The Pathology and Pathogenesis of Bluetongue

Full genome characterisation of bluetongue virus serotype 6 from the Netherlands 2008 and comparison to other field and vaccine strains

In mid September 2008, clinical signs of bluetongue (particularly coronitis) were observed in cows on three different farms in eastern Netherlands (Luttenberg, Heeten, and Barchem), two of which had been vaccinated with an inactivated BTV-8 vaccine (during May-June 2008). Bluetongue virus (BTV) infection was also detected on a fourth farm (Oldenzaal) in the same area while testing for export. BTV RNA was subsequently identified by real time RT-PCR targeting genome-segment (Seg-) 10, in blood samples from each farm. The virus was isolated from the Heeten sample (IAH "dsRNA virus reference collection" [dsRNA-VRC] isolate number NET2008/05) and typed as BTV-6 by RT-PCR targeting Seg-2. Sequencing confirmed the virus type, showing an identical Seg-2 sequence to that of the South African BTV-6 live-vaccine-strain. Although most of the other genome segments also showed very high levels of identity to the BTV-6 vaccine (99.7 to 100%), Seg-10 showed greatest identity (98.4%) to the BTV-2 vaccine (RSAvvv2/02), indicating that NET2008/05 had acquired a different Seg-10 by reassortment. Although Seg-7 from NET2008/05 was also most closely related to the BTV-6 vaccine (99.7/100% nt/aa identity), the Seg-7 sequence derived from the blood sample of the same animal (NET2008/06) was identical to that of the Netherlands BTV-8 (NET2006/04 and NET2007/01). This indicates that the blood contained two different Seg-7 sequences, one of which (from the BTV-6 vaccine) was selected during virus isolation in cell-culture. The predominance of the BTV-8 Seg-7 in the blood sample suggests that the virus was in the process of reassorting with the northern field strain of BTV-8. Two genome segments of the virus showed significant differences from the BTV-6 vaccine, indicating that they had been acquired by reassortment event with BTV-8, and another unknown parental-strain. However, the route by which BTV-6 and BTV-8 entered northern Europe was not established.

Agricultural diseases on the move early in the third millennium

With few exceptions, the diseases that present the greatest risk to food animal production have been largely similar throughout the modern era of veterinary medicine. The current trend regarding the ever-increasing globalization of the trade of animals and animal products ensures that agricultural diseases will continue to follow legal and illegal trade patterns with increasing rapidity. Global climate changes have already had profound effects on the distribution of animal diseases, and it is an inevitable reality that continually evolving climatic parameters will further transform the ecology of numerous pathogens. In recent years, many agricultural diseases have given cause for concern regarding changes in distribution or severity. Foot-and-mouth disease, avian influenza, and African swine fever continue to cause serious problems. The expected announcement of the global eradication of rinderpest is one of the greatest successes of veterinary preventative medicine, yet the closely related disease peste des petits ruminants still spreads throughout the Middle East and Asia. The spread of novel strains of bluetongue virus across Europe is an ominous indicator that climate change is sure to influence trends in movement of agricultural diseases. Overall, veterinary practitioners and investigators are advised to not only maintain vigilance against the staple disease threats but to always be sufficiently broad-minded to expect the unexpected.

Bluetongue virus infection alters the impedance of monolayers of bovine endothelial cells as a result of cell death

Bluetongue virus (BTV) is the cause of bluetongue, an emerging, arthropod-transmitted disease of ungulates. Bluetongue is characterized by vascular injury with hemorrhage, tissue infarction and widespread edema, lesions that are consistent with those of the so-called viral hemorrhagic fevers. To further investigate the pathogenesis of vascular injury in bluetongue, we utilized an electrical impedance assay and immunofluorescence staining to compare the effects of BTV infection on cultured bovine endothelial cells (bPAEC) with those of inducers of cell death (Triton X-100) and interendothelial gap formation (tissue necrosis factor [TNF]). The data confirm that the adherens junctions of BTV-infected bPAECs remained intact until 24h post-infection, and that loss of monolayer impedance precisely coincided with onset of virus-induced cell death. In contrast, recombinant bovine TNF-alpha caused rapid loss of bPAEC monolayer impedance that was associated with interendothelial gap formation and redistribution of VE-cadherin, but without early cell death. The data from these in vitro studies are consistent with a pathogenesis of bluetongue that involves virus-induced vascular injury leading to thrombosis, hemorrhage and tissue necrosis. However, the contribution of cytokine-induced interendothelial gap formation with subsequent edema and hypovolemic shock contributes to the pathogenesis of bluetongue remains to be fully characterized.

