A new study was the first to find in vitro evidence of chronic wasting disease (CWD) in samples of deer semen—the kind typically used for breeding purposes on deer farms. The study was published in PLOS One.
Tracy Nichols, PhD, an author of the study and a cervid health specialist staff officer with the United States Department of Agriculture (USDA), said the findings will not provoke a change in policy at this time. Instead they offer a necessary starting point in investigations concerning the transmission route of CWD, a fatal prion disease that affects cervids—members of the deer family.
"We need to know what the risk actually is, and if sexual transmission can occur," Nichols said. "We need someone to do artificial and natural pathway testing to get to the bottom of that question."
Evidence of CWD in sexual organs
The current study tested reproductive tissue from 21 white-tailed deer (WTD) bucks, including semen, testes stroma, and epididymis samples. Using an amplification assay, researchers tested for CWD prions, and evidence of CWD was seen in 9 of the animals who had known CWD infections.
"The data indicate that the presence of CWD prions in male sexual organs and fluids is prevalent in late stage, pre-clinical, CWD-infected WTD (80%-100% of the animals depending on the sample type analyzed)," the authors said. "Our findings reveal the presence of CWD prions in semen and sexual tissues of prion infected WTD bucks."
In about half of the nine positive specimens, the prions were in early stages, said Rodrigo Morales, PhD, of The University of Texas Health Science Center at Houston. Morales is a co-author of the study. This means if CWD is transmitted sexually, it could be done before an animal shows outward symptoms of the disease.
"We don't know at what level the prions could be transmitted, if they are transmitted sexually,” said Morales.
CWD is always fatal in cervids, and in the later stages produces notable symptoms, including stumbling, drooling, and a lack of fear of humans.
Little known about sexual spread of prion diseases
Morales also pointed out that CWD may be so infectious, sexual transmission may be a moot point.
"Animals in the same pen may be transmitting to each other, two females in the same pen could be transmitting to each other," he said.
Nichols explained that deer are remarkable social animals. "They lick each other, they go nose to nose," she said. She also said they regularly consume soil via grazing. CWD prions have been detected in soil, in animal urine, and in the carcasses of dead animals.
Morales said he was interested in breeding studies, which would show if a female deer's vaginal secretions can protect, or play host, to CWD prions.
Mark Zabel, PhD, the associate director of the Prion Research Center Colorado State University, said he was not surprised by the findings, and though intriguing, said it was too premature to draw conclusions from the study.
"We know almost nothing about sexual transmission and prion diseases," said Zabel.
Scrapie, a prion disease that affects sheep, was also found to exist in sheep semen, Zabel said, but it was never determined if the prions were passed during sexual intercourse.
For deer breeders, who can pay thousands of dollars for a single straw of deer semen to use in breeding, Zabel said the study should not cause panic.
"It's a wait-and-see approach," he said. "We don't want people to worry about testing and throwing straws away."
Epi and lab approach to research needed
Michael T. Osterholm, PhD, MPH, director of the University of Minnesota's Center for Infectious Disease Research and Policy, which publishes CIDRAP News, said the study should sound alarm bells for cervid farmers who have documented an introduction of CWD into their herds when it cannot be accounted for by animal movement.
"I have always been skeptical of cervid farmers who blame a positive CWD detection on contact with wild animals," said Osterholm. "Does transmission occur between animals on different farms that are shipping semen to closed farms that have no other explanation as to how the deer get infected?"
Osterholm, who led the August launch of a CWD Resource Center at CIDRAP, said research should now take both an epidemiologic and laboratory approach to the problem of potential sexual transmission.
Montana proposes increased harvest to limit CWD
In related news, the Montana Fish, Wildlife and Parks agency announced yesterday up to 13% of WTD in Libby, Montana could be infected with CWD. They said that's almost three times the agency's threshold for more aggressive management actions.
According to a story by Yellowstone Public Radio, the agency hopes to get permission from the state to sell an unlimited number of WTD tags, which would increase harvest activity outside of Libby.
Dec 30 PLoS One study
Jan 13 Yellowstone Public Radio story