1. Evaluating the Effectiveness of Using Syndromic Surveillance
to Identify a Neuro-Invasive Disease Outbreak in Los Angeles County
Curtis Croker MPH, Akbar Sharip MPH, Ferlie Villacorte MSN MPH, Mike Tormey MPH, Moon Kim MD MPH, Bessie Hwang MD MPH, Rachel Civen MD MPH, Raymond Aller MD
County of Los Angeles, Department of Health Services, Public Health, Acute Communicable Disease Control Program, Bioterrorism Preparedness and Response Unit
ABSTRACT
RESULTS
RESULTS (Continued)
RESULTS (Continued)
During the study period the surveillance system recorded approximately 12% of all ED visits in LA County (5 of 77 EDs).
The WNV-NI community wide outbreak was geographically dispersed and encompassed four of the five syndromic surveillance EDs (Figure 1).
Fifty-five percent of WNV-NI cases visited an ED in LA County (n=75) (Figure 3).
Six percent of WNV-NI cases visited a syndromic ED (n=8).
Two percent of WNV-NI cases were classified as neurological (n=3).
No neurological signals were generated during the outbreak period due to WNV-NI cases.
If chief complaints from all EDs were monitored with syndromic surveillance we would expect:
75 WNV-NI cases to be captured.
Less than one-half of these cases (<38) would be classified as neurological.
During the peak of the outbreak, less than one half of the 6 cases per day (<3) would be categorized as neurological.
It is unlikely that all six cases would present at the same ED.
Therefore, no signal would likely be produced by the syndromic surveillance system during this community wide WMV-NI outbreak, even with the participation of all EDs.
The objective of this investigation was to evaluate the effectiveness of monitoring emergency department (ED) chief complaints as an indicator for a Neuro-Invasive (NI) disease outbreak. A retrospective analysis of the West Nile Virus (WNV) community-wide outbreak that occurred in 2004 was performed, matching cases from the county’s vector control case database to the syndromic surveillance database. Based on the findings it is unlikely that syndromic surveillance would be able to effectively detect an outbreak with these characteristics: long duration, large geographical dispersion, and non-specific nature of patient chief complaints.
Figure 1. Distribution of West Nile Virus Neuro-Invasive Cases by Resident Address and Syndromic Surveillance Emergency Departments in Los Angeles County, 2004 (n=130*)
West Nile Neuro-Invasive Cases
Los Angeles County , 2003 (N=137)
INTRODUCTION
Not Admitted to ED
(n=62, 45%)
Admitted to ED
(n=75, 55%)
A syndromic surveillance system was established in Los Angeles (LA) County in November of 2001 to analyze ED chief complaints in select hospitals.
Chief complaints are analyzed and categorized daily into a syndrome (rash, respiratory, neurological, gastrointestinal); a signal is generated if a syndrome count exceeds its respective threshold.
To evaluate the performace of the system a disease event was chosen for review. Measures of interest included:
Frequency of ED usage.
Syndromic classification of ED cases.
Time required to seek medical attention.
Admitted to Non-
syndromic ED
(n=67, 49%)
Admitted to
syndromic ED
(n=8, 6%)
CONCLUSION
Classified as
Neurological
(n=3, 2%)
Monitoring ED chief complaints in LA County will unlikely serve as an effective indicator for an outbreak with characteristics like that of the WNV-NI community wide outbreak that occurred in This was due to the long duration of this outbreak, the non-specific nature of patient chief complaints, and the geographical dispersion of cases. Further review of known disease outbreaks and syndromic surveillance activity will provide a better understanding as to what types of disease outbreaks can be detected.
Figure 3. Flow Diagram of West Nile Neuro-Invasive Cases
Date of symptom onset to ED visit ranged from 0 to 62 days (Table 1).
Three cases were admitted to a hospital and released prior to their ED visits.
Four cases had multiple ED visits.
*Address information was incomplete for 7 cases
WNV-NI cases occurred over a three month period, and had a peak of 19 cases per week and six cases per day (Figure 2).
Syndromic WMV-NI cases had a peak of two cases per week and one case per day.
Table 1. West Nile Neuro-Invasive Cases Found in Syndromic Database
Obs Dx
ED #
Sx Week
Time from Sx to ED (days)
Chief Complaint
“Neuro“ Syndrome 1
Enceph 4 26
ENCEPHALOPATHY HX OF CAD Y 2
Mening 32 7
## ASEPTIC MENINIGIT 3 30 20
ALOC WEST NILE VIRUS ENCEPHALITIS 33
DEHYDRATION, FEVER OF UNKNOWN ORIGIN N 5 35
FEVER X 1 DAY (103.0) 6 31
FEBRILE FEVER 36
ALTERED LEVEL OF CON 8 62
PAIN LOWER EXTREMITY
Note:
Obs 2 ,3, 8 were admitted to the hospital prior to their ED visit date
Obs #2 had 2 ED visits (8/13/04, 12/15/04), first classified as neuro, second unrelated - first date used.
Obs #3 had 2 ED visits (8/2/04, 8/12/04), first was classified as unrelated and second classified as neuro -second date used
Obs #6 had 3 ED visits (8/6/04, 8/6/04, 9/3/04) , none of these were classified as neuro -first date used
Obs #8 had 2 ED visits (10/4 and 11/1) , both classified as unrelated- first date used
LIMITATIONS
METHOD
Determining ED usage by WNV-NI cases may be negatively biased due to 10% of cases having incomplete case histories.
The active ED syndromic surveillance data and the passive West Nile Virus case report data from the period June 1st through November 31st 2004 were reviewed.
Patient identifiers from WNV-NI case reports were linked to ED visits in the syndromic surveillance system.
Chief complaints, syndromic classification, and dates of interest (onset date, ED visit date) were reviewed.
Case reports were reviewed for mention of ED usage.
REFERENCES
Flaherty, J., Correlation of West Nile Virus Infection with Emergency Department Chief Complaints by using a Passive Syndromic Surveillance Model - Chicago, Illinois. MMWR. Sept. 2004; (53) 237.
ACKNOWLEDGEMENTS
Figure 2. West Nile Virus Neuro-Invasive Cases and Syndromic ED Visits in Los Angeles County, 2004 (n=136*)
Special thanks to Grace Run for the WNV-NI case map.
*Onset for 1 case unknown