​How are cases of Zika virus counted in Maryland?

On December 8, 2016, the Maryland Department of Health and Mental Hygiene (DHMH) started publicly reporting the combined total number of Maryland’s confirmed and probable cases of Zika virus disease (infected patients with symptoms) and infection (infected patients without symptoms). Cases are being classified according to the current case definition (https://wwwn.cdc.gov/nndss/conditions/zika/case-definition/2016/06/).   

Before December 8, DHMH reported numbers of confirmed cases of Zika virus disease (patients with Zika symptoms and laboratory tests that confirm Zika virus infection) according to the previous case definition (https://wwwn.cdc.gov/nndss/conditions/zika/case-definition/2016/02/26/).


Why did the number of cases of Zika virus in Maryland seemingly jump on December 8?

The current case definition allows DHMH to count persons with AND without symptoms and allows for two levels of classification (confirmed and probable). The previous definition only had a confirmed classification and did not include people without symptoms.  The result of using the current case definition is that it appears that the Zika case counts in Maryland rose substantially (110 total cases as of November 30 vs. 156 total cases as of December 8).  

This increase was an artificial result of the change in how cases are counted, and does not signify a sudden increase in travel-associated cases, nor does it signify local transmission of Zika virus.  The number currently being reported better reflects the impact of Zika virus on Maryland residents; however, four out of every five people infected with Zika virus do not show any symptoms.  The number of Marylanders who have been infected with Zika virus through travel is likely higher than the number presented here.​


Why does the number of cases of Zika virus in Maryland as reported on the DHMH website differ from the number reported on the CDC’s website?

CDC only reports cases with symptoms and does not include infected persons who do not have symptoms.   ​