Although the transfer of embryos is much less likely to result in disease transmission than the transport of live animals, the sanitary risks associated with embryo transfer continue to be the subject of both scientific investigations and adaptations of national and international legislation. Therefore, the implications are important for veterinary practitioners and livestock breeders. In vivo-derived and in vitro-produced embryos are widely used in cattle and embryos from other species, such as sheep, goats, pigs and horses, are also currently being transferred in fairly significant numbers. Bearing in mind the wide variety of embryos of different species and the correspondingly large number of viruses that are of concern, it is expedient at this time to look again at the importance of the zona pellucida (ZP) as a barrier against viruses and at the susceptibility or otherwise of embryonic cells to viral infection if ever they are exposed. For embryos with an intact ZP, viral infection of the embryo is unlikely to occur. However, the virus may stick to the ZP and, in this case, International Embryo Transfer Society (IETS) washing procedures in combination with trypsin treatment are mandatory. A caveat is the fact that currently more and more types of embryos are becoming available for transfer and scientific data cannot be extrapolated from one species to another. These topics are discussed in the present review.

Summary Three sows were fed 8.5 mg. dichlorvos (in a slow release preparation) per kg. body weight per day from day 41 to day 70 of pregnancy. Blood cholinesterase levels in the sows were markedly depressed by treatment, and had still not returned to normal at term (115 days). Piglets, removed by hysterectomy at term, appeared clinically and morphologically normal, but the weights of some of their organs (brain, thymus, thyroid, liver and sternocephalic muscle) were heavier, whilst weights of other organs (pituitary, spleen and eyes) were lighter than those of piglets from untreated sows. The effects of dichlorvos appear to be quite different from those of the chemically related organophosphorus compound trichlorphon which, if fed to sows in mid-pregnancy, is teratogenic, causing severe cerebellar hypoplasia with congenital ataxia in the progeny. Zusammenfassung Perorale Wirkung von Dichlorvos bei Schweinen während der mittleren Graviditätsphase Drei Schweinen wurden täglich vom 41. bis 70. Tag der Gravidität 8.5 mg/kg Körpergewicht Dichlorvos (2,2-Dichlorvinyldimethylphosphat, DDVP) in einer langsam absorbierbaren Form im Futter verabreicht. Das Präparat bewirkte eine deutliche Aktivitätsabnahme der Blutcholinesterase, welche zur Zeit der Geburt (115. Graviditätstag) noch immer nachzuweisen war. Die durch Hysterektomie am Ende der Gravidität gewonnenen Ferkel waren zwar klinisch und morphologisch normal, hingegen waren einige Organe (Gehirn, Thymus, Schilddrüse, Leber, M. sternocephalicus) schwerer, während andere Organe (Hypophyse, Milz, Augen) im Vergleich zu Ferkeln unbehandelter Muttertiere leichter waren. Die Effekte von DDVP unterscheiden sich erheblich von denjenigen des Trichlorphon, eines chemisch verwandten Organophosphats, welches bei Verabreichung während der mittleren Trächtigkeitsphase bei den Ferkeln eine zerebellare Hypoplasie mit kongenitaler Ataxie hervorrufen. Résumé Action perorale de Dichlorvos chez des porcs au milieu de la période de gestation On a donné à 3 porcs sous forme lentement absorbable dans un aliment quotidiennement du 41e au 70e jour de gestation 8,5 mg/kg de poids de Dichlorvos (2,2-dichlorvinyldiméthylphosphate). La préparation a provoqué une nette diminution de l'activité de la cholinestérase sanguine qui fut encore plus marquée au moment de la mise bas (115e jour de gestation). Les porcelets nés par hystérectomie en fin de gestation furent cliniquement et morphologiquement normaux; quelques organes (cerveau, thymus, glande thyroïde, foie, M. sternocephalicus) furent plus lourds et d'autres organes (hypophyse, rate, yeux) plus légers que ceux de porcelets provenant de truies non traitées. Les effets du DDVP se différencièrent nettement de ceux du Trichlorphon, un organophosphate chimiquement apparenté, dont l'absorption au milieu de la gestation a provoqué une hypoplasie du cervelet avec ataxie congénitale chez les porcelets. Resumen Efecto peroral de Diclorovos en cerdas durante la fase media de gravidez Se administró diariamente a tres cerdas desde el día 41° hasta el 70° de gravidez 8,5 mg./kg. peso en vivo de Diclorovos (dimetilfosfato de 2,2 diclorovinilo, DDVP) en una forma absorbible lentamente, con el pienso. La especialidad causó la disminución manifiesta en la actividad de la colinesterasa en sangre, la cual aún se podía poner en evidencia en el momento del parto (día 115° de gestación). Los lechones logrados mediante histerectomía al final de la gestación eran normales clínica y morfológicamente, mientras que algunos órganos (cerebro, timo, tiroides, hígado, m. esternocefálico) pesaban algo más, a la vez que otros órganos (hipófisis, bazo, ojos) eran más livianos en comparación con los lechones procedentes de parideras no tratadas. Los efectos del DDVP son bien diferentes de los producidos por Triclorofon, un compuesto organofosforoso allegado químicamente, el cual ocasiona en los lechones, si se administra durante la fase media de gestación, una hipoplasia cerebelar con ataxia congénita.

