hospital episode statistics provide information on hospital care in national health system hospitals in england . no case definition is available for viral encephalitis , but diagnosis is generally based upon clinical evidence of viral encephalitis and available confirmatory laboratory data as recorded in the patient s medical record at the time of discharge . the diagnosis is recorded by using the world health organization international classification of disease ( icd ) codes . these codes allow for specific ( clinical and laboratory evidence ) and nonspecific ( only clinical evidence ) diagnoses . we analyzed reports of hospital admissions for all adults and children ( < 17 years of age ) from april 1 , 1989 , to march 31 , 1998 . cases of viral encephalitis were identified by icd ninth edition ( icd-9 ) codes for admissions in 19891995 and 10th edition ( icd-10 ) codes for admissions in 19951998 . patient records were also extracted if any of seven diagnostic codes mentioned a diagnosis of viral encephalitis ( table 1 ) . since no icd-9 code has been defined for vzv or adenovirus encephalitis , any vzv or adenovirus infection code accompanied by a code for nonspecific viral encephalitis before 1995 was included in the analysis . multiple episodes for one patient were considered to be a single infection if < 1 month elapsed between episodes . the second data source was laboratory reports sent to the communicable disease surveillance centre from the public health laboratory service ( phls ) , national health system , and private laboratories in england . all laboratory reports of isolates of viruses known to cause encephalitis from january 1 , 1990 , to december 31 , 1998 , were extracted from labbase ( the phls laboratory network electronic database ) . from these reports data extracted included laboratory reports in england with specimen date between january 1 , 1990 , and december 31 , 1998 , of adenoviruses , herpes , vzv , cytomegalovirus , lymphocytic choriomeningitis virus , epstein - barr virus , measles , mumps , or rubella detected in the cerebrospinal fluid , or adenovirus , herpes , vzv , cytomegalovirus , lymphocytic choriomeningitis virus , epstein - barr virus , measles , mumps , or rubella in any specimen with an encephalitis diagnosis in the feature or comment field . an additional data source was the notifications of infectious disease system ( noids ) ( available from : url : www.phls.co.uk/facts/noids/noid.htm ) . clinically diagnosed cases of viral encephalitis are reported weekly as acute , infectious encephalitis to consultants in communicable disease control and collated by the communicable disease surveillance centre . data collected included the week of the report and patients age , sex , and local health authority of residence . reports from 1989 through 1998 were available for analysis . information about deaths attributed to viral encephalitis from january 1 , 1993 , to december 31 , 1998 , in england was available from office of national statistics data held by the communicable disease surveillance centre . we investigated trends in hospitalizations of viral encephalitis by fiscal year , using 1995 mid - year resident population estimates for england to calculate rates . single- and multi - variable regression was used to compare rates by fiscal year , age group ( < 1 , 14 , 59 , 1016 , 1724 , 2534 , 3544 , 4554 , 5564 , and > 65 years of age ) , sex , and regional health authority of hospital ( anglia and oxford , north west , south and west , trent , north thames , northern and yorkshire , south thames , and west midlands ) . we initially investigated evidence of clustering of case - patients with a nonspecific diagnosis by comparing weekly totals of hospitalizations of nonspecific viral encephalitis to the weekly overall mean number during the study period . possible clusters were considered to have occurred when the weekly total was > 2.58 and > 3.3 standard deviations from the mean , corresponding to 99.5% and 99.95% upper prediction limits , respectively . in addition , we used poisson regression to calculate an upper weekly and monthly prediction limit ( at 99.5% and 99.95% ) for each hospital s district health authority . any weekly or monthly count greater than or equal to the limit in each district was flagged as a possible outbreak . we compared noids and laboratory reports to hospital episode statistics to determine the level of underreporting in each . hospital episode statistics provide information on hospital care in national health system hospitals in england . no case definition is available for viral encephalitis , but diagnosis is