With the strains of PRRSV currently circulating in South-East Asia, what mortality level is to be expected in an unvaccinated herd? Has the highly pathogenic PRRS virus epidemic burned out? Are the less virulent strains back?
PRRSv diversity in South-East Asia
Having more than 3,000 ORF5 gene sequencing data in my personal archive, both PRRSv-1 and PRRSv-2 are co-existing in all Thai swine herds. More than 10 herds in Vietnam, Cambodia, and the Philippines were tested, and both PRRSv types were found.
Asian PRRSv-1 strains taxonomy
For PRRSv-1, Thai isolates together with Vietnamese, Cambodian, and the Philippine isolates are mainly grouped in subtype 1, and further evolved into 3 clades including clades A, D and H (Stadejek et al., 2008).
Asian PRRSv-2 strains taxonomy
For PRRSv-2, Thai, Vietnamese, and Cambodian isolates are mainly grouped into 2 lineages including lineages 5, sublineage 5.1 and 8, sublineage 8.7 HP (Shi et al., 2010).
Isolates in sublineage 8.7 HP are highly pathogenic (HP) PRRSv-2.
Although isolates in sublineage 8.7 HP are still genetically related to their ancestral strains from China, disease severity has been less compared to that of the first emergence.
PRRSv in the South-East Asia & Mortality
Mortality due to PRRSv in unvaccinated herds is varied, but is normally higher than that of vaccinated herds, regardless of the type of PRRSv modified live vaccine used.
In vaccinated herds, mortality is less than 5%, but increases to 10% or more in non-vaccinated pigs.
If you want to know more about the PRRSv distribution or taxonomy, please visit the section “PRRS virus” of this site.
Swine Viral Evolution and Vaccine Research Unit, Chulalongkorn University, Thailand.