What are the symptoms of Marek's disease in chickens?
Marek’s Disease (MD) is a highly contagious viral disease in poultry, characterized by T-cell lymphomas and peripheral nerve enlargement. Standard diagnostic criteria include case history, clinical signs, gross necropsy, and histopathology.
Marek’s Disease (“MD” or “fowl paralysis“), prevalent in gallinaceous birds, is transmitted to chicks via a herpes virus. Like all herpes viruses, once it enters a bird’s body, it infects the bird for life. The herpes virus remains viable and active in environments with temperatures between 20 to 25 degrees Celsius for over 12 months, making it difficult to control. The percentage of flock involvement with clinical signs of Marek’s Disease depends on the virus strain and bird breed. Laying hens (especially Leghorns) are more susceptible than broiler breeds.
Transmission and Epidemiology of Marek's Disease in Poultry: A Comprehensive Overview
The initial clinical outbreaks of Marek’s Disease (MD) were documented in turkeys in France, Israel, and Germany, characterized by a mortality rate of 40% to 80% [MOU1] due to tumor development, occurring between 8 and 17 weeks of age. Marek’s Disease Virus (MDV) can induce disease in poultry of any age, including embryos within eggs. However, due to their early slaughter age, broiler chickens are less susceptible to MD.
The most frequently reported age of MDV infection in various references is 3 weeks, with some studies indicating 6 weeks.
The incidence of Marek’s Disease in commercial poultry flocks exhibits considerable variability and is influenced by the following factors:
- Virus strain and dose
- Age at initial exposure
- Maternal antibody titers
- Host sex and genetic predisposition
- Vaccine virus strain and dose
- Multiple environmental stressors
Transmission and Spread of MD's Disease in Poultry: A Comprehensive Analysis
Marek’s Disease Virus (MDV) can manifest wherever poultry rearing occurs. The herpes virus responsible for marek’s disease is highly contagious and rapidly disseminates throughout unvaccinated flocks. Infected poultry shed the virus for the duration of their lifespan. The virus is released from feather follicle dander and readily spreads through dust and dander, infecting poultry via inhalation. Additionally, the virus contaminates litter (poultry feces), equipment, and poultry farm personnel clothing.
This virus can remain viable for several months, or even over a year, at room temperature, even after depopulation or removal of all infected poultry. Marek’s Disease Virus is not vertically transmitted (from mother to offspring). Furthermore, it is not transmissible to humans.
In situations where poultry housing facilities are in close proximity, it is imperative to ensure that new flocks are fully vaccinated and that poultry chicks are maintained in strict quarantine. It is also crucial to emphasize that Marek’s Disease can be transmitted and spread through personnel handling, including via hands, clothing, footwear, hair, and skin. Therefore, strict adherence to disinfection protocols and clothing changes is mandatory.
Diagnosis and Clinical Signs of Marek's Disease in Poultry: A Comprehensive Overview
Marek’s Disease (MD) is a lymphomatous and neuropathic disease caused by an alphaherpesvirus. Diagnosis is based on clinical signs, characteristic lesions, and histopathological examination of tissues or biopsy. Most veterinary diagnostic laboratories can investigate this disease in necropsy samples. Definitive diagnosis is based on tumor identification, not merely infection.
Classical MD can occur at any time, sometimes even immediately after hatching. Clinical signs include leg and wing paralysis, accompanied by peripheral nerve enlargement. However, nerve involvement is not always observed, particularly in adult birds. In severe cases, MD can lead to increased mortality in poultry aged 1 to 2 weeks, especially if they lack maternal antibodies. Depending on the MDV strain, lymphomatous lesions can develop in various organs, including gonads, liver, spleen, kidneys, lungs, heart, proventriculus, and skin.
Clinical signs of Marek’s Disease depend on the tissues targeted by the virus. In the classical form, MD causes inflammation and tumors in the nerves, spinal cord, and brain. This leads to leg and wing paralysis and may also cause head tremors. Birds with Marek’s Disease either succumb to the disease, become trampled, or develop severe skin lesions. Poultry infected with Marek’s Disease rarely recover.
