Marek's Disease and Tumoral Varicellosis (Marek's Disease Virus - MDV)

Marek's Disease (MD) is a highly contagious, lymphoproliferative and neuropathic disease of domestic chickens (Gallus gallus domesticus) caused by the Marek's Disease Virus (MDV), a cell-associated alphaherpesvirus of the genus Mardivirus, species Gallid alphaherpesvirus 2 (GaHV-2). The disease is characterized by T-cell lymphomas, mononuclear cell infiltration of peripheral nerves, and visceral tumors, leading to immunosuppression, paralysis, and high mortality. MDV is ubiquitous in commercial poultry flocks worldwide, and despite widespread vaccination, virulent pathotypes continue to emerge, causing significant economic losses due to mortality, carcass condemnation, and reduced egg production. The disease primarily affects chickens, but natural infection has been reported in quail, turkeys, and pheasants, though clinical disease is rare in these species. MD is classified into four clinical forms: classical (neurological), acute (visceral), ocular, and cutaneous. The disease is of paramount importance in commercial layers, broiler breeders, and backyard flocks, while broilers are less commonly affected due to their short lifespan, but early infection can lead to immunosuppression and secondary infections. MD is a WOAH-listed disease, and vaccination is the cornerstone of control, but vaccine breaks and emerging very virulent plus (vv+) strains necessitate continuous surveillance and biosecurity.

The causative agent is Marek's Disease Virus (MDV), a cell-associated alphaherpesvirus belonging to the genus Mardivirus, subfamily Alphaherpesvirinae, family Herpesviridae. Three serotypes are recognized: serotype 1 (GaHV-2) includes oncogenic strains (e.g., JM, GA, RB-1B, Md5, CVI-988/Rispens), serotype 2 (GaHV-3) includes non-oncogenic strains (e.g., SB-1, HPRS-24), and serotype 3 (MeHV-1) is the herpesvirus of turkeys (HVT, strain FC-126). MDV serotype 1 is the primary pathogen, with pathotypes classified as mild (mMDV), virulent (vMDV), very virulent (vvMDV), and very virulent plus (vv+MDV) based on their pathogenicity in vaccinated chickens. The viral genome is a linear double-stranded DNA of approximately 174-184 kbp, encoding over 100 genes, including oncogenes such as Meq (Marek's EcoRI-Q) and vIL-8, which are critical for tumorigenesis. The virus is enveloped, but in the environment, it is cell-associated and labile; however, it can survive in feather follicle epithelium (FFE) as cell-free infectious virus, which is the primary source of horizontal transmission. The virus is highly cell-associated in vitro and in vivo, except in the FFE where it becomes enveloped and infectious. MDV is resistant to many disinfectants but is inactivated by heat (56°C for 30 minutes), lipid solvents, and common disinfectants such as sodium hypochlorite and quaternary ammonium compounds.

MD occurs worldwide in commercial and backyard poultry, with a higher prevalence in layers and breeders due to their longer lifespan. The disease is transmitted horizontally via inhalation of infectious dust and dander contaminated with cell-free virus from feather follicles. The virus is shed continuously from infected birds, and once introduced into a flock, it spreads rapidly, with a morbidity of up to 100% and mortality ranging from 10% to 80% in unvaccinated flocks, depending on the viral pathotype and host genetics. In vaccinated flocks, mortality is typically less than 5%, but vaccine breaks can lead to higher losses. The incubation period is 4-6 weeks, but clinical signs may appear as early as 3 weeks in highly susceptible birds. Age at exposure is critical: chicks infected within the first days of life are most susceptible, while older birds are more resistant. Housing systems with high stocking density, poor ventilation, and inadequate biosecurity increase the risk of early exposure. The disease is more common in deep litter and free-range systems where feather dander accumulates. Seasonal variations are not significant, but stress factors such as molting, co-infections, and poor nutrition can exacerbate the disease. Egg production in layers can drop by 10-30% in affected flocks, and FCR in broilers may increase by 5-10% due to immunosuppression and secondary infections. Wild birds are not significant reservoirs, but mechanical transmission via insects and fomites is possible.

