Marek’s disease control takes careful attention to detail

Isabel M. Gimeno, DVM, professor, North Carolina State University speaks to Poultry Health Today.
calendar icon 13 June 2022
clock icon 2 minute read

Marek’s disease has become increasingly complex, and achieving good control may require revamping the vaccination strategy, according to Isabel M. Gimeno, DVM, professor, North Carolina State University.

The evolution of Marek’s disease virus (MDV) has increased the pathogen’s virulence. Besides the tumors MDV is known for producing, the virus can lead to a multitude of other problems including neuritis, atherosclerosis, panophthalmitis and immunosuppression, she said.

MDV also appears to be widespread. In an unpublished study, Gimeno found that, in the US, 100% of broiler chickens were positive for oncogenic MDV at 7 days of age. In addition, the pathotypes of the disease can differ completely from farm to farm, Gimeno said at a forum sponsored by Zoetis.

Although control of tumors has improved, managing MDV-induced immunosuppression remains especially challenging, she said.

“We have three types of immunosuppression that are completely unrelated to each other,” she said. “The first happens when the virus gets into the chickens via the respiratory tract and replicates in the lymphoid organs about 5 to6 days after infection (called transient or early immunosuppression). Then the virus goes into latency, but at about 3 weeks, it will reactivate. In case of challenge with a very virulent plus (vv+)MDV, reactivation is associated with immune-system dysregulation and immunosuppression (called permanent or late immunosuppression due to reactivation). Reactivation might be followed by tumors and associated immunosuppression (called permanent or late immunosuppression due to tumors).

Early immunosuppression

Early immunosuppression causes massive destruction of cells by the virus and reduced response of lymphocytes, Gimeno explained. There are bursa and thymus atrophy and clear necrosis, along with cellular and humoral immunosuppression.

Maternal antibodies against MDV protect completely against early immunosuppression whenever chickens are challenged with a virulent (v)MDV. However, such protection is partial when the challenge virus is a very virulent (vv)MDV and very poor if the challenge virus is vv+MDV. Early immunosuppression can be controlled by vaccination.

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