Mareks Disease

 Introduction

Marek's disease is a Herpes virus infection of chickens, and rarely turkeys, seen worldwide. Since the 1990s highly virulent strains have become a problem in North America and Europe. This disease is the most important disease of unvaccinated backyard chickens.

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 replicates initially in the respiratory tract in the lungs and B cells. It then enters feather follicles and produces virus-laden dander that is shed into litter. Eventually the virus reaches T cells, causing proliferation without virus protein production. T cells localize in brain or nerves causing encephalitis or 'floppy broiler syndrome'. If the animal survives this far, then the end stage of the disease is massive visceral tumors. 

Morbidity is around 10-50% and mortality can reach up to 100%. Mortality in an affected flock typically continues at a moderate or high rate for quite a few weeks. Affected birds are more susceptible to other diseases, both parasitic and bacterial.

Clinical Signs

Signs include paralysis of legs, wings and neck and weight loss. The bird will be dull and lethargic. It may have a grey iris or an irregular pupil size. There may be vision impairment and skin around feather follicles appear raised and roughened.

Diagnosis

History (unvaccinated) and clinical signs can provide a presumptive diagnosis of the disease. Differentials should be tested for and excluded, such as avian lymphoid leukosis, botulism, deficiancy of thiamine and deficiancy of calcium, phosphorus or vitamin D (particularly in birds at the start of lay).

Post mortem can also be performed as part of the diagnostic procedure and lesions seen at this may include the presence of grey-white foci of neoplastic tissue in liver, spleen, kidney, lung, gonads, heart, and skeletal muscle as well as thickening of nerve trunks and loss of striation. Microscopically one will see lymphoid infiltration, which is polymorphic.

Treatment and Control

Control measures include excellent hygiene and management systems e.g. 'all in/ all out' production and removal of litter between batches of chicks.

Vaccines are avaliable and used in the UK. The protocol is to administer the vaccine 'in ovum' at 18 days or at one day old. New virulent strains have evolved in recent years, which is leading to ineffective vaccines.

References

Merck & Co (2008) The Merck Veterinary Manual (Eighth Edition), Merial
Jordan, F, Pattison, M, Alexander, D, Faragher, T, (1999) Poultry Diesease (Fifth edition), W.B. Saunders
Randell, C.J, (1985) Disease of the Domestic Fowl and Turkey, Wolfe Medical Publication Ltd
Saif, Y.M, (2008) Disease of Poultry (Twelfth edition), Blackwell Publishing