MDV enters the host via the respiratory tract, where it is taken up by lung macrophages and dendritic cells. The virus undergoes an early cytolytic infection in lymphoid organs, particularly the bursa of Fabricius, thymus, and spleen, leading to necrosis and immunosuppression. This phase occurs 3-6 days post-infection. Subsequently, the virus establishes a latent infection in CD4+ T cells, which is the key to oncogenesis. The oncogene Meq transforms these T cells, leading to uncontrolled proliferation and the formation of lymphomas in visceral organs, nerves, skin, and eyes. The latent infection is reactivated in the feather follicle epithelium, where the virus replicates and is shed as cell-free infectious virus. The cytolytic infection causes atrophy of the bursa and thymus, leading to immunosuppression and increased susceptibility to secondary infections such as colibacillosis and coccidiosis. The lymphoproliferative phase results in tumor formation, which can cause organ dysfunction, nerve enlargement, and paralysis. The neurological signs are due to infiltration of peripheral nerves by lymphocytes, leading to demyelination and axonal degeneration. Ocular lesions result from lymphocytic infiltration of the iris and optic nerve, causing blindness. The disease progression is influenced by the viral pathotype, host genetics, and vaccination status.

Intrinsic factors include genetic susceptibility of the chicken line; some lines are more resistant to MDV-induced tumors. Age at exposure is critical, with chicks under 1 week of age being highly susceptible, while older birds develop resistance. Immunosuppression due to concurrent infections (e.g., infectious bursal disease virus, chicken anemia virus) or stress (e.g., high production, poor nutrition) increases susceptibility. Extrinsic factors include poor biosecurity, high stocking density, inadequate ventilation leading to high ammonia levels, and contaminated litter. Vaccination failure can occur due to improper vaccine handling, administration, or immunosuppression. The presence of very virulent plus (vv+) strains can overcome vaccine immunity. Maternal antibodies can interfere with vaccination if the vaccine is given too early. Environmental factors such as poor hygiene and inadequate cleaning and disinfection between flocks contribute to virus persistence.

Clinical signs vary depending on the form of the disease. The classical form presents with progressive paralysis of the legs and wings, often with one leg extended forward and the other backward (sciatic nerve involvement). Birds may show torticollis, dyspnea, and diarrhea. The acute form is characterized by depression, anorexia, and sudden death, with visceral tumors found at necropsy. Ocular form causes iris discoloration (gray eye), irregular pupil, and blindness. Cutaneous form presents with feather follicle tumors, appearing as raised nodules on the skin. In layers, egg production drops, and egg quality may deteriorate. In broilers, the disease may be subclinical, but immunosuppression leads to poor performance and increased mortality due to secondary infections. The incubation period is 4-6 weeks, and clinical signs may appear as early as 3 weeks in highly susceptible birds. Mortality can be high in unvaccinated flocks, reaching 80% in severe outbreaks.

Differential diagnoses include: 1) Avian leukosis (caused by avian leukosis virus, ALV) - similar visceral tumors, but ALV is transmitted vertically and causes tumors in older birds (>16 weeks), and lacks nerve involvement. 2) Reticuloendotheliosis (REV) - causes lymphomas and immunosuppression, but is less common and can be differentiated by PCR. 3) Lymphoid leukosis - similar tumors, but occurs in older birds and is transmitted vertically. 4) Newcastle disease (ND) - respiratory and neurological signs, but ND causes more severe respiratory signs and is caused by a paramyxovirus. 5) Avian encephalomyelitis (AE) - neurological signs in young chicks, but AE is caused by a picornavirus and does not cause tumors. 6) Vitamin E/selenium deficiency - causes encephalomalacia and muscular dystrophy, but no tumors. 7) Bacterial infections such as colibacillosis can cause neurological signs due to meningitis, but are differentiated by culture and absence of tumors. 8) Coccidiosis can cause diarrhea and poor performance, but no neurological signs. 9) Marek's disease vaccine reactions can cause tumors if the vaccine is contaminated with virulent MDV. 10) Other causes of paralysis such as trauma or botulism. Definitive diagnosis is based on histopathology, virus isolation, and PCR.

