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CBN Report: Epidemic of Mumps in Iowa

By Eric Toner, M.D., April 11, 2006

The nation’s largest outbreak of mumps since 1988 is currently underway in Iowa and is spreading to adjacent states. As of April 5, more than 300 cases had been reported in Iowa, 55 of which were reported in the past week [1], as compared with a yearly average of 3 cases in Iowa and 265 cases nationwide since 2001. The epidemic appears to have started at a university in eastern Iowa in December 2005. A handful of cases have been detected in neighboring Illinois, Nebraska, Minnesota, Kansas and Missouri [2]. The virus has been cultured and identified as genotype G, a strain that is included in the MMR vaccine. MMR is required of all school children in the U.S. and normally is administered in 2 doses, at 12 to 15 months and 4 to 6 years of age.

According to the MMWR dispatch of March 30, 2006, of the 219 initial cases reported in Iowa, the median patient age was 21 years (range: 3 to 85 years), with 48% of patients aged 17-25 years; 30% were known to be college students. Vaccine status was documented for 133 of the patients.

Table 1: Vaccine Status of Iowa Mumps Patients for Whom Receipt of MMR Vaccine had been Documented*

No. of Doses Received

No. of Patients

Received 2 doses

87 (65%)

Received 1 dose

19 (14%)

No vaccine

8 (6%)

*N=133; vaccine status not documented in 19 (14%) of patients.

The most common symptoms reported by 114 of the patients are detailed in the table below. In addition, 6 (5%) patients reported complications (e.g., orchitis), and 1 suspected case of encephalitis is being investigated.

Table 2: Symptoms Reported by Iowa Mumps Patients*


No. Reporting


94 (83%)

Submaxillary/sublingual gland swelling

46 (40%)


41 (36%)

Sore throat

36 (32%)

*N = 114

As of March 28, 2006, investigators had determined that only 36 (16%) of the 219 cases could be definitively linked (i.e., a source of infection was identified), suggesting frequent unapparent transmission [3]. The source of the Iowa epidemic is unknown; however, it is speculated that it was brought to the U.S. by an exchange student.

The Iowa outbreak follows on the heels of an epidemic of genotype G mumps in the United Kingdom last year. That outbreak peaked in the spring, and involved 56,000 people, most of whom were adolescents or young adults aged 15 to 24 years. Only 3% of the British patients had received 2 doses of the MMR vaccine [5].

The UK epidemic has been epidemiologically linked to a mumps outbreak that occurred in July 2005 in a summer camp in New York, where mumps was transmitted to 31 campers and staff members by a 20 year old unvaccinated camp counselor from the UK. The majority of cases (61%) were staff members, whose average age was 21. Most of them (77%) had not had 2 doses of the MMR vaccine [4].

While the cause of the Iowa outbreak has not yet been determined, it is likely to be attributed to a combination of factors, including: incomplete vaccination, waning immunity among those vaccinated as children, and increased potential for spread of respiratory pathogens in crowded settings such as college dormitories. To date, there has been no evidence indicating that this strain has become less susceptible to the current vaccine. If that is true, then this outbreak should be controllable by the usual means: high vaccination rates, isolation of the sick, contact tracing and social distancing. Most people born before the vaccine was licensed in 1967 were exposed to the virus in childhood and are likely to have lifelong immunity. For more information about mumps, consult the CDC’s website.


  1. Jones, T. Mumps cases roll across Iowa. Chicago Tribune. April 5, 2006. Available at: Accessed April 6, 2006.
  2. Bavley, A. Increase in mumps a concern in region. Kansas City Star. April 6, 2006.
  3. Mumps Epidemic--Iowa, 2006. MMWR 2006;55; (dispatch) 1-3. Available at: Accessed April 6, 2006.
  4. Mumps Outbreak at a Summer Camp--New York, 2005. MMWR 2006;55;175-177. Available at: Accessed April 6, 2006.
  5. Mumps Epidemic–United Kingdom, 2004-2005. MMWR 2006;55;173-175. Available at: Accessed April 6, 2006.