The investigation of Porcine Reproductive and Respiratory Syndrome virus (PRRSV) outbreaks routinely involves PRRSV gene sequencing and comparison to other local strains. Considerable effort is made to characterize the disease causing abilities of various families of PRRSV that are active within a region. Knowing the disease causing capabilities of the PRRSV that you are dealing with can help in deciding the best course of action for disease management. From time to time we do see that even though some PRRSV strains are very closely related or “homologous” they do not seem to cause the same level of disease. In general, the variation in the severity of disease associated with infection by any particular PRRS virus can be associated with a variety of host and virus factors. Some pigs are genetically more capable of controlling the infection. Some farms have a greater number of problematic concurrent diseases that can increase mortality rates. Researchers wanted to examine the possibility that two closely related PRRSV isolates had differences in their genetic makeup that could explain the difference in severity of disease. In 2017, two PRRSV variants were identified. One variant came from a farm that was experiencing severe abortion while a second variant came from a clinically healthy farm. The viruses were isolated and denominated as XJ17-5 and JSTZ1712-12.
The researchers found the following:
- Genomic sequencing showed that both were 14,960 base pair (bp) in length and shared 99.45% nucleotide identity (Close enough that we would usually say that they were the same virus).
- Both viruses were classified as the Highly Pathogenic (HP-PRRSV) subtype and belonged to a branch of the genetic tree along with other isolates containing the same 150-amino-acid deletion in nsp2.
- Pathogenic analysis showed that
- XJ17-5 is highly virulent causing 60% mortality
- JSTZ1712-12 is not virulent for piglets.
- Fragment comparisons identified 34-amino-acid differences between XJ17-5 and JSTZ1712-12 that might be associated with the distinct virulence
Take Home Message
Even with two PRRSV isolates that are 99.45 similar some very small genetic differences can create very big differences in disease causing abilities of the individual viruses.
Submitted by George Charbonneau, DVM
Ref: Chen N, Ye M, Huang Y, Li S, Xiao Y, Li X, Li S, Li X, Yu X, Tian K, Zhu J.Identification of Two Porcine Reproductive and Respiratory Syndrome Virus Variants Sharing High Genomic Homology but with Distinct Virulence.Viruses. 2019 Sep 18;11(9). pii: E875. doi: 10.3390/v11090875