Prevalence of diphtheria and tetanus antibodies among adults in Singapore: a national serological study to identify most susceptible population groups

BACKGROUND

In view of waning antitoxin titres over time after the last vaccine dose against diphtheria and tetanus, we determined the immunity levels in adults to identify most susceptible groups for protection in Singapore.

METHODS

Our study involved residual sera from 3293 adults aged 18-79 who had participated in a national health survey in 2010. IgG antibody levels were determined using commercial enzyme-linked immunosorbent assay.

RESULTS

Overall, 92.0% (95% confidence interval [CI]: 91.1-92.9%) had at least basic protection against diphtheria (antibody levels ≥0.01 IU/ml), while 71.4% (95% CI: 69.8-72.9%) had at least short-term protection against tetanus (antibody levels >0.1 IU/ml). The seroprevalence declined significantly with age for both diseases; the drop was most marked in the 50- to 59-year age group for diphtheria and 60- to 69-year age group for tetanus. There was a significant difference in seroprevalence by residency for diphtheria (92.8% among Singapore citizens versus 87.1% among permanent residents; P = 0.001). The seroprevalence for tetanus was significantly higher among males (83.2%) than females (62.4%) (P < 0.0005).

CONCLUSIONS

It may be of value to consider additional vaccination efforts to protect older adults at higher risk for exposure against diphtheria and tetanus, particularly those travelling to areas where diphtheria is endemic or epidemic.

Readiness exercise to combat avian influenza

AIM

To examine the readiness of our hospital for the potential pandemic threat of avian influenza, we developed and implemented simulation case scenarios in our hospital.

METHODS

Two volunteers, who assumed the identity of 'actual' patients, were trained to simulate acute respiratory symptoms following a visit to an avian influenza-affected area, and their identities and locations were kept confidential prior to the readiness exercise. A team of auditors was stationed at high-risk areas to assess adherence to the use of personal protective equipment (PPE) and infection control procedures.

RESULTS

A total of 324 healthcare workers and 84 administrators participated in this hospital-wide exercise. Following disclosure of their symptoms, the 'patients' were masked and isolated in negative-pressure rooms. A quarantine order was enforced on 38 inpatients and 45 healthcare workers who were present in the affected wards at the time of the exercise, which mandated the use of PPE. Although all affected healthcare workers were competent in the use of PPE, we observed breaches in PPE and isolation procedures in eight medical and nursing students, and 10 healthcare attendants. The exercise concluded after H5N1 tests returned negative.

CONCLUSION

We recommend the use of case simulation as an effective means of assessing potential breaches in infection control procedures.

A national study of the epidemiology of pneumococcal disease among hospitalised patients in Singapore: 1995 to 2004

INTRODUCTION

Infections with Streptococcus pneumoniae cause significant morbidity and mortality. In this study, we describe the epidemiology of pneumococcal disease based on hospitalisation rates for all age groups in Singapore. This is important for evaluating prevention and control strategies of pneumococcal disease.

METHODS

We conducted a retrospective study of hospitalisation cases admitted to all public and private hospitals from 1995 to 2004. 4,275 hospitalisation records were extracted, based on the International Classification of Diseases, Ninth Revision (ICD-9) codes for pneumococcal disease. We analysed the demographics, type of pneumococcal disease, length of stay and case fatality of these cases.

RESULTS

Our study showed that the mean annual hospitalisation rate for pneumococcal disease was 10.9 per 100,000 population from 1995 to 2004. The mean annual hospitalisation rate was highest in the young and the elderly.

CONCLUSION

Baseline information on the epidemiology of pneumococcal disease is important for the formulation and evaluation of a national prevention and control programme.

