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Marek’s disease (MD) is a lymphoproliferative disease caused by an alphaherpesvirus that affects many types of poultry, but chickens most severely. Young chickens, whose immune systems are still under development, are most susceptible. The virus is widespread, found in all parts of the world, and without any control measures it is capable of causing significant economic losses in commercial poultry flocks. It causes lesions and varying rates of mortality, but even in the absence of clinical signs, the disease's immunosuppressive effects can lead to poor growth and performance.
Disease Management Approaches
Marek's disease cannot be treated post-infection, but vaccination can prevent and minimize disease impact.* The virus is highly contagious, transmitted through bird dust and dander, and can survive for long periods in farm and field environments despite strict sanitation. Once infected, birds remain lifelong carriers and continue to shed the virus (even after vaccination), so ongoing vaccination compliance is critical to prevent new infection.
The MD virus has many strains grouped into three serotypes, with the most virulent categorized as serotype 1. Non-virulent strains are designated serotype 2, while serotype 3 denotes turkey strains. The most widely used Marek's vaccine is the herpesvirus of turkeys (HVT), which is non-pathogenic in chickens and therefore safe, but also closely enough related to the MD virus (serotype 3) to stimulate an immune response. For longer-living birds (layers and breeders) a more complete program generally includes at least one other vaccine of another serotype (1 or 2) to protect against the most virulent strains. Multivalent vaccines combining more than one serotype are also available.
Thanks to nearly universal vaccination of longer-lived birds (layers and breeders), today MD is currently well-controlled in most parts of the world, although the industry does sometimes see "breaks" in protection, especially from human error during complex vaccine preparation or injection processes at the hatchery. Early protection is important, so hatchery vaccination has become the standard protocol, but there are still other factors to consider in a Marek's program: whether to vaccinate shorter-lived birds, which serotypes to include, emerging strain threats, and so on.
We are a historical pioneer in Marek's disease control: Merial's U.S. avian operations began as one of the earliest commercial producers of MD vaccines,* helping establish our longstanding expertise in this area. Today, our portfolio of products** for hatchery administration covers all MD serotypes and includes:
- VAXXITEK® HVT + IBD, our globally marketed,*** advanced two-in-one vector vaccine that uses the HVT (serotype 3) virus as a vector to also carry protection against infectious bursal (Gumboro) disease (IBD)
- CRYOMAREX RISPENS or RISPENS, our multi-region product line of live, attenuated Rispens (serotype 1) strains, some in combination with other serotypes (brand name varies by market**)
- A selection of other live frozen vaccines for all three MD serotypes, both singly and in combination products
Merial's MD portfolio aligns with our immune foundation approach: vaccinate early, at the hatchery, against the most serious immunosuppressive diseases to optimize poultry health and therefore productivity. Brand and product availability vary across markets, but we are always evolving our offerings to address new challenges and changing epidemiologic conditions.
Our customer solutions also include the expertise of our Veterinary Services teams, who keep a close eye on the global landscape of Marek's disease. We develop customized vaccination programs for each customer's unique needs, whether aligned with established protocols or addressing new field challenges. And Merial's Vaccination Technologies & Services (VTS) group provides innovative equipment, support, and training for successful hatchery administration of your plan for MD control.
More About Marek's Disease*
Named after the Hungarian veterinarian József Marek who discovered it, Marek's disease was first described in 1907 and is widely found in chickens all over the world. After reaching near-epidemic levels in the 1960s, vaccination was introduced in 1970 and helped aid in disease prevention and control, although in the 1980s and '90s more virulent strains began to appear, especially in North America and Europe.
The disease is characterized by lesions in peripheral nerves and other tissues. Clinical manifestations vary widely, depending on virus strain (categorized into three serotypes); bird age, genetics, and conditions; environmental factors; and more. MD can be difficult to differentiate from other diseases, manifesting as a variety of syndromes with wide-ranging signs, including:
- Peripheral nerve lesions, which may cause limb paralysis or persistent neurologic disease
- Blindness or other eye abnormalities, caused by lymphocytic infiltration of the iris
- Visceral lymphomas
- Skin lesions around the feather follicles
- Listlessness, wasting, and early mortality
Mortality rates are generally much higher in layer birds. In broilers, skin nodular lesions are a significant cause of carcass condemnation. Even in the absence of clinical symptoms, Marek's disease can significantly damage chickens' immune systems as the virus attacks T lymphocytes. The resulting immunosuppression results in poor growth and performance, causing major economic loss:
- Reduced broiler carcass quality and lack of uniformity increases production costs
- Egg production is reduced in layers
- Birds are more prone to secondary infections and less responsive to other vaccines, leading to increased medication costs
*Merial produces and markets several vaccines against Marek’s disease. However, be aware that NOT all aspects of the disease mentioned on this page are addressed by a vaccine. Always consult the product label for exact vaccine indications.
**Many of Merial’s avian vaccines are only marketed and available in certain countries, sometimes under different trade names. Speak to your Merial representative or contact us to find out what’s available in your region.
***A vaccine product sold under the VAXXITEK® HVT + IBD trade name is available in more than 75 countries worldwide.
Scientific Literature & Other Materials
Want to dig deeper? Browse published research on a range of poultry diseases and products, plus our equipment brochures, website articles, and more.
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