ARS Keeps Up Fight Against
Marek's disease is a
naturally occurring, cancer-like disease that can lead to death or production
losses in chickens. It is generated by a herpesvirus that suppresses the immune
system and causes abnormal cell growth in nerves and tumors in the major
internal organs. ARS scientists are at
the forefront of efforts to wipe out this costly disease of commercial layers
scientist Richard Witter and his colleagues at the ARS Avian Disease and
Oncology Laboratory (ADOL) in East
Lansing, MI, first discovered in the 1960s the herpesvirus that causes Marek's
disease, and were the first to develop a vaccine in 1970.
Although there are
now several vaccines that have substantially reduced losses, they are not 100
percent effective. New and more pathogenic viral strains are continually
Once a chicken is
infected with Marek's disease, it will most likely die. Young chickens are most
susceptible, but older, unvaccinated birds develop the disease, too. Hens do
not pass the disease to their offspring through their eggs. The virus is shed
from feather follicles. Chickens become infected when they inhale the virus
through dander in their environment. Economic losses are estimated to be about
$160 million a year in the United States.
In 2000, ARS scientist Lucy Lee and her colleagues at ADOL
sequenced the genetic code of the virus that causes the disease (see
magazine story). With this genetic code, they are now studying the
molecular mechanisms by which it causes the disease and hope to create new
vaccines. Another ARS team in New York sequenced another strain, as well as a
harmless variant in turkeys that is used to vaccinate chickens.
ARS researchers have
different approaches in several disciplines to combine their strengths and gain
a better understanding of how the virus interacts with its host. They are
inoculating test chickens with genetically altered viruses to see if they will
either cause disease symptoms or stimulate a response in the immune system.
Armed with this information, they are designing recombinant vaccines that
protect against very virulent strains.
vaccines, breeders must also follow proper management and hygiene procedures
and have resistant stocks to prevent Marek's disease. That's why ADOL
researchers are developing inbred chicken lines with disease-resistant traits
linked to one or more genes, which helps them isolate and identify genes with
disease resistance. They want to identify genes, proteins and biological
pathways associated with immune responses. They've identified several
Marek's-disease-resistant genes and immunological responses that confer
In related work, ARS
Geneticist Hans H. Cheng and other ADOL researchers are part of an
international team mapping the chicken genome. A complete map (anticipated for
early 2004) would provide researchers the tool to develop chickens with
resistance to diseases, and Marek's is the first one targeted.
Aly M. Fadly, East Lansing, MI
ARS has teamed-up with Purdue University to find
more humane methods
of rearing poultry used in food production systems.
An ARS researcher and a Mississippi State
University researcher received a grant to further study a method they developed
to track pathogen
infections in live pigs.
Ground was recently broken on an
upgrade of an ARS forage and livestock research facility in Oklahoma.
ARS researchers are studying how
stable flies have
moved from being pests mainly on cattle in the barnyard to become problems in
pastures and rangeland areas as well.
new approach to
experimental vaccines has been developed by ARS scientists against exotic
Newcastle disease in poultry.