Oyster-associated outbreaks of Norovirus gastroenteritis in Singapore

Outbreaks of gastroenteritis associated with the consumption of raw imported half-shelled frozen oysters occurred in Singapore between 16 Dec 2003 and 04 Jan 2004. A total of 305 cases were reported with clinical symptoms of diarrhoea (94%), abdominal cramps (72%), vomiting (69%) and fever (54%). The median incubation period was 30.8h and the duration of illness was 2-3 days. The overall relative risk of oyster consumption was 14.1 (95% CI: 8.3-24.0, P<0.001). Stool and oyster samples tested negative for common bacterial pathogens, including Vibrio parahaemolyticus. However, stool samples were positive for the presence of Norovirus group II RNA via RT PCR while oyster samples indicated the presence of Norovirus particles by electron microscopy. The clinical and epidemiological features were suggestive of Norovirus gastroenteritis and were subsequently confirmed by laboratory tests of stools and implicated oysters. Steps have been taken to ensure that food outlets do not thaw frozen oysters and serve them raw.

Hemagglutinin-neuraminidase sequence and phylogenetic analyses of mumps virus isolates from a vaccinated population in Singapore

During 1999-2000, a sustained mumps outbreak in the highly vaccinated population in Singapore was attributed to vaccine failure associated with the Rubini vaccine strain. To explain this phenomenon, the complete nucleotide and amino acid sequences of the hemagglutinin-neuraminidase (HN) gene of eight mumps virus isolates from patients with parotitis in Singapore were determined and compared with those of known vaccine strains. Phylogenetic trees constructed on the basis of HN nucleotide and amino acid sequences showed that the Singapore mumps virus isolates were more closely related to the Urabe strain and belonged to a different cluster from the Rubini and Jeryl-Lynn strains. The Rubini vaccine showed only approximately 93% nucleotide and approximately 96% amino acid sequence similarity to Urabe and Singapore isolates. Compared with the vaccine strains, six of the eight isolates lacked the extracellular glycosylation site at residues 400-402. Other significant amino acid disparities (e.g., at residue 354) may also affect the antigenic properties of the HN protein. These findings suggest that the evolution and adaptation of the currently circulating mumps virus strains in the community has led to the emergence of genetically distinct viral strains. The low vaccine efficacy of the Rubini strain represents a major reason for the recent mumps resurgence and failure of mumps immunization in Singapore.

Stroke disease management--a framework for comprehensive stroke care

Disease management is an approach to patient care that coordinates medical resources for the patient across the entire healthcare delivery system throughout the lifetime of the patient with the disease. Stroke is suitable for disease management as it is a well-known disease with a high prevalence, high cost, variable practice pattern, poor clinical outcome, and managed by a non-integrated healthcare system. It has measurable and actionable outcomes, with available local expertise and support of the Ministry of Health. Developing the programme requires a multidisciplinary team, baseline data on target populations and healthcare services, identification of core components, collaboration with key stakeholders, development of evidence-based clinical practice guidelines and carepaths, institution of care coordinators, use of information technology and continuous quality improvement to produce an effective plan. Core components include public education, risk factor screening and management, primary care and specialist clinics, acute stroke units, inpatient and outpatient rehabilitation facilities, and supportive community services including medical, nursing, therapy, home help and support groups for patients and carers. The family physician plays a key role. Coordination of services is best done by a network of hospital and community-based care managers, and is enhanced by a coordinating call centre. Continuous quality improvement is required, with audit of processes and outcomes, facilitated by a disease registry. Pitfalls include inappropriate exclusion of deserving patients, misuse, loss of physician and patient independence, over-estimation of benefits, and care fragmentation. Collaboration and cooperative among all parties will help ensure a successful and sustainable programme.