Summary Vitamin A was incorporated at 300 times the minimum recommended level into the feed of 2 sows (A and B) from 21 d. prior to mating to term, and into the feed of 2 others (C and D) from 50 d. after mating to term. An untreated sow (E) was kept as a control. Minor skin changes were noted in the treated sows but gestation and farrowing were otherwise uneventful. Litter sizes and birth weights were within the usual range and, apart from one piglet (ex litter B) with duplication of the lower jaw and some piglets with splayleg, the majority were clinically normal at birth. However, one piglet (ex litter A) was stillborn and 3 piglets (ex litters A, B and D) died within the first 24 h. Remaining piglets were killed 24 h. after birth and then all were subjected to detailed post-mortem examination. Significant enlargement of the heart was observed in many piglets from treated sows and weights of certain other organs were different from those in controls. Haemopoietic centres were much more numerous in the livers of piglets from sows C and D than in piglets from the other groups. Radiographic examination did not reveal abnormalities of bone development in any of the piglets. Hepatic levels of vitamin A in piglets from the treated sows were significantly higher than those of control piglets. It is concluded that even when fed for prolonged periods at exceptionally high levels vitamin A is unlikely to produce serious embryotoxic or teratogenic effects in pigs. Zusammenfassung Wirkung sehr hoher alimentärer Vitamin-A-Dosen bei Mutterschweinen Bei zwei Schweinen (A u. B) wurde vom 21. Tag vor dem Decken bis zur Geburt im Futter eine Vitamin A-Menge verabreicht, die 300mal größer war als die empfohlene Minimaldosis. Bei zwei anderen Schweinen (C u. D) wurde die gleiche Vitamin A-Dosis vom 50. Tag nach dem Decken bis zur Geburt verabreicht. Ein weiteres Schwein (E) diente als Kontrolle. Bei den behandelten Tieren wurden geringe Hautveränderungen beobachtet, die Gravidität und die Geburt zeigten hingegen keine Besonderheiten. Wurfgröße und Geburtsgewichte waren normal. Abgesehen von einem Ferkel (aus dem Wurf des Mutterschweines A) mit einer Verdoppelung des Unterkiefers und einigen Ferkeln mit Abduktionen der Hintergliedmaßen (Splayleg), zeigte die Mehrzahl der Neugeborenen keine Abnormitäten. Ein Ferkel (aus Wurf A) wurde totgeboren, und drei Ferkel (aus den Würfen A, B und D) starben postpartal innerhalb 24 Stunden. Die restlichen Ferkel wurden 24 Stunden nach der Geburt getötet und einer exakten Sektion unterzogen. Viele Ferkel der behandelten Schweine zeigten eine Herzvergrößerung, und auch das Gewicht anderer Organe wich von dem der Kontrollen ab. Hepatische Blutbildungsherde traten bei Ferkeln der Schweine C und D in vermehrter Zahl auf. Röntgenuntersuchungen ergaben keine Hinweise auf Störungen der Knochenentwicklung. Der Vitamin-A-Gehalt in der Leber der Ferkel von behandelten Schweinen war signifikant höher als bei den Kontrollferkeln. Folgerung: Langdauernde Verabreichung von sehr hohen Vitamin-A-Dosen haben bei Schweinen wahrscheinlich weder embryotoxische noch teratogene Wirkungen. Résumé Actions de très fortes doses alimentaires de vitamine A chez des truies Une dose de vitamin A 300 fois plus élevée que la quantité minimale a été distribut́e dans la nourriture à deux truies (A et B) à partir du 21e jour avant la saillie jusqu'à la mise à bas. Une dose identique de vitamine A a été distribut́e à deux autres truies (C et D) à partir du 50e jour après la saillie. Une autre truie (E) a servi de témoin. On a observé de légères lésions cutanées chez les animaux traités, la gestation et la mise à bas s'étant déroulées normalement. Le nombre des porcelets et le poids à la naissance furent normaux. Mis-à-part un double maxillaire inférieur chez un porcelet (nicht́e de la truie A) et quelques porcelets présentant un «splayleg», la plupart des porcelets furent normaux. Un porcelet (nichée A) est mort-né et 3 porcelets (nichées A, B et D) moururent 24 heures après la naissance. Les autres porcelets furent sacrifiés 24 heures après la naissance et autopsiés. Beaucoup de porcelets des truies traitées ont présenté une hyperhlasie du cœur et le poids d'autres organes différait de celui des contrôles. Des foyers hémorragiques hépatiques ont été observé chez des porcelets des truies C et D. Les examens radiologiques n'ont pas montré de lésions dans le développement du squelette. Le taux en vitamine A dans le foie des porcelets de truites traitées fut significativement plus élevé que celui des porcelets de contrôle. Conclusion: L'application durable de fortes doses de vitamine A chez des porcs n'a vraisemblablement pas d'action embryotoxiques ou tératogènes. Resumen Efectos de dosis alimenticias excesivas de vitamin A en parideras Con el pienso se administró a dos cerdas (A y B) desde el día 21° antes de la monta hasta el parto una cantidad de vitamina A 300 veces mayor que la dosis mínima recomendada. Se facilitó a otras dos cerdas (C y D) la misma cantidad de vitamina A desde el día 50° después de la monta hasta el parto. Otra cerda (E) fué utilizada como testigo. En los animales tratados se observaron modificaciones cutáneas escasas, mientras que la gravidez y el parto no mostraban ninguna particularidad. Eran normales el número de lechones nacidos en cada cría y los pesos de nacimiento. Prescindiendo de un lechón (perteneciente a la camada de la paridera A) con duplicación de la mandíbula y algunos lechones con abducciones de las extremidades posteriores, la mayoría de los recien nacidos no evidenciaba ninguna anormalidad. Un lechón (de la camada A) nació muerto y tres lechones (de las camadas A, B y C) murieron después del parto en el espacio de las 24 horas primeras. Los lechones restantes se sacrificaron a las 24 horas después del nacimiento y se sometieron a una disección detallada. Se observó en muchos lechones de las cerdas tratadas una hipertrofia cardíaca y que el peso de los órganos restantes difería del de los testigos. Los centros hematopoyéticos eran mucho más numerosos en los hígados de los lechones de las cerdas C y D. Los exámenes radiográficos no revelaron anormalidades en el desarrollo óseo. Los niveles hepáticos de vitamina A eran significantemente más altos que en los lechones testigos. Se saca en conclusión que la administración alimenticia de niveles elevados de vitamina A durante periodos prolongados no suele producir efectos embriotóxicos o teratógenos serios en los lechones.

