Resistance of colostrum-deprived domestic lambs to infection with deer adenovirus.

Seven colostrum-deprived, 3-4-wk-old Rambouillet-Hampshire lambs were inoculated via the mucous membranes with deer adenovirus (DAdV) and monitored for clinical signs for 21 d post-inoculation at which time animals were euthanized and postmortem examinations were performed. Pre-inoculation and post-inoculation serum samples were tested for antibodies to DAdV, ovine adenovirus 7, bovine adenovirus 7, and goat adenovirus 1. Evidence for DAdV infection was determined by virus isolation, PCR tests, and histopathology with immunohistochemistry tests for DAdV. No clinical signs or lesions consistent with adenoviral hemorrhagic disease (AHD) in deer were seen in the lambs, and the lambs did not seroconvert to DAdV. DAdV was not detected by PCR, virus isolation, or immunohistochemistry in any of the samples tested from the lambs. A positive control deer similarly inoculated with DAdV developed fatal AHD 1 wk post-inoculation. Our colostrum-deprived lambs did not become infected when inoculated with DAdV.

Interspecific transmission of small ruminant lentiviruses from goats to sheep.

This study was conducted in order to evaluate the transmission of caprine lentivirus to sheep using different experimental groups. The first one (colostrum group) was formed by nine lambs receiving colostrum from goats positive for small ruminant lentiviruses (SRLV). The second group (milk group) was established by nine lambs that received milk of these goats. Third was a control group, consisting of lambs that suckled colostrum and milk of negative mothers. Another experimental group (contact group) was formed by eight adult sheep, confined with two naturally infected goats. The groups were monitored by immunoblotting (IB), enzyme-linked immunosorbent assay (ELISA), agar gel immunodiffusion (AGID) and nested polymerase chain reaction (nPCR). All lambs that suckled colostrum and milk of infected goats and six sheep of the contact group had positive results in the nPCR, although seroconversion was detected only in three of the exposed animals, with no clinical lentiviruses manifestation, in 720 days of observation. There was a close relationship between viral sequences obtained from infected animals and the prototype CAEV-Cork. Thus, it was concluded that SRLV can be transmitted from goats to sheep, however, the degree of adaptation of the virus strain to the host species probably interferes with the infection persistence and seroconversion rate.

Interspecific transmission of small ruminant lentiviruses from goats to sheep

This study was conducted in order to evaluate the transmission of caprine lentivirus to sheep using different experimental groups. The first one (colostrum group) was formed by nine lambs receiving colostrum from goats positive for small ruminant lentiviruses (SRLV). The second group (milk group) was established by nine lambs that received milk of these goats. Third was a control group, consisting of lambs that suckled colostrum and milk of negative mothers. Another experimental group (contact group) was formed by eight adult sheep, confined with two naturally infected goats. The groups were monitored by immunoblotting (IB), enzyme-linked immunosorbent assay (ELISA), agar gel immunodiffusion (AGID) and nested polymerase chain reaction (nPCR). All lambs that suckled colostrum and milk of infected goats and six sheep of the contact group had positive results in the nPCR, although seroconversion was detected only in three of the exposed animals, with no clinical lentiviruses manifestation, in 720 days of observation. There was a close relationship between viral sequences obtained from infected animals and the prototype CAEV-Cork. Thus, it was concluded that SRLV can be transmitted from goats to sheep, however, the degree of adaptation of the virus strain to the host species probably interferes with the infection persistence and seroconversion rate.

Cuticular and internal chemical composition of biting midges Culicoides spp. (Diptera: Ceratopogonidae), potential vectors of viral diseases.

The chemical profile of the cuticle and internal tissues of four species of Culicoides have been studied for the first time by gas chromatography-mass spectrometry. The chemical composition of females of C. obsoletus s.l. and C. lupicaris, vectors of diverse viral diseases, have been compared with that of other biting midges, such as C. kibunensis and C. fascipennis, and the non-biting midge Forcipomyia bipunctata. A total of 61 compounds belonging to 8 major chemical classes were identified in cuticular and internal tissues in n-hexane extracts. The compounds include carboxylic acids (CAs) (C6-C20), with C16:0, C16:1 and C18:1 being dominant, branched hydrocarbons (C29 to C38 mono/di/trimethylalkanes), linear hydrocarbons (C15 to C33, mainly odd chain carbons), terpenes (geranylacetone, geranylgeraniol acetate, squalene, terpenic alcohol), steroids (cholesterol), aldehydes (C9-C10 and even chain C20 to C30), and esters. The chemical profile depends on the species and whether the extracts are external (cuticle) or internal. The contents of linear and branched hydrocarbons and aldehydes was high in cuticular extracts but practically absent in internal tissues, which were, in contrast, rich in CAs, terpenes and steroids. The results are discussed and compared with other Culicoides midges and mosquito-related species.

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.

Evidence of proviral clearance following postpartum transmission of an ovine lentivirus.