generally based upon clinical evidence of viral encephalitis and available confirmatory laboratory data as recorded in the patient s medical record at the time of discharge . the diagnosis is recorded by using the world health organization international classification of disease ( icd ) codes . these codes allow for specific ( clinical and laboratory evidence ) and nonspecific ( only clinical evidence ) diagnoses . we analyzed reports of hospital admissions for all adults and children ( < 17 years of age ) from april 1 , 1989 , to march 31 , 1998 . cases of viral encephalitis were identified by icd ninth edition ( icd-9 ) codes for admissions in 19891995 and 10th edition ( icd-10 ) codes for admissions in 19951998 . patient records were also extracted if any of seven diagnostic codes mentioned a diagnosis of viral encephalitis ( table 1 ) . since no icd-9 code has been defined for vzv or adenovirus encephalitis , any vzv or adenovirus infection code accompanied by a code for nonspecific viral encephalitis before 1995 was included in the analysis . multiple episodes for one patient were considered to be a single infection if < 1 month elapsed between episodes . the second data source was laboratory reports sent to the communicable disease surveillance centre from the public health laboratory service ( phls ) , national health system , and private laboratories in england . all laboratory reports of isolates of viruses known to cause encephalitis from january 1 , 1990 , to december 31 , 1998 , were extracted from labbase ( the phls laboratory network electronic database ) . from these reports data extracted included laboratory reports in england with specimen date between january 1 , 1990 , and december 31 , 1998 , of adenoviruses , herpes , vzv , cytomegalovirus , lymphocytic choriomeningitis virus , epstein - barr virus , measles , mumps , or rubella detected in the cerebrospinal fluid , or adenovirus , herpes , vzv , cytomegalovirus , lymphocytic choriomeningitis virus , epstein - barr virus , measles , mumps , or rubella in any specimen with an encephalitis diagnosis in the feature or comment field . an additional data source was the notifications of infectious disease system ( noids ) ( available from : url : www.phls.co.uk/facts/noids/noid.htm ) . clinically diagnosed cases of viral encephalitis are reported weekly as acute , infectious encephalitis to consultants in communicable disease control and collated by the communicable disease surveillance centre . data collected included the week of the report and patients age , sex , and local health authority of residence . reports from 1989 through 1998 were available for analysis . information about deaths attributed to viral encephalitis from january 1 , 1993 , to december 31 , 1998 , in england was available from office of national statistics data held by the communicable disease surveillance centre . we investigated trends in hospitalizations of viral encephalitis by fiscal year , using 1995 mid - year resident population estimates for england to calculate rates . single- and multi - variable regression was used to compare rates by fiscal year , age group ( < 1 , 14 , 59 , 1016 , 1724 , 2534 , 3544 , 4554 , 5564 , and > 65 years of age ) , sex , and regional health authority of hospital ( anglia and oxford , north west , south and west , trent , north thames , northern and yorkshire , south thames , and west midlands ) . we initially investigated evidence of clustering of case - patients with a nonspecific diagnosis by comparing weekly totals of hospitalizations of nonspecific viral encephalitis to the weekly overall mean number during the study period . possible clusters were considered to have occurred when the weekly total was > 2.58 and > 3.3 standard deviations from the mean , corresponding to 99.5% and 99.95% upper prediction limits , respectively . in addition , we used poisson regression to calculate an upper weekly and monthly prediction limit ( at 99.5% and 99.95% ) for each hospital s district health authority . any weekly or monthly count greater than or equal to the limit in each district was flagged as a possible outbreak . we compared noids and laboratory reports to hospital episode statistics to determine the level of underreporting in each . from april 1 , 1989 , to march 31 , 1998 , a total of 6,414 adults and children were hospitalized in england with a diagnosis of viral encephalitis ( table 2 ) , a figure that corresponds to an estimated annual rate of 1.5 cases per 100,000 population . a specific diagnosis was recorded for 2,574 patients , the largest proportion of which ( 52% ) had herpes infection . the