Early experiments suggested that scrapie transmission via sheep embryos was a possibility, and gave rise to much controversy. However, when account is taken of the complex genetic effects on ovine susceptibility to scrapie, and of the several different scrapie strains with different clinical and pathological effects, the overall conclusion now is that transmission of classical scrapie by embryo transfer is very unlikely if appropriate precautions are taken. Recent embryo transfer studies have confirmed this. Other studies in sheep have shown that from about the middle of pregnancy the placental trophoblast is liable to scrapie infection in genetically susceptible ewes if the fetus is also susceptible. Since the contrary is also true, use of resistant ewes as embryo recipients could add to the safety of the embryo transfer, at least for classical scrapie. There has been little recent research on scrapie transmission via semen in sheep, and, with hindsight, the early studies, though negative, were inadequate. There is scant information on scrapie transfer via goat semen or embryos, although one study did find that bovine spongiform encephalopathy (BSE) was not transmitted via goat embryos. In cattle it has been shown that, if appropriate precautions are taken, the risks of transmitting BSE via semen and in vivo-derived embryos are negligible, and this conclusion has gained worldwide acceptance. 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 transmission risks in other species, provides optimism that transmission of CWD via semen and embryos of deer is unlikely. The presence of TSE infectivity in blood and various other tissues of infected animals, particularly sheep, gives rise to concerns that certain biological products currently used in reproductive technologies, e.g. pituitary gonadotrophins for superovulation, and certain tissue and blood products used in semen and embryo transfer media, could carry TSE infectivity. Instruments such as laparoscopes used for insemination, and for collection and transfer of embryos, especially in small ruminants, are also a concern because effective decontamination can be very difficult.

