Marek's disease virus is ubiquitous in poultry, and recent research indicates highly virulent strains can persist for years on farms.
On one Iowa farm, for example, strains collected in 2011 and 2013 were closely related to an isolate that caused disease in 1999, according to a scientific article. That article, "Identification of Marek's disease virus genes associated with virulence of US strains," was published June 11 in the Journal of General Virology.
Researchers with the Department of Agriculture and Michigan State University analyzed genetic sequences from 70 strains of Marek's disease virus and identified sequences connected with virulence. Similarities among strains collected years apart indicate the virus is difficult to eradicate once established on a farm, according to the article and an announcement from the USDA Agricultural Research Service.
Marek's disease can be deadly to unvaccinated poultry. Vaccination prevents life-threatening tumors, but it doesn't stop virus replication, the article states. The virus's persistence in poultry may be related to a rise in virulence of the strains that were found in commercial flocks over the past 40 years, the article states.
But the article and announcement indicate the researchers found no recent increases in virulence. One of the authors, Dr. John R. Dunn of the ARS, said in the announcement that current farm practices and vaccination strategies may be reducing persistence of the virus in barns and selection pressure.
The Merck Veterinary Manual entry on Marek's disease in poultry, authored by Dr. Dunn, states that Marek's disease is in chicken flocks worldwide.
"Every flock, except for those maintained under strict pathogen-free conditions, is presumed to be infected," the entry states. "Although clinical disease is not always apparent in infected flocks, a subclinical decrease in growth rate and egg production may be economically important."