Pseudorabies virus variant in Bartha-K61-vaccinated pigs, China, 2012

Abstract

The widely used pseudorabies virus (PRV) Bartha-K61 vaccine has played a key role in the eradication of PRV. Since late 2011, however, a disease characterized by neurologic symptoms and a high number of deaths among newborn piglets has occurred among Bartha-K61-vaccinated pigs on many farms in China. Clinical samples from pigs on 15 farms in 6 provinces were examined. The PRV gE gene was detectable by PCR in all samples, and sequence analysis of the gE gene showed that all isolates belonged to a relatively independent cluster and contained 2 amino acid insertions. A PRV (named HeN1) was isolated and caused transitional fever in pigs. In protection assays, Bartha-K61 vaccine provided 100% protection against lethal challenge with SC (a classical PRV) but only 50% protection against 4 challenges with strain HeN1. The findings suggest that Bartha-K61 vaccine does not provide effective protection against PRV HeN1 infection.

Keywords: Bartha-K61 vaccine strain; China; immune evasion; pigs; pseudorabies virus; virulent; virus variant; viruses.

Figure 1

Cytopathic effect and morphology of pseudorabies virus strain HeN1. A) Uninfected control Vero cells. B) Pseudorabies virus–infected Vero cells. A) and B) Original magnification ×200. The cytopathic effect, which was characterized by reticulated cells, was observed 48 h after inoculation. Spherical virus particles without (C) or with (D) viral envelope were observed by electron microscopy. Scale bars indicate 500nm

Figure 2

Phylogenetic analysis and comparison, based on gE amino acid sequences, of pseudorabies virus (PRV) isolates. An unrooted tree was constructed from the aligned amino acid sequences of 39 PRV isolates. Black diamonds indicate 16 PRV isolates from China that were collected in 2012; these isolates belong to a relatively independent branch in the phylogenetic tree (A) and possess 2 aspartic acid (Asp, D) insertions (positions 48 and 492–495), which are highlighted in yellow (B).

Figure 3

Rectal temperatures and gE antibody levels of Bartha-K61–vaccinated pigs inoculated with pseudorabies virus strain HeN1. A) Rectal temperatures >40.5°C were defined as fever and typically occurred 2–6 days after inoculation. B) Pseudorabies virus gE–specific antibody development was monitored by use of a gE ELISA and reported as blocking ratios; a ratio <0.6 was considered positive.

Figure 4

Brain tissue of an unvaccinated control pig (A) and pig inoculated with pseudorabies virus strain HeN1 (B). Arrows indicate lymphocyte infiltration around the small blood vessels in the brain cortex. Original magnification ×100.

Figure 5

Neutralizing ability of antisera generated against pseudorabies Bartha-K61 vaccine to block wild pseudorabies virus strain infection. The neutralization titer to Bartha-K61 was 20- to 40-fold; the neutralization titers to pseudorabies virus SC and HeN1 strains were 10- to 15-fold and 10-fold, respectively. The virus neutralization assay was performed with antiserum from 5 individual piglets; error bars represent the SD of the 5 experiments.