Mumps, 2007

During 2007, 28 cases of mumps (0.54 per 100,000 population) were reported in Minnesota. Seven (25%) cases were laboratory confirmed, including one (14%) case confirmed by both positive mumps IgM serology and a demonstrated rise in mumps IgG between acute and convalescent serologic specimens, five (71%) cases confirmed by mumps IgM serology only, and one (14%) case confirmed by mumps virus isolation from a throat specimen. Four (14%) of the 28 total cases were epidemiologically linked to a laboratory confirmed case. Seventeen (61%) cases met the clinical case definition for mumps and were reported as probable cases. Nineteen (68%) cases were reported between January and April, following a multi-state resurgence of mumps in 2006. Minnesota reported 180 mumps cases in 2006. By comparison, a total of 29 mumps cases had been reported in Minnesota in 2000-2005, all of which were laboratory-confirmed.

Thirteen (46%) case-patients had a documented history of two doses of mumps-containing vaccine. Five (18%) case-patients had a history of one dose of mumps-containing vaccine. One (4%) case-patient had not received mumps-containing vaccine because they were younger than the recommended minimum age for vaccination. The other nine (32%) case-patients had no documented history of vaccination for mumps; four (44%) reported a history of mumps and were born before 1957 and three (33%) self-reported a history of receiving mumps-containing vaccine which could not be verified.

Case-patients ranged in age from 7 months to 62 years. Seventeen (61%) cases occurred in persons younger than 21 years of age; five (18%) cases occurred in persons 22 through 33 years of age; two (7%) cases occurred in persons 34 through 49 years of age; and four (14%) cases occurred in persons 50 years and older. The multi-state mumps resurgence in 2006 demonstrated that birth before 1957 does not correlate with immunity to mumps, suggesting that natural immunity to mumps wanes in the absence of exposure to wild virus. This observation was supported in 2007 in Minnesota, as four case-patients were born before 1957.

Six (21%) cases occurred among family members residing in two households. No source of infection was identified for the index cases or for the remaining 22 (79%) cases, demonstrating that asymptomatic infections occur and suggesting that mumps is under-diagnosed.

Mumps surveillance is complicated by nonspecific clinical presentation, asymptomatic infections in an estimated 20% of cases, and suboptimal sensitivity and specificity of laboratory testing. False-positive serologic assays for mumps have been reported due to parainfluenza infections (viruses 1 and 3). Therefore, both IgM and IgG serologic testing as well as viral culture should be performed on all suspect mumps cases as soon as possible after symptom onset. Specimens for viral culture include buccal and throat swabs, and should be collected during the first 5 days of illness. Occasionally, false-negative mumps IgM results occur when serum specimens are collected within 3 days after onset of parotitis. A second serum sample (collected 5-7 days after onset) is recommended in this situation.

• For up to date information see>> Mumps
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2007