Investigating bluetongue virus persistance in sheep

Summary

Early in 2003 a paper by Takematsu et al. (2003) describing BTV persistence in sheep was published in the

Journal of General Virology. Takematsu et al. suggested that BTV might persist in χδT cells in the skin of infected sheep even in the presence of neutralising antibodies and that virus replication was “induced” by midge feeding. The finding of BTV in the skin of sheep infected 9 weeks previously recalled the regulatory problems that were caused by previous reports describing persistence of the virus in a “persistently infected” bull. These reports were ultimately retracted. Such reports could lead to the reintroduction by regulatory agencies of international regulations banning movement of sheep and cattle with antibodies to BTV.

Experiments were designed to determine the veracity of these observations in sheep using both a laboratory-adapted strain of BTV1, as described by Takamatsu et al., and a wild type isolate of the same serotype. This comparison was necessary, as adaptation of field isolates of BTV to growth in tissue culture is known to alter the biological properties of the virus.

Sheep were inoculated with wild type and laboratory-adapted BTV1. Infection with BTV1 was confirmed by virus isolation in all inoculated sheep. The virus was isolated from blood for up to 14 days post inoculation. All inoculated sheep had seroconverted to BTV1 by 28 days post inoculation and remained seropositive until the final blood collection. Uninoculated control sheep remained seronegative throughout the experiment. Skin biopsies were taken at 14, 28, 42, 55, 56, 69, 70, 83 and 84 days post inoculation. Midges were fed on the sheep 14 days before inoculation and again prior to biopsy on days 56, 70 and 83. Biopsies were cultured for 7 days and the supernatant examined for the presence of BTV1 by virus isolation and BTV antigen capture enzyme-linked immunosorbent assay (BACE). A selected number were also tested by polymerase chain reaction (PCR). BTV1 was not identified in any sample. This work provided unequivocal evidence that bluetongue virus does not persist in the skin of previously infected sheep and is not reactivated by midge feeding.

Widespread international support for the concept of bluetongue virus persistence in animals could have a significant medium and long-term adverse impact on the Australian livestock industry. Regulatory authorities in bluetongue sensitive countries could respond by banning the movement of bluetongue susceptible animals with antibodies to BTV. The current negotiating strategy would become obsolete, trade gains made could be threatened and export opportunities could be severely affected. These export opportunities lie principally with bluetongue sensitive markets, as the majority of existing markets do not have bluetongue requirements.

Providing strong evidence that BTV does not persist in previously infected animals is essential to counteract published work suggesting that persistence does occur experimentally in sheep. Acceptance that BTV does not persist in animals should allow the movement of seropositive animals to bluetongue sensitive markets provided the animals are located in an area without current bluetongue activity for an accepted period prior to export.