1. AN INVESTIGATION INTO THE RISK
FACTORS CONTRIBUTING TO
STREPTOCOCCUS SUIS OUTBREAKS IN
NURSERY PIGLETS
Danielle Hopkins
Masters Student
Population Medicine
University of Guelph
2. What is Streptococcus suis?
• Considered to be one of the most important post weaning
pathogens
• 35 serotypes identified currently
• Opportunistic in nature
• Causes sporadic outbreaks in the nursery (0-5%)
• Typically following clinical signs associated with acute meningitis
• Occasionally causes severe outbreaks (>10%)
3. The Data:
• 300 sow farrow-to-finish
• All cause mortality recorded from
October 2011 to March 2012
Outbreak data set
Full Data Set
• 2779 Observations
• (TAR) Time at risk= 63 days
Sow Data
• 297 sows
• 15 removed
(missing data)
Pig Data
• 483 pigs died
• 107 removed
(died past TAR)
Figure 1: Farm experiencing outbreak
of. S. suis
Photo by: Danielle Hopkins
Merged + Expanded Data
4. Objective
Outbreak Data
I) Determine sow- and litter- level factors that were
associated with the hazard of dying due to S. suis
within the nursery
Figure 1: Outbreak data set demonstrating overall cause of mortality within the nursery
5. Factors of interest
S. suis
mortality
within the
nursery
Nursery
mortality
within the
same litter
Previous
litter
mortality
Parity
Number of
piglets
weaned Age of
weaning
Pre-weaning
mortality
Seasonal
effects
6. Factors of interest
S. suis
mortality
within the
nursery
Nursery
mortality
within the
same litter
Previous
litter
mortality
Parity
Number of
piglets
weaned Age of
weaning
Pre-weaning
mortality
Seasonal
effects
7. Final model based on Cox’s regression
Covariate HR P-value
Age of weaning 1.077 0.001
Age*TVC 0.996 0.001
Nursery mortality in
the same litter
9.21 0.001
Previous litter
mortality
0.337 0.024
Seasonal effects
** (October to
referent)
0.349 0.001
Number of piglets
weaned
0.913 0.001
*TVC- time vary coefficient
**January is referent category
8. Final model based on Cox’s regression
Covariate HR P-value
Age of weaning 1.077 0.001
Age*TVC 0.996 0.001
Nursery mortality in
the same litter
9.21 0.001
Previous litter
mortality
0.337 0.024
Seasonal effects
** (October to
referent)
0.349 0.001
Number of piglets
weaned
0.913 0.001
*TVC- time vary coefficient
**January is referent category
1
2
3
4
9. Age of weaning
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
1 6 11 16 21 26 31 36 41 46 51 56 61
HazardRatios(log_t)
Figure 1: Hazard ratio estimate if a hypothetical piglet's weaning age
was increased by 7 days controlling for the time varying affects of age
Time in the nursery (days)
10. Age of weaning
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
1 6 11 16 21 26 31 36 41 46 51 56 61
HazardRatios(log_t)
Figure 1: Hazard ratio estimate if a hypothetical piglet's weaning age
was increased by 7 days controlling for the time varying affects of age
Time in the nursery (days)
11. Figure 2: Graphical representation of overall mortality within litters organized by
the week of death upon entering the nursery--- prior to vaccination trial
12. Figure 2: Graphical representation of overall mortality within litters organized by
the week of death upon entering the nursery--- prior to vaccination trial
>70% mortality
occurrences in
1st 4 weeks
13. Nursery mortality within the same litter
Legend
Blue line: No other pig
within that litter died
Red Line: At least one
additional pig died within
a litter
Recall:
HR=9.21
P=0.001
Initial thoughts
-within pen transmission?
-similar passive immunity?
-similar pathogen exposure?
14. Previous litter mortality
Legend
Blue line: Sow with
previous litter having
0% mortality
Red Line: Sow with
previous litter
having>0% mortality
Recall:
HR=0.337
P=0.024
Initial thoughts
-Build up of immunity?
16. Conclusions
1. In this farm, nursery pigs experiencing increasing
levels of mortality until week 4 when the mortality rates
dramatically drop
• Similar pattern of mortality with S. suis? Serotype specific?
