Researchers examine how disease is transmitted in livestock

Using new molecular techniques, a team of scientists has shown how a largely eradicated livestock disease, brucellosis, has been transmitted among cattle, bison, and elk in the greater Yellowstone area. The research may have important implications in the management of disease outbreaks between livestock and wildlife.

The scientists involved investigated the wildlife-livestock transmission pathways of brucellosis, which often causes a termination of pregnancy in animals.

The disease was introduced to elk and bison in the greater Yellowstone area over the past century, likely through infected cattle. More recently, however, the disease is being transmitted from elk to cattle, undermining livestock control efforts.

“Bison were long believed to be the most important source of cattle infections of brucellosis, but we’ve found that elk are a surprisingly important component of the disease spread and maintenance in the greater Yellowstone area. We were trying to understand how this pathogen is moving and which animals are its ‘best’ hosts. By learning this we can find ways to eradicate this important wildlife and livestock disease,” researcher Jeffrey Foster said in a recent article.

“Think about this type of disease research the next time you have milk in your cereal or eat a bite of cheese. Brucellosis is among the most important livestock diseases worldwide but has been largely eradicated in the U.S. through decades of livestock health programs. Our work uses new molecular techniques of genomics to precisely track the spread of a pathogen, Brucella abortus, which causes the disease,” Fisher said. “These approaches we use can be applied to other pathogens to study newly emerging infectious diseases. Genomics can be a powerful tool for disease epidemiology,” he continued.

Scientists developed and analyzed a genomic dataset of Brucella abortus, which spanned 30 years and included samples from cattle, bison, and elk. The study examined the evolution, cross-species transmission history, and spread of the disease, which is found worldwide in humans and livestock.

Results indicate five genetically distinct groups of strains of B. abortus in the greater Yellowstone area that are likely due to historic cattle introductions. Four out of the five strains are now primarily associated with elk and originated from the Wyoming feeding grounds, where state and federal land managers provide feed for elk in the winter.

Two of these elk-associated strains have spread at about four to eight kilometers per year, leading the scientists to conclude that elk are the most likely source of current brucellosis outbreaks in livestock. The fifth genetically distinct strain originated and was mainly found in bison of Yellowstone National Park. This strain appeared to be spreading less rapidly, however.

Previously, it was not known whether elk could sustain the disease in the absence of bison or supplemental feed grounds, the article explains. This study shows that elk, in some areas distant from the feed grounds, have strains that are unrelated to bison, suggesting that management of bison and feed grounds may not affect brucellosis dynamics in these other elk populations, where the disease has been spreading.

In North America, the greater Yellowstone area is the last remaining reservoir of B. abortus. More than 20 cattle and farmed bison herds have been infected in Wyoming, Idaho, and Montana since 2002, and the presence of the disease within livestock results in additional testing requirements and trade restrictions.

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