Porcine Reproductive and Respiratory
Syndrome Virus (PRRSV) in GB pig herds
C M Evans G F Medley and L E Green
Ecology and Epidemiology Group, Department of Biological Sciences
C.M.Evans@warwick.ac.uk +44(0)2476 575874
What is PRRS?
Disease manifestations:
Project aims
• Inability to conceive
• To determine factors associated with PRRSV antibody positive
herds
• Abortion
• Early farrowing
• To determine factors associated with higher levels of antibody
in positive herds
• Birth of weak-born, still-born
and mummified piglets
• To examine transmission dynamics and patterns of exposure
Materials and methods
.
Data collected on farm
Mummified piglet
Stillborn piglet
• Respiratory disease in finishing pigs
Statistical modelling – Linear mixed models
• Data clustered within pens and within herds
• Blood samples were taken from 105 herds at the following ages: • Outcome variable = antibody titres of pigs
- 8 weeks - 10 pigs
• Management factors examined for their ability to explain:
- 14 weeks - 10 pigs
- Positive vs. negative herds (risks for introduction)
- Breeding sows parity 0 - 5+ - total of 30 pigs
• Herd positive if mean titre of at least one age group from
- Persistence in positive herds (High Ab titre)
PRRSV Ab ELISA >20 (manufacturer’s cut-off)
Results
• Questionnaires were sent to vets
3 level linear mixed model
Yijk = β Χ 0 + β Χ k + β Χ jk + ν k + υ jk + eijk
105 herds : 27 vaccinated (according to vet questionnaire results), 44 negative, 34 positive
Figure 1: Proportion of pigs seropositive by age and Figure 2: Log odds of herd positive by
vaccination policy (61 herds, 2901 pigs)
number of sows
Proportion of samples
seropositive
Log odds
-0.5
Table 1: Factors associated with introduction and persistence
Critical threshold
of approximately
200 sows
-1.00
-1.50
Introduction
(Odds ratios and
95% CI)
-2.00
0
200
400
600
800
Number of sows
Figure 3: Log odds of herd positive by
distance to the nearest unit
Linear association
between proximity
to nearest unit and
log odds of being
positive
• Active seroconversion between 8- and 14weeks of age
• Vaccinated herds had a higher proportion of
positive pigs than non-vaccinated (p<0.001)
Log odds
0
-1.00
-2.00
-3.00
0
5
10
Persistence
(Odds ratios and
95% CI)
> 200 sows
2.02 (1.34 – 3.04)
Distance to
nearest unit
0.91 (0.87 - 0.95)
0.92 (0.88 – 0.97)
Have
quarantine
facilities
0.72 (0.55 – 0.94)
0.73 (0.56 – 0.92)
Distance to nearest unit and having quarantine facilities
were associated with negative herds and with lower titres
in positive herds. Variables such as moving and mixing pigs
at different stages of production were not associated with
persistence in positive units.
15
Distance to nearest unit (miles)
Was purchasing stock enabling persistence? Case studies of antibody titre by age from individual farms
Figure 4: 2 herds PRRSV negative in young pigs and purchased pigs for breeding
160
160
Titre
60
10
40
8
week
14
week
Gilt
Parity
1
Parity
2
Parity
3
Parity
4
110
Titre
Purchased pigs. Purchased
antibody positive stock? Decline
in titre over time not indicative
of active viral circulation?
110
60
10
Parity
5
-40
8
week
14
week
Gilt
Parity
1
Parity
2
Parity
3
Parity
4
Parity
5
Purchased pigs. Purchased
virus-positive stock? Viral
circulation present? Clinical
signs currently being seen
(from vet questionnaire)
Figure 5: 2 herds PRRSV negative in young pigs and did not purchase pigs for breeding
160
160
60
10
-40
8
week
14
week
Gilt
Parity
1
Parity
2
Parity
3
Parity
4
Parity
5
Did not purchase pigs. Indicative
of viral circulation that had not
yet reached younger pigs? Or
fadeout occurring?
110
Titre
Titre
110
Did not purchase pigs. Indicative of
an outbreak that had occurred in
the past?
60
10
-40
8
week
14
week
Gilt
Parity
1
Parity
2
Discussion and further work
• Proximity to other pig units and purchasing pigs may be allowing persistence of virus in positive herds
• This study highlights the importance of isolating stock in quarantine facilities and the health status of incoming stock
• Purchasing stock may allow persistence of certain infectious pathogens
• Fadeout of PRRSV is possible
Further work: Do parameters from the literature support these data? To be investigated with mathematical modelling
Parity
3
Parity
4
Parity
5
Acknowledgements
To all those who assisted in data collection, to
all farmers who participated in the study, Sam
Mason for database help and to all members of
the Ecology and Epidemiology Group