Mareks Disease is a wide spread ,highly contagious viral disease of chicken causing paralysis of the leg ,wings and also cancer in various organs.it can also affect the birds nervous system .Death lose can be high and there is no satisfactory treatment ,but the disease can be prevented by vaccination ,large commercial hatcheries routinely vaccinate day-old chick, but some small flock owners may be unaware of the need for mareks vaccination. . Once the virus is introduced into a chicken flock, regardless of vaccination status, infection spreads quickly from bird to bird. Infected chickens continue to be carriers for long periods and act as sources of infectious virus. Shedding of infectious virus can be reduced, but not prevented, by prior vaccination
Affected birds are more susceptible to other diseases, both parasitic and bacterial. The route of infection is usually respiratory and the disease is highly contagious being spread by infective feather-follicle dander, fomites, etc. Infected birds remain viraemic for life. Vertical transmission is not considered to be important. 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). It is inactivated rapidly when frozen and thawed.
Clinical signs seen with MD can occur in chickens from 4 weeks of age, signs are most frequently seen between 12 and 24 weeks of age and sometimes Although later. Paralysis of legs, wings or neck are the obvious symptoms that alerts us to a chicken with Mareks Disease.
Mareks disease is a type of cancer that produces tumors in nerves .
Most of the affected birds will have some degree of lameness or paralysis, although chickens with the acute form may not show this condition. The symptoms of lameness and leg weakness without outstretching of the legs may appear with Mareks Disease after the onset of egg laying age and be confused with egg binding. Often this type of lameness is mistaken for an Ecoli or Salmonella-type joint infection, however there is no swelling or hot joints with Mareks Disease.This type of paralysis often occurs late Autumn or early Spring following a cold spell.
This form of the disease will cause clinical symptoms and eventual death of 10-15% of birds between 12-16 weeks of age but rarely above this level. The most common clinical sign is partial or complete paralysis of the legs and wings. The signs can vary from bird to bird depending on the involvement of the different nerves. When the nerves controlling the neck muscles are affected, signs such as bending of the head or torticollis are seen.
Birds with paralysis will always die often because they are unable to reach feed and water. It is helpful to perform post mortem examination on dead birds to confirm the diagnosis of Mareks disease. Here you should notice nerve enlargement, especially of the ischiatic nerve passing near the kidneys
Similarly, the involvement of the vagus nerve can result in the paralysis. Crop impactions and pendulous crops that are quite often seen in Show chickens are often attributed to Marek’s disease as the virus infiltrates the nerves of this region. Crop impaction may also be unrelated to Mareks Diseases when chickens of any age but mostly mature hens over-engorge on grass or other fibrous matter. The fibre in grasses accumulates in a entangled mass that prevents the crop from emptying properly causing it to distend and become impacted (see article in this section on Crop and Gizzard impactions)
- Grey-white foci of neoplastic tissue in liver, spleen, kidney, lung, gonads, heart, and skeletal muscle.
- Thickening of nerve trunks and loss of striation.
- Microscopically – lymphoid infiltration is polymorphic.
Marek’s Disease Control
There are no anti-viral drug available to treat Mareks Disease so we must look to understanding the nature of the disease itself in order to develop a cure.
The first step to managing this disease is to identify whether the virus is present in your family of birds as some families are resistant to the disease ,as the best way to prevent Marek’s Disease is to develop a family of birds that is naturally resistant against the virus. Acquire families of birds that are resistant to MD, because there is strong level of heritability for genetic resistance against MD. Alternatively, following an outbreak use strong male survivors as these birds have shown a resistance to MD. Developing a family of birds that is genetically resistant to Mareks Disease is by far the best way to cure the disease – but remember stress management measures are also a must!
Stress factors especially social stress is known to precipitate Mareks Disease – Social stress is any form of anxiety produced by overcrowding, poor sanitation, improper housing, other illnesses like worms, coccidiosis, lice and mites – or mixing older birds with young birds – that prevent the young birds from resting eating or drinking – basically preventing them for being happy. Stress management is the best way to control Mareks Disease in flocks that have previously experience Mareks diseases from time to time.
Genetic factors and age of initial exposure to the virus are important determinants of MD susceptibility. It causes most problems in pullet flocks under 16 weeks of age and cockerels are more resistant to Marek’s Disease. Symptoms of Marek’s Disease may also be seen in pullets near or soon after the onset of egg production at 5-6 months of age or older depending on breed of chicken.
Because there are no antiviral drugs to cure the disease there are two options open to you for preventing this disease – vaccination or to breed yourself a resistant family of chickens – whatever option you take good management practices must be followed to reduce stress, as ultimately Mareks Disease is stress induced.
Vaccination is good option for flocks or families who have experienced moderate to high levels of Marek’s Disease (MD) – or even in flocks that experience poor health for reasons that remain unclear as often in these flocks Mareks disease is the underlying cause of the problems as the virus prevents the immune system from responding to disease challenges so that coccidiosis and Ecoli infections as well as respiratory infections become more common and often there is a poor response of these diseases to treatment and they tend to recur.
Vaccination is given at one day of age because Marek’s Disease has a short incubation period of 2 weeks with signs of disease appearing in birds as young as 3 weeks of age – even though the disease is not spread through the egg and newborn chicks are protected by maternal antibodies for a few weeks.
The aim of vaccination is to prevent the virus from accumulating in the environment of the young chickens, as Marek’s Disease is highly contagious being spread through dander form feather follicles (feather dust) and transmitted by inhalation.
Vaccination is the only means for controlling Mareks Disease in infected flocks because there are no treatments that cure an infected bird.
Dr Samuel Aremu
FARMERGIANT NIGERIA LIMITED