Etiology of Marek's Disease in Poultry: A Comprehensive Overview
The causative agent of Marek’s Disease is Marek’s Disease Virus (MDV), which belongs to the Mardivirus genus within the Alphaherpesvirinae subfamily. Within the Mardivirus genus, three closely related species are recognized, representing three serotypes of Marek’s Disease Virus. Gallid alphaherpesvirus 2 (MDV serotype 1) encompasses all pathogenic MDV strains and is further categorized into pathotypes known as mild (m), virulent (v), very virulent (vv), and very virulent plus (vv+). Gallid alphaherpesvirus 3 (MDV serotype 2) and Meleagrid alphaherpesvirus 1 (turkey herpesvirus, MDV serotype 3) are non-pathogenic viral strains isolated from chickens and turkeys, respectively. They are commonly employed as vaccines against Marek’s Disease.
Strains of MDV in Poultry: Clinical Manifestations and Pathological Findings
In the classical or neural form of Marek’s Disease (MD), primarily characterized by neural involvement in poultry, mortality rates seldom exceed 10 to 15 percent and can occur over several weeks to months. Clinical signs of MD in the neural form include:
- Partial or complete paralysis of legs and wings.
- Enlargement of one or more peripheral nerves – the thickness of affected nerves increases two to three times the normal thickness. The appearance of the nerves is not normal and shiny, and the nerve may appear grayish or yellowish and sometimes edematous. The sciatic, brachial, and sometimes vagus nerves are most affected and are easily visualized during post-mortem examination.
In the lymphoproliferative form, typically diagnosed by visceral lymphoma in multiple organs, disease incidence of 10 to 30 percent in a flock is not uncommon, and outbreaks of up to 70 percent can occur. Mortality may increase rapidly over a few weeks and then cease, or it may continue at a steady rate or slowly for several months. Clinical signs in poultry with the lymphoproliferative form of MD include:
- Lethargy.
- Loss of balance.
- Weakness and signs of immunosuppression.
In the lymphoproliferative form, multicentric lymphoma affecting the liver, gonads, spleen, kidneys, lungs, proventriculus, and heart is observed. Lymphomatous lesions appear as distinct white masses or enlargement in the affected organ, primarily in the liver, spleen, and proventriculus. Lymphomas can also develop in the skin around feather follicles and in skeletal muscles.
Article: On Newcastle Disease with Paralysis Symptoms and Treatment of Newcastle Disease in Chickens
Clinical Diagnosis of Marek's Disease in Poultry: Signs and Symptoms
Clinical examinations are instrumental in the diagnosis of Marek’s Disease (MD). Clinical signs and necropsy findings are valuable diagnostic aids. The following is a comprehensive overview of clinical and post-mortem signs of Marek’s Disease:
- Neural Form of Marek’s Disease: The neural form of Marek’s Disease is characterized by one or more of the following signs:
- Progressive paralysis, typically in the legs. A bird with leg paralysis often extends one leg forward and places the other leg underneath itself. This results from lymphocyte infiltration of the sciatic nerve.
- Wing paralysis.
- Weight loss.
- Labored breathing.
- Diarrhea.
- Starvation and death due to the inability to access feed and water or being trampled by other birds.
- Ocular Form of Marek’s Disease: Gray eye color, misshapen iris, and even tumor presence in the eyes – due to lymphocyte infiltration of the eye.
- Cutaneous Form: Swollen feather follicles (bumps) on the skin, which can form crusty scabs.
- Weight Changes: Adult birds often become emaciated and weak, while young birds experience severe weight loss.
- Visceral Form of Marek’s Disease: Tumors in internal organs, including the heart, ovaries, liver, kidneys, and lungs. A consistent necropsy finding in birds is the enlargement of peripheral nerves. Various nerves, particularly the vagus, brachial, and sciatic nerves, are enlarged and have lost their striations. Nodular or diffuse lymphoid tumors are also observed in various organs, especially the liver, spleen, gonads, heart, lungs, kidneys, muscles, and proventriculus.
- The bursa is rarely involved in tumor formation and often becomes atrophic. When the bursa is involved, tumor cells typically appear in the interfollicular regions.
