Seroprevalence and risk factors associated with Visna/Maedi virus in semi-intensive lamb-producing flocks in northwestern Spain.

A cross-sectional study was carried out to determine Visna/Maedi virus (VMV) seroprevalence and risk factors in semi-intensive lamb-producing flocks as a prelude to establishing a monitoring program in northwestern (NW) Spain. A total of 15,155 serum samples were taken from 78 commercial flocks and were submitted to an indirect VMV ELISA. Association between potential risk factors and seroprevalence at the flock level was assessed using a multivariable logistic regression model. A Generalized Estimating Equations (GEE) model and Exhaustive Chi-squared Automatic Interaction Detector (CHAID) were used to determine the seropositivity against VMV at the individual animal level. Individual seropositivity was 24.8% while 52.6% of the flocks examined had a true seroprevalence ≥1%. Flock size and introduction of new animals in the flock were significantly associated with seropositivity at the flock level. Flock size, sheep-goat contact, type of housing of lambs prior to weaning and age were significantly associated with individual VMV seropositivity. Confinement of lambs in preweaning lamb groups and high sheep-goat contact, regardless of the low number of goats per flock, were risk factors associated with individual VMV seropositivity, suggesting that these two factors are crucial for VMV control in semi-intensive lamb-producing flocks. These factors should be considered for developing more efficient strategies that will reduce the rate of VMV transmission.

Maedi-Visna virus was detected in association with virally exposed IVF-produced early ewes embryos.

The objective of this study was to determine whether MVV can be transmitted by ovine embryos produced in vitro and whether the zona pellucida (ZP) provides any protection against MVV infection. Zona pellucida (ZP)-intact and ZP-free embryos, produced in vitro, at the 8-16 cell stage, were cocultured for 72h in an insert over an ovine oviduct epithelial cell (OOEC)-goat synovial membrane (GSM) cell monolayer that had been previously infected with MVV (K1514 strain). The embryos were then washed and transferred to either direct contact or an insert over a fresh GSM cell monolayer for 6 h. The presence of MVV was detected using RT-PCR on the ten washing fluids and by the observation of typical cytopathic effects (CPE) in the GSM cell monolayer, which was cultured for 6 weeks. This experiment was repeated 4 times with the same results: MVV viral RNA was detected using RT-PCR in the first three washing media, while subsequent baths were always negative. Specific cytopathic effects of MVV infection and MVV-proviral DNA were detected in GSM cells that were used as a viral indicator and cocultured in direct contact or as an insert with MVV-exposed ZP-free embryos. However, no signs of MVV infection were detected in cells that were cocultured with exposed ZP-intact or non-exposed embryos. This study clearly demonstrates that (i) in vitro, ZP-free, early ovine embryos, which had been exposed to 10(3) TCID(50)/m MVV in vitro, are capable of transmitting the virus to susceptible GSM target cells, and that (ii) the IETS recommendations for handling in vivo produced bovine embryos (use of ZP-intact embryos without adherent material and performing ten washes) are effective for the elimination of in vitro MVV infection from in vitro produced ovine embryos. The absence of interaction between ZP-intact embryos and MVV suggests that the in vitro produced embryo zona pellucida provides an effective protective barrier.

Detection of ovine lentivirus in the cumulus cells, but not in the oocytes or follicular fluid, of naturally infected sheep.

The aim of this study was to examine the Maedi-Visna virus (MVV) infection status of oocytes, cumulus cells, and follicular fluid taken from 140 ewes from breeding flocks. MVV proviral-DNA and MVV RNA were detected using nested-PCR and RT-PCR MVV gene amplification, respectively in the gag gene. Nested-PCR analysis for MVV proviral-DNA was positive in peripheral blood mononuclear cells in 37.1% (52/140) of ewes and in 44.6% (125/280) of ovarian cortex samples. The examination of samples taken from ovarian follicles demonstrated that 8/280 batches of cumulus cells contained MVV proviral-DNA, whereas none of the 280 batches of oocytes taken from the same ovaries and whose cumulus cells has been removed, was found to be PCR positive. This was confirmed by RT-PCR analysis showing no MVV-viral RNA detection in all batches of oocytes without cumulus cells (0/280) and follicular fluid samples taken from the last 88 ovaries (0/88). The purity of the oocyte fraction and the efficacy of cumulus cell removal from oocytes was proved by absence of granulosa cell-specific mRNA in all batches of oocytes lacking the cumulus cells, using RT-PCR. This is the first demonstration that ewe cumulus cells harbor MVV genome and despite being in contact with these infected-cumulus cells, the oocytes and follicular fluid remain free from infection. In addition, the enzymatic and mechanical procedures we used to remove infected-cumulus cells surrounding the oocytes, are effective to generate MVV free-oocytes from MVV-infected ewes.

Serological survey and epidemiological investigation of maedi-visna in sheep in Finland.

A survey for antibodies to maedi-visna virus (MV) in the Finnish sheep surveillance flocks was conducted in 1994. Examination of a total of 12931 serum samples from animals over 1 year of age from 545 flocks (81% of all flocks) revealed eight seropositive flocks and the subsequent epidemiological investigation yielded one additional seropositive flock, indicating a low prevalence of 1.6%. The infection was very probably imported from Sweden in 1981, but it was not detected until the survey was conducted 13 years later. The entire primary infection flock was slaughtered in 1995. 77% of the sheep were seropositive but the animals were clinically healthy and only one (5%) of the contact flocks of the primary infection flock had contracted the infection. This secondary infection flock, 77% of which was seropositive, was slaughtered in 1994; however, animals in this flock had respiratory problems and the lungs of three sheep showed typical MV lesions. Seven (24%) of its contact flocks had contracted the infection and these each had one or two seropositive animals except for one flock which had seven (18%) seropositive animals. The results show that the initial spread of MV can be insidious and wide before infection is revealed in surveys or any clinical cases are encountered.

[Continuation of the observation and serological investigation of a Maedi-Visna virus infected sheep flock from January 1990 to June 1996]. / Fortsetzung der Beobachtung und der serologischen Untersuchung einer Maedi-Visna-Virus-infizierten Schafherde von Jänner 1990 bis Juni 1996.

This paper describes the second part of a longtime-study, started in 1987. Serologic investigations for detecting antibodies against Maedi Visna-virus (MVV) were performed, involving an institute own sheep flock. The method used was the immunodiffusiontest. The flock consisted of different breeds and their offsprings. So far, the virus seems to persist in the herd. This work also shows the importance of the central role of the does for spreading the virus. Seroconversion was detected in a sheep at the age of 32 months. The mother of this sheep was a thoroughbred and MVV-negative mountain sheep. After removal of the animals with high antibody (ab)-titers, until the end of 1991, the percentage of seronegative sheep increased. Then seropositive sheep didn't show high ab-titers anymore. Since 1990 only offsprings increased the size of the herd. The health status of the flock was clinically inconspicuous. It can be concluded that in spite of good food quality, good hygiene, without culling positive animals and just giving away accidentally some sheep, no elimination of MVV was registered in the flock over a period of more than six years. There was only seen a reduction of seropositive animals. Single results of serological tests, without knowing the sheep and the serological status of the herd, could pretend a false negative status.