Preventing experimental vertical transmission of scrapie by embryo transfer

Animal prion diseases: A review of intraspecies transmission

Animal prion diseases are a group of neurodegenerative, transmissible, and fatal disorders that affect several animal species. The causative agent, prion, is a misfolded isoform of normal cellular prion protein, which is found in cells with higher concentration in the central nervous system. This review explored the sources of infection and different natural transmission routes of animal prion diseases in susceptible populations. Chronic wasting disease in cervids and scrapie in small ruminants are prion diseases capable of maintaining themselves in susceptible populations through horizontal and vertical transmission. The other prion animal diseases can only be transmitted through food contaminated with prions. Bovine spongiform encephalopathy (BSE) is the only animal prion disease considered zoonotic. However, due to its inability to transmit within a population, it could be controlled. The emergence of atypical cases of scrapie and BSE, even the recent report of prion disease in camels, demonstrates the importance of understanding the transmission routes of prion diseases to take measures to control them and to assess the risks to human and animal health.

How do PrPSc Prions Spread between Host Species, and within Hosts?

Prion diseases are sub-acute neurodegenerative diseases that affect humans and some domestic and free-ranging animals. Infectious prion agents are considered to comprise solely of abnormally folded isoforms of the cellular prion protein known as PrP^Sc. Pathology during prion disease is restricted to the central nervous system where it causes extensive neurodegeneration and ultimately leads to the death of the host. The first half of this review provides a thorough account of our understanding of the various ways in which PrP^Sc prions may spread between individuals within a population, both horizontally and vertically. Many natural prion diseases are acquired peripherally, such as by oral exposure, lesions to skin or mucous membranes, and possibly also via the nasal cavity. Following peripheral exposure, some prions accumulate to high levels within the secondary lymphoid organs as they make their journey from the site of infection to the brain, a process termed neuroinvasion. The replication of PrP^Sc prions within secondary lymphoid organs is important for their efficient spread to the brain. The second half of this review describes the key tissues, cells and molecules which are involved in the propagation of PrP^Sc prions from peripheral sites of exposure (such as the lumen of the intestine) to the brain. This section also considers how additional factors such as inflammation and aging might influence prion disease susceptibility.

Prenatal transmission of scrapie in sheep and goats: A case study for veterinary public health

Unsettled knowledge as to whether scrapie transmits prenatally in sheep and goats and transmits by semen and preimplantation embryos has a potential to compromise measures for controlling, preventing and eliminating the disease. The remedy may be analysis according to a systematic review, allowing comprehensive and accessible treatment of evidence and reasoning, clarifying the issue and specifying the uncertainties. Systematic reviews have clearly formulated questions, can identify relevant studies and appraise their quality and can summarise evidence and reasoning with an explicit methodology. The present venture lays a foundation for a possible systematic review and applies three lines of evidence and reasoning to two questions. The first question is whether scrapie transmits prenatally in sheep and goats. It leads to the second question, which concerns the sanitary safety of artificial breeding technologies, and is whether scrapie transmits in sheep and goats by means of semen and washed or unwashed in vivo derived embryos. The three lines of evidence derive from epidemiological, field and clinical studies, experimentation, and causal reasoning, where inferences are made from the body of scientific knowledge and an understanding of animal structure and function. Evidence from epidemiological studies allow a conclusion that scrapie transmits prenatally and that semen and embryos are presumptive hazards for the transmission of scrapie. Evidence from experimentation confirms that semen and washed or unwashed in vivo derived embryos are hazards for the transmission of scrapie. Evidence from causal reasoning, including experience from other prion diseases, shows that mechanisms exist for prenatal transmission and transmission by semen and embryos in both sheep and goats.