The diagnostic algorithm begins with flock history and clinical signs, including age, vaccination status, and mortality pattern. A thorough clinical examination of affected birds is performed, noting neurological signs, ocular lesions, and skin tumors. Gross necropsy is essential: examine peripheral nerves (sciatic, brachial) for enlargement and loss of striations, and visceral organs (liver, spleen, kidney, ovary, proventriculus) for tumors. Histopathology of affected tissues (nerves, tumors, skin) is confirmatory, showing pleomorphic lymphocytic infiltration and demyelination. Serology using ELISA or AGID can detect antibodies, but is not diagnostic alone. PCR on tumor tissue or feather follicle epithelium can detect MDV DNA and differentiate serotypes/pathotypes. Virus isolation in cell culture (chicken embryo fibroblasts or duck embryo fibroblasts) is possible but time-consuming. Immunohistochemistry can detect MDV antigens. The algorithm should include differential diagnosis to rule out other causes of tumors and neurological signs. Molecular characterization of the viral pathotype (e.g., by sequencing the Meq gene) is recommended for epidemiological studies.

Serology: ELISA can detect antibodies to MDV, but titers are not protective and may reflect vaccination or infection. AGID is a simple test for flock screening. HI test is not used for MDV. Molecular diagnostics: PCR (conventional and real-time) can detect MDV DNA in blood, tumors, and feather pulp. Real-time PCR can quantify viral load and differentiate serotypes. Virus isolation: MDV can be isolated from lymphocytes or tumor cells by co-cultivation with susceptible cells, but it is slow and requires expertise. Histopathology: Microscopic examination of nerves shows infiltration by lymphocytes and plasma cells, with demyelination. Tumors are composed of pleomorphic lymphoid cells. Immunohistochemistry: Detection of MDV antigens (e.g., pp38) in tissues. Blood chemistry: No specific changes, but may show elevated liver enzymes if hepatic tumors are present. CBC: May show lymphocytosis or leukocytosis. Coccidiosis lesion scoring is not applicable. Mycotoxin feed assays may be performed to rule out immunosuppression due to aflatoxins.

Radiography is not commonly used for MD diagnosis, but may show hepatomegaly or splenomegaly in cases with visceral tumors. Ultrasonography can detect abdominal tumors in live birds, but is not practical in commercial settings. Gross necropsy photography is essential for documentation: typical findings include enlarged sciatic nerves, gray-white tumors in the liver, spleen, kidney, ovary, and proventriculus. The bursa of Fabricius may be atrophied or contain tumors. Skin lesions appear as raised nodules. Ocular lesions show iris discoloration and irregular pupil. Imaging is not a primary diagnostic tool for MD.

Gross necropsy lesions: Enlargement of peripheral nerves (sciatic, brachial, vagus) with loss of cross-striations and yellowish discoloration. Visceral tumors are firm, gray-white, and may be diffuse or nodular, affecting the liver, spleen, kidney, ovary, proventriculus, and lungs. The bursa of Fabricius may be atrophied or contain tumors. Cutaneous tumors appear as feather follicle nodules. Ocular lesions include iris discoloration and irregular pupil. Histopathology: Peripheral nerves show infiltration by pleomorphic lymphocytes, plasma cells, and macrophages, with demyelination and axonal degeneration. Tumors are composed of a mixed population of lymphoid cells, including small lymphocytes, large lymphoblasts, and reticulum cells. Intranuclear inclusion bodies (Cowdry type A) may be seen in the cytolytic phase. Immunohistochemistry can demonstrate MDV antigens (e.g., pp38, Meq) in tumor cells. The presence of T-cell markers (CD4) confirms the T-cell origin of the tumors.

There is no specific antiviral treatment for MD. Control relies on vaccination and biosecurity. In an outbreak, supportive care may include vitamins (especially vitamin E and C) and electrolytes in drinking water to reduce stress. Antibiotics may be used to control secondary bacterial infections, but they do not affect the virus. In severe outbreaks, depopulation may be necessary to prevent spread. Vaccination is the primary preventive measure: vaccines include HVT (serotype 3), SB-1 (serotype 2), and CVI-988/Rispens (serotype 1). These are administered in ovo (18-19 days of incubation) or subcutaneously at day-old. In ovo vaccination is preferred for broilers, while day-old subcutaneous vaccination is common for layers and breeders. The Rispens vaccine is the most effective against vv+ strains. Biosecurity measures include strict all-in/all-out management, thorough cleaning and disinfection of houses, and control of feather dander. Vaccination programs should be tailored to the local challenge and pathotype.

The prognosis for individual birds is poor, as the disease is progressive and fatal. For the flock, the prognosis depends on vaccination status, viral pathotype, and management. In vaccinated flocks, mortality is usually low (<5%), but vaccine breaks can lead to higher mortality. Egg production may drop by 10-30% and may not fully recover. In unvaccinated flocks, mortality can reach 80%, and the flock may need to be depopulated. The long-term impact on FCR and growth performance can be significant due to immunosuppression. With proper vaccination and biosecurity, the disease can be controlled, but the virus remains in the environment.

