Cytopathic bovine viral diarrhea viruses (BVDV): emerging pestiviruses doomed to extinction

Abstract

Bovine viral diarrhea virus (BVDV), a Flaviviridae pestivirus, is arguably one of the most widespread cattle pathogens worldwide. Each of its two genotypes has two biotypes, non-cytopathic (ncp) and cytopathic (cp). Only the ncp biotype of BVDV may establish persistent infection in the fetus when infecting a dam early in gestation, a time point which predates maturity of the adaptive immune system. Such fetuses may develop and be born healthy but remain infected for life. Due to this early initiation of fetal infection and to the expression of interferon antagonistic proteins, persistently infected (PI) animals remain immunotolerant to the infecting viral strain. Although only accounting for some 1% of all animals in regions where BVDV is endemic, PI animals ensure the viral persistence in the host population. These animals may, however, develop the fatal mucosal disease, which is characterized by widespread lesions in the gastrointestinal tract. Cp BVD virus, in addition to the persisting ncp biotype, can be isolated from such animals. The cp viruses are characterized by unrestrained genome replication, and their emergence from the persisting ncp ones is due to mutations that are unique in each virus analyzed. They include recombinations with host cell mRNA, gene translocations and duplications, and point mutations. Cytopathic BVD viruses fail to establish chains of infection and are unable to cause persistent infection. Hence, these viruses illustrate a case of "viral emergence to extinction" - irrelevant for BVDV evolution, but fatal for the PI host.

Figure 1.

Genome organization of viruses from the genera flavivirus, pestivirus, and hepacivirus within the family Flaviviridae. The length of the viral RNA genomes are illustrated on top of each panel, and homologous regions between the genomes are indicated. Only the RNA of flaviviruses is capped at the 5′-end. The processing of the polyprotein into the individual structural and nonstructural proteins is indicated below. All the core proteins and the NS4A of flaviviruses are further processed at their C-terms (white boxes). For cytopathic pestiviruses, enhanced expression of NS3 is observed, whereas the uncleaved form, NS2-3, predominates in non-cytopathic pestiviruses. ns, non-structural; UTR, untranslated region. (A color version of this figure is available at www.vetres.org.)

Figure 2.

Phylogenetic analysis and classification of pestiviruses based on N^pro: The phylogenetic tree that represents the genetic relationship of pestivirus strains was constructed from the complete N^pro coding sequences except for the two strains Aydin/04 (EU930014) and Burdur/05 (EU930015). However, since constructing the tree with all sequences shortened to 420 nucleotides did not change the basic structure of the tree (data not shown), we included these strains in the analysis. Sequences were obtained from GenBank and the sequences of the Swiss BDV strains CH-BD3 (GU244490) and CH-BD4 (GU244489) were described by C. Reichert. The analysis was computed using the programs included in the GCG software package as PILEUP for the multiple sequence alignment and PAUP for calculating the distances by the Kimura 2-parameter method and constructing the tree according to the neighbor-joining method. Branch numbers (italics) indicate the percentage of 1 000 bootstrap replicates. Only bootstrap values over 95 are shown. Branch lengths are proportional to genetic distances, and the bar shown indicates 0.1 substitutions per site. The classification of the various pestiviruses was composed according to [23, 71, 94]. Virus strains within the “BVDV-3” group were named according to [56, 57], but they had previously also been labeled as “unclassified” [90] or “atypical” [55]. Virus strains within the chamois group in this paper were sometimes assigned to BDV-4 [3, 61, 71, 78, 93, 99], whereas the Tunisian isolates SN1T and BM01 were recently labeled as TSV (Tunisian sheep virus, [57]). (A color version of this figure is available at www.vetres.org.)

Figure 3.

Phylogenetic analysis and classification of BVDV based on N^pro: The phylogenetic tree that represents the genetic relationship of BVDV strains was constructed from 349 nucleotides of the N^pro coding sequences. The genetic analysis was calculated and the figure prepared as described for Figure 2. The sub-genotype BVDV type-1l was assigned according to [36, 39]. The strains So CP/75 and TR8/TR16, which were also labeled as BVDV-1l by [5, 102] and [104], were renamed as BVDV type-1n and -1p, respectively. BVDV sub-genotypes m, n, and o were named as in [70]. All other groups were assigned as suggested by [97, 98]. (A color version of this figure is available at www.vetres.org.)

Figure 4.

Persistent versus transient infections: BVDV causes two types of infection in vivo. Infection of a seronegative animal with BVDV of either biotype leads to a transient (acute) infection that usually causes no or mild clinical symptoms. The virus will be eliminated by the humoral and cellular immune response that will protect the animal from further infections. In contrast, infection with ncp, but not cp, BVDV between the second and fourth month of gestation may lead to persistent infection of the fetus. Infection later in gestation may lead to abortion, malformation, or to the birth of normal, immunocompetent calves. PI calves are immunotolerant to the infecting BVDV strain. These animals are born antibody negative and shed large amounts of virus during their entire lifetime. If the persisting ncp virus mutates into a cp biotype or if the PI animal is superinfected with an antigenically related cp virus, the PI calf will succumb to the lethal mucosal disease. (A color version of this figure is available at www.vetres.org.)