Mareks – old disease, new problems?

Mareks disease is a condition most producers have heard of, but newcomers to the industry may not have experienced the severe problems many of us older members of the industry recall.

This month’s article was prompted by a ripple of renewed interest in Mareks which seems to be associated with a slight increase in diagnosis over recent months.

So what is Mareks Disease?

Mareks disease is caused by a virus infection. The virus is a herpesvirus, being broadly in the same family as ILT in chickens and cold sores in people. This is a significant point, as with all three conditions one of the interesting aspects is so called ’latency’. This is where earlier infection, possibly unnoticed by birds or producers, can remain quiet (or latent) until some other stress or strain comes along and allows the herpesvirus to swing into action.

How will a producer know he has infected birds?

As a result of the latency, disease does not usually occur until the virus gets the upper hand through the effects of other diseases, poor hygiene or general stress in the flock.

Where did the disease come from?

The disease was first described by Josef Marek in 1907 as a condition causing paralysis of the legs and wings, due to inflammation of nerves. The condition was then known as ’fowl paralysis’. In the 1920s and 1930s an association with internal tumours (lumps and bumps in various organs such as liver, spleen, kidney and lungs) was made. The disease was later termed Mareks disease in honour of the early work of Josef Marek.

The disease became a significant limitation in the success of the developing layer and broiler industry in the 1960s, due to some very high mortality—up to 80%!

The appearance of successful live vaccines in the late 1960s was one of the most significant factors in enabling the poultry industry to develop to the extent it has today.

What can a producer expect to see if he has a problem?

Disease can manifest itself in a number of ways, related to when he flock became infected, general hygiene and biosecurity on the rearing and laying farms, the presence of other diseases, the vaccines used or the strain of virus involved.

The producer may suspect Mareks Disease if they identify paralysed birds; often with one leg trailing or wings drooped. In other cases there may simply be higher than expected mortality or a number of birds wasting away. When internal organs are affected so is the birds immune system. This increases the likelihood of outbreaks of other disease such as coccidiosis or heavy worm infestations.

Obviously none of these clues are particularly specific, so diagnosis is most usually made in the post mortem room, and confirmed by further laboratory tests on the affected organs or nerves.

So when should my birds be vaccinated?

Day old layer chicks will be vaccinated in the hatchery by injection (leg or neck muscle of the chick, or even injected directly into the egg in some countries). In the UK the best cover for the strains known to be present is by using the so called bivalent approach. This involves two vaccine strains, the older more classical THV (Turkey Herpes Virus strain), combined with the newer Rispens strain.

These vaccines have to be handled carefully due to their delicate nature, and each chick needs to receive its injection of a full dose. Hatcheries spend considerable time in ensuring vaccine is presented to the birds in the most efficient manner and most have very involved audit programmes to monitor this.

So if the vaccines and hatcheries are so good why do we still see some problems?

As producers know, no vaccine is foolproof or 100% effective and as so frequently pointed out in these columns, the art is to tip the balance in favour of the birds rather than the virus.

The first aspect is the use of the best and most effective vaccines, appropriately applied. The emergence of the second Rispens vaccine in the 1990s in the UK was in response to the appearance of newer, hotter Mareks field virus. As with so many viruses which seem to have the knack of trying to get around our best control measures, new field strains could develop which are not covered by our current vaccines. This situation is constantly being monitored.

Another important aspect is what sort of pressure is put on these vaccines from background Mareks virus challenge. This used to be a significant risk on multi-age, constantly populated rearing sites, which are now a rare source of point-of-lay pullets.

Even on single age batch rearing farms effective cleansing and disinfection with high quality disinfectants is an absolute must in dislodging this very persistent virus. The virus is excreted from infected birds in faeces and feather dander, so any surfaces, perches or equipment contaminated with dust or grease need particular attention. Keeping birds on range with buildings with lots of furniture clearly favours persistence of the virus.

Even small amounts of virus left over can penetrate a bird’s defences if she is weakened or sick through disease such as hefty red mite burdens, or if environmental conditions are poor, such as deficiencies in ventilation compromising the birds respiratory system.

Keeping all stresses and strains to an absolute minimum is another ’must’.

Conclusions

Maybe the recent rumours of increased Mareks disease problems are just that—rumours! However, disease surveillance—keeping an eye out for new or old diseases in your flocks—is a key to success on your own farm, as well as contributing to the well-being of the national flock.

This is another example of the importance of continuing to monitor diseases or deaths in your flock, however niggling, by submitting birds to your veterinary surgeon for investigation. Early intervention then allows effective control strategies and meaningful investigation of any breakdowns.