Atypical pestivirus and severe respiratory disease in calves, Europe.

To the Editor: The article by Decaro et al. () described an outbreak of severe respiratory disease in calves in Italy caused by an atypical bovine pestivirus. This report confirms our concern that this group of viruses is probably widespread () and present on >3 continents. Moreover, it demonstrates that atypical bovine pestiviruses are capable of causing disease in calves in the field and a clinical picture consistent with most bovine viral diarrhea virus (BVDV) infections that occur naturally or experimentally (,).

Some key issues remain unknown. The origin of the bovine pestivirus and the route of introduction into the herd are unclear, although phylogenies demonstrated a close relationship between this virus strain from Italy and atypical bovine pestiviruses from Brazil. Batches of fetal bovine serum from Brazil have repeatedly been found to be contaminated with atypical bovine pestiviruses (,), and there is a risk for contamination of vaccines with these viruses. Animal trade and vaccines should be considered when conducting further investigations into this outbreak.

The evolutionary relationship between the atypical and the recognized pestivirus species () needs to be clarified. Maximum-likelihood and Bayesian analyses of a concatenated dataset positioned atypical pestiviruses consistently in a clade sister to BVDV-1 and BVDV-2, and the larger clade was sister to pestivirus of giraffes (). The same pattern was observed in complete genome phylogeny ().

The term atypical is not informative, and a new name has been proposed for these bovine pestiviruses (). Outcomes of infections with these viruses are typical, but the viral antigens and phylogenies are unique.

In Response: We read with interest the comment by Liu et al. () to our article reporting overt disease in calves caused by infection with atypical or ‘Hobi’-like pestivirus (). Although we can rule out that vaccination was the source of infection because only killed vaccines were administered on the farm, the most recent findings about this emerging group of viruses deserves further discussion regarding their possible taxonomical, diagnostic, and prophylactic implications.

The association of ‘Hobi’-like pestivirus with clinical disease in cattle was confirmed by reports of experimental and natural infections (,). A cytopathogenic/noncytopathogenic ‘Hobi’-like virus pair was isolated from a diseased heifer, demonstrating that the new group of viruses includes 2 biotypes already known for bovine viral diarrheal viruses 1 (BVDV-1) and 2 (BVDV-2) ().

Although the clinical presentation might mimic that of extant BVDV-1/BVDV-2, distinctive features were also evident in the reported cases (,). At the antigenic and phylogenetic levels, ‘Hobi’-like strains showed distinct differences from the other bovine pestiviruses: they were at least as different from BVDV-1/2 as are border disease virus and classical swine fever virus (; N. Decaro et al., unpub. data). The antigenic and genetic uniqueness of ‘Hobi’-like viruses will likely lead to the definition of a new species within genus Pestivirus, but how this species should be designated remains unclear.

Some scientists have tentatively proposed to name the virus bovine viral diarrhea 3, but others believe this nomenclature would be problematic from regulatory and scientific standpoints (J. Ridpath, pers. comm.). Molecular assays standardized for BVDV-1/2 might not be able to detect ‘Hobi’-like strains because of the presence of mismatches in the oligonucleotide binding regions (). Prophylactic measures should take into account the circulation of ‘Hobi’-like pestiviruses in cattle herds. Whether commercial BVDV vaccines are effective against the emerging pestivirus is unknown, and requires future in-vivo cross-protection studies.