Background

Enterovirus 71 (EV71), which was firstly described in 1969 during an outbreak with central nervous system complications in California, causes a variety of neurological diseases, including aseptic meningitis, encephalitis, and poliomyelitis-like paralysis [1]. This virus is also one of only a few enterovirus serotypes most often associated with large outbreaks of hand, foot, and mouth disease (HFMD) [2, 3]. EV71 is associated with the fatal cases of brain stem encephalitis during large HFMD outbreaks in Malaysia (1997) [1) showed that the EV71 specific sequences were identified in all specimens of 0-4 days, and 5-8 days after onset. The positive rates of EV71 in throat samples dropped markedly to 42.86% during 9-12 days, and maintained at about 20-30% during 13 to 24 days after onset, while that in feces reduced to 71.43% during 9-12 days, and maintained roughly 20% afterwards till 37-40 days after onset. EV71 associated nucleotides were undetectable in respiratory samples fourth weeks after onset and almost undetectable in feces more than five weeks later. The longest EV71-postive time in throat swab was 24 days and that in feces was 42 days post-onset.

Figure 1
figure 1

The positive rates of EV71 nucleotides in samples from the definitely diagnosed HFMD patients. The positive rates of EV71 related nucleosides identified in serially collected throats and feces samples from the definitely diagnosed HFMD patients. Square hole represents RT-PCR positive rates from feces sample. Black square represents RT-PCR positive rates from throat swabs.

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To evaluate the negative conversion rate in clinical samples of EV71 nucleotides after disease, the time period of EV71 negative closest to that of EV71 positive was taken as the time of EV71 nucleotide negative conversion. Figure 2 showed that none of the tested cases converted to EV71 negative in throats or in feces in the first four days. In the time period of 5-8 days, only one feces showed EV71 negative conversion. EV71 negative conversion happened mostly in the time periods of 9-12 days and 13-16 days after onset. 48.48% throat swabs and 17.65% feces samples in the period of 9-12 days, as well as 12.12% throat swabs and 38.24% feces samples in the period of 13-16 days were EV71 negative, respectively. Afterwards, the EV71 negative conversion rates increased gradually in throats and feces, but EV71 nucleotides in feces samples lasted clearly longer than that in throats. No significant difference in persistence of EV71 nucleotides were analyzed between mild and severe cases, between genders and among the onset ages.

Figure 2
figure 2

The portions of negative conversion of EV71 nucleosides in the HFMD convalescent. The portions of negative conversion of EV71 related nucleosides in throats and feces samples from the HFMD patients in the convalescent.

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To figure out the possible different profiles of EV71 disappearance, persistent states of EV71 nucleotides in throats compared to feces of individual patient were analyzed. 36.36% (12/33) cases revealed almost same time period of EV71 nucleotide disappearance both in throats and feces, 39.39% (13/33) cases showed longer persistence of EV71 nucleotides in feces, and 21.21% (7/33) were longer in throats (Figure 3). Furthermore, the identifications of EV71 nucleotides in each time period of individual case were counted. Only two out thirty-three cases showed EV71 positive in the subsequent samples after previous negative in their serial throat samples, however, 6 cases were EV71 positive reversion in the serial feces samples.

Figure 3
figure 3

Comparison of the clearances of EV71 in the throat and feces specimen of individual cases. F: feces; T: throat. The right Y-axis represents the numbers of the cases and the left Y-axis represent the percentages.

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Discussion

HFMD has been an important public health concern worldwide, especially in the Asia-Pacific region. Up to now, more than 900,000 HFMD cases have been reported in mainland of China. Enteroviruses can be isolated from both the lower and upper alimentary tract and can be transmitted by both fecal-oral and respiratory routes. Fecal-oral transmission may predominate in areas with poor sanitary conditions, whereas respiratory transmission may be important in more developed areas. The relative importance of the different modes of transmission probably varies with the particular EV and environmental setting [1]. Like other infections of enteroviruses, e.g. Coxsackie's viruses and polioviruses, EV71-caused HFMD often occurred sporadically or epidemically, sometimes without clear transmission chain [2, 15]. One of the reasons is believed to be the wide and continuous circulation of EV71 among humans as well as environment [16, 17]. On the other hand, long time of virus persistence and shedding in the patients' secretions and excretions makes infectious sources be more abundant. Additionally, relatively stronger resistance of viruses in environment let the disease more easily transmit [18].

With the established nested RT-PCR, we prove a long persistence of EV71 nucleotides in the throats and feces of the patients with HFMD. Only about half of the patients show EV71 negative in their respiratory secretions and stools two weeks after onset. The traditional techniques for detecting and characterizing enteroviruses rely on the time-consuming and labor-intensive procedures of viral isolation in cell culture and neutralization by reference antisera [1, 19]. Virus persistent time in the throat in this study seems to be longer than that of previous data, mostly obtained from virus culture, which showed EV71 shedding up to 2 weeks [13]. Isolation of enteroviruses from specimens need appropriate cultured cell lines and suitable specimen. The best specimens for isolation of virus are, in order of preference, stool specimens or rectal swabs, throat swabs or washings, and cerebrospinal fluid. Throat swabs or washings and CSF are most likely to yield virus isolates if they are obtained early in the acute phase of the illness [19, 20]. Overall, the positive rate of virus isolation for enteroviruses from throat swabs in acute period is less than 50%. Even combined throat plus vesicle swabs enables the identification of virus increase, but still less than 70% [9, 13]. It is obvious that measurement of EV71 shedding merely based on virus culture will result in a portion of false negative. Certainly, identification of positive EV71 nucleotides by PCR in specimen does not indicate the presence of 100% live virus. Therefore, combination of the results from those two techniques may be more helpful for evaluating EV71 shedding. Additionally, detection of virus in a sample does not equal to being able to set up an efficient infection or transmission, which may be influenced by numerous factors, e.g virus load, exposing time and pathway, environmental and host situations.

In line with the concept that the EV shedding time from gastrointestinal tract usually longer than from respiratory tract [13], our study also illustrates a similar pattern that more than 20% cases maintain EV71 positive in the stool samples after clearance of EV71 nucleotides in their throats. It highlights a special requirement of decontamination for feces after recovery.

Conclusions

Excretion of EV71 may persist for months after infection, but most cases become negative 2 weeks after onset. Thus, the patients during the first 2 weeks should be at high risk to spread the pathogens.