This scientific review was prompted by recent legislation to curtail the use of semen from potentially virus-infected bulls to produce embryos for import into the European Union. From studies in laboratory animals, humans and horses, it is apparent that viruses may sometimes attach to, or be integrated into, spermatozoa, although in domestic livestock, including cattle, this seems to be a rare phenomenon, and carriage of virus through the zona pellucida into the oocyte by fertilising sperm has never been described in these species. Four specific viruses; enzootic bovine leukosis (EBLV), bovine herpesvirus-1 (BoHV-1), bovine viral diarrhoea virus (BVDV) and bluetongue virus (BTV), all of which tend to cause subclinical infections in cattle, but which can occur in bovine semen, are examined with regard to the risks that use of infected semen might lead to production of infected embryos. With regard to in vivo-derived embryos, when internationally approved embryo processing protocols are used, the risks from EBLV- and BTV-infected semen are negligible, and the same is almost certainly true for semen infected with BoHV-1 if the embryos are also treated with trypsin. For BVDV, there is insufficient data on how the virus is carried in semen and how different BVDV strains can interact with sperm, oocytes and embryos. There is a potential, at least, that in vivo-derived embryos resulting from infected semen might carry BVDV, although field studies so far suggest that this is very unlikely. With regard to in vitro-produced embryos, use of semen infected with any of the four viruses, with the probable exception of EBLV, will often lead to contaminated embryos, and virus removal from these embryos is difficult even when the internationally approved embryo processing protocols are used. However, it has never been demonstrated that such embryos have resulted in transmission of infection to recipients or offspring.

The foot-and-mouth disease (FMD) epidemic in the U.K. in 2007 highlighted the threat of infectious diseases to rare and valuable livestock and stimulated a renewed interest in biosecurity. Not all diseases resemble FMD, however; transmission routes and pathological effects vary greatly, so biosecurity strategies must take this into account. Realism is also needed as to which diseases to exclude and which will have to be tolerated. The aim should be to minimise disease generally and to exclude those diseases that threaten existence of the livestock, or preclude their national or international movement. Achieving this requires a team effort, bearing in mind the livestock species involved, the forming system ('open' or 'closed') and the premises. Effective biosecurity demands that practically every aspect of farm life is controlled, including movements of people, vehicles, equipment, food, manure, animal carcasses and wildlife. Above all, biosecurity strategies must cover the disease risks associated with moving the livestock themselves, and this will require quarantine if adult or juvenile animals are imported into the herd or flock. Reproductive technologies such as artificial insemination and embryo transfer offer much safer ways for getting new genetic materials into herds/flocks for breeding than bringing in live animals. Embryo transfer is especially safe when the sanitary protocols promoted by the International Embryo Transfer Society (IETS) and advocated by the Office International des Epizooties (OIE: the 'World Organisation for Animal Health) are used. It can also allow the full genetic complement to be salvaged from infected animals. Cryobanking of genetic materials, especially embryos, is another valuable biosecurity strategy because it enables storage for contingencies such as epidemic disease or other catastrophes. Reproductive technologies such as AI and embryo transfer offer safer ways to move and introduce new genetic materials for breeding, and transferring embryos is especially safe when the sanitary protocols promoted by the IETS and advocated by the OIE are used. Research has demonstrated that with those provisos the risks of transmitting catastrophic diseases like FMD and BSE by embryo transfer are exceedingly small. Another reproductive technology with major biosecurity benefits is embryo cryopreservation which can enable long-term storage of the full genetic complement of valuable livestock with minimal risks of any diseases being present. This paper focuses mainly on biosecurity aspects of moving or storing in vivo-derived embryos for which the disease control advantages are well researched. There is less confidence with regard to embryos produced in vitro, or by genetic manipulation and cloning, and it is evident that higher disease risks may occur with these. Fortunately, sanitary protocols set out in the IETS Manual (IETS, 1998) for in vivo-derived embryos can also be applied to advantage for these other reproductive technologies, and codes of practice for in vitro-produced and micromanipulated embryos have been published in the OIE Code (OIE, 2002).

