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 collaborating 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.

Healthy chickens.

"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.

Mycoplasmosis: Proteins and Persistence

Mycoplasmosis is a serious respiratory disease of chickens, caused by the pathogen Mycoplasma gallisepticum (MG). The U.S. poultry industry needs better tools to control the disease. That's the opinion of Scott Branton, a veterinary medical officer at the Poultry Research Unit (PRU) in Mississippi State, Mississippi. He and his colleagues are using genetic research to better understand the pathogen's virulence and develop more effective control strategies.

"The development of a novel, effective means of MG control is hampered by the lack of knowledge about MG and MG-induced pathogenesis and by limited availability of applicable tools," Branton says. But proteomic studies are changing that.

Chicks atop a picture of a genetic map of a chicken. The chicken genome has 39 pairs of chromosomes.

Branton and his colleagues at PRU and Georgia State University investigated the proteomes of two MG strains. Although the strains were closely related, earlier studies had shown that their proteomes differed significantly in virulence and persistence. Further studies showed that one of the strains was related to disease pathology, and the other was not virulent.

"The information in these studies could help explain phenotypic differences and could be applied toward the development of novel means of MG control," Branton says.

Fighting the Flu

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.

Newcastle Disease Nemeses

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.

For example, the SEPRL team has developed an improved assay to diagnose NDV in poultry and wild birds.

For more information about ARS research on zoonotic diseases, contact Cyril Gay, Leader of ARS National Program #103: Animal Health.

February 2008