An outbreak of multidrug-resistant Salmonella enterica subsp. enterica serotype Typhimurium, DT104L linked to dried anchovy in Singapore

Multidrug-resistant Salmonella enterica subsp. enterica serotype Typhimurium DT104L was first reported in Singapore from mid-July to mid-October 2000. Salmonella strains isolated from clinical laboratories were submitted to a reference laboratory for serotyping, phage-typing and pulsed-field gel electrophoresis (PFGE) using XbaI restriction endonuclease. An epidemiological investigation was conducted to determine the source of infection and mode of transmission using a structured questionnaire. A total of 33 cases involving mainly infants and toddlers were detected in the 3-month long outbreak. The outbreak strain was of the R-type ACGSTSu, i.e. resistant to ampicillin, chloramphenicol, gentamicin, streptomycin, tetracycline and sulphonamide. PFGE showed all isolates had an indistinguishable pattern, indicating a common source of infection. Consumption of imported dried anchovy was found to be the vehicle of transmission after adjusting for all confounding variables in the case-control study using stepwise logistic regression (OR 25.6; 95% CI 3.9-167.9; P = 0.001). Imported dried seafood should be properly processed, packed, labelled, and thoroughly cooked to prevent transmission of multidrug-resistant S. Typhimurium.

A survey of Nipah virus infection among various risk groups in Singapore

Following the Nipah virus (NV) outbreak in March 1999 in Singapore, a serological survey was undertaken to screen individuals potentially exposed to NV. Blood samples were tested for NV IgM, IgG and neutralizing antibodies. Twenty-two (1.5%) of 1469 people tested had antibodies suggesting NV infection. Although 12 of the 22 infected people (54.6%) were symptomatic, the remaining 10 (45.4%) were clinically well and had no past history of compatible pulmonary or neurological disease. Clinical and serological findings suggested three people had been infected with NV before the outbreak was recognized. All those who were infected were male abattoir workers. None of the people who had contact with horses, and no healthcare workers exposed to infected patients and their specimens had detectable antibodies. This study provides evidence that NV causes asymptomatic infection. All of the antibody positive individuals had direct contact with pigs and there was no evidence of human to human transmission.

Helicobacter pylori in familial clusters based on antibody profile

Studies have shown a high prevalence of Helicobacter pylori infection in close communities and that intrafamilial spread during early childhood may be a route of transmission. A total of 72 household members from 21 families were enrolled in this study. Sera from individuals showed 50/72 (69.4%) seropositive for IgG against H. pylori by ELISA. Western blots showed diversity in the protein profiles with molecular masses ranging from approximately 8 to 130 kDa. Cohen's kappa statistical analysis of the blot patterns showed that nine families demonstrated similar profiles (100%), while 4 other families showed varying similarities (17-50%). The results support the hypothesis of intrafamilial transmission of H. pylori. Furthermore, serological studies can be used as an effective approach to determine the familial status in relation to H. pylori infection.

Intra-familial evidence of horizontal transmission of hepatitis B virus surface antigen mutant G145R

OBJECTIVES

To provide intra-familial evidence on the horizontal transmission of hepatitis B virus surface antigen (HBsAg) mutant G145R.

METHODS

Serum samples from family members of 10 vaccinated infants who carried this G145R mutant were collected. The presence of the mutant was analysed by polymerase chain reaction (PCR) and sequencing.

RESULTS

The G145R mutant was identified in family members of three of the 10 infants. In family 1, the mutant found initially in child 1 was identified in another child and the father. In families 2 and 3, the G145R mutant detected previously in child 1 was detected in the father. Additional mutations in HBsAg were identified in at least two members in family 1 and 2, suggesting horizontal transmission of the mutant among them. The G145R mutant was found in samples with high levels of neutralizing antibody against HBV (anti-HBs). In addition, liver damage was seen in one G145R carrier infant.

CONCLUSIONS

The G145R mutant could be transmitted horizontally among family members, and this could occur in the presence of high levels of anti-HBs. Improvement of detection system for the G145R and other HBsAg mutant will be needed for their effective control.