PrP(Sc) detection and infectivity in semen from scrapie-infected sheep

A scrapie-positive ewe was found in a flock that had been scrapie-free for 13 years, but housed adjacent to scrapie-positive animals, separated by a wire fence. Live animal testing of the entire flock of 24 animals revealed seven more subclinical scrapie-positive ewes. We hypothesized that they may have contracted the disease from scrapie-positive rams used for breeding 4 months prior, possibly through the semen. The genotypes of the ewe flock were highly scrapie-susceptible and the rams were infected with the 'Caine' scrapie strain having a short incubation time of 4.3-14.6 months in sheep with 136/171 VQ/VQ and AQ/VQ genotypes. PrP(Sc) accumulates in a variety of tissues in addition to the central nervous system. Although transmission of prion diseases, or transmissible spongiform encephalopathies, has been achieved via peripheral organ or tissue homogenates as well as by blood transfusion, neither infectivity nor PrP(Sc) have been found in semen from scrapie-infected animals. Using serial protein misfolding cyclic amplification followed by a surround optical fibre immunoassay, we demonstrate that semen from rams infected with a short-incubation-time scrapie strain contains prion disease-associated-seeding activity that generated PrP(Sc) in sPMCA (serial protein misfolding cyclic amplification). Injection of the ovinized transgenic mouse line TgSShpPrP with semen from scrapie-infected sheep resulted in PrP(Sc)-seeding activity in clinical and, probably as a result of the low titre, non-clinical mouse brain. These results suggest that the transmissible agent, or at least the seeding activity, for sheep scrapie is present in semen. This may be a strain-specific phenomenon.

Factors influencing temporal variation of scrapie incidence within a closed Suffolk sheep flock

Several studies have shown that transmission of natural scrapie can occur vertically and horizontally, and that variations in scrapie incidence between and within infected flocks are mostly due to differences in the proportion of sheep with susceptible and resistant PRNP genotypes. This report presents the results of a 12-year period of scrapie monitoring in a closed flock of Suffolk sheep, in which only animals of the ARQ/ARQ genotype developed disease. Among a total of 120 of these, scrapie attack rates varied between birth cohorts from 62.5 % (5/8) to 100 % (9/9), and the incidence of clinical disease among infected sheep from 88.9 % (8/9) to 100 % (in five birth cohorts). Susceptible sheep born to scrapie-infected ewes showed a slightly higher risk of becoming infected (97.2 %), produced earlier biopsy-positive results (mean 354 days) and developed disease at a younger age (median 736 days) than those born to non-infected dams (80.3 %, 451 and 782 days, respectively). Taken together, this was interpreted as evidence of maternal transmission. However, it was also observed that, for the birth cohorts with the highest incidence of scrapie (90-100 %), sheep born to infected and non-infected dams had a similar risk of developing scrapie (97.1 and 95.3 %, respectively). Compared with moderate-attack-rate cohorts (62.5-66.7 %), high-incidence cohorts had greater numbers of susceptible lambs born to infected ewes, suggesting that increased rates of horizontal transmission in these cohorts could have been due to high levels of environmental contamination caused by infected placentas.

Evidence for maternal transmission of scrapie in naturally affected flocks

It has been known for many years that the offspring of scrapie affected ewes are at increased risk of developing scrapie but whether this is simply the result of an increased genetic susceptibility or transmission of infection has always been unclear. To contribute to clarify this we analysed the data collected in a detailed study of scrapie occurrence in a number of naturally affected commercial sheep flocks in Great Britain (GB) to investigate the association between PrP genotype and parental scrapie status and the incidence of scrapie. Our analyses confirmed the strong association between PrP genotype and the incidence of scrapie found in previous studies and a low incidence of scrapie in animals carrying the ARR allele and a high risk in homozygous VRQ animals. However, we also demonstrate an increased incidence of scrapie in the offspring of scrapie affected ewes controlling for the confounding effect of PrP genotype, but no increased scrapie incidence in the offspring of scrapie affected sires. Our results suggest that some of the increased incidence of scrapie in the offspring of scrapie affected ewes is the result of transmission of infection from mother to offspring. Our results confirm that a breeding policy aimed at decreasing the genetic susceptibility of the population should decrease the incidence of scrapie and that removing the offspring of scrapie affected animals from affected flocks could contribute to the control of this disease.

