A Short Review: Current awareness on the transmission of prion infection through semen and embryos in ruminants with emphasis on Chronic Wasting Disease (CWD).
R Greg Stewart, DVM MS PhD, Southern Veterinary Services, Inc, www.southernvet.co
Much rumor and innuendo are circling about the recent Chronic Wasting Disease (CWD) positives on a ranch in Mountain Home Texas. Because a pair of twin brothers and another pen mate from the same sire tested CWD positive, a “giant leap in science” has been made to assume that since these bucks were the product of artificial insemination with frozen semen that the semen was the mode of transmission. This assumption is, by all scientific evidence, false.
Let me address what is known scientifically and what we have learned through recent epidemiology gained when other herds have been depopulated. First, the USDA APHIS Program Standards on Chronic Wasting Disease completed in May 2014 state in Section 2.6 and on page 17 of the document that: “At this time there is no scientific evidence that germplasm (embryos or semen) may transmit the disease”. Second, the most studied transmissible spongiform encephalopathies (TSE) in animals are scrapie in sheep/goats and bovine spongiform encephalopathy (BSE) or Mad Cow in cattle. Let’s see what is scientifically known in these species.
In the book Prions: A Challenge for Science Medicine and the Public Health System (edited by F Holger et al, Second Edition, 2004), several detailed studies were reviewed to address the issue of transmission through germplasm (semen or embryos). Perhaps the best one was detailed on page 161 of the book. I will summarize it here. Semen from 13 bulls (8 of which were BSE positive) was used to breed 167 cows. Some of the cows carried their fetuses to term, and others were embryo flushed and the embryos either transferred to recipient cows or used in further lab studies. Some of the bred cows used in these experiments as negative controls were imported from New Zealand, a country known to be free of BSE. Here’s what they learned:
No BSE transmission was found in any of the embryos that were ground up and injected into susceptible mice brains. This is an abnormal route of infection but is very sensitive.
No BSE was found in any offspring from cows bred to have their own calves.
No BSE was found in any offspring from recipient cows carrying embryos bred with positive semen.
Embryos from positive cows were put into negative recipients. The animals were held for seven years. No BSE was found in this group.
The summary made by the editors states that there is no evidence of transmission by sire either by natural matings or through semen.
Furthermore, in the book Emerging and Exotic Diseases of Animals (edited by AR Spickler, 2010), “The presence of BSE has not been scientifically documented in milk, semen, or embryos.” And in the book, Bovine Medicine: Diseases and Husbandry of Cattle, (edited by AH Andrews et al, 2004), “No transmission of BSE has been documented either with contaminated semen or with semen from contaminated Bulls.”
In the sheep and goat analogue of this disease, scrapie, much is known. In a paper published in Reproduction (2008 March 135(3):415-418, “Semen from scrapie infected Rams does not transmit prion infection to transgenic mice.” Sarradin P, Melo S, Barc C, Lecomte C, et al.) “Scrapie is the most common transmissible TSE in livestock…Artificial insemination is widely used in modern farming, and as large amounts of prion protein have been found in sheep sperm membrane, epididymal fluid, and seminal plasma…horizontal transmission by this route has been hypothesized since no clear information has been obtained on possible sexual transmission of TSE. We therefore tested the contamination levels of semen from scrapie-infected rams at different stages of incubation, including the clinical phase of the disease. We report here that under our experimental conditions, ram semen did not transmit infectivity to scrapie-susceptible transgenic mice over expressing…the sheep prion (PRNP) gene. These results suggest that artificial insemination and natural mating have a very low or negligible potential for the transmission of scrapie in sheep flocks.”
In a paper published in Theriogenology (2008 Sept 15;70(5):725-745, “Risks of transmitting ruminant spongiform encephalopathies (prion diseases) by semen and embryo transfer techniques.” Wrathall AE, Holyoak GR, Parsonson IM, and Simmons HA) “Research on TSE transmission via reproductive technologies in deer has not yet been done, but information on the pathogenesis and epidemiology of chronic wasting disease (CWD) of deer, and on the transmission risks in other species, provides optimism that transmission of CWD via semen or embryos of deer is unlikely.”
So then, what has been learned from the recent quarantines and depopulations?
In the Iowa-Braake depopulations, three bucks that were produced by artificial insemination tested positive. One of these was a sire named “withheld by author” that tested negative at death (RGS unpublished personal communication). The source of the outbreak in Iowa was never confirmed. The facility had been CWD testing since 2002.
In an investigation in Wisconsin, three bucks were tested positive for CWD. One was produced by artificial insemination and two by live cover. No sires were the same. No mothers were the same (RGS unpublished personal communication). The source of the outbreak in Wisconsin was never confirmed. The facility had been testing since 2002.
Because we find twin brothers and another half brother positive in the same pen from the same sire is NOT evidence of semen transmission. Frequently when our practice serves the industry with reproductive techniques, it is common to artificially breed many does to the same semen in the same harem or pen. This makes semen utilization and the process more efficient. The transmission of the prion agents of TSEs via germplasm, either semen or embryos is a myth. Something much repeated with no evidence. There are however valid data on horizontal transmission.
Horizontal transmission with feces, saliva, and urine are researched and are proven modes of experimental transmission. Semen and embryo transmission are NOT proven modes of transmission. Just because he told you he was a French model and you meet him on the Internet does not mean it is so. Beware of Dr. Google and others who give free advice and counsel on the Internet and those who repeat it.