New Leads in Transmission of Vesicular Stomatitis

Research shows grasshoppers can pass the virus to cattle, but a common grasshopper pesticide also kills the virus on plants.

Vesicular stomatitis virus (VSV) is endemic in Mexico and causes sporadic outbreaks in the United States. Though rarely fatal, VSV causes physical discomfort in livestock, reduces production efficiency and may result in serious secondary infections. And because clinical signs in cattle and pigs are similar to those of foot-and-mouth disease (FMD), every outbreak must be closely monitored.

New research from ARS scientists in Wyoming could help prevent the spread of VSV. Barbara Drolet at the agency's Arthropod-Borne Animal Diseases Research Laboratory (ABADRL) in Laramie and Justin Derner at the ARS High Plains Grasslands Research Station in Cheyenne have shown that, under laboratory conditions, rangeland plants can harbor VSV and pass the virus to grasshoppers feeding on them. Though there are no reports to date of field rangeland-plant testing during outbreaks, the scientists showed that a common grasshopper pesticide also kills the virus on the plants.

Infected animals salivate heavily, shedding virus in the saliva, which results in animal-to-animal transmission. During an outbreak, producers try to control the spread of VSV by restricting animal movement, disinfecting all materials used, and limiting the animals' exposure to insects that transmit the virus.

Soil and plants have been thought to be sources of VSV, but because this has not been previously confirmed, current recommendations for VSV control do not include decontamination of corral soils and pastures.

Previous research by ABADRL and University of Wyoming scientists showed that, in grasshoppers, the virus can multiply and then infect cattle that eat the insects while grazing. That study prompted Drolet to investigate two assumptions made in the initial proposal of a grasshopper-cattle infection cycle: If infected animals shed the virus onto pasture plants as they graze, can the virus remain infectious on the plant surface? If so, will grasshoppers become infected by eating the contaminated plants?

To determine the window of opportunity for grasshoppers to ingest viable VSV from contaminated plants, Drolet and Derner selected 14 rangeland plant species that grasshoppers eat, exposed the plants to VSV in a laboratory setting, and measured virus survival over time.

"Several plant species harbored viable virus up to 24 hours in the lab," Drolet says. This is the first report demonstrating the stability of VSV on rangeland-plant surfaces.

The scientists then exposed two of the plant species to VSV and fed them to grasshoppers 24 hours later. The grasshoppers became infected, which supports the hypothesis that grasshopper-cattle-grasshopper transmission of VSV is possible.

The scientists next tested a common grasshopper pesticide and found that it could deliver a double punch if used during an outbreak in pastured animals: In addition to reducing the grasshopper population, the pesticide inactivated VSV on contact, thus potentially reducing a source of virus for grazing animals and any remaining grasshoppers.

"We haven't investigated the molecular mechanisms behind it," Drolet says. "But the results clearly show that this pesticide is lethal to VSV."

This knowledge could be useful in making disease-management decisions during future outbreaks.

Editor's Note: This research was published in the May 2009 issue of Applied and Environmental Microbiology. For more information about ARS cattle virus research, contact Cyril Gay and Eileen Thacker, leaders of ARS National Program #103, Animal Health.

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