Clinical and Epidemiologic Factors Associated with Confirmed Mumps Cases in Washington State, 2008–2013

BACKGROUND:   Determining the likelihood of mumps in patients with parotitis can be difficult due to poor sensitivity of serologic testing to rule-out the diagnosis in persons previously vaccinated; public health resources may be invested to an equal extent in patients with parotitis eventually classified as “suspect” to those classified as “confirmed”.  In addition to clinical presentation, other information, including travel history, epidemiologic linkage to a cluster or lab-confirmed case, number of doses of mumps-containing vaccine, a history of prior mumps disease, and virologic testing results from commercial laboratories can be used to help determine the resources needed to investigate reports of parotitis. The objective of this project was to determine clinical and epidemiologic factors associated with confirmed cases to better target public health laboratory resources to patients at highest risk for mumps. 

METHODS:   Clinical, epidemiologic, and laboratory data on patients reported with mumps during January 2008 through September 2013 were compiled from Washington State Department of Health line lists and current notifiable conditions database. Previously assigned case classifications were cross tabulated by exposure risks and laboratory testing to assess accuracy of classification, using the 2010 CSTE case definition for consistency.  Bivariate logistic regression was used to determine factors associated with confirmed vs. suspect case classification.

RESULTS:   This analysis included 33 “confirmed” cases, 3 “probable” cases, 154 “suspect” cases, and 36 cases where an alternate etiology was found.  All cases except one confirmed case were determined to be correctly classified. All confirmed cases of mumps disease and 139 (90%) suspect cases had known parotitis. Fifteen (45%) confirmed cases and only one (<1%) suspect case were epi-linked to someone with similar symptoms. Compared to suspect cases, confirmed cases were less likely to have received two or more mumps-containing vaccines (OR=0.05, P-value=0.0001), and more likely to have traveled (OR=4.91, P-value=0.0003). No significant differences were seen by age (OR=1.01, P-value=0.14), sex (OR=0.70, P-value=0.36), or self-report of prior mumps disease (OR=1.62, P-value=0.50).

CONCLUSIONS:   Based on findings from this analysis, patients with parotitis and a history of travel, epi-linkage, or receipt of less than two mumps-containing vaccines should be targeted for immediate public health interventions.  Given the limited sensitivity of available mumps laboratory tests, these findings emphasize the importance of considering the vaccination status, travel history, and any epidemiologic linkage, when allocating public health resources to investigation of a report of possible mumps.