Blood lead levels of a population group not occupationally exposed to lead in Singapore

A survey was conducted between 1995 and 1997 to assess the impact of introduction of unleaded petrol and other public health measures on the blood lead level of the population. The geometric mean blood lead level of 269 government employees as determined by graphite furnace atomic absorption spectroscopy, was 66.0 microg/l, much lower than that recorded before introduction of lead-free petrol. Using multiple regression analysis, factors significantly associated with blood lead levels were: exposure to traffic, age (>50 years) and active smoking. Passive smoking, exposure to recent paint work, consumption of alcohol and traditional medicine were found not to be significantly associated with the blood lead level.

Risk factors for Nipah virus infection among abattoir workers in Singapore

During 10-19 March 1999, 11 workers in 1 of 2 Singaporean abattoirs developed Nipah-virus associated encephalitis or pneumonia, resulting in 1 fatality. A case-control study was conducted to determine occupational risk factors for infection. Case patients were abattoir A workers who had anti-Nipah IgM antibodies; control subjects were randomly selected abattoir A workers who tested negative for anti-Nipah IgM. All 13 case patients versus 26 (63%) of 41 control subjects reported contact with live pigs (P=.01). Swine importation from Malaysian states concurrently experiencing a Nipah virus outbreak was banned on 3 March 1999; on 19 March 1999, importation of Malaysian pigs was banned, and abattoirs were closed. No unusual illnesses among pigs processed during February-March were reported. Contact with live pigs appeared to be the most important risk factor for human Nipah virus infection. Direct contact with live, potentially infected pigs should be minimized to prevent transmission of this potentially fatal zoonosis to humans.

Resurgence of mumps in Singapore caused by the Rubini mumps virus vaccine strain

The measles, mumps, and rubella vaccine containing the highly attenuated Rubini mumps virus strain conferred no protection against acute parotitis in vaccinated children in Singapore. Its introduction into the national childhood immunisation programme has resulted in a reduction in the seroprevalence of mumps to prevaccination levels.

Epidemiological surveillance of melioidosis in Singapore

During the period 1989 to 1996, a total of 372 cases of melioidosis, with 147 deaths, were reported, giving a mean annual incidence rate of 1.7 per 100,000 population and a case-fatality rate of 39.5%. Majority (89%) of the clinical cases were confirmed by culture of Burkholderia pseudomallei, while the others were presumptive cases based on a single blood specimen with an indirect haemaglutination (IHA) antibody titre of > or = 1:16. The highest incidence rate was reported in those aged 45 years and above (5.7 per 100,000 population), males (2.8 per 100,000 population), and Indian ethnic group (3.0 per 100,000 population). Cases were distributed throughout the island all year round. There was no correlation with rainfall. Most of the cases (77.4%) had other concurrent medical conditions, the most common being diabetes mellitus (57.5%). Factors significantly associated with a higher case-fatality rate were age (55 years and above), septicaemia, smoking history and heart or renal failure. The overall case-fatality rate has been declining from 60% in 1989 to 27% in 1996 due to a greater awareness among medical practitioners to diagnose and treat the disease early. The overall seroprevalence of IHA antibody (titre of > or = 1:16) among asymptomatic population groups was 0.2%. B. pseudomallei isolated from clinical specimens were sensitive to imipenem (100%), ceftazidime (99.1%), piperacillin (99.7%), ampicillin-clavulanate (98.5%), minocycline (97.4%), chloramphenicol (94.3%), doxycycline (94.3%) and tetracycline (93.9%). Of 395 samples of soil collected during epidemiological investigation of reported cases, 1.8% were positive for B. pseudomallei.

Epidemiology of sick building syndrome and its associated risk factors in Singapore

OBJECTIVES

To investigate the occurrence of sick building syndrome in a tropical city, and its relation to indoor air quality and other factors.

METHODS

2856 office workers in 56 randomly selected public and private sector buildings were surveyed. The study consisted of a self administered questionnaire assessing symptoms and perception of the physical and psychosocial environment, inspection of the building plans and premises, and measurement of temperature, relative humidity, respirable particles, chemicals, bioaerosols, and other variables.

