Challenges in Interpretation of Diagnostic Test Results in a Mumps Outbreak in a Highly Vaccinated Population

DISCUSSION

As in this outbreak, most cases of mumps in vaccinated populations have occurred in individuals vaccinated over 10 years previously and among large groups with close person-to-person contact, such as in teenagers and university-aged populations. The reason is unclear, but, as previously mentioned, waning immunity has been postulated as a possible cause (9, 10, 13, 14). One study, which found that some countries were more likely to experience outbreaks of mumps than others, concluded that administration of doses of a highly immunogenic vaccine 4 to 8 years apart appeared to be the best way to reduce the likelihood of mumps outbreaks (14). However, no correlation could be found between mumps seroprevalence and the likelihood of outbreaks. Several studies have brought into question whether measuring antibody levels following vaccination is an effective method of assessing resistance to vaccine-preventable viral infections (12, 20), against which a cell-mediated immunity response appears to provide protection in addition to humoral immunity (14, 21). It should be noted, however, that there does seem to be some correlation between levels of humoral antibody and the ability to resist infection (13). Nevertheless, the reported outbreaks in vaccinated populations may indicate that the immunological and virological features of such outbreaks need to be studied, with the aim of better explaining the reasons for the apparent vaccine failure, and thus to examine whether a review of the mumps vaccination schedule might be worth some consideration by public health authorities.

The investigation of this outbreak demonstrated the difficulties of using recognized diagnostic tests for mumps in individuals who have been immunized. Not surprisingly, since the majority of individuals included in the investigation were fully immunized, 22 of 27 serology specimens tested early after symptom onset—only 5 of which had been tested at >5 days, the latest Public Health Ontario (PHO)-recommended testing time for mumps virus IgG diagnostic serology—had elevated IgG levels to mumps virus (see Table S3 in the supplemental material [footnote]). In vaccinated individuals, IgG levels are usually already elevated; therefore, seroconversion usually cannot be demonstrated. This is especially so where an enzyme immunoassay (EIA) is used as the diagnostic serology test, as this does not enable accurate quantification of antibody responses (18). Detection of IgM also presents diagnostic difficulties in vaccinated individuals, as a demonstrable increase in IgM levels following infection is often delayed or altogether absent in such individuals, even among those with positive RT-PCR results (16, 18, 22). The fact that the three IgM-positive serology specimens (see Table S3 [footnote]) had all been collected at ≥8 days after the onset of symptoms may be an indication of the need for delayed specimen collection when using IgM detection as a diagnostic test for mumps virus infection in vaccinated individuals. However, the low percentage of positive tests (11.5% overall and only 1 of 6 RT-PCR-positive individuals who were tested for IgM), even with most of the individuals having parotitis and a known link to the outbreak, may show that a demonstrable IgM response is often altogether absent in cases in vaccinated individuals, even with delayed specimen collection, and that false-negative IgM test results (i.e., negative IgM results in mumps-infected cases) are very likely to occur in such cases (3, 16, 18).

Twenty-five percent of the buccal swabs and 21.4% of throat swabs collected from symptomatic individuals tested positive for mumps virus RNA by RT-PCR (Table S3). This test is considered the gold-standard for mumps diagnosis (3, 22). However, the success of detecting the virus depends on the technique used to collect and transport specimens as well as on the time of collection. PHO guidelines recommend collection of specimens within 3 to 5 days after onset of symptoms (17). However, the Vaccine-Preventable Diseases Surveillance Manual of the CDC recommends that specimens should ideally be collected within 3 days after symptom onset, especially in cases where individuals have been vaccinated (18, 22), and this recommendation is supported by published research (16). In this outbreak investigation, only 7 of the 29 symptomatic individuals tested gave positive results by RT-PCR. Whether this could have been due to low levels of virus in buccal and throat secretions among the cases (some of whom were tested beyond the CDC-recommended 3-day window), to the low sensitivity of the RT-PCR protocol relative to that used by the CDC (16), or perhaps to poor or inconsistent methods of specimen collection and/or transportation is unclear. There is some evidence that levels of virus in mumps-infected vaccinated individuals may be very low and that the virus may be only briefly present, even in cases with parotitis (22); in at least one study, this was apparent in both vaccinated and unvaccinated outbreak cases (16). This may explain the fact that fewer than half of the RT-PCR-positive specimens cultured in this investigation yielded virus (Table S3). The even lower yield of the urine specimens cultured for virus (3.3%) may also have been a result of this, although culture of mumps virus from urine is well recognized as being less likely to yield virus than culture from throat or buccal swabs (18).

