Marek’s disease is a highly contagious, tumor-causing viral condition caused by Gallid alphaherpesvirus 2 (Marek’s disease virus, or MDV). Before effective vaccination arrived in the 1970s, the disease cost US poultry producers an estimated $150 million annually (Kennedy et al., 2017). Mass vaccination has since contained it in high-income production systems, though the disease persists where programs are inconsistent. Named after Hungarian veterinarian József Marek, MDV belongs to the genus Mardivirus. Serotype 1 drives all clinical disease; serotypes 2 and 3, including herpesvirus of turkeys (HVT), are non-pathogenic and underpin commercial vaccines (WOAH, 2023).
How Marek’s Disease Spreads: Dander, Dust, and Droppings
Infection occurs when a bird inhales virus-laden particles shed from feather follicle epithelium. Infected dander persists for months in litter, making cleanout and extended downtime essential (MSD Veterinary Manual, 2025). Droppings are also a contamination route: infected birds shed virus via feces, which can reach water and feed troughs (Feathered Farm Life, 2025). Equipment, clothing, and vehicles are further vectors. MDV does not spread through eggs, but chicks in a previously infected house face immediate exposure (Morrow and Fehler, 2004).
Marek’s Disease Symptoms
Marek’s disease symptoms vary across five recognised clinical forms. The neurological form produces asymmetric limb paralysis. Sciatic involvement gives the „splits“ posture, one leg forward and one back (Colorado State University VDL, 2024). The visceral form causes tumors in the liver, spleen, and proventriculus, with mortality reaching 60 to 80 percent in unvaccinated flocks (WOAH, 2023). Ocular involvement turns the iris pale and distorted, impairing vision. Meanwhile, the cutaneous form creates firm nodules at feather follicles and is a frequent cause of condemnation at slaughter (MSD Veterinary Manual, 2025).
Immunosuppression underlies all forms. MDV damages T lymphocytes and the bursa of Fabricius, leaving birds vulnerable to coccidiosis, colibacillosis, and other secondary infections (Merck Veterinary Manual, 2025).
Marek’s Disease Treatment
There is no effective Marek’s disease treatment once clinical signs appear. Neoplastic transformation cannot be reversed. Affected birds are permanent viral shedders, prolonging house contamination (Merck Veterinary Manual, 2025). Depopulation is the standard commercial response. Welfare deteriorates rapidly, and the risk to neighbouring houses makes delay costly.
The Marek’s Disease Vaccine
The Marek’s disease vaccine was the first vaccine to prevent cancer in any species (Gimeno, 2008). It is „leaky“, meaning that it suppresses clinical disease but does not prevent infection or shedding (Bailey et al., 2020). Vaccination is most commonly given via injection into the breast muscle at day of hatch, but can also be administered in ovo at 18 to 19 days (WOAH, 2023). The Cobb Broiler Management Guide recommends at least 1,500 plaque-forming units of HVT for broilers on used litter or grown to heavy weights (Cobb-Vantress, 2021). Demanding scenarios use bivalent or trivalent combinations pairing HVT with Rispens/CVI988 and serotype 2 vaccines (Rosales, 2023).
Because vaccination does not prevent shedding, MDV continues to evolve under selection pressure. Decades of leaky-vaccine use have produced progressively more virulent strains (Bailey et al., 2020). All-in, all-out production, extended downtime, and thorough disinfection remain essential prevention tools, along with many other biosecurity protocols.
REFERENCES
1.) Bailey, R. I., Cheng, H. H., Tiley, K., Donalds, R., Taylor, L., van Hulten, M., and Kaufman, J. (2020). Pathogen transmission from vaccinated hosts can cause dose-dependent reduction in virulence. PLOS Biology, 18(3), e3000619. https://doi.org/10.1371/journal.pbio.3000619
2.) Cobb-Vantress (2021). Cobb Broiler Management Guide. Cobb-Vantress, Inc. Available at: https://www.cobb-vantress.com/resources/management-guides/
3.) Colorado State University Veterinary Diagnostic Laboratories (2024). Marek’s Disease. Colorado State University. Available at: https://vetmedbiosci.colostate.edu/vdl/colorado-avian-health-program/mareks-disease/
4.) Feathered Farm Life (2025). Protecting Flocks from Marek’s Disease with Effective Vaccination Strategies. Available at: https://featheredfarmlife.com/vaccination-for-mareks-disease-in-poultry/
5.) Gimeno, I. M. (2008). Marek’s disease vaccines: a solution for today but a worry for tomorrow? Vaccine, 26(Suppl 3), C31–C41. https://doi.org/10.1016/j.vaccine.2008.04.009
6.) Kennedy, D. A., Dunn, P. A., and Read, A. F. (2017). Modeling Marek’s disease virus transmission: a framework for evaluating the impact of farming practices and evolution. Epidemics, 21, 85–93. https://doi.org/10.1016/j.epidem.2017.07.003
7.) Merck Veterinary Manual (2025). Marek’s Disease in Poultry. Merck & Co., Inc. Available at: https://www.merckvetmanual.com/poultry/neoplasms-in-poultry/marek-s-disease-in-poultry
8.) Morrow, C. and Fehler, F. (2004). Marek’s disease: a worldwide problem. In: Davison, F. and Nair, V. (eds.), Marek’s Disease: An Evolving Problem. Elsevier Academic Press, London, pp. 49–61.
9.) MSD Veterinary Manual (2025). Marek’s Disease in Poultry. Merck & Co., Inc. Available at: https://www.msdvetmanual.com/poultry/neoplasms-in-poultry/marek-s-disease-in-poultry
10.) Rosales, A. G. (2023). Marek’s Disease Control in Broiler Breeds. The Poultry Site. Available at: https://www.thepoultrysite.com/articles/mareks-disease-control-in-broiler-breeds
11.) World Organisation for Animal Health (WOAH) (2023). Marek’s Disease. Terrestrial Manual, Chapter 3.3.13. WOAH. Available at: https://www.woah.org/fileadmin/Home/fr/Health_standards/tahm/3.03.13_MAREK_DIS.pdf