Editor — The potential of oral fluid to replace serum for specific antibody detection for use in evaluating population immunity levels for important vaccine-preventable virus infections was recently discussed in the Bulletin (1). The important role of genotyping virus strains in surveillance of vaccine-preventable diseases to identify "escape mutants" and to investigate epidemiologically linked cases is well established. We report here an investigation of the rate of the virus shedding in oral fluid in patients with serological evidence of recent measles, mumps and rubella (MMR). While other clinical specimens such as blood, nasopharyngeal aspirate (NPA), throat swab and urine are recommended for testing for this purpose, oral fluid offers considerable compliance advantages to the patient and is easy and economical to collect.

In all, 1047 oral fluid samples were tested by reverse transcription polymerase chain reaction (RT-PCR) assay for MMR: 966 samples were collected as part of the salivary surveillance scheme in the United Kingdom (2) and had been previously tested for antibody (3), and 81 samples were obtained specifically for molecular studies from confirmed cases. All samples were collected using the Oracol device (Malvern Medical Developments, Worcester, UK) (4).

The results (Table 1) support MMR virus genome detection by RT-PCR in oral fluid samples collected during the first 14 days after onset of symptoms. In contrast, MMR-specific IgM (3) may not be detected for several days after the onset of symptoms. The most important factor influencing the successful detection of viral genome by RT-PCR was the timing of sample collection, and detection rate decreased with time after onset of symptoms. A higher rate of RT-PCR positivity was also found in those samples which had not previously been used for detection of specific antibody, suggesting that oral fluids contain low levels of virus and that subjecting samples to cycles of freeze–thawing may damage genome copies. A further observation was that the proportion of oral fluid samples in which measles virus genome was detected was significantly higher than that for mumps and rubella (P<0.001). This suggests a longer lasting viraemia for measles, or that the immune system may take longer to clear measles than mumps or rubella viruses.

Oral fluid samples from isolated cases that are to be used for molecular studies should ideally be collected within 7 days of onset of illness and stored at – 70 °C until required for testing. During large outbreaks or epidemics where large numbers of samples may be collected this is not so critical, and samples obtained and used for other purposes (e.g. antibody detection) may also be used for molecular studies. However, the likelihood of detection of virus genome is much reduced.

We have shown that oral fluid samples are a safe and practical alternative to blood, NPA, throat swabs and urine for molecular studies involving MMR. Sequencing of the PCR amplicon also enables the characterization of viral pathogens without cell culture, which is less sensitive and can be difficult and time-consuming. These results, together with those on antibody detection, reinforce the value of oral fluid samples for surveillance of MMR and highlight the need for more detailed studies to exploit fully the use of this method in surveillance programmes.

L. Jin,¹ A. Vyse,¹ & D.W.G. Brown¹

Conflicts of interest: none declared.

1. Nokes DJ, Endquselassie F, Nigaru W, Vyse AJ, Cohen BJ, Brown DWG, et al. Has oral fluid the potential to replace serum for the evaluation of population immunity levels? A study of measles, rubella, and hepatitis B in rural Ethiopia. Bulletin of the World Health Organization 2001;79:588-95.

2. Gay N, Ramsay M, Cohen B, Hesketh L, Morgan-Capner P, Brown D, et al. The epidemiology of measles in England and Wales since the 1994 vaccination campaign. Communicable Disease Report. CDR Review 1997;7:R17-21.

3. Perry KR, Brown DW, Parry JV, Panday S, Pipkin C, Richards A. Detection of measles, mumps and rubella antibodies in saliva using antibody capture radioimmunoassay. Journal of Medical Virology 1993;40:235-40.

4. Brown DWG, Ramsay ME, Richards AF, Miller E. Salivary diagnosis of measles: a study of notified cases in the United Kingdom, 1991-3. BMJ 1994;308:1015-7.

¹ Enteric, Respiratory and Neurological Virus Laboratory, Central Public Health Laboratory, 61 Colindale Avenue, Colindale, London NW9 5HT, England. Correspondence should be addressed to Dr Jin at this address (email: ljin@phls.org.uk).