RESULTS

Symptoms typical of the sick building syndrome were reported in 19.6% of the respondents. Multivariate modelling substantiated contributions associated with low thermal comfort, high work related stress, too much noise, a history of allergy or other medical conditions, poor lighting, young employees, and female sex. Measurements of indoor air quality or ventilation were not found to be reliable predictors of the symptoms.

CONCLUSION

The survey confirmed the presence of sick building syndrome and its risk factors in the tropics. A biopsychosocial approach to the problem involving symptomatic treatment, environmental control, good ergonomic design, and stress management is recommended.

Sick building syndrome: an emerging stress-related disorder?

BACKGROUND

This study examined the role of work-related psychosocial stress in the aetiology of sick building syndrome and tested the hypothesis that in buildings with no recognized environmental problems, health complaints typical of the syndrome were primarily stress-related.

METHOD

A case-control study used data from confidential questionnaires to assess symptoms and perception of the physical and psychosocial environment among 2160 subjects in 67 offices. Working conditions were also inspected and indoor air quality monitored.

RESULTS

We found an incremental trend in prevalence of sick building syndrome among office workers who reported high levels of physical and mental stress, and decreasing climate of co-operation. This association was confirmed after multivariate adjustment for significant personal and environmental exposure factors. Using a subsample, we further modelled interaction between stress and the other covariates but none achieved statistical significance.

CONCLUSION

Our study confirmed stress to be a significant and independent determinant of the health complaints, and that symptoms compatible with the sick building syndrome in many cases were stress-related. Our findings underscore the importance of personal and organizational stress management to prevent ill health at the office.

Prevention and control of hepatitis B virus infection in Singapore

Hepatitis B virus (HBV) accounted for 24% to 54% of the reported acute viral hepatitis cases in Singapore from 1982 to 1996. The prevalence of HBV infection, as indicated by the presence of markers of HBV, increased from 9.3% in children below 5 years of age to 54.6% in adults above 55 years. The overall hepatitis B surface antigen (HBsAg) prevalence was 5.7% for males and 3.4% for females, with the highest rate among the Chinese. About 39% of the HBsAg carriers were hepatitis B 'e' antigen positive. The main mode of transmission during the first year of life was perinatal, with 43% of the babies born to HBsAg-positive mothers developing the carrier state. Horizontal transmission within the infected household was significantly associated with sharing of personal and household articles. Based on the findings of seroprevalence surveys in various population groups and clinical trials on the safety, immunogenicity and efficacy of various doses and schedules with the plasma-based and yeast-derived hepatitis B vaccines in newborn babies, a national childhood hepatitis B vaccination programme was formulated and implemented in phases, starting with babies born to carrier mothers on 1 October 1985 and finally extending to all newborns on 1 September 1987. The hepatitis B prevention and control programme has been successful. During the period 1994 to 1996, more than 90% of children completed the full schedule of immunisation by below one year of age, and 85% had evidence of vaccination at school entry at age six. Follow-up of 2 cohorts of vaccinated children showed that perinatal transmission has been reduced by 80% to 100%. Horizontal transmission has also declined through other public health measures. The efficacy of the hepatitis B vaccine and the adequacy of reduced doses in the long-term protection of chronic carrier state have been shown in children and adults. The incidence of acute hepatitis B has declined from 10.4 per 100,000 in 1985 to 4.8 per 100,000 in 1996. There is a noticeable reduction in HBsAg prevalence in selected population (school children, national servicemen and antenatal women). The age-standardised incidence rate of primary liver cancer among males had also dropped from 27.8 per 100,000 per year during 1978 to 1982 to 19.0 per 100,000 per year during 1988 to 1992.

