Risk factors and dynamics of bovine TB:
Preliminary results of a cohort study in south west England
Ana Ramirez-Villaescusa, Ecology & Epidemiology
1.Background
2. Objectives
• Investigate the role cattle to cattle transmission and management play in the introduction and
• Bovine tuberculosis is an infectious disease caused by Mycobacterium bovis.
• The potential of transmission between cattle should be considered in the process of control
and eradication of this increasing disease.
• Approximately 20,000 cattle per year have been slaughtered in the past few years due to this
disease. There is not vaccination available yet.
• Cattle to cattle transmission has been demonstrated both in experimental and field
investigations.
maintenance of bovine tuberculosis on cattle farms in Britain.
3. Materials and Methods
•Questionnaire data from 148 farms was collected between June 2003 – February 2004.Data refers to
period between October 2001- June 2003.
•Data was stored in Access Database and analysed using S-Plus.
• Several factors such as spread of slurry, reduced use of hay, dairy as opposed to beef as
type of herd and large herd size, have been identified in the past as predisposing risk
factors.
•Sources of data are farmers interviews (questionnaire, building survey), TB test data from VLA
and cattle movement data from BCMS.
4. Results
4.1. Survival Analysis:
4.2. Examples using Kaplan-Meier Survival Curves
• Survival analysis has been done to observe patterns of potential risk factors for
TB breakdown associated with practices and farm management.
all farms
censored
95% confidence interval
break_ins and outs
1.0
0.8
0.8
• Period of observation of breakdowns: October 2001 - December 2005. Data last
updated in November 2004.
only break_outs
censored
only break_ins
all farm s
Type of contacts w ith other cattle:
1.0
Survival curve for all farms
0.6
0.2
• Survival times are censored when failure has not been observed at the end of
the observation period or farm was lost to follow up in the study period.
0.4
0.4
0.6
Probability of survival
failure not observed
0.2
HR=2.93
• The hazard ratio (HR) is the product of a baseline hazard and an exponential
function of a series of predictor variables.
0.0
0.0
p=0.0028
0
200
400
600
800
1000
0
m ore than 200
censored
between 101-200
between 51-100
up to 50
all farms
0.8
• Currently being done.
• As a rule, risk factors which were biologically sound and statistically significant
and protective at univariable level within the created risk factor group have been
chosen as baseline.
• In a second stage, variables are being analysed controlling for herd type and
size. Controlling for frequency of testing will also be done.
0.6
Probability of survival
0.6
p=0.0014
0.2
p<0.0001
0.0
0.0
0.2
Survival
0.4
HR=2.18
HR=2.81
0
0
Multivariable Analysis
1000
not spread all year
censored
spread all year
all farm s
p=0.002
200
• Kaplan-Meier survival curves presented here show the survival time to first
failure (breakdown). X-axis represents the time interval between failure/end of
observation period and time when entered the study.
• Where Hazard Ratios (HR) values are not given, P values were not significant at
univariable level. In each, the baseline is taken as the logical opposite of the
plotted risk.
800
0.8
HR=0.24
• 137 identified risk factors have been analysed at univariable level.
• It is a plot of the survival function where all the predictor variables equal zero.
600
Tim e (days)
1.0
Herd size
Univariable analysis
Kaplan-Meier Survival Curve
400
Manure spread all year
1.0
• HR expresses the change of a unit change in the predictor variable on the
frequency of the outcome. It remains constant over the period of time.
• Hazard ratios have been calculated for each variable, the baseline being the
rest of the variables within the same risk factor group.
200
Time (days)
0.4
• Explanatory variables are binary.
Probability of survival
lost to follow up
• Outcome variable: time to first breakdown since Oct’01 on unrestricted farms.
Farms were restricted in Oct’01 entered the study when became unrestricted.
400 Time (days) 600
800
200
1000
400
600
Tim e (days)
800
1000
4.3. Some univariable results
Risk factor group
risk factor
herd size
HR
coef
low 95% CI
up 95% CI
p value
25
0.24
-1.43
0.0969
0.594
0.002
>200 cattle
46
2.81
1.03
1.78
4.44
0.0002
type herd
dairy
57
2.22
0.798
1.41
3.5
0.0006
location
south Devon
80
0.71
-0.345
0.45
1.11
manure origin
poultry
14
2.23
0.803
1.17
4.26
0.015
manure type
whole slurry
68
1.73
0.55
1.25
2.41
0.0011
manure spread
all year
36
2.18
0.78
0.45
1.35
0.0014
replacements type
steers
40
1.73
0.546
1.07
2.78
disease present
rotavirus
18
2.42
0.883
1.33
4.4
0.0039
disease present
lameness
93
1.94
0.662
1.18
3.18
0.0085
0.0047
disease present
up to 50 cattle
# observ.
herd size
0.14
0.025
bvd
27
2.12
0.754
1.26
3.58
disease present
johne's disease
13
2.36
0.858
1.21
4.61
0.012
disease present
stillborns
43
1.88
0.633
1.17
3.02
0.0087
contacts other cattle
breaks-in only
10
2.93
1.08
1.45
5.93
0.0028
contacts other cattle
bull hired
50
0.6
-0.507
0.361
1.01
0.052
bedding type
barley straw
91
1.44
0.368
0.884
2.36
0.14
5.Conclusions and Future Work
Preliminary results have shown that certain farm practices, such as herd size, spread and type of manure and having contact with other cattle, could increase the risk of having a TB breakdown.
Multivariable analysis will be completed.
Assessment of interactions between animal groups in the herd will be done next. For this, information about fields, buildings and density of animal groups will be used.
Acknowledgements
Supervisors: L.E.Green & G.F.Medley, All the participating farmers, DEFRA for funding, The TB Project Team: Patricia Findlay, Rod Fleming, Fiona Lang, Sam Mason, Genevieve Perkins, Anna Thomas, Jo Wright