The foot and mouth disease (FMD) epidemic in the UK in 2001 highlighted the threat of infectious diseases to rare and valuable livestock and stimulated a renewed interest in biosecurity and conservation. However, not all diseases resemble FMD: their transmission routes and pathological effects vary greatly, so biosecurity strategies must take this into account. Realism is also needed as to which diseases to exclude and which will have to be tolerated. The aim should be to minimise disease generally and to exclude those diseases that threaten the existence of livestock or preclude their national or international movement. Achieving this requires a team effort, bearing in mind the livestock species involved, the farming system ('open' or 'closed') and the premises. Effective biosecurity demands that practically every aspect of farm life is controlled, including movements of people, vehicles, equipment, food, manure, animal carcasses and wildlife. Above all, biosecurity strategies must cover the disease risks associated with moving the livestock themselves and this will require quarantine if adult or juvenile animals are imported into the herd or flock. The present paper emphasises the important role that reproductive technologies, such as artificial insemination and embryo transfer, can have in biosecurity strategies because they offer much safer ways for getting new genetic materials into herds/flocks than bringing in live animals. Embryo transfer is especially safe when the sanitary protocols promoted by the International Embryo Transfer Society and advocated by the Office International des Epizooties (the 'World Organisation for Animal Health') are used. Embryo transfer can also allow the full genetic complement to be salvaged from infected animals. Cryobanking of genetic materials, especially embryos, is another valuable biosecurity strategy because it enables their storage for conservation in the face of contingencies, such as epidemic disease and other catastrophes.

Semen from 13 bulls, eight with clinical bovine spongiform encephalopathy (BsE), was used to artificially inseminate (AI) 167 cows with clinical BSE, and their resultant embryos were collected non‐surgically seven days after Al. The viable and non‐viable embryos with intact zonae pellucidae were washed 10 times (as recommended by the International Embryo Transfer Society) then frozen. Later, 587 of the viable embryos were transferred singly into 347 recipient heifers imported from New Zealand, and 266 live offspring were born of which 54.1 per cent had a BSE‐positive sire and a BSE‐positive dam. The recipients were monitored for clinical signs of BSE for seven years after the transfer, and the offspring were monitored for seven years after birth. Twenty‐seven of the recipients and 20 offspring died while being monitored but none showed signs of BSE. Their brains, and the brains of the recipients and offspring killed after seven years, were examined for BSE by histopathology, PrP immunohistochemistry, and by electron microscopy for scrapieassociated fibrils. They were all negative. In addition, 1020 non‐viable embryos were sonicated and injected intracerebrally into susceptible mice (20 embryos per mouse) which were monitored for up to 700 days, after which their brains were examined for spongiform lesions. They were all negative. It is concluded that embryos are unlikely to carry BSE infectivity even if they have been collected at the end‐stage of the disease, when the risk of maternal transmission is believed to be highest.

