Broiler Health Research: U.S. researchers are shedding light on health issues for broilers

In all animals, obesity results from an energy imbalance that occurs
when more food energy (calories) is consumed than the body actually
needs.

CHICKS AND THE GENOME Chicks atop a picture of a genetic map of a chicken. The chicken genome has 39 pairs of chromosomes.

In all animals, obesity results from an energy imbalance that occurs when more food energy (calories) is consumed than the body actually needs. The excess energy is stored mostly as fat. Over the years, poultry breeders have bred chickens that grow faster and produce more meat in response to a growing worldwide consumer demand. But modern broiler/breeder chickens don’t adequately balance their feed consumption to match their energy requirements. When these birds are given unrestricted access to feed, they will overeat and become obese.

An important biochemical pathway was discovered in animals that maintains energy balance in the body. A key component of the pathway is an enzyme called “AMP-activated protein kinase” (AMPK), which helps to regulate both energy use by individual cells and food intake by the animal. This pathway is currently being studied in chickens and turkeys.

Animal scientists Monika Proszkowiec-Weglarz and Mark Richards in the ARS Growth Biology Laboratory at Beltsville, Maryland, along with research leader John McMurtry and Penn State University collaborator Ramesh Ramachandran, recently identified and sequenced the genes responsible for the AMPK pathway in birds and showed that they function in different tissues throughout the body of the broiler chicken.

According to Proszkowiec-Weglarz, AMPK plays a central role in sensing cellular energy levels. It begins a series of events that affect food intake and metabolism of fat, carbohydrate, and protein. “AMPK is really a molecular fuel gauge and a master metabolic regulator in cells,” she says. “It responds to fluctuations in the levels of cellular energy and of specific extracellular nutrients and hormones.”

Cells obtain energy from conversion of adenosine triphosphate (ATP) to adenosine monophosphate (AMP). “AMPK senses the ratio of AMP to ATP and works to raise the level of ATP within cells” says Proszkowiec-Weglarz. “Our goal is to learn how the AMPK pathway functions in birds to achieve energy balance, so growers can efficiently produce chickens of optimal weight while minimizing excess fat.   

GENETICS AND HEALTH
Genetic research is for the birds – and it's a good thing, too, as scientists with the Agricultural Research Service (ARS) are using their knowledge of avian genetics to improve the health of poultry. Genetic information is leading to better vaccines and breeding techniques to protect poultry against health threats such as exotic Newcastle, avian influenza, Marek’s disease, coccidiosis, and mycoplasmosis.

CONTROLLING COCCIDIOSIS
Coccidiosis is an avian disease that costs the poultry industry an estimated $350 million annually. Researchers in the Animal Parasitic Diseases Research Unit, part of the Henry A. Wallace Beltsville Agricultural Research Center (BARC) in Maryland, have been investigating Eimeria, the intestinal protozoans responsible for the disease.

The team, which includes microbiologist Mark Jenkins, zoologist Ray Fetterer and molecular biologist Kate Miska, is conducting research to improve the detection and characterization of the
parasites that cause coccidiosis and to improve the effectiveness of poultry
vaccines.

The researchers have already developed a method to quickly identify Eimeria species in poultry litter – information that is essential for targeted vaccine application. Their work highlighted the need for a vaccine to control E. praecox, a species for which no broiler vaccines currently exist, even though it occurs in U.S. poultry operations. The BARC researchers have also identified proteins that may contribute to immunity and susceptibility, and they’re colla-borating with university colleagues to develop a novel Eimeria vaccine.

“Our research uses a multifaceted approach that combines pathogen genomics with biochemical and immunological techniques,” Jenkins says. The resulting tools could have significant benefits for U.S. poultry, and for those who produce and consume it.

MARKERS AND MAREK’S
Resistance or susceptibility to diseases such as Marek’s disease virus (MDV) is an inherited trait. Many poultry breeding programs have attempted to breed resistance to MDV, a devastating disease that causes tumors in chickens. By using marker-assisted selection, poultry breeders have improved the speed and accuracy of traditional breeding methods in recent years. Genetic studies have contributed not only to prevention of Marek’s disease, but to treatment as well.

At the Avian Disease and Oncology Laboratory at East Lansing, Mich., geneticist Hans Cheng and his colleagues have developed a clone of a virulent and infectious MDV genome that is contained within a bacterial artificial chromosome, which they will use to investigate the functions of specific genes. This information can be used to improve MDV treatment.

“It will enable us to associate specific genes and variations within these genes to variations in phenotypes,” Cheng says. “For example, we could identify what genetic changes lead to greater virulence. Then we could develop vaccines that target the individual genes involved.”

Cheng’s genetic studies have also shed light on the nature of MDV. In one study, Cheng and his colleagues observed an unusual immune response in MDV-infected chicken cells. Unlike other viruses, MDV leads to an increased presence of an antigen known as “MHC class II” on the cell surface.
“Most of the time, viruses try to fly under the radar without attracting the immune system’s attention,” Cheng says. Increasing the presence of MHC class II proteins, however, attracts T cells to the area. But it does so without harming the virus. The ARS team suspects that MDV may upregulate MHC class II in order to manipulate the T cells, which it uses to spread to new cells. Further research is required to confirm the accuracy of this hypothesis.

Scientists at the Southeast Poultry Research Laboratory (SEPRL) in Athens, Ga., are determining the gene sequences for two major poultry respiratory diseases. Avian influenza virus (AIV) and Newcastle disease virus (NDV) target chickens, turkeys, and wild aquatic birds. A recent study compared North American H5N1 subtype viruses to the pathogenic viruses that have been causing trouble in Asia.
“The analysis revealed that the North American viruses are nonvirulent and distantly related to the viruses in Asia, with 75 to 79 percent similarity between the genes for the North American and Asian viral surface proteins,” says microbiologist Erica Spackman.

Spackman and her colleagues are investigating how AIV spreads, but further research is needed to determine how genes are involved in the process. Initial studies show a complex relationship between the virus and its avian hosts, with the disease’s progression affected by the interaction of multiple gene products. A related project, in which scientists are sequencing the complete genomes of 1,000 AIV specimens, could help by significantly expanding the existing information on AIV sequence data.

Genetics research can also identify potential problems before they become problematic. While examining the virulence and diversity of NDV, SEPRL scientists identified 11 previously undescribed viral genotypes.

“Some of the wildlife viruses were found to be closely related to isolates from U.S. live bird markets and genetically distinct from NDV used in live vaccines,” says microbiologist Claudio Afonso.

This, coupled with the discovery that the USDA real-time assay used to identify NDV was unsuccessful at identifying class I viruses, suggests that the virus could circulate undetected between wild birds and poultry. This information can help scientists, poultry industry professionals, and policymakers decide how to best use their resources.