Marek’s Disease (MD) Overview
Marek’s disease (MD) is a widespread, highly contagious, tumor-causing viral disease caused by the Marek’s disease virus (MDV), also known as Gallid herpesvirus 2 (GaHV-2). Among susceptible birds, MDV infection causes lymphoproliferative and neurologic disease in chickens, turkeys, and Japanese quail, resulting in the formation of malignant T cell lymphomas. The only current method of prevention of MD in chickens is by vaccination. However, the vaccine doesn't protect the bird from becoming infected with the virus, it only inhibits the onset of tumor formation, but not that of the virus infection on its own. Thus, mixing vaccinated birds with non-vaccinated birds will put those which have not received the vaccine at risk of becoming infected with MDV.
What Causes Marek's Disease
MDV is a type of herpesvirus. Herpesviruses are double-stranded DNA viruses which got their name from the Greek word herpes, έρπης, meaning "creep". A common feature of herpesviruses is latency, which is defined by viral infection without replication or production of infectious virions. This group of viruses are known for gravitating towards (and replicating in) lymphoid tissue---where in humans and other mammals, this refers to the lymph nodes. Chickens do not possess lymph nodes in confined areas of their bodies like mammals, instead their lymphoid tissue is spread throughout the body.
MDV shares molecular properties with several other herpesviruses causing lymphomas---such as:
- Epstein-Barr virus (EBV): Also known as human herpesvirus 4, EBV is found worldwide and is one of the most common viruses that affects humans. EBV can cause infectious mononucleosis, also called mono, and other illnesses in humans.
- Kaposi-Sarcoma associated virus (KSHV): Also known as human herpesvirus 8 (HHV8). KSHV infection causes Kaposi sarcoma (KS), which affects the cells that line human's blood and lymphatic vessels (known as endothelial cells), eventually leading to the development of cancer cells. Many people can be infected with KSHV without developing KS.
- Herpes Simplex Virus (HSV): HSV infection causes oral and genital herpes in humans---these are described in two types of HSV called HSV-1 (oral herpes), and HSV-2 (genital herpes).
- Varicella Zoster Virus (VZV): VZV infection causes Shingles outbreaks in humans.
How Marek's Disease is Transmitted
Birds become infected with MDV through exposure to infectious MDV virions in feather dander from infected birds. The virions mature in the feather follicle cells, where they are later shed into the environment. Many seemingly healthy birds are actually infected with MDV, regularly shedding the infectious MDV virions into their surrounding environment. Both infected (this includes vaccinated birds) and recovered chickens are lifelong carriers of the MDV, and will continue to shed the virus in their feather dander and oral and nasal secretions for life. Therefore, it is important to know if any of the birds in your flock are infected, not to "cull" them but simply to keep in mind that if you bring any new chickens to the premises, that they are going to be at risk of becoming infected with MDV if they are kept in the same environment and did not receive the vaccination. MDV is not transmitted through the parents to the egg.
The virus has been known to survive for at least one year
in the environment. MDV infections can be detected through virus isolation and by the demonstration of the presence of viral antigens or antibodies, which can be accomplished by testing feather follicles.
Clinical Signs of Marek's Disease
Only a small percentage of chickens infected with MDV will develop clinical form of the disease. When chickens do develop clinical signs of MD, they usually develop when the bird is between 2 to 4 months of age. This is because onset of clinical signs is usually triggered by a stress, such as that each young bird's body goes through during hormonal growth changes ( start of egg laying in hens or crowing in roosters). This is why there are sometimes incidents where chickens older than 4 months of age develop signs of MD. It is actually just a delay in the presentation of signs, where they appear fine until a stressful event occurs (predator attack, rough handling, inner flock fighting, change of ownership or management, extreme weather changes, high parasite loads, etc.), causing them suddenly go lame. This initial lameness is often mistaken as an injury, which progressively worsens until the bird can barely or is unable to walk. These birds may eventually develop the classic MD paralysis pose, where one leg is positioned straight out in front of their bodies, with the other straight out behind. Sometimes they may appear as if they are using their wings in an attempt to balance themselves.
Different Forms of Marek's Disease
MD can develop in several different forms with the most common being a lymphoproliferative syndrome, associated with the growth of lymphomas throughout the bird's body. MD most commonly results in the enlargement of the peripheral nerves, which become infiltrated with immature mononuclear cells.
|Form||Gross pathology||Clinical signs|
|Neurological form ||Enlarged or inflammation of the peripheral nerves. In some cases may be 2-3 times normal size with loss of crossstriations and gray or yellow discoloration.||Incoordination or stilted gait as an initial first sign, followed by progressive, paralysis of one or more legs (often presents as one leg stretched forward and the other leg backwards), wings (wing drooping), or crop (gasping and darkening or purplish coloring of comb resulting from lack of oxygen).|
|Cutaneous (skin) form||Associated with enlarged feather follicles, often scattered or clustered together, especially on the legs, shanks and feet||Reddened, bloody looking shanks |
|Ocular (eye) form||Ocular lesions||Gray, often misshapen or shrunken iris or eyes with unequally-shaped pupils. Sometimes conjunctivitis and edema are observed. Chickens often progress to develop blindness in one or both eyes.|
|Transient paralysis (TD)||Vasogenic brain edema||Incoordination (ataxia), partial to complete paralysis lasting only temporary (1 to 2 days), afterwhich the chicken recovers. However many of these cases result in chickens developing clinical MD several weeks later.|
|Visceral (internal) form||Lymphomas occur in visceral organs||Often chickens will die suddenly without any apparent cause. In other cases, when signs do appear they may include depression and stunted growth.|
How Marek's Disease is Diagnosed
There are a number of laboratory tests, offered by different diagnostic testing facilities, that can be done to find the Marek's disease virus (MDV) in chickens. Tests for MDV can be done using different types of samples, including blood, tissue, and feathers. In the United States, North Carolina State University Poultry Tumor Lab
has an extensive list of available tests.