remaining 223 included patients with encephalitis associated with measles , mumps , rubella , exotic viruses ( arboviruses ) , and lymphocytic choriomeningitis virus infections . other  and  unspecified  are included because both groups existed from 1995 to a total of 2,734 cases were diagnosed in children ( including 35 in neonates ) , corresponding to an estimated annual rate of 2.8 per 100 000 children and accounting for 43% of all hospitalizations of viral encephalitis . an estimated 21% declining trend occurred in the rate over the 9 years , although this decline is not significant ( p=0.065 ) . over half ( 51% ) of the hospitalizations of children occurred for those < 5 years of age , with the highest rate ( 8.7/100 000 ) for infants < 1 year of age . the rate remained highest for infants throughout the 9 years of the study ; however , we found some variation between the age groups in the time trends that approached significance ( p=0.063 ) , with the largest decline in adolescents ages 1016 years ( estimated to be 37% ) and the smallest in toddlers 14 years of age ( estimated to be 4% ) . little difference appeared in the rate in male children compared to female children overall ( 2.9 and 2.8/100,000 children ; p=0.23 ) ; furthermore , the trend over the 9 years did not differ by sex ( p=0.63 ) . in adults , 3,680 patients diagnosed with encephalitis were hospitalized , which approximates an annual rate of 1.1 per 100 000 adults with no overall significant trend over time ( p=0.17 ) . no significant variation appeared in the rate between the age groups overall , but the highest rate occurred in those aged 1724 years at approximately 1.2 per 100,000 . the change in the incidence of hospitalized case - patients over time varied between age groups ( p=0.0016 ) ; this change was mostly attributable to an increase of 46% in the rate in elderly case - patients ( > 65 years of age ) with no significant changes in other age groups . we found no difference in the overall rate of male case - patients compared to female case - patients ( 1.1 and 1.0/100,000 adults ; p=0.12 ) ; furthermore , the trend over the 9 years did not differ by sex ( p=0.74 ) . the rate between the regional health authority of hospitals did not vary significantly overall ( p=0.55 ) ; however , the largest proportion of cases ( 16% ) and highest rate ( 1.63/100 000 population ) were hospitalized in the north thames region . the trend over the 9 years did not significantly vary by regional health authority ( p=0.13 ) . the proportion of cases with a specific diagnosis was significantly lower in children ( 33% ) than adults ( 45% ) ( p<0.001 ) . in children , this proportion varied throughout the study ( p<0.001 ) ; it was at a minimum of 23% during 19921993 and a maximum of 50% during 19961997 . children with a specific diagnosis were significantly younger than those without ( mean age of 5 vs. 6.1 years ; p<0.001 ) but did not differ in terms of sex . in adults , those with a specific diagnosis were significantly older compared to those without ( mean age 49 vs. 43 years ; p<0.001 ) but did not differ in terms of sex . the proportion of all cases with a specific diagnosis varied significantly by region , ranging from 34% in trent to 47% in northern and yorkshire ( p<0.001 ) . the regions of northern and yorkshire had the highest proportion of herpes encephalitis ( 28.35% ) ; in comparison , trent had 21.4% , and north west ( the region with the lowest proportion ) had 18% . in hospitalized case - patients , the severity of cases was compared by using a crude case - fatality rate and mean length of stay ( table 3 ) . the overall case - fatality rate was 6.5 per 100 cases , and the mean length of stay was 17.5 days . considerable variation appeared in case - fatality rate and mean length of stay among the different diagnoses . herpes encephalitis had the highest overall case - fatality rate and mean length of stay and was considered to be the most severe diagnosis . in children , the overall case - fatality rate was 2.3 per 100 cases ( 95% confidence interval [ ci ] 1.8 to 3 ) , 1.9 per 100 cases of herpes ( 95% ci 0.7 to 4 ) , and 2.1 per 100 cases of viral encephalitis without a specific diagnosis ( 95% ci 1.5 to 2.8 ) . the mean length of stay for children was 10.9 days , increasing to 11.6 days for patients without a diagnosis specified and 13.2 days for patients with herpes encephalitis . in adults , the case - fatality rate was higher than children at 9.7 per 100 cases ( 95% ci 8.8 to 10.7 ) , 12.5 in herpes encephalitis ( 95% ci 10.4 to 14.3 ) , and 8.5 per 100 cases without a diagnosis specified ( 95% ci 7.2 