Association between the genetic similarity of the open reading frame 5 sequence of Porcine reproductive and respiratory syndrome virus and the similarity in clinical signs of Porcine reproductive and respiratory syndrome in Ontario swine herds

Abstract

A study of Ontario swine farms positive for Porcine reproductive and respiratory syndrome virus (PRRSV) tested the association between genetic similarity of the virus and similarity of clinical signs reported by the herd owner. Herds were included if a positive result of polymerase chain reaction for PRRSV at the Animal Health Laboratory at the University of Guelph, Guelph, Ontario, was found between September 2004 and August 2007. Nucleotide-sequence similarity and clinical similarity, as determined from a telephone survey, were calculated for all pairs of herds. The Mantel test indicated that clinical similarity and sequence similarity were weakly correlated for most clinical signs. The generalized additive model indicated that virus homology with 2 vaccine viruses affected the association between sequence similarity and clinical similarity. When the data for herds with vaccine-like virus were removed from the dataset there was a significant association between virus similarity and similarity of the reported presence of abortion, stillbirth, preweaning mortality, and sow/boar mortality. Ownership similarity was also found to be associated with virus similarity and with similarity of the reported presence of sows being off-feed, nursery respiratory disease, nursery mortality, finisher respiratory disease, and finisher mortality. These results indicate that clinical signs of PRRS are associated with PRRSV genotype and that herd ownership is associated with both of these.

Figure 1

Generalized additive models (GAMs) for the association between percent sequence similarity and similarity of the clinical signs abortion and stillbirth when all virus types are included (top 2 graphs) and when only wild-type viruses are included (remaining 4 graphs). The 95% confidence intervals (CIs; shaded areas) for the top 4 graphs were generated by normal approximation. The dashed line indicates the logit of the proportion of pairs of herds reporting clinical similarity; this line is considered to be the null hypothesis of the model, where the odds of clinical similarity are no greater than chance alone. The lowest 2 graphs are shown with a 95% simulation envelope (shaded areas) based on 5000 random model permutations; this region is considered to be consistent with the null hypothesis.

Figure 2

Models for the association between percent sequence similarity and the similarity of 5 other clinical signs. These GAMs include only wild-type viruses and are shown with a 95% simulation envelope based on 5000 random model permutations; this region is considered to be consistent with the null hypothesis.

Figure 3

Example of GAM predictions and CIs for the relationship between sequence similarity and similarity of nursery respiratory disease when ownership similarity is included in the model.