Porcine Reproductive and Respiratory Syndrome Virus, Thailand, 2010–2011

Dachrit Nilubol

, Thitima Tripipat, Tawatchai Hoonsuwan, and Khampee Kortheerakul

Author affiliations: Author affiliation: Chulalongkorn University, Bangkok, Thailand

Abstract

Characterization of porcine reproductive and respiratory syndrome virus (PRRSV) isolates from pigs in Thailand showed 30-aa discontinuous deletions in nonstructural protein 2, identical to sequences for highly pathogenic PRRSV. The novel virus is genetically related to PRRSV from China and may have spread to Thailand through illegal transport of infectious animals from bordering countries.

Porcine reproductive and respiratory syndrome (PRRS) has a substantial economic effect on the swine industry worldwide. PRRS virus (PRRSV), a member of the family Arteriviridae, is the etiologic agent of the syndrome. PRRSVs are divided into 2 distinct genotypes: type 1 and type 2. The genotypes have a similar genomic organization, and 10 open reading frames (ORFs) have been identified (1–3). Nonstructural protein 2 (Nsp2) and ORF5 are the most variable regions (4,5), coding for replicase protein and neutralizing epitope, respectively.
In general, PRRSV causes a disease characterized by reproductive failure in sows and respiratory infection in growing pigs. However, in June 2006, a disease characterized by high fever and associated with a high mortality rate emerged in the People’s Republic of China (PRC), resulting in the death of >20 million pigs (6). The disease, referred to as porcine high fever disease (PHFD), was caused by a new PRRSV variant with a unique hallmark: 2 discontinuous 30-aa deletions in Nsp2. The variant, identified as a highly pathogenic (HP) PRRSV, has subsequently become endemic in PRC (7), and it has spread to other countries, including Vietnam (8) and Lao People’s Democratic Republic (Lao PDR) (9).
It is thought that HP-PRRSV spread to Thailand early in 2010. Pigs on a small farm in Nong Khai, a border province in northeastern Thailand located near Lao PDR, showed signs of illness identical to those for PHFD. Within 2 weeks of the initial outbreak, similar clinical features were observed in pigs on 19 small farms in a nearby village. Since then, pigs exhibiting similar clinical signs have been observed in >100 herds in >20 provinces throughout Thailand. The causative agent was isolated from sick pigs and determined to be PRRSV.
To further our knowledge about PRRSV in Thailand, we genetically characterized partial Nsp2 and complete ORF5 genes of PRRSV isolates. In addition, we determined sickness and mortality rates on affected farms.