to 9.7 ) . the mean length of stay for adults was 22.4 days , which increased to 30 days for those with herpes or  other  diagnoses compared with 17 days for cases without a diagnosis specified . ci , confidence interval ; vzv , varicella - zoster virus 1 ; lcmv , lymphocytic choriomeningitis virus . for nonspecific cases of viral encephalitis , the frequency of hospitalizations ( noted by week ) indicated that some clustering of undiagnosed viral encephalitis had occurred during the 9 years of study since the upper prediction limits at 99.5% and 99.95% were exceeded in 6 weeks . three of the six clusters occurred closely together in 1993 ( weeks 45 , 47 , and 51 ) , which suggests an overall period of higher incidence during this time . the poisson regression model suggested clustering of encephalitis of unknown cause in district health authorities , and the total number of cases occurring in a week was greater than the prediction limit at 99.95% in three districts . the model did not identify any clusters when the period was extended to those occurring within 1 month . significant underreporting of viral encephalitis was seen in the routine systems when compared to hospital episode statistics . a total of 215 cases of acute infectious encephalitis were identified through noids between april 1 , 1989 , and march 31 , 1998 , compared to 6,414 identified in hospital episode statistics . this disparity implies that almost all ( 97% ) hospitalized cases were not formally reported . a total of 599 cases of viral encephalitis were identified through the laboratory reporting system from january 1 , 1990 , to december 31 , 1998 . fifty nine per cent of these cases were herpes ( 86 herpes simplex virus 1 , 48 herpes simplex virus 2 , and 219 subtype not known ) , 21% varicella and 4% adenovirus . the remaining 16% of cases included 26 cytomegalovirus , 15 epstein - barr virus , 20 measles virus , and 13 mumps . of herpes encephalitis , 19% ( 66/353 ) of cases occurred in children and 4.8% ( 17/353 ) in neonates . of herpes encephalitis cases in neonates that were typed , seven were herpes simplex virus 2 and two were herpes simplex virus 1 . the number of laboratory reports of viral encephalitis increased from 52 in 1990 to 80 in 1991 , falling to 55 in 1993 , and rising again to a peak of 104 in 1998 . excluding those cases for which no specific icd-9 or icd-10 code exists and which could not have been identified in hospital episode statistics , 558 cases of viral encephalitis were reported through the laboratory system from january 1 , 1990 , to march 31 , 1998 . this total represents approximately 71% underreporting when compared to 1,867 hospitalizations of viral encephalitis with a specific infection reported ( table 4 ) . this underreporting varied by virus , ranging between 28% and 100% he s , hospital episode statistics ; herpes , herpes simplex virus ; vzv , varicella - zoster virus 1 ; lcmv , lymphocytic choriomeningitis virus . viral encephalitis was given as the final underlying cause of 622 deaths certified from january 1 , 1993 , to march 31 , 1998 . in that comparative period , hospital episode statistics showed 259 deaths , only 42% of those formally certified and reported to the office of national statistics . most ( 483/622 ) certified deaths from viral encephalitis did not have a specific diagnosis ; however , 139 deaths did . we compared the 139 deaths to the 112 identified in hospital episode statistics for which a specific diagnosis was recorded ( table 4 ) . from april 1 , 1989 , to march 31 , 1998 , a total of 6,414 adults and children were hospitalized in england with a diagnosis of viral encephalitis ( table 2 ) , a figure that corresponds to an estimated annual rate of 1.5 cases per 100,000 population . a specific diagnosis was recorded for 2,574 patients , the largest proportion of which ( 52% ) had herpes infection . the remaining 223 included patients with encephalitis associated with measles , mumps , rubella , exotic viruses ( arboviruses ) , and lymphocytic choriomeningitis virus infections . other  and  unspecified  are included because both groups existed from 1995 to a total of 2,734 cases were diagnosed in children ( including 35 in neonates ) , corresponding to an estimated annual rate of 2.8 per 100 000 children and accounting for 43% of all hospitalizations of viral encephalitis . an estimated 21% declining trend occurred in the rate over the 9 years , although this decline is not significant ( p=0.065 ) . over half ( 51% ) of the hospitalizations of children occurred for those < 5 years of age , with the highest rate ( 8.7/100 000 ) for infants < 1 