Marek’s Disease: The Basics

What is Marek’s Disease?

Marek’s disease is a highly contagious viral illness that primarily affects chickens. It is caused by a herpesvirus, specifically the Marek’s disease virus (MDV), and is one of the most common and devastating poultry diseases worldwide. While chickens are the primary host, the virus does not affect humans or other animals.

Marek’s disease is often characterized by paralysis, tumors, weight loss, immune suppression, and in severe cases, sudden death. It’s especially concerning because once the virus enters a flock, it can persist for life in carrier birds and the environment, making it nearly impossible to eradicate.

While Marek’s disease cannot be cured, it can be managed with good biosecurity practices, vaccination, and proper flock management.

Different Forms of Marek’s Disease

Marek’s disease manifests in four primary forms, each targeting different parts of the bird’s body. The severity and symptoms depend on the form of the disease and the immune status of the affected bird.

1. Nerve Form (Classical or Neurolymphomatosis)

This is the most common and recognizable form of Marek’s disease and is caused by inflammation or tumors in the nerves.

Symptoms:

• Paralysis of Legs or Wings: One leg may extend forward while the other stretches backward (a characteristic posture known as the “splits”).
• Wing Droop: Birds may have difficulty lifting one or both wings.
• Neck Paralysis: The neck may become twisted or limp (torticollis).
• Loss of Balance: Birds may stagger, fall, or be unable to stand.
• Delayed Reflexes: Affected birds may respond slowly to stimuli.

These symptoms can progress to complete paralysis, leaving the bird unable to move, eat, or drink.

2. Visceral Form (Tumors in Internal Organs)

This form is caused by tumors growing in the internal organs and is often fatal.

Symptoms:

• Sudden Death: Birds may die with no prior signs of illness.
• Weight Loss: Birds may waste away even if they appear to be eating.
• Lethargy: Birds may seem listless or tired.
• Digestive Issues: Tumors in the liver, spleen, or intestines can lead to diarrhea and poor digestion.

Affected Organs:

• Liver
• Spleen
• Heart
• Kidneys
• Intestines

This form of Marek’s is often diagnosed post-mortem during a necropsy when tumors are visible in the affected organs.

3. Ocular Form (Eye Marek’s Disease or Gray Eye)

This rare form of Marek’s disease affects the eyes, leading to vision impairment or blindness.

Symptoms:

• Gray or Milky Eyes: The iris may lose its normal color and appear cloudy.
• Irregular Pupil Shape: The pupil may become constricted or unevenly shaped.
• Blindness: Affected birds may become completely blind and disoriented.
• Reduced Response to Light: The bird may not react to bright light or movement.

4. Cutaneous Form (Skin Lesions)

The cutaneous form of Marek’s is less common and primarily affects the skin and feather follicles.

Symptoms:

• Bumps or Nodules: Small tumors may appear at the base of feather follicles.
• Scabby Lesions: Scabs or crusty areas may develop, often near the vent or under the wings.
• Feather Loss: Birds may lose feathers in affected areas.

While this form is less severe than others, it can still contribute to poor flock health overall.

How Marek’s Disease Spreads

Understanding how Marek’s spreads is crucial to managing and preventing outbreaks in your flock. The Marek’s disease virus (MDV) spreads through several pathways, with infected birds being the primary source of transmission.

1. Feather Dust and Dander

The most common way Marek’s disease spreads is through infected feather dust and dander. The virus replicates in the feather follicles of infected birds, which release viral particles into the environment.

• Inhalation: Healthy chickens become infected when they inhale dust or dander that contains the virus.
• Persistence in Dust: Feather dust contaminated with Marek’s can remain infectious for months, even under normal environmental conditions. This is why regular cleaning and dust control are critical in a Marek’s-positive flock.

2. Direct Contact

While inhalation is the primary mode of transmission, direct contact with infected birds or their secretions can also spread the disease. This includes:

• Saliva and Feces: The virus is shed in the saliva and droppings of infected birds, contaminating shared food, water, and surfaces.
• Feather Shedding: Direct contact with feathers or skin from infected birds can expose healthy chickens to the virus.

3. Indirect Transmission

The virus can also spread indirectly when contaminated objects or people move between coops. For example:

• Shared Equipment: Feeders, waterers, and cleaning tools can transfer the virus if not properly sanitized.
• Visitors: People who visit multiple flocks without proper biosecurity can carry contaminated dander on their clothing, shoes, or hands.
• Wild Birds: While wild birds are not as common a vector, they can occasionally introduce Marek’s disease into your flock by carrying contaminated material.

4. Vertical Transmission (Not Possible)

It’s important to note that Marek’s disease is not passed directly from hen to egg. However, chicks can be infected immediately after hatching if they are exposed to contaminated dander in the brooder or environment.