fort Dodge Animal Health, explains the background to the project.

"We had been aware for some years that this 'double vaccination' was done in some countries, and field experience seemed to indicate that it contributed to increased protection. However, no scientific work could explain these field observations.

"In 2002, we developed, in collaboration with Compton IAH (Venugopal Nair and Sue Baigent), a research project on Marek's disease. One of the achievements was the development of a PCR method that allowed us to understand what happens in vaccinated birds in terms of vaccine replication. This tool was obviously the best one to use to try to get an answer on the issue of double vaccination, among others vaccination protocols.

"We spent four years collecting samples on pullet farms after vaccination with different protocols, and it appeared that giving two injections of CVI-HVT vaccine increased the level of replication of the vaccine in the birds. Of course, when collecting these samples, we had several groups from same genetic line, in same farm, from same hatchery to allow us to compare the results.

"We then ran a lab study with Compton, and we showed that protection against challenge is very closely correlated with vaccine replication in the birds. In other words, the criteria we were measuring provided an excellent indicator for protection.

"At the end of 2007, we decided to run another lab study measuring this effect in laboratory conditions. I am still working on the results. However, it seems to indicate that as in field conditions, the double vaccination increases the protection of the birds."

Intervet/Schering Plough Animal Health Field Trials Results Expected in Near Future

Intervet/Schering-Plough Animal Health also believes that there can be an advantage from double vaccination.

"Revaccination at the hatchery or on-farm at delivery may work because it increases the proportion of birds that have sufficient vaccine virus in them to develop adequate immunity against fields challenge. Revaccination also helps in situations where a certain percentage of the birds is missed in the first round because of human or machinery error.

"At this time our Marek's disease vaccines are used in the UK for broiler breeders only and they seem to be working well. We believe that these vaccines – assuming that they are stored, mixed and administered correctly – provide adequate protection against field Marek's challenge even in a single dose.

"There is now a very useful tool - PCR - on the horizon which will enable us to monitor vaccinated flocks for vaccination accuracy. We are doing field trials with this tool and results will be published in the near future."

* "We agree entirely with Olivia Potter's comments, that defeating Marek's requires a partnership approach"

Spokesperson for Intervet/Schering-Plough Animal Health

Industry Must Remain Vigilant

For the present, it seems that the industry has succeeded in controlling Marek's. But it has been a hard battle and the events of the past few years have left everybody keenly aware of the need to remain vigilant.

One of the biggest fears is of increased virulence. This is being carefully monitored, and Intervet/Schering-Plough Animal Health confirms that there is no immediate cause for alarm.

"We have not seen any evidence yet that the virulence of field Marek's viruses in the UK or in Europe has increased and therefore that a new vaccine or vaccines would be needed."

A definite 'positive' that has emerged from this brief investigation into how the free-range industry has dealt with Marek's is the way the different sectors have united in their efforts to overcome a problem that threatened to put livelihoods at risk. It is good to see all sectors prepared to share information and compare experiences.

A final observation from Intervet/Schering-Plough Animal Health puts this into a nutshell. "We agree entirely with Olivia Potter's comments, that defeating Marek's requires a partnership approach in which hatcheries, pullet rearers, laying farms and vaccine manufacturers all work together with equal responsibility."

Further Reading

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Find out more information on Marek's disease by clicking here.

July 2008