Article: the Chicken’s Digestive System: A Key to Understanding and Identifying Poultry Diseases
Differential Diagnosis of MD in Poultry: Distinguishing Similar Diseases
Tumors induced by Marek’s Disease Virus (MDV) may resemble tumors caused by retroviral pathogens such as avian leukosis virus (ALV) and reticuloendotheliosis virus (REV), necessitating careful differentiation. Unlike the uniform cell population observed in lymphoid leukosis (LL), Marek’s Disease lymphomas consist of diverse pleomorphic lymphoid cells. This means the lymphoid cell population in LL lymphomas is more homogenous compared to MD lymphomas, which exhibit greater cellular diversity. Lymphomas refer to tumors in the bird’s body.
Furthermore, lymphoid leukosis and reticuloendotheliosis typically manifest in birds older than 16 weeks, while Marek’s Disease commonly affects younger chicks. Additionally, paralysis is a prominent clinical sign in Marek’s Disease but is not typically observed in the other two diseases. However, nerve enlargement and T-cell lymphomas in visceral organs are common features shared by all three diseases.
Prevention of Marek's Disease: Strategies for Safeguarding Your Poultry Flock
The most reliable method of prevention is vaccination of day-old chicks. Birds should be vaccinated before exposure to the virus. Immunity develops within 4 to 7 days post-vaccination. During this period, strict quarantine measures are essential. If personnel must move between different poultry houses, they should shower and change clothing.
It is important to note that Marek’s Disease vaccination is primarily administered to breeder and layer chicks, as the disease is less prevalent in broiler chickens due to their shorter lifespan.
The optimal method for chick vaccination is in the hatchery itself. However, if you intend to vaccinate the chicks yourself, adhere to the following guidelines for proper Marek’s Disease vaccination:
- The vaccine must be refrigerated before use – never allow the vaccine to warm up during storage, as this will inactivate it.
- Wash hands thoroughly before vaccination.
- Use sterile gloves and needles.
- Follow the manufacturer’s instructions diligently.
- When reconstituting the vaccine powder with diluent, ensure the diluent temperature is within the recommended range. As the vaccine is live, extreme temperatures or prolonged exposure to air will render it ineffective.
- The reconstituted vaccine should be administered within two hours of preparation. Do not store unused vaccine.
- As Marek’s Disease vaccines are typically sold in large quantities, they are not suitable for small-scale or backyard poultry farming. Therefore, it is crucial to source day-old chicken from reputable commercial hatcheries that provide vaccination services.
Furthermore, before the arrival of new chicks, thoroughly remove all residual materials from previous flocks and disinfect all equipment and the poultry house.
Allow at least one week between the departure of the previous flock and the arrival of the new chicks.
Commonly used Marek’s Disease vaccines include:
- Turkey Herpesvirus (HVT, naturally non-pathogenic Meleagrid alphaherpesvirus 1)
- SB-1 or 301 B/1 (naturally non-pathogenic Gallid alphaherpesvirus 3)
- CVI988/Rispens (attenuated Gallid alphaherpesvirus 2)
HVT recombinant vaccines contain genes from other poultry viruses. These include Newcastle disease virus, infectious bursal disease virus, or infectious laryngotracheitis virus, and they are gaining popularity. These recombinant vaccines provide protection against both Marek’s Disease and the inserted virus. Bivalent vaccines comprising HVT and SB-1 or 301 B/1 strains of Gallid alphaherpesvirus 3 offer enhanced protection against virulent Marek’s Disease Virus strains. CVI988/Rispens, an attenuated Marek’s Disease Virus strain, appears to be the most protective commercially available vaccine. It is often administered in combination with HVT.
HVT recombinant vaccines contain genes from other poultry viruses. These include Newcastle disease virus, infectious bursal disease virus, or infectious laryngotracheitis virus, and they are gaining popularity.
In ovo vaccination, administered to embryos on the 18th day of incubation.
Common Misconceptions About Marek's Disease in Poultry: Debunking Myths
Due to the fact that some Marek’s Disease vaccines are derived from diseased turkeys, some believe that co-mingling turkeys with chicken flocks will naturally expose the chicks to the herpes virus, thereby conferring immunity. Turkey Herpesvirus (HVT) is a strain related to Marek’s Disease and is indeed administered to chicks as a Marek’s Disease vaccine. However, while other avian species may exhibit some resistance to HVT, co-mingling turkeys with chickens does not provide protection against other Marek’s Disease Virus strains.