Failure to transmit scrapie infection by transferring preimplantation embryos from naturally infected donor sheep

The objective of the study was to examine whether or not the preimplantation embryo can act as a carrier of classic scrapie infection. The study was carried out on quarantined premises with sheep of highly susceptible scrapie genotypes. Uninfected embryos, collected from New Zealand-derived Suffolk ewes, were surgically transferred into recipient ewes that were also of New Zealand origin. Seventeen negative control lambs were born on the study premises from these embryo transfers. Thirty-nine experimental lambs were from embryos collected from naturally infected donor ewes. The experimental lambs were also born on the study premises after their surgical transfer into recipient ewes of New Zealand origin. These embryos had been collected from donor ewes in a scrapie-infected flock where the ewes were clinically sick with scrapie or developed clinical scrapie after embryo collection. All lambs were confirmed as scrapie susceptible of the ARQ/ARQ genotype. Twenty-eight experimental animals survived to the end point of the study at 5 yr of age with a mean survival of 1579 d. In the negative control group, 12 of 17 sheep survived to 5 yr of age with a mean survival of 1508 d. Postmortem examinations were carried out on all animals derived by embryo transfer, and in none was histologic or immunohistochemical evidence of scrapie found. In contrast, in the originating flock the majority of scrapie cases occurred in ARQ/ARQ genotyped animals where a 56% mortality from scrapie had been recorded in animals of this genotype. Thus, the study provides no evidence for transmission of scrapie and reinforces published evidence that vertical transmission of scrapie may be circumvented by embryo transfer procedures.

Relative risks and approaches to biosecurity in the use of embryo technologies in livestock

Embryo technologies have been integrated into production systems for a variety of livestock species. As relates to transmission of infectious diseases, our working hypothesis has been that use of embryo transfer for distribution of germ plasm within and between herds and flocks is likely safer than the movement of postnatal animals. Indeed, research and experience generally have been supportive of this hypothesis. However, the relative risks of transmitting infectious agents via embryo transfer vary among donor species. Further, different methods of producing embryos appear to present different risks. This paper provides a comparative overview of the risks of transmitting infectious diseases via transfer of both in vivo- and in vitro-derived embryos in common domesticated livestock species. Also discussed are universal approaches to biosecurity in embryo production and transfer.

Disinfection procedures for controlling microorganisms in the semen and embryos of humans and farm animals

Semen and embryos generated by assisted reproductive techniques (ARTs) may be contaminated with numerous microorganisms. Contamination may arise from systemic or local reproductive tract infections in donors or the inadvertent introduction of microorganisms during ARTs, and may lead to disease transmission. This review describes sanitary procedures which have been investigated to ascertain whether they are effective in rendering semen and embryos free of pathogenic microorganisms, including internationally adopted washing procedures, which can be supplemented by antibiotics and enzymatic treatments. Other methods include treatment with antibodies or ozone, photoinactivation, acidification, and the use of novel antiviral compounds. In conclusion, despite the wide range of antimicrobial procedures available, none can be recommended as a universal disinfection method for rendering semen and embryos free from all potentially pathogenic microorganisms. However, some procedures are unsuitable, as they can compromise the viability of semen or embryos. In humans, washing by the gradient centrifugation method appears to be effective for reducing the microbial population in semen and is harmless to the spermatozoa. A useful procedure for embryos involving multiple washes in sterile medium has much to commend it for the prevention of disease transmission; furthermore, it is recommended by the International Embryo Transfer Society (IETS).

Expression of the prion protein gene (PRNP) and cellular prion protein (PrPc) in cattle and sheep fetuses and maternal tissues during pregnancy

