Healthy Animals 41
ARS Scientists Work to Reduce Spread of Cattle Viruses
More milk, less
calves. ARS scientists have discovered why Holsteinsbred to
produce more milkare less fertile than before breeding efforts were
stepped up to increase dairy production.
chicken. Technology developed by ARS researchers that automatically
scans poultry carcasses for contamination has been successfully tested in a
commercial poultry plant.
ewes. Artificial insemination techniques that work well with cattle and
swine can be difficult or costly to perform in sheep, but help's on the way,
thanks to ARS studies.
flow. Doppler technologythe very same technology used by
meteorologists to track thunderstormsis being used by ARS scientists to
better understand the rate at which fescue toxicosis restricts blood flow in
coli. Immunizing calves with either of two forms of a vaccine newly
developed by ARS researchers might reduce the spread of sometimes deadly
Escherichia coli O157:H7 bacteria.
bugs. Plywood-shelved carts that are used to transport eggs into
processing plants can harbor Enterobacteriaceae, according to a
microbial survey conducted by ARS scientists.
screwworms. Transgenic screwworms developed by ARS researchers could
set the stage for new, improved methods of eradicating the pest based on the
sterile insect technique.
Viral infections in cattle can be costly for producers. Two such
virusesbovine 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 710 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 cattleeven 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 dont 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
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.
Immunizing young animals against bovine viral diarrhea is
important to reducing losses. Here, microbiologists John Neil and Julia Ridpath
vaccinate calves with a commercial available vaccine to study the immune
response it generates in the animals.
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 agencys 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
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.
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We havent investigated the molecular mechanisms behind it,
Drolet says. But the results clearly show that this pesticide is lethal
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
Thacker, leaders of ARS National Program #103, Animal Health.