This paper considers whether transmissible spongiform encephalopathies (TSEs or prion diseases) could be spread by artificial insemination, embryo transfer and other more advanced reproductive technologies which are used for genetic improvement and also for purposes such as production of recombinant drugs for medical use. Although the technologies are most used in cattle, they are increasingly used in sheep, goats and deer as well, all of which can be naturally affected by TSEs. In general, provided appropriate precautions are taken, the risks of TSE carriage specifically by the gametes (spermatozoa and oocytes) or by in-vivo-derived embryos per se appear to be negligible, but further research, some of which is already in progress, will be helpful to give assurance on this point. Greater concerns relate to the many biological products that are used in the technologies, e.g. pituitary hormones used for the superovulation of donors, and various tissues and blood products used in semen and embryo culture/transport media, some of which have the potential to carry TSE infectivity if derived from infected animals. The myriad instruments and items of technical equipment that are used also give cause for concern because if they become contaminated with TSEs they may, due to their construction, be impossible to sterilise properly.

Two-hundred-and-fifteen embryos recovered from 76 donor ewes from flocks endemically infected with sheep pulmonary adenomatosis (SPA) and mated with uninfected rams were transferred to 131 uninfected recipients under strict sanitary conditions using International Embryo Transfer Society protocols. The recipients and their progeny were kept in a closed, isolated SPA-free flock. Thirty-eight of 51 progeny from SPA-positive donors and 55 of 74 progeny from donors in which no lesions of SPA were detected survived for at least five years after birth. In a similar study 11 embryos from four uninfected donors mated to an SPA-infected ram were transferred to seven recipients, and four of five progeny born to four recipients survived for at least five years. No evidence of SPA was found in the recipients or their progeny by embryo transfer in either study. On the basis of clinical and pathological criteria, it is concluded that embryo transfer can be used to provide an effective barrier against the transmission of SPA from donors from infected flocks, whether or not the parents show clinical signs of the disease.

This paper addresses the risks involved when bovine embryos are moved internationally and, specifically, the possibilities of transmitting foot-and-mouth disease, bluetongue and vesicular stomatitis by embryos originating from an area in South America. The risk scenario pathway was divided into three phases for analysis. The first phase dealt with the potential for embryo contamination which depends on the disease situation in the exporting country and/or region, the health status of the herds and the donor cows from which the embryos are collected, and the pathogenetic characteristics of the specified disease agent. The second phase covers risk mitigation by use of internationally accepted standards for processing of embryos, and the third phase encompassed the risk reductions resulting from post-collection surveillance of the donors and donor herds, and also from testing of embryo-collection (flushing) fluids for the disease agent. Quantitative risk analysis showed that under the circumstances specified in the paper, the risk of transmission of foot-and-mouth disease and vesicular stomatitis by embryos would be likely to be less than 1 in 100 billion (10−11.0) and 1 in 100 million (10−8.0), respectively. The values for bluetongue were 1 in 30000 (10−4.2) when embryos were collected in the vector season and 1 in 1 million (10−6.0) in the season with low vector activity. These risk values were influenced by the incidence of each disease in the area of origin and the ease with which clinical signs can be recognised. Competent embryo processing according to procedures recommended by the International Embryo Transfer Society were also of great importance. The analysis showed that the reasons for the low levels of risk of transmission differed for each of the three diseases. In the case of bluetongue, vector ecology was of major importance.

Guidelines for the safe international movement of livestock embryos are provided in the International Animal Health Code of the Office International des Epizooties, and recommendations for embryo processing, based on numerous research papers on embryo-pathogen interaction studies, are given in the Manual of the International Embryo Transfer Society. Risk assessment is the logical extension of these approaches, since it provides veterinary authorities with a complete package of information on which to base their import/export decisions. Risk assessment includes evaluation of disease prevalence, effectiveness of Veterinary Services and competence of the embryo collection team. It also takes account of the epidemiology and pathogenesis of the disease concerned. The application of risk assessment for embryo movement is illustrated in this paper by comparisons of the probabilities of transmitting foot and mouth disease, bluetongue and vesicular stomatitis by bovine embryos. The risk scenario pathway was divided into three phases for analysis. The first phase deals with the potential for embryo contamination, which depends on the disease situation in the exporting region, the health status of donor herds and donor cows, and on the pathogenetic properties of the disease agent. The second phase covers risk mitigation by use of the internationally accepted standards for embryo processing, and the third phase considers the risk reductions resulting from post-collection surveillance of donors and donor herds, and also from testing of embryo-collection (flushing) fluids for the disease agent. It was evident from this assessment that low risks of transmitting disease by international movement of bovine embryos depend initially on a low disease incidence in the exporting region and on easily recognisable disease signs. Competent embryo processing was also of great importance, and in the case of bluetongue, vector ecology had a major influence. In addition to providing a logical basis for import/export decisions, risk assessment is useful for evaluating the potential outcome of new research and for assessing the safety of the movement of embryos of other species for which little or no research information is available on embryo-pathogen interactions.

