Study looks at Bluetongue virus transmission, infection dynamicsWritten by Gayle Smith
When Bluetongue virus strikes livestock, the results can be devastating for ranchers. But thanks to University of Wyoming research, a better understanding of the virus may be on the way.
Myrna Miller with the Wyoming State Veterinary Lab is conducting a research project that will determine the effects of climate variables and maternal antibodies on the natural transmission of Bluetongue virus in cattle. Through her research, Miller hopes to determine the annual infection rate, when transmission starts in the season, the intensity of transmission and whether it changes over the season.
A look at Bluetongue
Bluetongue virus is recognized in Wyoming as a hemorrhagic disease of wildlife. From the genus Culicoides, the virus is transmitted by the biting midge, also known as no-see-ums. The virus strikes seasonally, mostly during late summer and early autumn.
“It usually stops with the onset of freezing weather,” Miller says.
The severity of Bluetongue varies from year to year. According to Miller, it can depend upon geographic location, abundance of insect vectors, strain of virus, existing herd immunity and genetic variations in the host.
Miller says since 2000, 10 new strains of Bluetongue have been identified in the U.S.
Whitetail deer are among the wildlife most prone to the disease.
During an outbreak, deer may be found dead, lying near the edge of the water, Miller says. When they contract the disease, their lungs will fill with fluid, and hemorrhages start under the skin and near the heart. The mortality rate amongst whitetail deer can be up to 90 percent.
Pronghorn antelope, mule deer, elk, sheep and cattle can also contract the virus but typically get sick to a lesser extent, Miller explains.
“This disease is not transmitted animal to animal but by an insect bite,” she continues.
Miller notes, “One of the questions I want to address through my research is if this virus is transmitted at a lower level before large outbreaks occur. Outbreaks can be severe or virtually unrecognized.”
In cattle, for example, the virus typically isn’t recognized, but Miller continues, “However, sheep can get very sick, depending upon the population and population immunity.”
In sheep, the Bluetongue virus is also known as sore muzzle, muzzle disease and pseudo foot-and-mouth disease, because it causes a sore mouth.
Once they contract the virus, animals may stop eating, develop lesions in their mouth and have swelling, excessive salivation, lameness and even reproductive failure, the research expert says.
“Cattlemen who try to export seedstock and have their cattle test antibody positive will be prevented from exporting semen and frozen embryos,” she notes.
In Europe, a new strain of Bluetongue has been recognized that causes reproductive failure in cattle.
“Bluetongue is an expanding disease. The warming climate may cause the vector to spread into climates where it was previously too cold for it to exist,” she explains.
Through her research, Miller hopes to document the current extent of the virus and its seasonal transmission dynamics. She would also like to determine if the severity of the disease is higher during years when the temperature is hotter.
Another important part of her research is determining what antibodies are present once an animal is exposed to the virus.
“The surviving animals develop antibodies that will last their lifetime,” she says.
Studies of the virus have shown a higher incidence in eastern Wyoming counties and the Big Horn Basin.
“In the eastern counties, 80 percent of the cattle and sheep in the study tested serum positive. Once we have a population that is antibody positive, they are the most resistant, and their young get antibodies from the mother’s milk that will protect them from the disease the first four to five months of their life,” she says.
Miller would like to compare these resistant-born animals to those that are born antibody negative.
The study design Miller will be using will follow a group of cattle over three seasons.
“I will be looking at groups of calves that are maternal antibody positive and negative, then compare the time of onset of the infection and the amount of virus in their system. I also plan to trap some of these bugs and test them for the virus,” she says.
She also hopes to identify the current types of virus circulating.