Issue 31, October 2007
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Keeping Zoonotic Diseases in Check

Anthrax. Plague. Rabies. What do these devastating diseases have in common? They're all zoonoses––diseases that can be transmitted between humans and animals. Zoonotic diseases can create serious problems for agricultural production­––not only by harming animal and human health, but by decreasing efficiency and disrupting trade.

To help control and eradicate the most serious zoonotic diseases affecting U.S. livestock, researchers with the Agricultural Research Service (ARS) are analyzing genomes, improving diagnostic platforms, studying vectors, developing vaccines and investigating the immunological responses of various animals. Below are five examples of how ARS efforts have helped reduce the impact of these diseases on the United States' agricultural industry.

Brucellosis

Brucellosis, caused by the Brucella bacteria, can induce fever, debilitation and miscarriages in humans, livestock and wild animals. Brucella can spread via physical contact or through consumption of unpasteurized milk products.

A key element of protecting livestock from brucellosis is eliminating infection in wild animals––such as bison, elk and wild swine––that share their grazing land. Although cooperative federal and state programs have nearly eliminated brucellosis in U.S. livestock, outbreaks in wild animals pose a potential threat.

One wildlife vaccination method involves firing projectile vaccines. Steven Olsen, a veterinary medical officer at the ARS National Animal Disease Center (NADC)––part of the National Centers for Animal Health––at Ames, Iowa, collaborated with scientists at Colorado State University in Fort Collins to develop an improved vaccine pellet for bison.

They found that placing the vaccine RB51 in a gel protected the vaccine's live bacteria, leading to more successful vaccinations.

NADC researchers are also evaluating novel vaccine strains––one for bison and elk and one for swine. One vaccine, developed at Virginia Tech in Blacksburg, Va., compared favorably with commercially available vaccines. Another also showed promise when administered orally or via injection.

Leptospirosis

Leptospirosis––which affects over 500,000 people globally every year––has a large host range and is one of the world's most common zoonotic diseases. It is transmitted via contact with contaminated body fluids containing spiral-shaped Leptospira bacteria. Mild cases resemble flu, but serious infections can lead to death.

Richard Zuerner, a NADC microbiologist, and his ARS colleagues have helped sequence the genomes of several Leptospira strains. Genomic studies conducted with collaborators at Monash University and the University of Queensland––both located in Australia––revealed changes in one serovar of L. borgpetersenii that affects cattle. Such changes could eventually limit the pathogen's ability to survive outside a mammalian host, which would reduce its effectiveness at spreading leptospirosis.

In a related study, Zuerner and his colleagues inspected tissue samples from infected animals to see if a specific bacterial protein formed when animals were infected with Leptospira. They found a protein consistent with LipL46: a "lipoprotein"––a membrane protein linked directly to fat molecules––identified by collaborators at UCLA. Further tests will determine whether the protein is expressed during infection or at another time.

This bison is part of a 13-head herd involved in a brucellosis vaccine study at the National Animal Disease Center in Ames, Iowa.

Tuberculosis

The prevalence of bovine tuberculosis (TB) in the United States has decreased greatly since eradication efforts were first introduced in 1917. Unfortunately, the persistence of the infection in large dairy herds, imported cattle and wild animals has prevented its complete elimination.

In a biosafety level 3 facility at NADC, veterinary immunologist Ray Waters, veterinary pathologist Mitchell Palmer and molecular immunologist Tyler Thacker have evaluated the efficacy of existing vaccines and improved diagnostic assays, contributing to a significant reduction in the prevalence of TB in recent decades.

Many of their efforts target the deer––both wild and captive––that transmit it to cattle. In one project, the team collaborated with Chembio Diagnostic Systems, Inc. (Medford, N.Y.) to develop a rapid diagnostic test for the detection of TB in deer. The test uses a whole-blood sample and takes 10-15 minutes to process. The team also worked with

Prionics AG (Zurich, Switzerland) to develop improved antigens to use in TB diagnostic assays. By improving existing tools and developing new control measures, the NADC scientists have helped reduce the spread of bovine tuberculosis throughout the United States.

Rift Valley Fever

Rift Valley Fever (RVF) is a potentially fatal insect-borne disease of humans and animals, such as cattle, sheep, camels and goats. Abortions and loss of young animals are associated with RVF outbreaks. The risk of severe disease and death is much higher for livestock than for humans, but the symptoms––which include fever, aching muscles and vomiting––are unpleasant for both.

Scientists at the Arthropod-Borne Animal Diseases Research Laboratory (ABADRL) at Laramie, Wyo., are working with domestic and international collaborators to protect our nation's agricultural animals against the disease. Although RVF has not yet been introduced into the United States, it is considered a significant disease threat to U.S. livestock. The ABADRL team is cooperating with the Department of Homeland Security to assess potential vaccines for the RVF virus. In addition, the scientists are developing and evaluating diagnostic tests for RVF virus antigens.

Veterinary immunologist Ray Waters feeds a pair of white-tailed deer.

In 2006, a team established by entomologist Kenneth J. Linthicum, director of the ARS Center for Medical, Agricultural and Veterinary Entomology in Gainesville, Fla., produced a model that successfully predicted a Rift Valley fever outbreak in sub-Saharan Africa several months in advance. The early warning allowed organizations to advise at-risk countries to increase surveillance and insect control.

Bovine Spongiform Encephalopathy

Bovine spongiform encephalopathy (BSE, or "mad cow disease") is a deadly neurological disorder characterized by abnormally folded prions––proteins that occur naturally in mammals. In humans, the related encephalopathy, caused by the ingestion of BSE-contaminated bovine products, is known as variant Creutzfeldt-Jakob disease. In both cattle and humans, the disease causes neurological dysfunction and, eventually, death.

The United States has had only three confirmed cases of BSE. NADC scientists confirmed the first case in December 2003, and ARS scientists at the U.S. Meat Animal Research Center in Clay Center, Neb., traced its origin to Canada. Between June 2004 and July 2006, over 759,000 cattle have been tested for BSE, two of which tested positive. Laboratory tests showed that the manifestation of BSE in the second and third cases, which originated in the United States, differed significantly from the first. Veterinary medical officer Juergen Richt and NADC colleagues helped identify and describe the atypical cases.

In related work, Richt and his colleagues evaluated eight "prion protein knock-out" Holstein calves developed in collaboration with Hematech, Inc., a pharmaceutical research company based in Sioux Falls, S.D. These were the first cattle ever produced to lack the prion protein involved in the pathogenesis of BSE. Brain material derived from these animals was not able to support BSE-amplification when tested in an in vitro BSE amplification assay. The scientists determined that the health and growth rate of these unique animals, which will be invaluable for future BSE studies, compared to those of normal Holsteins.

Zoonotic diseases are one of several challenges facing U.S. livestock, but thanks to these and other research efforts in the ARS Animal Health National Program, our agricultural animals are among the healthiest in the world.

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