Mumps is an acute viral infection that is spread by respiratory droplets. Parotitis occurs in 30–40% of infected people.1, 2, 3 Myalgia, malaise, fever, and headache can also occur.4 Serious complications, such as orchitis, oophoritis, meningitis, encephalitis, and deafness, were common in the pre-vaccine era but are now rare.5 The incubation period ranges from 12–25 days, but parotitis typically develops 16–18 days after mumps exposure.4
In the USA, the incidence of mumps decreased by 99% after the introduction of vaccination in 1967,5, 6 but outbreaks still occasionally occur. In 2006, a resurgence of mumps in the USA was observed primarily among college students (aged 18–22 years) with high coverage of two-dose measles, mumps, and rubella (MMR) vaccination.7, 8, 9 Since then, outbreaks in settings with high vaccination coverage have been reported, primarily among young adults, nationally and globally.10, 11, 12, 13, 14
Between 1946 and 1958, the USA tested 67 nuclear weapons in the northern Marshall Islands, with a cumulative nuclear yield that was 7200 times more powerful than the atomic weapons used in World War 2.15 An international agreement with the USA, the Compact of Free Association, was signed in 1986 to settle all claims arising from the nuclear testing. The Compact of Free Association allows Marshallese people to freely travel to the USA as migrants, requiring a passport but not visas or immigration health screening; however, they are not offered full rights of US citizenship.15
Marshallese people have been migrating to the USA since the 1980s in increasing numbers to work (primarily in the poultry industry), educate their children, and seek new opportunities.16 This increasing migration has been driven by rising tides from climate change, which are predicted to make the Marshall Islands uninhabitable in the next 20–30 years.17 Springdale, Arkansas, hosts the largest population of Marshallese people in continental USA, likely 8000–14 000 inhabitants.16
Marshallese culture has been characterised as very close-knit, community-oriented, matriarchical, and matrilineal. Marshallese people traditionally live in densely populated accommodation, often with 12–20 people in a three-bedroom house. They have some of the highest rates of infectious and chronic diseases in the world, including tuberculosis, leprosy, invasive pneumococcal disease, perinatal hepatitis B infection, syphilis, and diabetes.15, 18 Some Marshallese people and medical providers have expressed concern that there might be long-lasting health effects from the nuclear testing. Marshallese adults often maintain a strong belief in traditional healers and distrust in western medicine. Illness is often stigmatised and seen as a sign of weakness or divine punishment.18 Individuals routinely present late in the natural history of disease after traditional healing interventions have failed. In Arkansas, Marshallese people are ineligible for health insurance coverage through Medicaid or Medicare, so care is often sought in an emergency department at high cost and with little emphasis on continuity of primary care.19 Marshallese adults in Arkansas typically speak Marshallese and interpreter services are extremely limted. Health literacy among Marshallese adults is generally below basic.20 These issues, in conjunction with a pervasive distrust of the US Government, a lack of familiarity with the US health system, and poverty, lead to a situation wherein the opportunity for, and consequences of, disease spread are high.
Research in context
Evidence before this study
We initially searched PubMed without language restrictions up to Aug 6, 2018, using the search terms “mumps outbreak” and “Marshallese”, and did not identify any relevant studies. We then repeated the search using the terms “mumps outbreak” and “highly vaccinated” and identified numerous studies describing mumps outbreaks among various highly vaccinated populations, including studies that had also assessed the effectiveness of a third dose of measles–mumps–rubella (MMR) vaccine. Most studies described outbreaks among highly vaccinated college-aged populations. In one study, Nelson and colleagues described a mumps outbreak in a highly vaccinated island population, which had similar characteristics to our outbreak and had also been controlled with a third dose of MMR vaccine. They concluded that high household crowding indices and high student contact rates were the major contributors to the intensity and persistence of the outbreak.
Added value of this study
We describe the second largest outbreak of mumps in the USA in the past 30 years. The majority of affected individuals were Marshallese. More cases were identified among school-aged children than among college-aged students. Despite the large number of cases, the proportion of patients who reported complications was low. We also observed unusual outcomes, including recurrent parotitis and prolonged viral shedding, both of which have not been greatly studied. To control the outbreak, we used unique response strategies in terms of communication, partnership, and vaccine clinics, including distributing outbreak doses of MMR vaccine in school settings that met Centers for Disease Control and Prevention criteria for a third-dose clinic.
Implications of all the available evidence
Multiple outbreaks with mumps genotype G, including this one, bring into question whether current vaccine effectiveness might be lower for genotype G than for other genotypes or that waning immunity has a part in mumps resurgence. This outbreak highlighted the need for response strategies tailored to the affected populations and consideration of social, cultural, and political factors in controlling transmission.
From 2000 to 2015, Arkansas Department of Health, Little Rock, AR, USA, investigated one to six cases of mumps each year; none were in Marshallese people. On Aug 8, 2016, the department received notification that a Marshallese adult residing in Springdale, Arkansas, had developed parotitis 3 days earlier and was confirmed to have mumps. A rapid increase in case counts prompted them to request assistance from the US Centers for Disease Control and Prevention (CDC) on Sept 2, 2016. The outbreak quickly spread in the Springdale community and expanded into the larger northwest Arkansas community, peaking in November, 2016. By August, 2017, incidence rates had decreased to zero. Here, we describe the epidemiology of this outbreak from Aug 5, 2016, to Aug 5, 2017.