Lentiviral transmission by transfer of infected colostrum and/or milk is considered to be highly efficient. In this study, postpartum transmission of ovine progressive pneumonia virus (OPPV) from 10 naturally infected ewes to their 23 lambs was followed from the perinatal period throughout a four-year period. The lambs were allowed to suckle from their dam from birth through 32 weeks of age. Virus was tracked by virus isolation, quantitative PCR (qPCR), and anti-OPPV antibody responses as measured by cELISA. Cell-associated OPPV was isolated from colostrum/milk cells in 7 out of 10 ewes and provirus envelope (env) loads ranged 8 to 10(5) copies/mug DNA in colostrum/milk cells from the 10 ewes using qPCR. Provirus env loads were also detected in the peripheral circulation of 21 lambs at 8 weeks and two lambs at 22 weeks. The qPCR product at 8 weeks was confirmed as the transmembrane (tm) gene of OPPV by cloning and sequencing. Both cELISA titers ranging from 325 to 3125 and cross-neutralizing antibody titers ranging from 6 to 162 to seven different OPPV strains were found in the colostrum of the 10 ewes. Furthermore, cELISA titers in serum from lambs remained detectable through 32 weeks following the clearance of provirus at 24 weeks. After 32 weeks, both provirus and anti-OPPV antibody responses have subsequently remained undetectable through 4 years of age. These data suggest the clearance of cell-associated lentiviruses from lamb circulation after passive transfer of antibody via colostrum.

Seven new ovine progressive pneumonia virus (OPPV) field isolates from Dubois Idaho sheep comprise part of OPPV clade II based on surface envelope glycoprotein (SU) sequences.

Seven new ovine progressive pneumonia virus (OPPV) field isolates were derived from colostrum and milk of 10 naturally OPPV-infected sheep from the US Sheep Experiment Station in Dubois, Idaho, USA. Sixteen sequences of the surface envelope glycoprotein (SU) from these seven Dubois OPPV field isolates and SU sequence from OPPV WLC1 were obtained, aligned with published SRLV SU sequences, and analyzed using phylogenetic analysis using parsimony (PAUP). Percent nucleotide identity in SU was greater than 95.8% among clones from individual Dubois OPPVs and ranged from 85.5 to 93.8% between different Dubois OPPV clones. SU sequences from Dubois OPPVs and WLC1 OPPV had significantly higher percent nucleotide identity to SU sequences from the North American OPPVs (85/34 and S93) than caprine-arthritis encephalitis virus (CAEVs) or MVVs. PAUP analysis also showed that SU sequences from the Dubois OPPVs and OPPV WLC1 grouped with other North American OPPVs (85/34 and S93) with a bootstrap value of 100 and formed one OPPV clade II group. In addition, Dubois and WLC1 SU amino acid sequences had significantly higher identity to SU sequences from North American OPPVs than CAEV or MVV. These data indicate that the seven new Dubois OPPV field isolates along with WLC1 OPPV are part of the OPPV clade II and are distinct from CAEVs and MVVs.

Effect of passive transfer of maternal immune components on infection with ovine herpesvirus 2 in lambs.

OBJECTIVE: To define the role of passively tranferred immunity in protection against early infection with ovine herpesvirus 2 (OvHV-2) in lambs. ANIMALS: 15 adult sheep and 34 lambs. PROCEDURES: 2 groups of animals were used, including 15 lambs born to OvHV-2-free ewes and 19 lambs born to OvHV-2-positive ewes. After nursing colostrum, all lambs and their dams were introduced into a flock positive for OvHV-2. Blood was obtained from the lambs every 2 weeks and examined by PCR assay and competitive inhibition ELISA. RESULTS: None of the animals had positive results by PCR analysis for samples obtained approximately 2 weeks after introduction into the flock. In the group of lambs from OvHV-2-infected ewes, 5 of 19 had positive results at 1 month of age and 17 of 19 by 5 months of age. In the group of offspring from OvHV-2-negative ewes, only 1 of 15 had positive results at 1 month of age, and the number reached 12 of 15 by 5 months of age. All lambs in both groups had positive results by 6 months. An active antibody response to the virus was detected in animals within 3 weeks after viral DNA became detectable in the blood. CONCLUSIONS AND CLINICAL RELEVANCE: Analysis suggests that passively transferred immunity does not play an important role in the delay of infection with OvHV-2 in lambs. Age also does not seem to influence susceptibility. The rate of infection in young lambs may simply be a reflection of the intensity of viral exposure in their environment.

The role of lambs in louping-ill virus amplification.

In some areas of Scotland, the prevalence of louping-ill virus has not decreased despite the vaccination of replacement ewes for over 30 years. The role of unvaccinated lambs in viral persistence was examined through a combination of an empirical study of infection rates of lambs and mathematical modelling. Serological sampling revealed that most lambs were protected by colostral immunity at turnout in May/June but were fully susceptible by the end of September. Between 8 and 83% of lambs were infected over the first season, with seroconversion rates greater in late rather than early summer. The proportion of lambs that could have amplified the louping-ill virus was low, however, because high initial titres of colostral antibody on farms with a high force of infection gave protection for several months. A simple mathematical model suggested that the relationship between the force of infection and the percentage of lambs that became viraemic was not linear and that the maximum percentage of viraemic lambs occurred at moderately high infection rates. Examination of the conditions required for louping-ill persistence suggested that the virus could theoretically persist in a sheep flock with over 370 lambs, if the grazing season was longer than 130 days. In practice, however, lamb viraemia is not a general explanation for louping-ill virus persistence as these conditions are not met in most management systems and because the widespread use of acaracides in most tick-affected hill farming systems reduces the number of ticks feeding successfully.