year of age . the rate remained highest for infants throughout the 9 years of the study ; however , we found some variation between the age groups in the time trends that approached significance ( p=0.063 ) , with the largest decline in adolescents ages 1016 years ( estimated to be 37% ) and the smallest in toddlers 14 years of age ( estimated to be 4% ) . little difference appeared in the rate in male children compared to female children overall ( 2.9 and 2.8/100,000 children ; p=0.23 ) ; furthermore , the trend over the 9 years did not differ by sex ( p=0.63 ) . in adults , 3,680 patients diagnosed with encephalitis were hospitalized , which approximates an annual rate of 1.1 per 100 000 adults with no overall significant trend over time ( p=0.17 ) . no significant variation appeared in the rate between the age groups overall , but the highest rate occurred in those aged 1724 years at approximately 1.2 per 100,000 . the change in the incidence of hospitalized case - patients over time varied between age groups ( p=0.0016 ) ; this change was mostly attributable to an increase of 46% in the rate in elderly case - patients ( > 65 years of age ) with no significant changes in other age groups . we found no difference in the overall rate of male case - patients compared to female case - patients ( 1.1 and 1.0/100,000 adults ; p=0.12 ) ; furthermore , the trend over the 9 years did not differ by sex ( p=0.74 ) . the rate between the regional health authority of hospitals did not vary significantly overall ( p=0.55 ) ; however , the largest proportion of cases ( 16% ) and highest rate ( 1.63/100 000 population ) were hospitalized in the north thames region . the trend over the 9 years did not significantly vary by regional health authority ( p=0.13 ) . the proportion of cases with a specific diagnosis was significantly lower in children ( 33% ) than adults ( 45% ) ( p<0.001 ) . in children , this proportion varied throughout the study ( p<0.001 ) ; it was at a minimum of 23% during 19921993 and a maximum of 50% during 19961997 . children with a specific diagnosis were significantly younger than those without ( mean age of 5 vs. 6.1 years ; p<0.001 ) but did not differ in terms of sex . in adults , those with a specific diagnosis were significantly older compared to those without ( mean age 49 vs. 43 years ; p<0.001 ) but did not differ in terms of sex . the proportion of all cases with a specific diagnosis varied significantly by region , ranging from 34% in trent to 47% in northern and yorkshire ( p<0.001 ) . the regions of northern and yorkshire had the highest proportion of herpes encephalitis ( 28.35% ) ; in comparison , trent had 21.4% , and north west ( the region with the lowest proportion ) had 18% . in hospitalized case - patients , the severity of cases was compared by using a crude case - fatality rate and mean length of stay ( table 3 ) . the overall case - fatality rate was 6.5 per 100 cases , and the mean length of stay was 17.5 days . considerable variation appeared in case - fatality rate and mean length of stay among the different diagnoses . herpes encephalitis had the highest overall case - fatality rate and mean length of stay and was considered to be the most severe diagnosis . in children , the overall case - fatality rate was 2.3 per 100 cases ( 95% confidence interval [ ci ] 1.8 to 3 ) , 1.9 per 100 cases of herpes ( 95% ci 0.7 to 4 ) , and 2.1 per 100 cases of viral encephalitis without a specific diagnosis ( 95% ci 1.5 to 2.8 ) . the mean length of stay for children was 10.9 days , increasing to 11.6 days for patients without a diagnosis specified and 13.2 days for patients with herpes encephalitis . in adults , the case - fatality rate was higher than children at 9.7 per 100 cases ( 95% ci 8.8 to 10.7 ) , 12.5 in herpes encephalitis ( 95% ci 10.4 to 14.3 ) , and 8.5 per 100 cases without a diagnosis specified ( 95% ci 7.2 to 9.7 ) . the mean length of stay for adults was 22.4 days , which increased to 30 days for those with herpes or  other  diagnoses compared with 17 days for cases without a diagnosis specified . ci , confidence interval ; vzv , varicella - zoster virus 1 ; lcmv , lymphocytic choriomeningitis virus . for nonspecific cases of viral encephalitis , the frequency of hospitalizations ( noted by week ) indicated that some clustering of undiagnosed viral encephalitis had occurred during the 9 years of study since the upper prediction limits at 99.5% and 99.95% were exceeded in 6 weeks . three of the six clusters occurred closely together in 1993 ( weeks 45 , 47 , and 51 ) , which suggests an overall period of higher incidence during this time . the