Most of the individuals who were confirmed and suspect cases in this outbreak (i.e., 88.5% of confirmed cases, including all of the laboratory-confirmed cases, and 100.0% of the suspect cases) experienced parotitis. As usual in mumps outbreaks, parotitis with no other apparent cause was considered to be the main indication of whether an individual might be considered to be a possible case of mumps, although a few individuals with more general respiratory symptoms but no parotitis were also tested for infection. It is well documented that only about two-thirds of individuals with mumps develop parotitis (15); therefore, it is very likely that up to one-third of all infected cases in this outbreak may have remained unidentified and undiagnosed within the community. The identification of mumps cases was further complicated by the fact that this outbreak occurred in spring, when influenza virus and other respiratory pathogens were still circulating in the community, albeit at lower levels than they would have been earlier in the year. This was illustrated by the identification of influenza B virus and RSV, respectively, in two of four individuals tested for other respiratory pathogens. Both individuals had parotitis but were excluded as cases and remained unclassified because of the presence of other respiratory pathogens. Parotitis in cases of influenza B virus and RSV infection is likely relatively rare compared to parotitis associated with mumps; there appear to be few published reports, if any, of these viruses being isolated from individuals with parotitis. In one study in Spain where specimens from 101 patients with parotitis were tested for several viruses, including influenza virus and RSV, only one patient tested positive for RSV, and no evidence of influenza B virus was found in any of the specimens (23). During the investigation described here, the decision was made to adhere to the criteria specified in the case definitions and thus allow those two individuals to remain unclassified (i.e., noncases); however, it is recognized that it is quite possible that they were in fact mumps cases. Further, a limitation of the investigation and of the classification of cases was that only four individuals were tested for pathogens other than mumps virus, so this criterion could not be applied across all individuals in a standardized manner.

Thus, the presence of other respiratory pathogens in the community and the fact that many individuals with mumps do not experience parotitis, in addition to the relatively low sensitivity of the various diagnostic tests in vaccinated populations, all made the detection and classification of cases in this outbreak very difficult. Moreover, media coverage of the outbreak may have resulted in an increase in the presentation of individuals with parotitis, and also of those with no parotitis and only general respiratory symptoms, to health care providers. This in turn may have increased not only the ability of the health unit to capture true cases but also the likelihood that some of the test-negative immunized individuals with parotitis, even with identified links to the outbreak, may not have been true cases of mumps. As discussed above, the poor performance of diagnostic tests in cases in vaccinated individuals presented a barrier to ruling out the latter.

This outbreak investigation illustrated the challenges of creating an effective case definition for an outbreak of mumps in a highly vaccinated population, in contrast to definitions for use in unvaccinated or undervaccinated populations, where more conservative criteria may suffice. Although the definition was revised and broadened as the investigation proceeded and more information became available, it is very likely that at least some true cases may have remained unreported to Public Health or may have been incorrectly classified as noncases by Public Health. If so, then there might have been individuals with existing cases of mumps in the community who were not advised to self-isolate at any point. For example, any infected individuals who had been fully vaccinated and experienced general respiratory symptoms but not parotitis may not have been considered possible mumps cases and therefore may never have been reported to Public Health. Furthermore, as discussed previously, it is possible that the two individuals reported to Public Health who were ruled out as cases of mumps because of having tested positive for other respiratory pathogens were in fact mumps cases. Although serious complications of mumps infection, especially in vaccinated populations, are relatively rare, there could possibly be negative consequences if such misclassification is repeatedly allowed to occur in such outbreaks over the course of time. With the benefit of hindsight, based on the diagnostic uncertainty illustrated in this instance, it may be advisable in mumps outbreaks in highly vaccinated populations to create an even broader definition for suspect cases than that used in this outbreak, based on either general respiratory symptoms or parotitis, and identified direct or indirect link(s) to an outbreak exposure, and not, as was done in this outbreak, excluding immunized individuals with negative laboratory results or individuals with identified infections with other respiratory pathogens. The presence or absence of epidemiological links should be determined as accurately as possible by repeated interviewing of individuals whenever a new source of potential exposure has been identified. The use of this broader case definition for suspect cases would mean that any individuals potentially infected with mumps virus, even those without parotitis, those testing positive for other respiratory pathogens, and those with negative mumps diagnostic test results, would be advised to self-isolate, reducing the risk of further spread of infections within the community. In addition, to assist in identifying symptomatic individuals for appropriate follow-up by public health authorities, active communication to area physicians in the event of a confirmed case or identified cluster is advisable, with a caution against ruling out mumps as a differential diagnosis based solely on client immunization history, absence of parotitis, or negative diagnostic test results.