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Reassessment of a large-scale
syphilis epidemic: using an
estimated infection date
Schumacher CM, Bernstein KT, Zenilman JM, Rompalo AM
Baltimore City Health Department
Johns Hopkins University
Introduction
• Timely surveillance and early identification of
syphilis outbreaks crucial to Elimination Plan
• Epidemic curves illustrate disease dynamics
– Traditionally defined by date health department
receives notice of infection
– Approach does not account for lag time between date
of infection and date of report
• Hypothesis: Date of infection is more accurate
depiction of syphilis dynamics
Introduction, cont’d
• Large outbreak in
Baltimore City,
Maryland provided
model for evaluation
of infection date curve
Source: CDC. MMWR. March 2, 1996 45 (8):166-169.
Methods
• Records of early syphilis cases (primary,
secondary, early latent) reported to Baltimore
City Health Department January 1994 and June
2003
• Stratified by sex and disease stage
• 2 epidemic curves
– Date case received by BCHD (report date)
– Estimated date of infection (Infection Date)
• Infection Date = Diagnosis Date – Median incubation time
– 45 days primary
– 60 days secondary
– 183 days early latent
Results
• 8409 syphilis cases reported to Baltimore
City Health Department (BCHD)
• 7806 (92.8%) diagnosed with Primary,
Secondary or Early Latent Syphilis
• 7663 (98%) included in Final Analysis
– Exclusions
• 19 (0.2%) missing sex
• 1 (0.01%) missing report date
• 123 (1.6%) missing diagnosis date
Results (cont’d)
1. Reports fail to account for large increases in
infections during development period
2. Report curves do not follow shape or
appropriate lag-times during epidemic period
3. Reports underestimate infections during
development period
4. Reports overestimate infections during
epidemic period
1. Failure to account for increases
in infections, P&S syphilis, males
Development Period
Primary and Secondary Syphilis in Males, Baltimore City, Maryland
Primary and Secondary Syphilis in Males
1993 - 1999
Baltimore City, Maryland 1995
+29%
140
80
120
70
Number of Cases
Number of Cases
90
100
80
60
40
-51%
60
50
40
30
20
10
20
0
0
1
1
2
3
1993
4
1
2
2
3
1994
4
1
2
3
1995
4
3
1
2
3
4
1995
1996
Year/Quarter
1
4
2
3
1997
4
1
2
3
1998
Quarter/Year
Report Date
Report Date
Infection Date
Infection Date
For display purposes, data was restricted to years 1993 - 1999
4
1
2
3
1999
4
1. Failure to account for increases in
infections, Early Latent Syphilis,
males
Development Period
Early Latent Syphilis
in Males,
Baltimore
Early Latent
Syphilis
in Males City, Maryland
1993 - 1999
180
160
140
120
100
80
60
40
20
0
+40%
140
Number of Cases
Number of Cases
Baltimore City, Maryland 1995
1
2
3
1993
120
100
-48%
80
60
40
20
0
4
1
2 13
1994
4
1
2
32 4
1995
1
2
3
43
1996
1995
Quarter/Year
Year/Quarter
1
2
3
1997
Report
Date InfectionInfection
Date
Report Date
Date
4 41
2
3
1998
4
1
2
3
1999
4
2. Report Curve does not reflect
shape of infection curve
Epidemic Period
Primary and Secondary
in Males,
Baltimore
City, Maryland
Primary andSyphilis
Secondary
Syphilis
in Males
Baltimore
Maryland
1993City,
- 1999
Number
NumberofofCases
Cases
140
120
120
100
100
80
80
60
60
40
40
20
20
0
0
1
2 3
1
1993
4
1
2 3 4
2
1994
1996
1 2 3
3
1995
4
1
4
2
3
4 1
1
1996
2
3
4
2
1997
Quarter/Year
Year/Quarter
Report Date
Report Date
Infection Date
Infefction Date
1
2
3 4
3
1998
1997
1
2
3
4
1999
4
3. Reports underestimate Infections
during development period
• P&S, males
– 1Q 1995 – 4Q 1995, 279 infections
– 2Q 1995 – 1Q 1996, 232 reports (83%)
• Early Latent, males
– 1Q 1995 – 4Q 1995, 386 infections
– 3Q 1995 – 2Q 1996, 330 reports (85%)
4. Reports overestimate infections
during epidemic period
• P&S, males
– 3Q 1996 – 2Q 1997, 370 infections
– 4Q 1996 – 3Q 1997, 404 reports (109%)
• Early Latent, males
– 2Q 1996 – 1Q 1997, 507 infections
– 4Q 1996 – 3Q 1997, 534 reports (105%)
Findings
• Lag-time bias may be present when defining
epidemic period based on date of report
– Ascertaining changes in demographics and social
factors between pre-epidemic and epidemic periods
provides insight into causes and control methods
