In this paper, we report the number of tests positive for mumps in South Africa between 2012 to 2017. Most of these cases were reported by the private sector laboratories and occurred mostly in the 1–4 and 5–9 age groups. This age distribution is consistent with what has been reported in other countries during the pre-MuCV era, with most of the infections reported in children below 10 years of age (1). The mumps incidence in our setting was found to be lower than that reported in other countries in the pre-vaccine era throughout the study period, most likely reflecting under-reporting in the public sector. However, under-reporting in the public sector is expected since MuCV has not been introduced into the country’s Extended Immunization Programme (EPI) and is also not under surveillance. However, the few positive results from the public sector may represent unidentified large outbreaks.
The recently reported resurgence of mumps infections has been found amongst adolescence and young adults in overcrowded and semi-closed settings such as communes, colleges and camps (8–10). In the United States (US), military recruits, a sub-population that has previously been associated with mumps outbreaks, were found not to be involved in the resurgence of mumps infections reported between 1998–2007 in the US (8). This was associated with the decision in 1991, to introduce the MMR vaccine amongst recruits irrespective of previous vaccination status. Although this finding could strengthen a case for booster doses in older age groups, particularly those at high risk such as college students, antibody titres have been found not to be durable, with titres returning to pre-MMR3 dose levels one year after vaccination in individuals between 18–24 years in a non-outbreak setting (19). A booster dose of the mumps vaccination is currently recommended only in the setting of an outbreak (9,10,21). Although protective antibody levels against mumps infections are not well-defined, suggested causes of the resurgence of infections have included waning immunity over time due to a lack of a durable T-cell mediated response, as well as antigenic differences between vaccine and circulating mumps strains, (1, 4, 7–10, 21–24). As such, the mismatch between vaccine and circulating mumps stains has also prompted the consideration of a polyvalent vaccine (1, 25)
In our study, most of the samples submitted for MuV testing were CSF and blood specimens. However, information on the clinical presentation of the patients from whom specimens were collected was not available. One study conducted in Gauteng Province in South Africa used CSF samples from patients who had clinical presentation of central nervous system disease (meningitis, encephalitis or other febrile illness with focal neurological signs) to determine the presence of MuV and to characterise the strains, if found (12). The study findings showed a low frequency of MuV-associated CNS disease [3/260 (1.2%)], and phylogenetic analysis of one detected strain showed that it was a Jeryl-Lynn or RIT4385 vaccine-like strain. A suggestion made by the authors was the establishing of a MuV surveillance programme in the country.
Our finding of a male predominance with regards to infections is similar to what has been reported in other studies (26, 27). This has been associated with immunological differences between males and females, where females have been shown to have a stronger T-helper1 cell (Th1) immune response, as well as having persistent and higher antibody levels compared to males (24, 28). Orchitis has been reported to be the most common complication of mumps infection, and this may also explain the male bias seen in the results (6). Males have also been found to have an increased risk of complications that occur less commonly following mumps infections such as mumps-associated meningitis and encephalitis (6, 28).
The seasonal pattern of mumps infections differs by country, with this difference attributed to variations in meteorological, environmental and seasonal exposure factors (29). Mumps virus survival and virulence has also been reported to be supported by increasing humidity and temperatures (29). In our study, we found that the infections tended to peak in the October-November months, which represents spring in South Africa. This is consistent with the seasonal pattern that has been found in other settings, even though the months representing spring in the Northern Hemisphere may differ (26, 27, 30, 31).
Although Gauteng, Western Cape and KwaZulu Natal were seemingly the most affected provinces, these regions were also over-represented in the analysis, with the highest number of samples having been obtained from these provinces. This spatial variation in the findings may also be due to the differential availability of laboratory services in these provinces compared to others in the country, as well as the fact that two of these provinces, Gauteng and Kwa-Zulu Natal, were the most populous provinces in the country throughout the study period.
A strength of our study is that we analysed data from both the public and private health sectors. However, our study also has several limitations. Firstly, missing data could not be accounted for and information on risk factors was not available since secondary data was analysed. Secondly, 50% of mumps infections present non-specifically or with respiratory symptoms, while 20–30% of infections are reportedly asymptomatic or have mild symptoms (1, 21). These cases may therefore not present at health facilities and would therefore not be included in the data reviewed. Therefore, cases of mumps infection in whom mumps was only diagnosed clinically without being investigated using laboratory testing would not have been accounted for in our study. Also, mumps was not a notifiable disease in South Africa at the time that this paper was written. Thus, case-based data that could have supplemented the laboratory-based data were not available. The above-mentioned limitations may also account for the small numbers of mumps test requests, particularly from the public health sector, where mumps infections are likely to be diagnosed based on clinical presentation rather than by laboratory testing. Thirdly, we were not able to comment on mumps-related complications in our setting because information on clinical presentation was not included in the analysed data. Increased age is associated with more severe disease in many childhood diseases, and in mumps infections, this increased risk occurs more commonly in males compared to females (10, 28) Fourthly, differential availability of laboratory services across the provinces may also have had an impact on the completeness of the analysed data. The estimates of acute infections presented may be an underestimation of the true burden of mumps disease and may explain why the cumulative incidence found in our study was lower than the incidence of ≥ 100 cases/100000 that has been reported in the pre-vaccine era in other settings.