Surveillance of legionellosis and Legionella bacteria in the built environment in Singapore

During the period 1986 to 1996, a total of 258 sporadic cases of community-acquired legionellosis was reported, giving a mean annual morbidity rate of 0.9 per 100,000 population. Majority of the cases (91.5%) was serologically diagnosed by the indirect fluorescent antibody (IFA) test against Legionella pneumophila based on single sera with a titre of > or = 1:1024. The others were confirmed by a four-fold increase in IFA titre to a minimum of 1:128 (7.4%) and by culture (1.1%). Cases were reported predominantly among males, ethnic Indians, the elderly and those with concurrent medical conditions. The overall case-fatality rate was 14.7%, with the rate significantly higher among those aged > or = 45 years (21%), smokers (20.4%) and those with concurrent medical conditions (22.5%). Seroepidemiological studies among the healthy population based on an IFA titre of > or = 1:32 showed a prevalence of between 10.3% and 21.9%. Of the various occupational groups investigated, high seroprevalence rates were found in foreign construction workers (52.6%) and occupants of a building investigated for an unusual building-related illness (76.1%). Environmental surveillance of Legionella bacteria showed that 36% of cooling towers, 15% to 19% of decorative fountains and waterfalls and 2% of spa pools were positive. The majority (85%) of environmental isolates was identified as Legionella pneumophila: 54.8% of serogroup 1 and 5% of Pontiac subtype.

A nationwide outbreak of coconut-associated paratyphoid A fever in Singapore

A large outbreak of 167 cases of paratyphoid A was reported in Singapore during the period 15 February to 4 May 1996. Cases were distributed all over the country and no specific food establishment was implicated. The attack rate was highest among the Indian ethnic group between the ages of 25 and 34 years. A case-control study showed that consumption of a variety of food items in which imported deshelled coconut was used as an ingredient was statistically significantly associated with the illness (P < 0.001). As soon as the vehicle of transmission was suspected, the import of deshelled coconut was banned. No further cases were reported four weeks after the ban. The outbreak highlighted the need to maintain a high degree of vigilance against the introduction of infectious diseases through imported food.

Local transmission of Plasmodium vivax malaria in Singapore

A localised outbreak involving 19 cases and two carriers of vivax malaria was reported at Dairy Farm from May to June 1996. Epidemiological investigations showed that the outbreak was caused by the introduction of two foreign workers with imported relapsing vivax malaria into this malaria-receptive area. Transmission of infection occurred amongst 17 other foreign workers residing in the locality and two local residents staying at a condominium nearby. The outbreak was rapidly brought under control through active case finding, isolation and treatment of infected persons, epidemic vector control measures, and improved drainage to prevent Anopheles maculatus breeding. Were it not for the very thorough epidemiological investigations in this episode, foreign workers and local residents with a recent travel history would have been misclassified as imported and the outbreak missed. Medical practitioners should always consider the possibility of malaria in a foreign worker presenting with fever and notify the case as soon as the disease is suspected.

Dengue--a re-emerging infectious disease in Singapore

Despite its well-established integrated nationwide Aedes mosquito control programme which incorporates source reduction, public health education and law enforcement, Singapore has not been spared from the regional resurgence of dengue. The disease incidence has been increasing from 9.3 per 100,000 in 1988 to 102.7 per 100,000 in 1996 at the time when the Aedes house index (HI) has dropped to around 1% from > 25% in the 1960s. Majority of the cases reported from 1990 to 1996 were dengue fever (DF); dengue haemorrhagic fever (DHF) constituted only 6.7%. The case-fatality rate was 0.1% with 13 (81.3%) of 16 serologically confirmed deaths above 19 years of age. The median age has shifted from 14 years in 1973 to 27 years in 1996. The proportion of primary infections also increased from about one-third in 1990 to nearly half in 1996. All four dengue serotypes have been detected from infected persons with dengue 2 predominating in 1990, 1991 and 1993, dengue 3 in 1992 and 1994 and dengue 1 in 1995 and 1996. The disease incidence was significantly correlated with Aedes aegypti HI and residents of compound houses had a significantly higher rate of infection as well as a higher morbidity rate compared with dwellers of high-rise public housing estates. Seroprevalence surveys confirmed the low level of dengue transmission. The immunity level of the general population has been declining with only 6.4% of children and young adults below 25 years of age possessing haemagglutination-inhibition antibody to dengue 2. It would appear that the successful vector control programme over the last two decades has brought about a paradoxical situation in that outbreaks tend to occur more frequently and with even greater intensity because of the low herd immunity of the population. Until the dengue vaccine is commercially available for mass immunisation of the population, community-based integrated control of Aedes aegypti remains the key to the prevention and control of DF/DHF.

