The mumps outbreak that started in Iowa has spread to Illinois, Minnesota, Nebraska, South Dakota, and Wisconsin, an epidemiologist with the Iowa Department of Public Health said today.
Meghan Harris, M.P.H., said all states bordering Iowa -- with the exception of Missouri -- have reported cases of mumps. Moreover, said Harris, the end is no where in site.
"You can expect the numbers that we report tomorrow to be up [from cases reported March 30] and the numbers that will be reported Thursday are likely to be higher again," she said in an interview.
The Iowa Department of Public Health said will update mumps information tomorrow and again on Thursday.
The 245 confirmed, probable, or suspected cases of mumps reported to the Iowa Department of Public Health already represent the nation's largest mumps outbreak in 17 years.
Early on most of the Iowa cases were reported in college-age students. The mean age at onset was 21, and almost 35% of the cases occurred in 19-year-olds. But Harris said that no single college or university was implicated in the outbreak.
"We can assume that this outbreak is associated with some aspects of college life such as living in dorms or mixing together at the same clubs," she said. The best explanation, according to Harris, is that college students "in general have a higher risk of sharing saliva."
The probable cases of mumps were those that met the clinical case definition but had no confirmed serologic or virologic testing. Suspect cases were those with positive IgM serology and confirmed cases were those that met clinical definition that were also laboratory-confirmed or epidemiologically linked.
Follow-up reports have been completed on 136 cases of which 66% were in patients who had received two doses of the measles/mumps/rubella vaccine, which is considered sufficient to guarantee full immunity.
Blaise Congeni, M.D., director of infectious diseases at Akron Children's Hospital in Ohio, said that with any vaccine there is always a chance of primary vaccine failure, "meaning that the vaccine never achieved conversion, or the possibility of secondary vaccine failure, meaning that immunity wanes over time."
The current recommendation is that a first dose of measles-mumps-rubella (MMR) vaccine be administered at 12 to 15 months with a second dose at four to six years, but Dr. Congeni said that "if the second dose is missed it can also be administered during adolescence."
Based on published reports, Dr. Congeni speculated that secondary vaccine failure may be a factor in the mumps outbreak, but Harris said that theory failed to pan out after investigation. "There is no correlation between time since immunization and outbreak," she said. "There are cases among people who were immunized two years ago and other who were immunized 23 years ago. There is also no correlation with lot number of the vaccine, manufacturer, or physician who gave the immunization."
Dr. Congeni also speculated that it may be difficult to identify an index case because the primary symptom of mumps -- parotitis -- is often not fully evaluated by clinicians. "If the patient's record indicates that he or she has been received two MMR doses, the assumption would be that this is caused by another virus," he said. If the symptoms abate, the episode would be little more than a notation in a medical record.
Harris agreed that Dr. Congeni's theory best explains what has happened in Iowa, especially since the mumps strain is the same one that caused outbreaks in England over the past few years. That suggests that mumps arrived with a recent visitor from Britain, but "because these symptoms are often overlooked -- especially in people who have received the required vaccinations -- we doubt we will ever identify the index case," she said.
Dr. Congeni said, however, that a mumps outbreak does not carry the potential for public health disaster that would be associated with a measles or rubella outbreak.
"There is a small risk that some kids will develop viral meningitis and orchitis is a concern, but we know that orchitis doesn't cause sterility," Dr. Congeni said. As of March 30, 5% of cases included orchitis among symptoms and one case of encephalitis was being investigated.
Iowa Mumps Outbreak Spreads Quickly to Other States
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