2. Pigs are more likely to have mortality if at least one
other pig from the litter died
• Similar exposure characteristics, within litter spread of S. suis?
3. Pigs are less likely to have mortality if a sow had
mortality associated with S. suis in their previous litter
• Build up of immunity?
17. Limitations
• 483 mortalities were experienced during the 6-month
duration of the trial
• 13 of those were submitted for post-mortem analysis and
confirmation of S. suis infection
• Confirmation via clinical signs may not have been 100% accurate?
• 5% of sows didn’t have information on parity or pre-
weaning mortality
• Beneficial to explored these risk factors further
18. Future Steps
• Steps to reduce clinical
cases of S. suis
• Isolate and treat sick pigs/
remove dead pigs from pen
• All-in-all-out to reduce
pathogen load
• Decrease stress within the
nursery
• Reduce overcrowding
• Maintain constant temperature
• **Vaccinate sows or piglets
***Vaccine efficacy against S. suis still under debate
Figure 2: “Future Steps” piglet
Retrieved from: https://www.culturecraze.com
19. Acknowledgements
Advisory Committee
Dr. Robert Friendship
Dr. Zvonimir Poljak
Dr. Vahab Farzan
The farm and farmers that participated in the
study
Funding via Swine Improvement Porc, and
the University of Guelph/OMAFRA Research
Partnership
Photo credit and fellow Streptococcus suis
masters student: Emily Arndt
Editor's Notes
I aquired data from a 300 sow farrow to finish operation that was experiencing a severe outbreak of S. suis. All cause-mortality was recorded from october 2011 to march 2012,
That this is all cause morality data, however the deaths recorded were documented to have clinical signs for acute meningitis and very characteristic of S. suis. In addition a subset of samples were collected from post mortems or clinically ill pigs and S. suis was the confirmed cause of death. I also have an image here that I took from a sick pen of pigs experiencing acute meningitis due to s. suis to show the severe impact an outbeak can have on farm
I had two data sets where one was sow-level data and the other was pig level data that recorded the mortalities during this time period, However the mortalities before and after the nursery were removed as we wanted to focus on nursery level risk factors
These two data sets were then merged together and expanded based on the number of pigs born within a litter, in order to get information on both pigs that survived and pigs that did not survive within the nursery
I ended up with 2779 observations or pigs within the study, and the TAR was 63 days or 9 weeks which was the time the piglets spent in this specific farms nursery
We then preformed all statistical analysis or cox’s hazard regression analysis in stata 14
The main objective:
This image
These are all the risk factors of interest, Ill just go over the first two in a bit more detail as we generated these risk factors from the data and they might not be completely intuitive, so starting from the left and working to the right we have nursery mortality within the same litter which means if one pig experiences mortality within the litter what is the hazard another pig will experience mortality from the same litter, then previous litter mortality is for the sows that had two litters within the study period, if their first litter experience mortality, what is the hazard that their second litter would also experience mortality. Then we also have …...
Factors based on previous literature surrounding s.suis mortaltiy and the information available in the data sets
Also interested in the potential interactions and confouding effects of these factors
Preweaning mortality not directly related to mortality in the nursery but may be an indicator of litter health
The first factor I would like to talk about is Age of Weaning. Now this one is not completely intuitive because of its time varying coefficient so I created this hypothetical model to help illustrate the point. Essentiall this model shows the hazard of mortality if you increased the weaning age of a piglet by 7 days.
As you can see up until approximately day 22, there is an increased hazard of mortality due to s. suis with this increased weaning age, then it transitions to having a protective affect or a hazard ratio below 1. Now to explain the reason for this I going to show you a graph containing the pattern of mortality within a litter
Here you can see the timing of mortality for the entire 6th month trial. Week 0 being when a piglet enters the nursery to week 9 being their final week within the nursery
And if we look at the results it appears that almost 70% of the mortalities occur within the first week, meaning that if a piglet survived past this 4th week they seemed to be able to survive the duratino of the nursery. Therefore, this age of weaning concept may be due to this common pattern of mortality.
Moving onto nursery mortality within the same litter
Season may have had confounding affects on the data as the colder months experiences higher levels of mortality than the warmer months, which has also been documented in previous literature.
Site 1 or 2 papers supporting this evidence of spatial spread, gottshcalk?
