The Threat of Marek’s Disease Virus Is Expanding The virus responsible for this lymphoproliferative disease is growing more virulent and evading control by available vaccines

D espite the attention being paid to avian influenza and its potential to cause a pandemic among humans, the threat of avian diseases extends well beyond the influenza virus. Of particular interest to poultry producers are foodborne infectious bacteria such as Salmonella and Campylobacter spp., as well as several viruses, including those responsible for causing Exotic Newcastle Disease, avian leukosis, and Marek’s disease (MD). Indeed, Marek’s disease virus (MDV), an oncogenic avian herpesvirus that causes the lymphoproliferative disease characteristic of MD, is becoming more virulent and evading control by available vaccines—thus posing an increasing challenge for poultry producers. Poultry plays an important global role nutritionally and economically, making the control of these and other bird diseases an increasingly important challenge. Poultry raised for meat— mainly chickens, turkeys, and ducks, but also other specialty bird species—and for eggs is an important source of protein for humans worldwide. Because of its increasing market share and low production costs, economists predict that poultry will overtake pork by the year 2020 as the leading meat consumed across the world. In the United States, poultry accounts for 41% and thus is already the top meat consumed, and it is the thirdlargest agricultural commodity overall, topping every plant species, including corn. American consumer demand leads to the processing of 1 million birds and the consumption of 10 million eggs on average per hour! Meeting this amazing demand for meat and eggs challenges producers. To improve the economic efficiency of rearing birds and producing eggs, the poultry industry has bred special chickens and also developed methods for greatly increasing the numbers of birds being produced (Fig. 1).