ASSOCIATION BETWEEN GENETIC DIVERSITY OF PRRS VIRUS AND THE SPATIAL, TEMPORAL AND
TRADE PATTERNS IN THE UNITED STATES
Beatriz Martínez-López (1), Alda F. A. Pires (1,2), Dale Polson(3),
(1)
(2)
Center for Animal Disease Modeling and Surveillance (CADMS) – VM: Medicine & Epidemiology, UC Davis, US,
VM: Population Health & Reproduction, UC Davis, US (3)Boehringer-Ingelheim Vetmedica, Inc, US
Introduction
Since its first description in the US in the late 80s,
porcine reproductive and respiratory syndrome virus
(PRRSV) is still one of the main causes responsible
for large economic losses in the US swine industry
-estimated in more than $664 million per year (1)- and
worldwide. PRRSV is a highly variable RNA virus of
the family arteriviridae that is classified into genotypes
by restriction fragment and length polymorphism
(RFLP) (2). While the direct and indirect transmission
of PRRSV has been demonstrated under
experimental conditions, little observational studies
have been conducted to quantify the role of each
transmission route under diverse epidemiological
conditions (i.e., different site types, farm sizes, animal
movement patterns or nearby farm densities).
Certainly a better knowledge of the dynamics of
PRRSV direct or indirect transmission under field
conditions will inform decisions regarding the
implementation of specific intervention strategies (e.g.,
increase of biosecurity, pre-movement testing, area
vaccination, air filtration, etc.) to more cost-effectively
prevent and control PRRSV.
Here we proposed the use of multilevel models in
combination with phylogenetic, space-time and social
network analyses (SNA) to get further insights into the
PRRSV spread dynamics and transmission routes
under diverse epidemiological conditions.
Materials and methods
More than 200 PRRSV sequences (ORF 5 region)
collected over 3 years from PCR-positive field
samples were used to estimate the genetic diversity
of PRRSV. More than 150,000 animal movement
records during the same period of time were used to
evaluate the animal movement patterns and
characteristics. The spatial location of sites (i.e.,
geographical coordinate), the time when the PRRSV
sequence is collected as well as other on site
characteristics (i.e., type of farm, etc.) were used to
compute the spatial and temporal distance and to
evaluate differences among types of sites.
A multilevel regression model was used to quantify
the association between the pairwise genetic distance
of two isolates (ij) belonging to the same genotype,
the spatial and temporal proximity and the frequency
and characteristics of swine movements (i.e.,
bidirectional number of incoming and outgoing
shipments, pairwise comparison of the farm centrality
measures and of the number of animals moved).
Phylogenetic analysis and pairwise comparisons
were done with Disease BioPortal® and SNA metrics
and multilevel modeling in R-language v.3.1.1 (R
Core Team, 2014).
Results
Model results show that PRRSV isolates are spatially
and temporally clustered [Spatial and Temporal
distance_ij] and differ with the type of site [Diff_site_
type_ij] (Table 1). Similarly, there was a significant but
negative association (OR<1) between the genetic
diversity of two PRRSV isolates and sharing pigs
[Movs_between_ij] or having high centrality measures
[indegree_ij and betweenness_ij] in sites i and j.
Table 1. Multilevel logistic regression model results
Predictor
OR
Spatial_distance_ij
Temporal_distance_ij
Diff_site_type_ij
Indegree_ ij
Movs_between_ij
Betweenness_ ij
1.09
1.11
1.10
0.89
0.60
0.92
Lower
95%CI
1.06
1.09
1.03
0.86
0.53
0.89
Upper
95% CI
1.12
1.12
1.18
0.92
0.66
0.94
Discussion
We used phylogenetic, space-time and social network
analyses in combination with multilevel regression
modeling to evaluate the association between
PRRSV genetic diversity (which is a proxy for multiple
PRRSV wild-type infections or high PRRSV evolution
rate) and space, time and pig movement
characteristics in swine production systems of US.
Results strongly suggest evidence of PRRSV
transmission occurring by pig movements between
sites. We found also a significant, but smaller,
association between genetic distance and space-time
distance, suggesting area spread among herds. This
may be associated to aerosol transmission or other
factors clustered in space. Results are in agreement
with hypothesis and previous studies conducted in
Canada (3, 4).
Acknowledgements
This work was possible thanks to funds provided by
Boehringer Ingelheim. We thank Zack Whedbee, for
his valuable technical assistance.
References
1. Holtkamp et al (2013). J. Swine Health Prod. 21,
72-84.
2. Wesley et al (1992). J. Vet. Diagn. Investig. 10,
140-144.
3. Rosendal et al (2014a). BMC Vet Res. 10:83.
4. Rosendal et al (2014b). Transbound Emerg Dis. doi:
10.1111/tbed.12253