poisson regression model suggested clustering of encephalitis of unknown cause in district health authorities , and the total number of cases occurring in a week was greater than the prediction limit at 99.95% in three districts . the model did not identify any clusters when the period was extended to those occurring within 1 month . significant underreporting of viral encephalitis was seen in the routine systems when compared to hospital episode statistics . a total of 215 cases of acute infectious encephalitis were identified through noids between april 1 , 1989 , and march 31 , 1998 , compared to 6,414 identified in hospital episode statistics . this disparity implies that almost all ( 97% ) hospitalized cases were not formally reported . a total of 599 cases of viral encephalitis were identified through the laboratory reporting system from january 1 , 1990 , to december 31 , 1998 . fifty nine per cent of these cases were herpes ( 86 herpes simplex virus 1 , 48 herpes simplex virus 2 , and 219 subtype not known ) , 21% varicella and 4% adenovirus . the remaining 16% of cases included 26 cytomegalovirus , 15 epstein - barr virus , 20 measles virus , and 13 mumps . of herpes encephalitis , 19% ( 66/353 ) of cases occurred in children and 4.8% ( 17/353 ) in neonates . of herpes encephalitis cases in neonates that were typed , the number of laboratory reports of viral encephalitis increased from 52 in 1990 to 80 in 1991 , falling to 55 in 1993 , and rising again to a peak of 104 in 1998 . excluding those cases for which no specific icd-9 or icd-10 code exists and which could not have been identified in hospital episode statistics , 558 cases of viral encephalitis were reported through the laboratory system from january 1 , 1990 , to march 31 , 1998 . this total represents approximately 71% underreporting when compared to 1,867 hospitalizations of viral encephalitis with a specific infection reported ( table 4 ) . he s , hospital episode statistics ; herpes , herpes simplex virus ; vzv , varicella - zoster virus 1 ; lcmv , lymphocytic choriomeningitis virus . viral encephalitis was given as the final underlying cause of 622 deaths certified from january 1 , 1993 , to march 31 , 1998 . in that comparative period , hospital episode statistics showed 259 deaths , only 42% of those formally certified and reported to the office of national statistics . most ( 483/622 ) certified deaths from viral encephalitis did not have a specific diagnosis ; however , 139 deaths did . we compared the 139 deaths to the 112 identified in hospital episode statistics for which a specific diagnosis was recorded ( table 4 ) . the public health impact of viral encephalitis in england can not be clearly defined with existing data sources . during our study of viral encephalitis in england , we found that more cases of viral encephalitis were identified in hospital episode statistics than in routine surveillance systems . most cases requiring hospitalization are viral encephalitis with unknown etiology ; of those cases with a specific diagnosis recorded , herpes is the most common cause . adults account for the majority of case - patients , and rates have increased in those aged > 65 years . our analysis of cases of viral encephalitis without a specific diagnosis in hospital episode statistics identified clusters that would have otherwise gone undetected . the coding system used for the hospital diagnoses recorded in hospital episode statistics changed from icd-9 to icd-10 beginning on april 1 , 1995 . in some cases , diagnoses became less specific and in some cases more specific ( e.g. , herpes simplex virus was coded to herpes while vzv encephalitis had a specific icd-10 code from 1995 ) . in this study , we made some effort to compensate by including hospitalized cases with a diagnosis of chickenpox and an unspecified viral encephalitis infection . the change in coding did not appear to cause any major changes in the total number of hospitalized viral encephalitis cases . why are so few diagnoses in hospital episode statistics specific ? whether an appropriate investigation has been carried out , the extent to which a specific diagnosis is sought , and the quality of medical records affect the recording of specific diagnoses in hospital episode statistics . appropriate investigation is limited by the diagnostic techniques available ; virus isolation from cerebrospinal fluid or the brain can delay a diagnosis because culturing is slow , sensitivity may be poor , and obtaining specimens may require special techniques . determining the virus causing the encephalitis may not be seen as essential if all patients