This paper reviews current knowledge on transmission of scrapie and bovine spongiform encephalopathy (BSE) by semen and embryos.In sheep, in particular, it is difficult to distinguish between the genetic transmission of susceptibility to scrapie and vertical transmission of the infection.Nevertheless, there is evidence that vertical transmission of infection does occur, probably across the placenta, but none to suggest a significant scrapie risk from semen.Two teams have studied scrapie transmission from experimentally infected sheep using embryo transfer.Whereas one team found no evidence fortransmission,the results from the other team suggest that embryos, even after washing, might carry the disease into the offspring.In regard to goats, although genetic differences in susceptibility exist, they are much less obvious than in sheep.There is no evidence for vertical transmission or for transmission through semen and embryos.With regard to BSE, although it appears that genetic differences in susceptibility are absent or unimportant, some recent work does suggest that the disease may be passed from cow to calf.The route of transmission and stage or stages when this takes place are unclear, however.In conclusion, despite growing evidence to indicate that scrapie and BSE are unlikely to be transmitted through semen and embryos, more research is needed to confirm this.Furthermore, until all possibility of risk is ruled out, risk reduction methods must be considered, especially when semen and embryos are being imported into countries where the diseases do not exist.

In vitro production of embryos is the latest technology to be applied in commercial livestock breeding. Knowledge of interactions between infectious agents and in vitro-derived embryos is limited. This paper gives evidence of pertinent differences between in vivo and in vitro-derived bovine embryos and summarizes research on IVF embryo-pathogen interactions. Sanitary controls for the production of specific-pathogen-free IVF embryos are discussed, and some epidemiologically relevant questions are posed. Since the “2 types” of embryos and their production methods differ, the risks of disease transmission through transfer of each type of embryo must be assessed independently. Also, sanitary controls must be designed to insure that infectious agents do not affect the results of research utilizing IVF, and that application of IVF for commercial production of calves will not lead to a new mode of transmission of infectious diseases.

The possibility that infectious diseases could be transmitted between animals, farms or countries when embryos are transferred is a justifiable concern, and, because of the risks, regulatory authorities are obliged to apply a variety of necessary constraints that may seem frustrating and expensive for embryo transfer practitioners. The International Embryo Transfer Society (IETS), through its Import/Export Committee, has worked strenuously for over a decade to reduce the need for expensive testing of donors, and to help formulate safe, practical protocols for embryo movements, based on embryo handling, that are now found in the Office International des Epizootics (OIE), International Animal Health Code and the IETS Manual. The purpose of this paper is to provide an up-date on knowledge of embryo/pathogen interactions, and to discuss the potential risks of disease transmission through transfer of in vivo-produced embryos and how these risks may be avoided. Quantitative risk assessment is now used increasingly by regulatory authorities in the development of international embryo certification requirements, and embryo transfer practitioners have an important role to play in this expanding science.

A diagnosis of congenital goitre was confirmed histologically in piglets which were born hairless and swollen, and with significantly enlarged thyroid glands. The iodine content of the thyroid glands and the serum total thyroxine concentrations were very low. No evidence was found of iodine deficiency or significant goitrogenic activity in the diet fed to the sows. An investigation of the parentage of six affected litters revealed that they all had one or other of two boars as the sire, grandsire or great grandsire. The suspicion of an inherited disorder was confirmed when a test mating of a suspect carrier boar and sow resulted in the birth of two affected piglets. The ratio of the numbers of affected and unaffected piglets was statistically consistent with an autosomal recessive mode of inheritance.