Marek’s disease is a highly contagious viral disease that affects poultry worldwide. It poses a significant threat to the poultry industry, leading to substantial economic losses due to reduced productivity and increased mortality. This detailed guide aims to provide veterinarians, veterinary students, poultry farmers, and researchers with in-depth information on Marek’s disease, covering its etiology, epidemiology, pathogenesis, clinical signs, lesions, prevention, and control measures.

What is Marek’s Disease?

Marek’s disease, also known as avian neurolymphomatosis, is caused by an alpha-herpesvirus known as Marek’s disease virus (MDV). The disease is characterized by the development of tumors and nerve damage in chickens, leading to various clinical symptoms. Over the years, different strains of MDV have evolved, with some being more virulent and causing more severe disease.

Etiology

Marek’s disease is caused by Marek’s disease virus, a highly contagious oncogenic herpesvirus. The virus primarily affects chickens, but it can also infect other avian species. There are three serotypes of MDV:

• Serotype 1: Pathogenic strains that cause the disease.
• Serotype 2: Non-pathogenic strains isolated from chickens.
• Serotype 3: Herpesvirus of turkeys (HVT), which is used in vaccines.

Epidemiology

Marek’s disease is prevalent worldwide, particularly in areas with intensive poultry farming. It affects chickens of all ages, although young birds are more susceptible. Factors such as farm management, biosecurity practices, and vaccination strategies influence the prevalence and severity of the disease. The virus is shed from infected birds through feather dander, which can contaminate the environment and serve as a source of infection.

Predisposing Factors

Several factors can increase the susceptibility of chickens to Marek’s disease:

• Genetics: Some chicken breeds are more resistant to MDV, while others are more susceptible.
• Age: Young chickens, especially those between 2-16 weeks old, are more vulnerable.
• Stress: Factors such as overcrowding, poor nutrition, and concurrent infections can weaken the immune system.
• Immunosuppression: Birds with compromised immunity are at higher risk of developing severe disease.

Transmission

Marek’s disease is transmitted primarily through inhalation of infectious feather dust or dander shed by infected birds. The virus can survive in the environment for several months, making it difficult to eradicate from contaminated farms. Vertical transmission (from hen to egg) does not occur; the virus is transmitted horizontally from bird to bird.

Clinical Signs and Lesions

Clinical signs

• Neurological Form: Paralysis of legs, wings, or neck, leading to the inability to move or eat.
• Ocular Form: Changes in eye color (gray eye), leading to partial or complete blindness.
• Cutaneous Form: Skin lesions characterized by enlarged feather follicles.
• Visceral Form: Tumors in internal organs such as the liver, spleen, and kidneys.

Postmortem Lesions

Upon postmortem examination, birds affected by Marek’s disease typically present with:

• Enlarged nerves: Sciatic, brachial, and vagus nerves appear thickened and swollen.
• Tumors in organs: Visceral tumors in the liver, spleen, heart, and kidneys.
• Eye lesions: Lymphocytic infiltration in the iris, leading to graying of the eye.
• Skin lesions: Thickening of the skin at the feather follicles.

Pathogenesis

The pathogenesis of Marek’s disease involves several stages:

Early Infection: The virus infects the respiratory tract and disseminates through the blood.

Latent Infection: The virus remains dormant in the lymphoid tissues, avoiding the immune response.

Reactivation: Under stress or immunosuppression, the virus reactivates, causing clinical disease and tumor development.

Oncogenesis: The virus induces the transformation of T lymphocytes, leading to lymphoma formation in various organs.

Prevention and Control

Prevention and control involve a combination of vaccination, biosecurity, and management practices:

Vaccination: The most effective measure is vaccination since administering vaccines at hatch actively prevents the onset of clinical disease.

Biosecurity: Implementing strict biosecurity measures helps to actively prevent the introduction and spread of MDV.

Genetic Selection: Breeding for resistant lines of chickens.

Management Practices: Reducing stress, ensuring proper nutrition, and maintaining good farm hygiene.

Vaccination Schedule

Treatment

There is no effective treatment for Marek’s disease once clinical symptoms appear. Therefore, the focus must be on prevention through vaccination and biosecurity measures. While supportive care, such as providing adequate nutrition and managing secondary infections, may improve the bird’s quality of life, it will not cure the disease.

Conclusion

Marek’s disease poses a significant threat to the poultry industry; however, effective vaccination, proper management, and strict biosecurity measures actively minimize its impact. By remaining informed and taking proactive steps, the poultry community can reduce the risks associated with the disease, ultimately enhancing the health and productivity of their flocks.

Add also: Newcastle Disease in Chickens

References

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