Monitoring for Local Transmission of Zika Virus using Emergency Department Data

Abstract

ObjectiveCase and cluster identification of emergency department visitsrelated to local transmission of Zika virus.IntroductionThe first travel-associated cases of Zika virus infection in NewYork City (NYC) were identified in January 2016. Local transmissionof Zika virus from imported cases is possible due to presence ofAedes albopictus mosquitos. Timely detection of local Zika virustransmission could inform public health interventions and mitigateadditional spread of illness. Daily emergency department (ED) visitsurveillance to detect individual cases and spatio-temporal clusters oflocally-acquired Zika virus disease was initiated in June 2016.MethodsED visits were classified into two Zika syndromes based onchief complaint text and the International Classification of Diseasesversion 9 and 10 diagnosis codes for patients≥6 years old: 1) feverand 2) Zika-like illness. Zika-like illness was defined as visits withmention of Zika; symptoms of rash, fever, and either joint pain orconjunctivitis; diagnosis of Guillain-Barré syndrome; or diagnosis ofrare and non-endemic arboviral infection.We applied the prospective space-time permutation scan statistic1in SaTScan daily since June 2016 to the fever syndrome, selectedas a single representative symptom, to detect clusters by hospital orzip code of patient residence. The maximum spatial cluster size is20% of observed visits, and the maximum temporal cluster size is 14days – reflecting the incubation period.2The study period is 90 days.Statistical significance is determined using Monte Carlo simulations(N=999). Any cluster with a recurrence interval≥365 days issummarized in a map and line-list of contributing visits. The mapdepicts the zip codes of the cluster with an overlay of census tracts athighest risk for human importation of Zika virus, as estimated by azero-inflated Poisson regression model developed at NYC DOHMHthat is updated regularly to reflect the most recent available data onconfirmed cases.Zika-like illness syndrome visits are output in a daily line-list.DOHMH staff contact the EDs that patients visited to determinetravel to Zika-affected country, clinical suspicion of Zika infection,and laboratory testing.ResultsDuring June 1–August 16, 2016, we observed a mean of 253(range: 202-299) ED visits for the fever syndrome per day. Sixteenspatio-temporal fever syndrome clusters have been detected. Of these,2 clusters were during testing and optimization of scan parameters,13 were due to data quality issues, and 1 was dismissed due to thelarge geographic range of the cluster, spanning 3 boroughs.During June 1–August 16, 2016, we observed a mean of 2.7(range: 0-7) ED visits for the Zika-like illness syndrome. Daily countsranged from 0-3 visits from June 1-June 16 and 1-7 visits since June16. Nineteen visits that occurred from July 31-August 4 were furtherinvestigated to establish a protocol for follow-up. Of those, elevenpatients reported recent travel to countries with local transmission,one had travel over 3 months ago and an alternate diagnosis, six hadunknown travel history due to incomplete follow-up, and one reportedno travel. The one without travel had a diagnosis inconsistent withZika virus disease. Subsequently, analysts contacted EDs only for thesubset of Zika-like illness syndrome visits with no indication of travelor without an alternate discharge diagnosis. Findings from this effortwill be presented.ConclusionsThe fever syndrome provides a means to monitor for clusters usingED data. Prospective cluster detection signal volume was manageableand has not identified clusters requiring additional investigation.The Zika-like illness syndrome can be used for case finding.Contacting EDs helps to supplement information missing in thesyndromic system, such as travel history as well as Zika testing anddiagnosis. As Zika-like illness syndrome counts are low and diseaseis emergent, contacting EDs is feasible and helpful in ruling out localZika virus transmission. No visits or clusters to-date have indicatedlocal transmission.