In fact, it may expose them to common chicken diseases such as mycoplasmosis and blackhead disease.
Furthermore, some believe that foregoing vaccination will allow subsequent generations of chicks to develop genetic resistance to Marek’s Disease. To date, there is no scientific evidence supporting this claim. If this were the case, poultry breeding companies would have already produced Marek’s Disease-resistant chicken strains.
Another common misconception is that vaccinating a portion of the flock will induce herd immunity throughout the entire flock. This is a false belief. If the entire flock is not vaccinated with the full recommended dose, the flock will remain susceptible to Marek’s Disease.
Transmission Pathways of Marek's Disease in Poultry: Understanding the Spread
Marek’s Disease is a highly contagious viral disease transmitted through direct bird-to-bird contact, contact with contaminated litter, soil, clothing, footwear, equipment, and dander. Chicks are also exposed to Marek’s Disease through inhalation of airborne dust and dander originating from feather follicles of infected poultry or contaminated equipment and personnel.
Strategies to Prevent MDV in Your Poultry Flock
Purchase only vaccinated chicks. If unvaccinated chicks are acquired, administer the Marek’s Disease vaccine promptly at one day of age. Isolate the chicks from other birds until they develop full immunity (at least two weeks). To prevent Marek’s Disease outbreaks in your poultry flock, adhere to the following guidelines:
- Thoroughly disinfect all equipment before the arrival of chicks.
- Implement a one-week downtime between flocks to eliminate potential contaminants. During this period, thoroughly disinfect the poultry house.
- Maintain proper hygiene and ventilation within the poultry house.
- Implement robust biosecurity measures.
- Minimize personnel movement in and out of the quarantine area. If personnel must move between different poultry houses, they should change clothing.
- Closely monitor the flock and remove any suspected birds.
Detecting Marek's Disease in Your Poultry Flock: Identifying Key Indicators
If neural involvement is present, you will observe paralysis in the legs and wings of the affected birds. Additionally, Marek’s Disease can be diagnosed through laboratory testing or, more commonly, through necropsy (post-mortem examination) of a deceased bird, in conjunction with evaluating the flock’s history and observed clinical signs.
Will All My Birds Die if They Contract Marek's Disease?
With proper flock vaccination, Marek’s Disease outbreaks are typically prevented. In vaccinated flocks, only a small percentage of birds may become infected. However, in unvaccinated flocks, mortality rates can be significant, potentially exceeding 50%. Nevertheless, mortality rates can vary depending on the specific viral strain.
Poultry Susceptibility to MD: Which Type of Chicken is Most at Risk?
As previously mentioned, Marek’s Disease can occur at any age, but the highest incidence is typically observed around the 6th week of life. Broiler chicken are usually ready for slaughter from 38 days of age onwards, meaning the likelihood of them contracting the disease is relatively low. However, the risk is significantly higher for layer and breeder chickens, which have a lifespan of approximately two years.
Is MDV Vaccination Necessary?
Marek’s Disease vaccination is strongly recommended for layer and breeder chickens. As previously discussed, due to flock susceptibility and the virus’s resilience at ambient temperatures, it is crucial to vaccinate your flock. Furthermore, consider the potential for subsequent flocks housed in the same facility to contract the virus if the initial flock is infected. Therefore, take vaccination seriously. However, Marek’s Disease vaccination is generally not required for broiler chickens.
How Do I Treat Marek's Disease? What is the Cure for Marek's Disease?
As explained thoroughly above, there is no treatment for Marek’s Disease. The only way to prevent infection in the flock is through vaccination of day-old chicks.
What Should I Do If My Flock Tests Positive for Marek's Disease?
Consult with a poultry specialist or veterinarian immediately. Quarantine the entire flock and separate sick birds from healthy ones. After depopulation, thoroughly disinfect and clean the entire poultry house and equipment. Ensure proper ventilation to remove contaminated dust, dander, and feathers.
Is There a Risk of MD Transmission to Humans?
No, Marek’s Disease Virus is not transmissible to humans.
Refrences:
