Isolation of measles virus from a naturally-immune, asymptomatically re-infected individual

Background: The changing epidemiology of measles with mild measles cases increasingly being recognised in previously-vaccinated individuals, suggests that more asymptomatic or subclinical cases might be occurring. Although this has been clearly documented in previously-vaccinated individuals, the frequency of these asymptomatic infections in individuals previously naturally-infected with measles is not known. Also, it is not known whether these asymptomatic or mildly-infected individuals who do not display the full range of clinical signs of measles are capable of transmitting the virus to other susceptible persons. Objectives: To demonstrate the isolation of measles virus (MV) from previously, naturally-immune individuals asymptomatically infected with measles while in close contact with acutely infected family members and to document the secondary immune responses (SIR) associated with asymptomatic measles infection. Study Design: Throat swab and urine specimens from five acute measles cases and their family contacts, taken within 5 days of onset of rash in each acute case, were used to isolate MV by tissue culture. Positive tissue culture results were confirmed by indirect immunofluorescence (IF) staining. Measles specific antibodies (IgG and IgM), IgG urea avidity and measles-neutralising antibodies were measured in the one family (index family) where an asymptomatic measles infection of a contact was demonstrated. Results: The acutely infected patient in the index family (T1/96) had a measles-neutralising antibody titre of < 1:10, measles IgG urea avidity of 24% and MV was isolated and confirmed by IF from urine and throat swab specimens. T1/96 represents acute measles infection after primary vaccine failure because he had a clear history of being vaccinated against measles as a child. MV was also successfully isolated from throat swab and urine specimens from the other four acute cases and from the urine but not the throat swab of an asymptomatically infected family contact in the index family (mother, T2/96). T2/96 had a history of natural measles infection as a child approximately 50 years ago. In addition to detectable MV in urine this contact also had a SIR with a rise in measles specific neutralising antibody titre. No virus was isolated from the other contact in the index family (father, T3/96) or from the contacts of the other four acute cases examined. Conclusions: This is the first report of a confirmed asymptomatic MV infection, by MV isolation and IF testing and a concurrent SIR, in a previously naturally-immune contact of an acute case. The importance of these findings to the epidemiology and control of MV as well as the diagnostic value of MV urine isolation and IF confirmation for mild or asymptomatic cases must be examined further.