Effects of housing on the incidence of visna/maedi virus infection in sheep flocks.

The incidence of seroconversion to visna/maedi virus (VMV) infection and its relationship with management and sheep building structure was investigated in 15 dairy sheep flocks in Spain during 3-7years. Incidence rates were 0.09 per sheep-year at risk in semi-intensive Latxa flocks and 0.44 per sheep-year at risk in intensive Assaf flocks and was greatest for the one year old Assaf replacement flock. Separate multivariable models developed for replacement and adult flocks indicated that in both cases seroconversion was strongly associated to direct contact exposure to infected sheep and to being born to a seropositive dam. The latter effect was independent of the mode of rearing preweaning and the risk of seroconversion was similar for sheep fed colostrum and milk from a seropositive or a seronegative dam. These results are further evidence of the efficiency of horizontal VMV transmission by close contact between sheep and also suggest a inheritable component of susceptibility and resistance to infection. In contrast, indirect aerogenous contact with seropositive sheep was not associated with seroconversion as evidenced in replacement sheep housed in separate pens in the same building as adult infected sheep for one year. Consequently, VMV may not be efficiently airborne over short distances and this is important for control of infection. Moreover, there was no relationship between seroconversion and shed open areas. The latter could be related to having examined few flocks in which high infection prevalence dominated the transmission process while ventilation, may depend on a variety of unrecorded factors whose relationship to infection needs to be further investigated.

PCR detection of colostrum-associated Maedi-Visna virus (MVV) infection and relationship with ELISA-antibody status in lambs.

A recent large-scale experimental study showed that bottle-feeding ovine colostrum from seropositive ewes results in high MVV-seroconversion in lambs. In contrast, relatively few lambs that naturally suckled colostrum from seropositive dams seroconverted as a result of it. Furthermore, lambs fed uninfected bovine colostrum readily seroconverted when mixed with ovine-colostrum lambs indicating that horizontal MVV transmission between lambs was efficient. MVV-infection was further investigated in the same samples using two PCR tests targeting sequences in the long-terminal repeats (LTR) and POL MVV genes. PCR-tests confirmed previous serological findings. However, the LTR-PCR was more sensitive and allowed detecting infection earlier than the other tests, including 5-8% of new-born lambs from seropositive dams, providing more evidence that prenatal MVV-infection may be more important than considered. The degree of agreement between PCR and antibody tests in individual samples was low up to 6 months of age and moderate at 10 months-old. Nine percent of lambs were always PCR-negative but seroconverted and 19% of lambs were PCR-positive at least once and did not seroconvert. However, seroconversion was associated with increasing number of times lambs were PCR-positive and ovine colostrum-fed lambs were more frequently PCR-positive than other lambs. The significance of these findings in terms of MVV-infection, epidemiology and control is discussed.

Desarrollo de una técnica de PCR para detectar virus maedi-vinsa (VMV) libre e integrado en células y aplicación al estudio de la infecci´n calostral en corderos / Development of a PCR test to detect cell-free and -integrated maedi-visna virus (mvv) and it’s use to investigate colostral infection in lambs

Se desarrolló una técnica de PCR para detectar virus maedi-visna (VMV) libre e integrado en células, seestudio su concordancia con una PCR-LTR y un ELISA de anticuerpos de probada eficacia en muestras decalostro de ovejas infectadas y se emplearon los tres ensayos para investigar la infección calostral por VMV encorderos. Los cebadores se diseñaron en una región conservada en las seis secuencias de VMV disponibles enGenBank y amplifican un producto de 744 pares de bases (pb). Se realizaron 856 ensayos incluidos 283 con lagag, e independientemente de la PCR empleada se observó una buena correlación entre la presencia de viruslibre e integrado y esto es novedoso y plantea la importancia relativa de ambas formas víricas en la infección porVMV. En cambio, la concordancia entre las PCRs y el ELISA fue solo moderada y se corroboró que a menudono se detecta VMV en animales seropositivos. Las PCRs detectaron VMV en la mayoría de calostros ingeridospor corderos que posteriormente a los 10 meses de edad fueron seropositivos y además la gag y en menormedida la LTR, también en algunos calostros de corderos que fueron seronegativos, probablemente porquetomaron menos cantidad de calostro que los seropositivos. Además de aportar mas evidencia de la asociaciónpositiva entre la infección por VMV y el volumen de calostro con VMV ingerido, este resultado sugiere que lagag es más sensible que la LTR en esta matriz

A PCR test was developed to detect cell-free and cell-integrated maedi-visna virus (MVV), its degree ofagreement with an LTR-PCR and an antibody ELISA of proven efficacy was analysed in colostrum samplesand all three tests were employed to investigate colostral MVV infection in lambs. Primers were designed fromgag gene sequences homologous in the six MVV sequences presently in GenBank, and amplify a 744bp fragment.856 assays were carried out including 283 with the new gag-PCR and a good correlation was observedbetween the presence of cell-free and -integrated MVV. This is novel and questions the relative role of the twoviral forms in MVV infection. Instead, the correlation between PCR and ELISA results was only moderateand provided further evidence that MVV detection may fail in infected animals. The PCRs detected MVV incolostrum ingested by most lambs that later tested seropositive at 10 months-old and additionally, the gag-PCRand to a lesser extent the LTR-PCR, detected MVV in colostrum taken by lambs seronegative at 10 months-oldmost likely because they ingested less colostrum. As well as providing further evidence of the positive associationbetween MVV infection and volume of MVV-containing colostrum ingested, this result suggest that thegag-PCR developed is more sensitive than the LTR-PCR used in this study

Relative contribution of colostrum from Maedi-Visna virus (MVV) infected ewes to MVV-seroprevalence in lambs.

Maedi-visna virus (MVV) seroprevalence associated with consumption of colostrum from seropositive ewes was investigated in 276 housed lambs from birth to 300 days-old. At birth, lambs were allocated to five experimental groups according to the maternal MVV-serological status, source and mode of feeding colostrum (bovine or ovine and bottle fed or suckled from the dam) and type of horizontal MVV-exposure (raised with the dam or separately with other lambs). The risk of being seropositive at 300 days-old was associated with feeding ovine colostrum from seropositive ewes and increased with intake of bottle-fed ovine colostrum and was higher in lambs separated from their dams and raised with other experimental lambs compared to lambs raised with their dams. Approximately 75-87% of ELISA-positive results in lambs that had ovine colostrum was attributable to colostrum itself. However, approximately only 16% of naturally raised and 29-61% of bottle-fed ovine colostrum lambs were ELISA-positive as a result feeding ovine colostrum. These results confirm that ovine colostrum from seropositive ewes can be a major source of MVV but its overall contribution to seroprevalence in natural conditions is relatively low, and shows that horizontal MVV transmission can be an important source of infection in new-born lambs.