After an outbreak, the flock should be monitored for clinical signs and mortality. Serological monitoring using ELISA can assess antibody levels, but it is not a reliable indicator of protection. PCR testing of feather pulp can detect viral shedding. Cleaning and disinfection of the house is critical: remove all litter, wash with detergent, and disinfect with an effective disinfectant (e.g., sodium hypochlorite, quaternary ammonium). Allow the house to be empty for at least 2 weeks. Implement strict biosecurity to prevent introduction of the virus. For replacement pullets, ensure they are vaccinated with an appropriate vaccine (e.g., Rispens) and that the vaccine is properly handled and administered. Monitor for vaccine breaks and adjust the vaccination program if necessary. Regular audits of biosecurity and vaccination practices are recommended.

Pearls: 1) Always examine the sciatic nerves in any bird with paralysis; enlargement is pathognomonic for MD. 2) In layers, a sudden drop in egg production with no other signs may be due to MD. 3) Vaccination is effective but does not prevent infection; it prevents tumor formation. 4) The bursa of Fabricius is atrophied in MD, unlike in infectious bursal disease where it is enlarged initially. 5) Feather follicle epithelium is the only site where cell-free virus is produced; thus, feather dander is the main source of transmission. Pitfalls: 1) Do not confuse MD with avian leukosis; ALV causes tumors in older birds and is transmitted vertically. 2) Do not rely solely on clinical signs; confirm with histopathology and PCR. 3) Do not vaccinate with HVT alone in areas with vv+ strains; use Rispens or a combination. 4) Do not ignore biosecurity; vaccination alone is not sufficient. 5) Do not use antibiotics to treat MD; they are ineffective against viruses.

There is no specific antiviral drug for MD. Supportive therapy includes: 1) Vitamins: Vitamin E (100-200 IU/L drinking water) and Vitamin C (1 g/L drinking water) for 3-5 days to reduce stress. 2) Electrolytes: Commercial electrolyte solutions (e.g., 1 kg/1000 L drinking water) for 3-5 days. 3) Antibiotics for secondary bacterial infections: Amoxicillin (10-20 mg/kg body weight, PO, BID, or 250 mg/L drinking water for 3-5 days), Oxytetracycline (10-20 mg/kg, PO, or 200-400 mg/L drinking water for 3-5 days), Tylosin (25 mg/kg, PO, or 500 mg/L drinking water for 3-5 days), Tilmicosin (75 mg/L drinking water for 3 days), Enrofloxacin (10 mg/kg, PO, or 50-100 mg/L drinking water for 3-5 days, where legal), Florfenicol (20-30 mg/kg, PO, or 400 mg/L drinking water for 3-5 days). Withdrawal times must be observed. Anticoccidials are not indicated unless coccidiosis is present. Vaccines: HVT (serotype 3) and SB-1 (serotype 2) are given in ovo or subcutaneously at day-old. CVI-988/Rispens (serotype 1) is given subcutaneously at day-old. Dosages are per manufacturer's instructions. In ovo vaccination is performed at 18-19 days of incubation using a 0.05 mL dose. Day-old vaccination is 0.2 mL subcutaneously. Booster vaccination is not typically needed.

Landmark studies include the development of the first MD vaccine (HVT) by Churchill and Biggs in 1967, which demonstrated the feasibility of vaccination against a herpesvirus-induced tumor. Subsequent studies by Witter and colleagues characterized the emergence of vvMDV and vv+MDV strains, leading to the development of the Rispens vaccine, which remains the gold standard for protection against vv+ strains. The molecular basis of oncogenesis has been extensively studied, with the Meq oncogene identified as a key player. Field trials have shown that in ovo vaccination with HVT provides early protection and is cost-effective for broilers. Meta-analyses have confirmed that vaccination reduces mortality and tumor incidence, but does not prevent infection or shedding. The AAAP and WVPA have published consensus guidelines on MD diagnosis and control, emphasizing the importance of biosecurity and vaccination. The WOAH/OIE lists MD as a notifiable disease, and guidelines for surveillance and control are provided. Recent research focuses on novel vaccines, including recombinant vaccines, and on understanding the immune response to MDV.