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.

Age-specific seroprevalence of Border disease virus and presence of persistently infected sheep in Basque dairy-sheep flocks

Using p125/p80 antibody and antigen-ELISA tests, age-specific seroprevalence and presence of persistently infected (PI) sheep were investigated in six commercial latxa dairy-flocks, housed for variable periods. The flocks all had a recent history of Border disease (BD). Every flock included seropositive sheep and seven 0.5-3-year-old PI sheep were detected in two of four flocks tested. Age-specific antibody patterns differed according to the presence or absence of PI sheep in the flock. In flocks free of PI sheep, seroprevalence was 6-13% in 1-year-old sheep and 42-93% in older sheep. In contrast, seroprevalence was 67-99% in sheep raised with PI sheep for at least 1 year and 29-33% in replacement 0.5-0.6-year-old sheep (including a PI sheep) indicating that Border disease virus (BDV) transmission in Basque dairy-flocks can be relatively slow. Moderate seroprevalence in young replacement sheep should not discourage further testing to detect PI sheep, and our results highlight the risk of failing to achieve "natural vaccination" prior to pregnancy by mixing PI sheep with BDV-unexposed ewes.

Single tube nested PCR for the detection of Toxoplasma gondii in fetal tissues from naturally aborted ewes

A single tube nested polymerase chain reaction (PCR) assay targeting the multicopy 18S-5.8S rRNA internal transcribed spacer (ITS1) region has been developed for the diagnosis of Toxoplasma gondii-induced abortion in ovine fetal tissues. In all, 145 ovine fetal samples including brain, spleen, lung, liver, kidney, placenta and fetal fluids from 53 fetuses and stillborns of 32 farms in Northern Spain were analyzed. Thirty-six samples belonging to nine fetuses and one stillborn lamb were T. gondii PCR-positive. Although T. gondii DNA was amplified from different types of tissues, brain was the tissue with the highest detection rate. All animals that had histopathological lesions associated to T. gondii infection were positive by PCR. In addition, four fetuses whose histological examination was hindered by autolysis were PCR-positive. Results obtained by PCR and indirect fluorescent antibody test (IFAT) showed good correspondence, demonstrating the diagnostic value of the two techniques. However, PCR has the advantage over serology in its ability to diagnose T. gondii infection at earlier stages of gestation when the fetus is not yet immunocompetent and in lambs that have taken colostrum. Once other abortifacient agents are ruled out, PCR detection of the ITS1 region in fetal tissues is a valuable and relatively rapid technique for the diagnosis of ovine abortion caused by T. gondii.