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July 21, 2025Marek’s Disease in Poultry – A Comprehensive Overview
What Is Marek’s Disease?
Marek’s Disease (MD) is a viral, lymphoproliferative, and neuropathic disease primarily affecting chickens, and occasionally other poultry such as turkeys and quails. It is caused by the Marek’s disease virus (MDV), a highly contagious herpesvirus known as Gallid alphaherpesvirus 2.
This disease is present worldwide wherever chickens are raised. Unless flocks are maintained under strict pathogen-free conditions, they are likely to be exposed to MDV.
Pathogenesis & Virology
The progression of MDV infection occurs in several phases:
Initial cytolytic phase – Early infection in B lymphocytes, leading to cell destruction.
Latency phase – The virus persists silently in the host’s cells.
Second cytolytic phase – Causes immune suppression.
Proliferative phase – T-cell tumors form in various organs.
The feather follicle epithelium acts as the main viral reservoir, releasing infectious particles into the environment via dander and dust. These particles can remain infectious for months.
Epidemiology & Transmission
Hosts: Chickens are most commonly affected, but turkeys, quail, and peafowl may also be infected.
Spread: Through inhalation of contaminated dander or dust, direct contact between birds, and via contaminated shoes, clothing, or equipment.
Environmental Survival: The virus can persist in poultry houses for months, sometimes years.
Age Susceptibility: Most common in birds aged 6–20 weeks, but cases can occur as early as 3 weeks or as late as 1 year.
Prevalence: Clinical disease rates range from 1% to over 50% depending on strain virulence, breed susceptibility, and environmental conditions.
Clinical Signs & Lesions
Marek’s Disease presents in several forms, each with distinct symptoms:
Neurological Form:
Unilateral leg or wing paralysis (“splits” posture).
Twisted neck (torticollis) or incoordination.
Crop-emptying problems and breathing difficulty if certain nerves are affected.
Ocular Form:
Changes in iris color or shape.
Blindness in one or both eyes.
Visceral Form:
Tumors in liver, spleen, kidney, heart, ovaries, and other organs.
Weight loss, lethargy, depression, and reduced appetite.
Cutaneous Form:
Enlarged feather follicles, especially on the legs.
General Signs:
Loss of appetite, pale combs, dehydration, diarrhea, or sudden death without warning.
Diagnosis
Veterinarians rely on:
Clinical history and visible symptoms such as paralysis or blindness.
Post-mortem findings, including nerve enlargement and internal tumors.
Histopathology to confirm T-cell tumors and rule out similar diseases like lymphoid leukosis.
Advanced testing such as PCR or immunohistochemistry for definitive diagnosis.
Economic & Global Impact
Marek’s Disease is one of the most common poultry diseases worldwide, capable of causing:
High mortality rates, sometimes 40–80% in severe outbreaks.
Decreased growth rates and reduced egg production, even in subclinical cases.
Significant economic losses for both smallholder and commercial poultry farmers.
Summary Table
| Category | Details |
|---|
| Causative Agent | Gallid alphaherpesvirus 2 (MDV-1) |
| Hosts | Chickens (primary), sometimes turkeys and quail |
| Transmission | Inhalation of dander/dust, contaminated environment, fomites |
| Durability | Virus remains infectious for months in the environment |
| Affected Age | 3 to 20 weeks (peak), can vary |
| Disease Forms | Neurological, ocular, visceral, cutaneous |
| Main Symptoms | Paralysis, blindness, tumors, weight loss |
| Diagnosis | Clinical signs, necropsy, histopathology, PCR |
| Impact | High mortality, production loss, global spread |
Conclusion
Marek’s Disease is a major threat to poultry health, with its ability to cause tumors, paralysis, and blindness, while persisting in the environment for long periods. Understanding how it spreads, its signs, and how it affects your flock is key to effective prevention and control
