Marek’s Disease in Poultry
Introduction
Marek’s Disease is a highly contagious, oncogenic viral disease of chickens characterized by T-cell lymphoma formation, nerve damage, and immunosuppression. It is one of the most economically important neoplastic diseases in poultry production worldwide due to mortality, reduced productivity, and condemnation at slaughter.
Etiology
Causative Agent
Marek’s Disease is caused by Marek’s Disease Virus (MDV), also known as Gallid alphaherpesvirus 2, a member of the genus Mardivirus, family Herpesviridae.
Virus Characteristics
- Enveloped double-stranded DNA virus
- Highly cell-associated in infected birds
- Very stable in feather dander in the environment
- Oncogenic (induces tumor formation)
Epidemiology
Species Affected
- Chickens (primary host)
- Occasionally affects turkeys (rare and less severe)
Age Susceptibility
- Most severe in young chickens (2–16 weeks of age)
- Chicks infected early in life are at highest risk of tumor development
Transmission
- Inhalation of infected feather follicle dander (primary route)
- Horizontal transmission between birds
- Virus persists in poultry house dust and litter
Risk Factors
- Unvaccinated flocks
- High-density production systems
- Poor ventilation and hygiene
- Genetic susceptibility of certain breeds
Pathogenesis
Initial Infection
The virus enters via the respiratory route and infects lymphoid tissues, especially T lymphocytes.
Phases of Infection
- Cytolytic phase: early destruction of lymphocytes causing immunosuppression
- Latent phase: virus persists in T cells without clinical signs
- Transforming phase: neoplastic transformation of T cells leading to lymphoma
Target Tissues
- Peripheral nerves → inflammation and enlargement
- Visceral organs (liver, spleen, gonads) → tumors
- Skin and feather follicles → tumor formation
Clinical Signs
Neurological Form
- Paralysis of legs, wings, or neck
- Unilateral leg paralysis (“split leg”)
- Ataxia
Visceral Form
- Weight loss
- Depression
- Enlarged organs due to tumors
Ocular Form
- Gray or irregular pupil (“gray eye”)
- Blindness
Cutaneous Form
- Nodules in skin and feather follicles
Summary for Practitioners
Marek’s disease should be suspected in unvaccinated flocks with paralysis, weight loss, and tumors in multiple organs, especially in birds between 6–20 weeks of age.
Postmortem Findings
Gross Lesions
- Enlarged peripheral nerves (sciatic, brachial)
- Multifocal tumors in liver, spleen, kidneys, gonads
- Asymmetrical enlargement of affected nerves
- Atrophy of thymus and bursa in some cases
Microscopic Lesions
- Lymphoid tumors composed of T lymphocytes
- Nerve demyelination and lymphoid infiltration
- Neoplastic transformation of lymphoid cells
Diagnosis
Clinical Diagnosis
Based on paralysis, nerve enlargement, and tumor formation in young chickens.
Laboratory Diagnosis
- Histopathology of tumors and nerves
- PCR for viral DNA detection
- Virus isolation (less commonly used)
Differential Diagnosis
- Lymphoid leukosis
- Avian leukosis virus infection
- Nutritional deficiencies causing paralysis
- Botulism (acute flaccid paralysis)
Summary for Practitioners
Differentiation from lymphoid leukosis is essential, as both diseases cause tumors but differ in age of onset and nerve involvement.
Treatment
General Approach
There is no treatment for Marek’s disease.
Management
- Culling of affected birds
- Proper disposal of carcasses
- Improved biosecurity and hygiene
Summary for Practitioners
Treatment is not possible. Prevention is the only effective control strategy.
Control and Prevention
Vaccination
- Live attenuated vaccines (e.g., turkey herpesvirus vaccine)
- Administered at day-old in hatcheries
- Does not prevent infection but prevents tumor formation
Biosecurity
- Cleaning and disinfection of poultry houses
- Control of dust and feather dander
- All-in/all-out production systems
Genetic Resistance
- Some chicken lines have higher genetic resistance
Summary for Practitioners
Vaccination at hatchery level combined with strict hygiene is the cornerstone of Marek’s disease control.
Zoonotic Importance
Marek’s disease is not zoonotic and does not infect humans.
Economic Importance
Marek’s disease causes major economic losses due to mortality, reduced growth, tumor condemnation at slaughter, and vaccination costs.
Summary
Marek’s disease is a highly contagious oncogenic herpesvirus infection of chickens characterized by lymphoid tumors, nerve damage, and immunosuppression. Prevention through vaccination and biosecurity is essential for control.
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