diagnosed with viral encephalitis are routinely given acyclovir , regardless of the virologic diagnosis . regional variation in the proportion of hospitalizations without a specific diagnosis a nonspecific diagnosis is recorded in hospital episode statistics if the laboratory results arrive after discharge . during the period of study , the rapid and highly sensitive pcr test was not widely used in england ; however , as the test becomes more widely available , the proportion of cases with a specific diagnosis in hospital episode statistics may increase . in this study , we found that adults with a specific diagnosis were older than those without a diagnosis , which is likely to reflect the age distribution of herpes , vzv , and other specific viral encephalitis diagnoses . a lower case - fatality rate , however , and mean length of stay in hospital suggests that these infections without a specific diagnosis are less severe . with the emergence of new diseases and availability of vaccines for some viruses , determining , routine data sources have been used alongside hospital episode statistics to determine the likely cause of hospitalizations without a specific diagnosis ( 26 ) . to minimize overestimating the public health cost of viral encephalitis , multiple consultant episodes of care for a patient were counted as a single infection if < 1 month elapsed between each . hospital episode statistics do not include strict case definitions , which could also lead to overestimating . for example , between january 1 , 1990 , and march 31 , 1998 , a total of 71 cases of measles were reported in hospital episode statistics , most in children < 5 years of age . these cases are unlikely to be true measles encephalitis since measles was rare in the 1990s following the introduction of the combined measles , mumps , and rubella vaccine in 1988 ( 27 ) . furthermore , none of the laboratory reports of measles encephalitis in this study were in infants , and only three were in children ( aged 7 , 11 , and 14 years ) . cases have been included in the laboratory data based on a single high serum antibody titer . had a stricter case definition been used , as has been suggested for measles ( 28 ) , even fewer cases of viral encephalitis would have been identified in the laboratory system . hospital episode statistics identified a lower number of deaths from viral encephalitis than the number reported routinely to the office of national statistics . deaths in hospital episode statistics are known to be incomplete since patients with encephalitis may die after discharge , and the death may be recorded in an episode that does not include encephalitis as a diagnosis and was thus missed in the data extraction ( 29 ) . some epidemics of encephalitis in england are undetected in routine surveillance systems and undiagnosed in hospital episode statistics . this situation suggests the potential for another emerging infectious disease , such as west nile infection , to occur in the u.k . ; existing routine surveillance systems would be incapable of detecting this ( 3032 ) . levels of disease in england are not as high as reported in a similar study in the united states ( 33 ) ; however , the proportion without a specific diagnosis is comparable . to improve surveillance , clinicians must be encouraged to report viral encephalitis , although notifications are not specific . other existing surveillance systems , such as the british pediatric surveillance unit ( available from : url : http://www.bpsu.inopsu.com/ ) , could be modified to include a specific diagnosis of viral encephalitis . alternatively , a sentinel surveillance system using existing laboratory networks and offering screening for a wide range of viruses could provide more accurate and timely data .
we analyzed hospitalizations in england from april 1 , 1989 , to march 31 , 1998 , and identified approximately 700 cases , 46 fatal , from viral encephalitis that occurred during each year ; most ( 60% ) were of unknown etiology . 
 of cases with a diagnosis , the largest proportion was herpes simplex encephalitis . 
 using normal and poisson regression , we identified six possible clusters of unknown etiology . over 75% of hospitalizations 
 are not reported through the routine laboratory and clinical notification systems , resulting in underdiagnosis of viral encephalitis in england . 
 current surveillance greatly underascertains incidence of the disease and existence of clusters ; in general , outbreaks are undetected . 
 surveillance systems must be adapted to detect major changes in epidemiology so that timely control measures can be implemented .