It is easy to get distracted by the headline-grabbing poultry health issues of avian influenza, Newcastle disease and salmonella. While these may well be the most pressing disease problems for our industry, the danger is that concentrating all our time and resources on them tends to ignore diseases that may be having a chronic and significant impact on performance and profitability, day in and day out.
With outbreaks of Marek's disease occurring in 2007, this article provides a timely reminder to producers that Marek's disease is a hidden menace. It is caused by a very resistant and hence very persistent herpesvirus which causes infection in chickens and, to a lesser extent, turkeys.
Control of Marek's disease infection depends on a combination of effective vaccination and high-quality cleansing and disinfection of poultry houses. Any deficiencies in either area can tip the balance in favour of the virus rather than the bird.
Marek's disease was first described over 80 years ago as a condition causing paralysis of the legs and wings due to inflammation of the nerves. The condition was then known as fowl paralysis. In the 1920s and 1930s, an association with internal tumours in various organs such as liver, spleen, kidneys and lungs was made. The disease became a significant limitation to the success of the developing layer and broiler industries in the 1960s due to some very high mortality rates of 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 that it has today.
The disease can manifest itself in a number of ways related to the type of bird, when the flock became infected, general hygiene and biosecurity on the rearing and laying farms, the presence of other diseases and the vaccines used or the strain of virus involved.
Signs and symptoms
In laying stock, producers may suspect Marek's disease if they identify paralysed birds; often with one leg trailing or wings dropped. In other cases, there may simply be higher than expected mortality or a number of birds wasting away.
In broilers, losses are mostly due to high condemnation rates at slaughter predominantly from the cutaneous (skin) form of Marek's disease. In free-range broilers or birds taken to heavier weights, mortality may result from internal tumours, predominantly of liver and spleen.
In turkeys, acute mortality can be associated with internal tumours of liver and spleen often where turkeys have been reared in the same accommodation as infected chickens.
Even where there is no acute mortality, the effects on internal organs can lead to significant damage to the immune system. This can be associated with suboptimal performance and clinical outbreaks of other diseases such as coccidiosis, heavy worm infestations or viral diseases such as Gumboro disease. The impaired immune system is also unable to respond to routine vaccinations and hence will leave flocks unable to resist a whole range of disease challenges which affect performance.
All this means that if the background challenge from Marek's disease virus is allowed to build up, performance can suffer even without a specific diagnosis of clinical Marek's disease being made.
This vicious circle continues and a "silent infection" build-up can put pressure on any Marek's disease virus vaccination programme. It is thought that this phenomenon has led to the regular emergence of more and more virulent pathotypes of Marek's disease virus since vaccines first appeared in the 1970s. Researchers then had to work hard to develop increasingly effective vaccines to keep ahead of the virus and prevent clinical outbreaks.
Vaccination is far from simple. Due to the persistence of the virus in the environment, vaccination is a race against time to get the bird to produce a protective response before it meets field infection. These requirements and the nature of the virus mean that protection can only be achieved by physically injecting chicks with live vaccine. This is done either on one-day olds by intramuscular injection into the leg or neck, or increasingly into the unhatched embryo by injecting hatching eggs at transfer.
The successful response to vaccination depends on choosing the correct vaccine strain, correct preparation of this delicate vaccine and its accurate administration to each and every chick. Even then, it is possible to overcome the protection offered by the vaccine by exposing vaccinated birds to field virus too early or in such large amounts that the vaccine response is overwhelmed.
To gain the benefit of all this hard work it is important to reduce the exposure of young birds to virulent field virus. Newly hatched chicks should be placed in thoroughly cleaned and disinfected houses that are well separated from houses with older chickens. This can be achieved by using a structured biosecurity programme concentrating on terminal cleansing and disinfection using products of proven virucidal activity.
In infected birds, the virus is excreted in feather dander and dust and therefore environmental contamination of all surfaces and litter can be very high from infected flocks. Furthermore, since the virus survives at ambient temperature for a long time (65 weeks) when cell associated and is resistant to some disinfectants (quaternary ammonium and phenol) the selection of an appropriately virucidal disinfectant is important.
It is therefore essential that first-stage cleaning is capable of removing all organic material from surfaces using good physical cleaning and degreasing agents to lift material from perches, nests and other wooden surfaces.
Vaccination and hygiene for control
Marek's is a viral disease that can cause significant economic damage to poultry production through a variety of direct and indirect effects.
Vaccination alone will not control all these effects. This stubborn virus can survive in the environment for 65 weeks. Feather follicles in the skin are the site of viral replication resulting in feather, dander, dust and litter containing the virus which is then spread by the airborne route.
Good hygiene practices that prevent "carry-over" of Marek's organisms, thus ensuring that each new crop gets a completely clean fresh start, in combination with effective vaccination are key to controlling the disease.