CHICAGO – The number of mumps cases in Iowa has jumped to at least 300 in the past week and the infection has shown up in neighboring states, puzzling health officials who said Tuesday they still do not know the cause of the epidemic, the nation’s biggest outbreak of mumps since 1988.
The number of confirmed and suspected cases in Iowa represents a more than 10-fold increase since the last week of February.
Fifty-five new cases have been reported in the past week alone.
For epidemiologists, it is a stunning number of cases in a state that, in recent years, has averaged three cases per year. Iowa’s outbreak has already exceeded the average annual number of mumps cases nationwide – 265 a year since 2001, according to the federal Centers for Disease Control and Prevention.
Illinois, meanwhile, has reported only a handful of cases, mostly in counties near Iowa.
The mumps vaccine is considered to be about 95 percent effective, meaning that 5 percent of those who get the vaccine don’t develop the proper immunity. Officials insist there is no wider problem with the vaccine. If that was the case, they say, many more people would be getting infected with mumps.
But officials have no explanation as to why the infection is growing as quickly as it is, especially in the Dubuque area, which has reported more than one-third of Iowa’s cases. There is plenty of inconclusive evidence and speculation – that there is a strain of the virus from England and that college students who live in close quarters are driving the epidemic.
“We don’t have any concrete answers,” said Meghan Harris, surveillance epidemiologist for the Iowa Department of Public Health, in Des Moines. “We expected the numbers to rise, but there is no way to predict how high the numbers will go.”
A spokeswoman for the CDC in Atlanta said the agency, which has a team of investigators in Iowa, is monitoring mumps activity in other states but has not found numbers that begin to approach Iowa’s.
Much of the focus on the potential cause centers on college campuses, where the first of the cases were discovered late last year.
College students account for about 21 percent of the cases, according to state health records released Tuesday. The median age of those infected is 21.
“Our best understanding is the exposure is initially occurring in college settings . . . people living in close proximity and fraternizing,” Harris said. “Those are the kinds of things that spread a disease.”
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Mumps is a virus that is spread most often by coughing and sneezing. People diagnosed with mumps are advised to stay home and rest for five days or until the symptoms leave.
The strain of the Iowa infection resembles the outbreak that struck the United Kingdom recently, producing some 56,000 cases of mumps. That epidemic – like Iowa’s – had a high attack rate on young adults.
Neighboring states have reported mumps cases – two in Nebraska and one in Minnesota. It is not clear, though, whether these cases are directly linked to the Iowa outbreak. One confirmed case comes from Missouri, but a spokesperson for the Missouri Department of Health and Senior Services said there is no evidence that case is linked to the Iowa infection.
So far Illinois has had eight confirmed cases and seven more possible patients, most of them in the northwest corner of the state, near Iowa.
“It’s a little higher than usual,” said Melanie Arnold, a spokeswoman for the Illinois Department of Public Health.
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In Iowa, uneven immunization histories appear to be playing a role in the spread of mumps. In 1977, Iowa law required one dose of measles, mumps and rubella vaccine for entry into public schools. The law changed in 1991, mandating two doses. Statistics released Tuesday show that 68 percent of those infected received two doses and 13 percent received only one. The dosage received by the remaining 19 percent of the 300 diagnosed is, for now, unknown until investigators track down all of the medical records.
“That means there is a population of adults and young adults in high school and college who only had a single dose,” said Dr. Martin Myers, director of the National Network for Immunization Information, in Galveston, Texas.
“Once you get a proportion of the population below a certain threshold, then you can have sustained transmission . . . you can have it spread through the community,” Myers said.
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Nearly all – 98 percent – of school-aged children in Iowa have received the two doses of the vaccine, Harris said. The state has no records on how much of the overall population has received both doses.
Arnold said Illinois schoolchildren are required to receive two doses of mumps vaccine; most get the shots before they enter kindergarten.
The infection has spread rapidly. A month ago mumps had been detected in 11 Iowa counties, all but two of them in the eastern part of the state. Nearly four times that number of counties are now affected, stretching across the state. Thirty-two counties have reported five or fewer cases. Polk County, which includes Des Moines and is the most populous county, has six cases.
Kevin Teale, a spokesman for the Iowa Department of Public Health, said investigators do not know why there are so many cases in Dubuque County. Officials in that county’s health department referred all questions regarding the outbreak to state authorities.
Loras College in Dubuque has reported more than a dozen cases. When asked if the presence of several college campuses in Dubuque County might be a factor in the high number of cases there, Teale declined to speculate, saying, “We’re focused now on slowing the increase in cases and will look at how it may have started later.”
(Chicago Tribune staff reporter Jeremy Manier contributed to this report.)
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