Is the zona pellucida an efficient barrier to viral infection?

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.

Atypical scrapie in sheep from a UK research flock which is free from classical scrapie.

Background: In the wake of the epidemic of bovine spongiform encephalopathy the British government established a flock of sheep from which scrapie-free animals are supplied to laboratories for research. Three breeds of sheep carrying a variety of different genotypes associated with scrapie susceptibility/resistance were imported in 1998 and 2001 from New Zealand, a country regarded as free from scrapie. They are kept in a purpose-built Sheep Unit under strict disease security and are monitored clinically and post mortem for evidence of scrapie.

Risks of transmitting ruminant spongiform encephalopathies (prion diseases) by semen and embryo transfer techniques.

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.

Evaluation of risks of viral transmission to recipients of bovine embryos arising from fertilisation with virus-infected semen.

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.

Biosecurity strategies for conserving valuable livestock genetic resources.

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.

Studies of embryo transfer from cattle clinically affected by bovine spongiform encephalopathy (BSE).

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 AI. 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.

Prevention of transmission of sheep pulmonary adenomatosis by embryo transfer.

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.

A quantitative assessment of the risk of transmission of foot-and-mouth disease, bluetongue and vesicular stomatitis by embryo transfer in cattle.

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.

Risks of transmitting scrapie and bovine spongiform encephalopathy by semen and embryos.

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.

The risks of disease transmission by embryo transfer in cattle.

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.

An inherited congenital goitre in pigs.

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.

Serological reactions to Brucella species in British pigs.

Until and including 1987 diagnostically significant serological titres to swine brucellosis had occurred in the serum agglutination test (SAT) or the complement fixation test (CFT), ie, > or = 100 iu or > or = 20 icftu, respectively, almost every year since reliable records began, but usually only about 0.05 per cent in the SAT and 0.005 per cent in the CFT.

The importance of the zona pellucida for disease control in livestock by embryo transfer.

From the point of view of disease risk, the movement of livestock by embryo transfer is undoubtedly much safer for trading than the movement of live animals or semen. Nevertheless, strict governmental control by veterinary certification of health of embryos is still vital. In cattle, sheep and pigs, unlike laboratory species such as the mouse, infectious agents do not appear to pass through the zona pellucida (ZP) into the embryo proper.

Porcine cytomegalovirus (PCMV) in early gestation.

Following intranasal exposure to PCMV at or within 48 h of coitus transplacental infection occurred in two groups of gilts. Five out of 22 embryos were infected in the first group but only 2 out of 63 in the second. A more rapid immune response as measured by circulating antibody was probably instrumental in abrogating infection in the second group.

Field trials of an inactivated, oil-emulsion porcine parvovirus vaccine in British pig herds.

Inactivated porcine parvovirus vaccines have been available commercially in Britain since 1984 and are now widely used in breeding herds. To investigate their value in cost benefit terms an oil-emulsion vaccine developed at Weybridge was used in trials on 1243 gilts in 12 herds during the period 1984 to 1986. In each herd approximately half the gilts were given the vaccine before breeding and the remainder were left unvaccinated.

Superinfection with porcine cytomegalovirus initiating transplacental infection.

Four sows with circulating antibody were exposed to porcine cytomegalovirus. Virus was detected in 8 of 24 foetuses by immunofluorescence and/or virus isolation from 2 sows with low levels of antibody. In 6 of the infected foetuses, the virus was in capillary endothelium and macrophages of the lung, and was associated with interlobular oedema in 2 of these.

Maternally-derived antibodies to porcine parvovirus and their effect on active antibody production after vaccination with an inactivated oil-emulsion vaccine.

Two sows which had been vaccinated with an oil-emulsion porcine parvovirus vaccine, and had developed high haemagglutination-inhibiting antibody levels to the virus, farrowed three successive litters each, a total of 74 piglets. Serum samples from these piglets were tested for haemagglutination-inhibiting antibody at birth, three and 17 days after birth, and at monthly intervals thereafter to study the decline of maternally-derived antibody. Regression curves were constructed from the data to show the projected pathway (mean and 95 per cent tolerance limits) of the decline of maternally-derived antibody.

Efficacy of porcine parvovirus vaccines.

Three inactivated porcine parvovirus vaccines were tested for efficacy in 66 susceptible gilts. The gilts were challenged with virulent virus on the 40th day of gestation. All the vaccines provided excellent protection against fetal mortality despite insignificant serological responses to one of them.

Seasonal variations in serum progesterone levels in pregnant sows.

In the autumn and winter of 1979 and the spring and summer of 1980 serum samples were taken from approximately 20 sows at between 40 and 90 days of gestation in each of seven commercial herds. In most of these herds progesterone concentrations were significantly lower in the autumn than in the other three seasons. Between June 1980 and June 1981 every pregnant sow in a further herd of 250 sows was sampled at 25 to 30 days and at 70 to 91 days of gestation.