Apoptosis and immuno-suppression in sheep infected with bluetongue virus serotype-23

The role of apoptosis in pathogenesis of bluetongue (BT) has been suggested from various in vitro studies. However, to date, no clear data are available regarding BTV-induced apoptosis and its consequences in natural host, sheep. In the present study, bluetongue virus (BTV)-induced apoptosis was studied in sheep blood and splenic mononuclear cells by analyzing annexin(+)-propidium iodide(-) early apoptotic cells, DNA ladder pattern, and caspase-3 gene expression. The onset of apoptosis and lymphocyte depletion in viraemic phase and IFN-alpha response indicated the involvement of BTV and IFN-alpha in the pathogenesis of BT. The development of Pasteurella pneumonia in 4 of 7 infected sheep during the experiment pointed to possible BTV-induced immuno-suppression and predisposition to secondary microbial infections. These results have significant implications not only in understanding immuno-pathological consequences but also in studying interactions of BTV with host cells.

Re-emergence of bluetongue, African horse sickness, and other orbivirus diseases

Arthropod-transmitted viruses (Arboviruses) are important causes of disease in humans and animals, and it is proposed that climate change will increase the distribution and severity of arboviral diseases. Orbiviruses are the cause of important and apparently emerging arboviral diseases of livestock, including bluetongue virus (BTV), African horse sickness virus (AHSV), equine encephalosis virus (EEV), and epizootic hemorrhagic disease virus (EHDV) that are all transmitted by haematophagous Culicoides insects. Recent changes in the global distribution and nature of BTV infection have been especially dramatic, with spread of multiple serotypes of the virus throughout extensive portions of Europe and invasion of the south-eastern USA with previously exotic virus serotypes. Although climate change has been incriminated in the emergence of BTV infection of ungulates, the precise role of anthropogenic factors and the like is less certain. Similarly, although there have been somewhat less dramatic recent alterations in the distribution of EHDV, AHSV, and EEV, it is not yet clear what the future holds in terms of these diseases, nor of other potentially important but poorly characterized Orbiviruses such as Peruvian horse sickness virus.

Effects of bluetongue virus infection on sperm quality in bulls: A preliminary report

During the recent bluetongue virus (BTV) outbreak in Germany, semen quality in bulls naturally infected with BTV-serotype 8 was evaluated. Bluetongue status was assessed by both serology and polymerase chain reaction (PCR). Six bulls became BTV-PCR positive between September and November 2007 without showing clinical signs. Between April and May 2008, all six bulls were PCR negative but remained seropositive. Semen data from non-infected test bulls recorded between 2006 and 2007 were matched for season and age and used as controls. BTV-8 infection had no effect on sperm volume and concentration, but reduced sperm motility was seen after thawing (January-August 2008: 44.1 ± 12.7% vs. 58.0 ± 7.9% in the uninfected bulls; P < 0.001). Malformed sperm in both in fresh and thawed semen from BTV-positive animals was above the 20% permitted maximal limit from December 2007 to February 2008. Infection with BTV-8 transiently impaired semen quality in bulls.

Real-time quantitative reverse transcription-PCR assays specifically detecting bluetongue virus serotypes 1, 6, and 8

Bluetongue virus (BTV) is a major pathogen of ruminants. Especially serotypes 1, 6, and 8 are of concern to veterinary authorities in central Europe. This article describes highly sensitive real-time reverse transcription-PCR assays directed to BTV genome segment 2 for specific detection of BTV-1, -6, or -8 in animal samples.