Issue 41, April 2010
About this Newsletter

ARS Scientists Work to Reduce Spread of Cattle Viruses

Viral infections in cattle can be costly for producers. Two such viruses—bovine viral diarrhea virus (BVDV) and vesicular stomatitis virus (VSV)—cause outbreaks in the United States that leave animals with symptoms that can reduce production efficiency. However, the viruses’ impact or the way they spread among animals is not always straightforward.

Cattle Value Reduced After Virus Exposure

Fever, pneumonia, diarrhea and compromised immunity are among the telltale signs of infection with the group of viruses that cause bovine viral diarrhea, an economically significant disease that affects cattle herds throughout the world. Calves exposed to a BVDV in utero may develop persistent infections and shed the virus throughout their lives. Post-birth exposure to BVDV usually leads to acute infections that last 7–10 days.

With lifelong compromised health, persistently infected (PI) cattle are obviously a drain on economic resources, but they may be even more costly than previously assumed. A collaborative study involving scientists from the Agricultural Research Service (ARS) shows that PI cattle can actually decrease the profitability of surrounding cattle—even those that never develop clinical disease. This work was published in the January 2009 issue of the American Journal of Veterinary Research.

PI cattle have higher mortality rates and lower production efficiency than other cattle. But the economic consequences of BVDV don’t end there, according to a study initiated by veterinary consultant Bill E. Hessman of the Haskell County Animal Hospital in Sublette, Kansas. In collaboration with ARS and university colleagues, Hessman showed that after exposure to PI cattle, non-PI cattle had higher morbidity rates and lower production efficiency than cattle with absolutely no exposure to PI animals.

Microbiologist Julia Ridpath at the ARS National Animal Disease Center in Ames, Iowa, helped design and analyze the study, which was conducted in a newly constructed feedlot. The collaborators tested 21,743 calves as they entered the feedlot. They identified PI animals, characterized the BVDV strains present, and tracked the spread of strains within and between pens.

Some pens held one or more PI cattle. Others had no PI cattle, but were adjacent to infected pens. The remaining pens neither held infected cattle nor adjoined infected pens.

The scientists found that the mortality rates were 25.6 percent for PI cattle and 2.4 percent, overall, for non-PI cattle. Of the non-PI cattle, those that were exposed to PI cattle had a mortality rate of 3.6 percent, and those that had no exposure had a mortality rate of 1.7 percent.

The higher mortality and morbidity rates due to PI exposure have been reported previously. But this study was one of the first to compare performance outcomes, such as production efficiency, of PI-exposed animals and non-PI-exposed animals.

Production efficiency, based on the ratio of feed intake to weight gain, for PI-exposed animals was less than half that of non-PI-exposed animals. This is a significant observation for livestock producers because it demonstrates that the economic damage incurred by exposure to PI animals is not limited to increased treatment costs. Even PI-exposed animals that remained clinically healthy gained weight less efficiently than non-PI-exposed animals.

Based on this study, estimated economic losses caused by exposure to PI cattle could be between $40 and $90 per animal, due to increased mortality and morbidity and decreased performance.

New Leads in the Vesicular Stomatitis Virus Transmission Cycle

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, 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. 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.