• Using infection date as timeframe of epidemic
removes bias due to incubation time of disease
stage and time between diagnosis and reporting
Findings, cont’d
• Difference of curves in 1995 show
reporting not prompt after diagnosis
– Timely reporting necessary to find and treat
potential contacts before contacts become
infectious
– Delayed reporting further impedes Health
departments ability to reach contacts, allowing
for epidemic propagation
Findings cont’d
• Report overestimation and overlap of
curves during epidemic period likely due to
increased physician awareness and more
intense case seeking
Limitations
• Those in highest risk populations likely not
included
– Should not bias results since missing from
both curves
– Effect on either curve unknown
• Effect of disease stage misclassification
also unknown
Conclusions
• Using estimated date of infection as epidemic
timeframe more accurate depiction
– Understanding community dynamics at time of
transmission may be more useful in determining
causes and methods of control especially when
overlapping epidemics present
• Comparison on two curves can serve as check
on communication between providers and health
departments
Recommendations
• With electronic data, algorithm relatively
easy, fast and inexpensive
• Health departments should consider using
estimated dates of infection as timeframe
for epidemic investigations
P&S Syphilis in Males
Primary and Secondary Syphilis in Males
Baltimore City, Maryland
January 1993-June 2003
140
Number of Cases
120
100
80
60
40
20
0
1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2
1993
1994
1995
1996
1997
1998
1999
Quarter/Year
Report Date
Infection Date
2000
2001
2002
2003
P&S Syphilis in Females
Primary and Secondary Syphilis in Females
Baltimore City, Maryland
1993 - 1999
Number of Cases
120
100
80
60
40
20
0
1
2
3
1993
4
1
2
3
1994
4
1
2
3
1995
4
1
2
3
4
1996
1
2
3
1997
Quarter/Year
Report Date
Infection Date
4
1
2
3
1998
4
1
2
3
1999
4
Development Period
P&S in Females
Development Period
Primary and Secondary Syphilis, Females
Baltimore City, Maryland
80
+18 %
Number of Cases
70
60
50
- 42%
40
30
20
10
0
1
2
3
1995
Year/Quarter
Report Date
Infection Date
4
Epidemic Period
P&S in Females
Epidemic Period
Primary and Secondary Syphilis in Females
Baltimore City, Maryland
Number of Cases
120
100
80
60
40
20
0
1
2
3
1996
4
1
2
3
4
1997
2
3
1998
Year/Quarter
Report Date
1
Infection Date
4
Early Latent Syphilis in Males
Number of Cases
Early Latent Syphilis in Males
Baltimore City, Maryland
1993 - 1999
180
160
140
120
100
80
60
40
20
0
1
2
3
1993
4
1
2
3
1994
4
1
2
3
1995
4
1
2
3
4
1996
1
2
3
1997
Quarter/Year
Report Date
Infection Date
4
1
2
3
1998
4
1
2
3
1999
4
Epidemic Period
Early Latent in Males
Number of Cases
Epidemic Period
Early Latent Syphilis in Males
Baltimore City, Maryland
180
160
140
120
100
80
60
40
20
0
1
2
3
4
1
1996
2
3
1997
Year/Quarter
Report Date
Infection Date
4
Early Latent Syphilis in Females
Number of Cases
Early Latent Syphilis in Females
Baltimore City, Maryland
1993 - 1999
160
140
120
100
80
60
40
20
0
1
2
3
1993
4
1
2
3
1994
4
1
2
3
1995
4
1
2
3
4
1996
1
2
3
1997
Quarter/Year
Report Date
Infection Date
4
1
2
3
1998
4
1
2
3
1999
4
Development Period
Early Latent in Females
Number of Cases
Development Period
Early Latent Syphilis in Females
Baltimore City, Maryland
100
90
80
70
60
50
40
30
20
10
0
+ 96%
-46%
1
2
3
1995
Year, Quarter
Report Date
Infection Date
4
Epidemic Period
Early Latent in Females
Epidemic Period
Early Latent Syphilis in Females
Baltimore City, Maryland
160
Number of Cases
140
120
100
80
60
40
20
0
1
2
3
1996
4
1
2
3
4
1997
2
3
1998
Year/Quarter
Report Date
1
Infection Date
4
Reports underestimate infections
during development period
• P&S Females
– 1Q 1995 – 4Q 1995, 204 infections
– 2Q 1995 – 1Q 1996, 152 reports (75%)
• Early Latent, Females
– 1Q 1995 – 4Q 1995, 301 infections
– 3Q 1995 – 2Q 1996, 258 reports (86%)
Reports overestimate infections
during epidemic period
• P&S, Females
– 3Q 1996 – 2Q 1997, 323 infections
– 4Q 1996 – 3Q 1997, 346 reports, (107%)
• Early Latent, Females
– 1Q 1996 – 3Q 1997, 797 infections
– 3Q 1996 – 1Q 1998, 862 reports, (108%)