We investigated the expression of prion protein gene both on mRNA and protein levels in bovine and ovine female reproductive organs during gestation and various tissues of their fetuses. The fetal tissues of both species included brain, cotyledon, heart, intestine, kidney, liver, lung, and muscle. In cattle, prion protein gene (PRNP) transcripts were detected by semiquantitative RT-PCR in reproductive tissues such as ovary, oviduct, endometrium, myometrium, follicles, and granulosa cells. In various tissues of 2-month-old fetuses, higher expression levels were found in brain and cotyledon compared to the other tissues. To detect the expression of the gene transcript in in vivo preimplantation embryos and 1-month-old fetuses, real-time PCR was performed showing that the level of gene expression in zygote stage was significantly higher (p < or = 0.05) than that of the other stages. Sheep were categorized as resistant (RI) or high susceptible (R5) to scrapie according to their PRNP genotype. In both genotype groups, the PRNP mRNA was detectable in all tissues studied including ovary, oviduct, endometrium, myometrium, and caruncle of ewes and all tissues of 2-month-old fetuses of both groups. Comparison between reproductive organs demonstrates the highest expression level in caruncle tissue of R1 ewes, whereas the level was high in brain and low in liver of both R1 and R5 fetuses. In addition, real-time RT-PCR was performed in immature oocytes, mature oocytes, in vivo embryos at morula stage, and 1-month-old fetuses. The results showed that the relative expression levels of the ovine PRNP mRNA in mature oocytes and morula stage embryos were significantly lower than those in immature oocytes and 1-month-old fetuses (p < or = 0.05). Western blot analyses revealed the immunoreactive bands corresponding to the cellular prion protein (PrPc) in all maternal and fetal tissues examined of both cattle and sheep. Moreover, immunohistochemical staining implicated localization of the PrPc in ovarian cortex and ovarian medulla of both species. However, PrPc was not detected in oocyte, granulosa cells, theca cells, and corpus luteum in this study.

Epidemiological implications of the susceptibility to BSE of putatively resistant sheep

The experimental infection of sheep with bovine spongiform encephalopathy (BSE) by the oral route and the likelihood that sheep were fed BSE-infected meat and bone meal has led to extensive speculation as to whether or not sheep are naturally infected with BSE. In response, the UK government has initiated the National Scrapie Plan (NSP), an ambitious £120 million per year project to create a BSE- and scrapie-resistant national sheep flock, by selectively breeding for a genotype of sheep believed to be resistant to both diseases. This genotype has recently been shown to be susceptible to BSE by intracerebral (i.c.) inoculation. Should these sheep be sufficiently susceptible to BSE via natural transmission, the NSP might fail. Here we estimate the susceptibility of this genotype to horizontal (sheep-to-sheep) transmission of BSE by comparison with more extensive oral and i.c. exposure data for other sheep genotypes. We show that a previous estimate of the risk of BSE transmission to sheep via the feedborne route remains robust. However, using a mathematical model for the within-flock transmission of BSE, we show that, while the best estimate indicates that the NSP should be successful, current data cannot exclude the failure of the NSP.

Scrapie and experimental BSE in sheep

Scrapie is a natural disease of sheep, but it can also be successfully transmitted between sheep by experimental inoculation. Although BSE is primarily a disease of cattle, it has also infected humans (causing vCJD) and, in addition, can be transmitted orally to sheep bringing concerns that BSE might naturally have infected the UK sheep population. Because of this, scrapie and BSE are being compared and studied in detail in sheep. PrP genotype controls sheep susceptibility and resistance to scrapie and to BSE, and deposition of the disease-associated PrP(Sc), used as a marker of infection, has the potential to act as a means of identifying BSE-infected animals and describing different pathogenesis mechanisms. Sheep orally dosed with BSE show signs of infection in their blood and this model is of major importance in the study of the safety of blood products for use with human beings.

Polymorphic distribution of the ovine prion protein (PrP) gene in scrapie-infected sheep flocks in which embryo transfer was used to circumvent the transmissions of scrapie

The genetic sequence of the ovine prion protein (PrP) gene between codons 102 and 175 with emphasis on ovine PrP gene codons 136 and 171 was determined, and the polymorphic distribution of the ovine PrP gene in the scrapie-exposed Suffolk embryo donors and offspring from these donors that were transferred to scrapie-free recipient ewes was investigated in this study. The most common genotype was AA(136)QQ(171) (70% and 63% in the donor and offspring flocks, respectively), which is considered a high risk genotype in US Suffolk sheep. Although embryos were collected from scrapie-positive donors and many embryos had the high risk genotype, no scrapie occurred in the resulting offspring. Based upon the results of this study, we conclude that vertical transmission of scrapie can be circumvented using embryo transfer procedures even when the offspring have the high risk genotype.