Role of imported ruminants in the genetic heterogeneity of pestivirus strains in Northern Ireland

Objectives

The genus pestivirus within the family Flaviviridae includes bovine viral diarrhoea virus (BVDV) types 1 and 2, border disease virus (BDV) and classical swine fever virus. The two recognised genotypes of BVDV are divided into subtypes based on phylogenetic analysis. The genetic heterogeneity of pestiviruses from clinical samples collected in Northern Ireland between 2008 and 2011 was investigated and compared to those previously described in Northern Ireland and the Republic of Ireland. In addition, the role of cattle and sheep importation in expanding the genetic diversity of these isolates was explored.

Materials and Methods

A91 BVDV positive samples from 88 herds submitted to AFBI between 2008 and 2011 were selected for further sequencing. In addition, RNA from samples from 839 bovine and 4,437 ovine animals introduced to the region during 2010 and 2011 were tested for the presence of pestiviral RNA with an reverse-transmission polymerase chain reaction (RT-PCR) assay (VetMAX Gold BVDV, Life Technologies) and the positive samples selected for sequencing. A 288 base pair portion of the 5’ untranslated (UTR) region was amplified by RT-PCR and the product sequenced. The sequences were aligned and compared with the corresponding sequences of a number of reference strains and with 25 previously reported sequences.

Results

The analysis indicated that the predominant subtype circulating between 1999 and 2010 was BVDV-1a (86 samples out of 91, 94.5%). Five out of the 91 samples clustered close to reference strains in subtype BVDV-1b (5.5%). 18 out of the 839 samples from bovine animals introduced to Northern Ireland gave a positive result (2.14%, Ct<37) and 8 an inconclusive result (0.9%, Ct ≥37) comprising a total of 24 animals. A true prevalence of BVD virus positive bovine animals within those introduced was calculated as 2.9% (1.7-4%). Sequence data were obtained for eight of these samples, all of which clustered close to reference strains in subtype BVDV-1b. No positive results were obtained with the ovine samples.

Conclusions

Since only the BVDV-1a subtype was detected in samples collected between 1968 and 1999, the present study suggests that at least one new subtype (BVDV-1b) has been introduced to Northern Ireland between 1999 and 2010. BVDV-1b was also the only strain detected in imported cattle. This finding highlights the potential for the importation of cattle to introduce new strains. In contrast, the risk associated with the introduction of sheep was found to be minimal.