A shipyard outbreak of salmonellosis traced to contaminated fruits and vegetables

A large outbreak of food poisoning occurred in September 1996 and involved at least 116 workers at a shipyard in Jurong. Salmonella weltevreden was isolated from the stool specimens of 24 hospitalised cases and three food handlers, giving a total of 27 bacteriologically confirmed cases (25 symptomatic and two asymptomatic) in this outbreak. Based on a case-control analysis, a number of food items consumed on 23 and 24 September 1996 were implicated in the outbreak, viz. a vegetable dish (P < 0.01), watermelon juice (P < 0.01) and cut watermelon (P < 0.001), papaya (P < 0.001) and pineapple (P < 0.05). The median incubation period was 38 hours and the median duration of illness was two days. Four samples of cut fruits obtained from a food stall at the canteen were found to be positive for Salmonella weltevreden. (The antibiogram of all the strains isolated from food samples and stool cultures was similar, consistent with a common source of infection.) Extensive investigation suggested that contamination may have occurred through unauthorised use of industrial water for washing the fresh produce. Early recognition of the outbreak and prompt implementation of control measures prevented further spread of infection.

An institutional outbreak of Salmonella enteritidis in Singapore

A large outbreak of food poisoning occurred in Singapore in March 1995 when a total of 188 inmates in an institution was taken ill. Salmonella enteritidis was isolated from the stool cultures of 35 inmates (16 symptomatic and 19 asymptomatic). All the isolates were of the serotype profile 0:1, 9, 12 and H:g, m (antigen phase I); all were sensitive to ampicillin, ceftriaxone, chloramphenicol, co-trimoxazole and ciprofloxacin. Plasmid profile analysis and restriction enzyme fragmentation patterns (REFPs), as generated with EcoRI and HindIII, of a 60 kb plasmid obtained from these isolates were all identical, confirming that the outbreak resulted from a single source of infection. Stratified statistical analysis of food-specific attack rates strongly implicated imported canned luncheon pork consumed by the inmates on 26 March 95 as the single most probable cause of the food poisoning [p < 10(6), Mantel-Haenszel weighted odds ratio (OR) = 14.33; 95% confidence interval (CI) = 6.20-33.15]. The median incubation period of this outbreak was 19.3 hours and the median duration of illness was three days. The outbreak was rapidly brought under control through prompt implementation of epidemic control measures which comprised active search for diarrheal cases, rectal swabbing of asymptomatic inmates, isolation of those found to be infected, and maintenance of a high standard of personal, food and environmental hygiene.

Molecular epidemiology of hepatitis B virus vaccine variants in Singapore

Perinatal infection with variants of hepatitis B virus occurs despite combined immunoprophylaxis with hepatitis B immunoglobulin and currently licensed plasma-derived and recombinant yeast hepatitis B vaccines. Several variants have been detected during a large study of infants born to carrier mothers in Singapore. The most frequent variant was a virus in which a single amino acid substitution Gly to Arg occurred at amino acid position 145 of the outer protein coat of the virus. Similar mutations have been described in Italy, Japan, the USA and a number of other countries. The emergence of antibody escape mutants is a cause for concern for the detection of virus and possibly for future immunization programmes.

Prevalence of hepatitis B virus (HBV) infection in Singapore men with sexually transmitted diseases and HIV infection: role of sexual transmission in a city state with intermediate HBV endemicity

STUDY OBJECTIVES

To describe the prevalence of hepatitis B virus (HBV) infection in patients with sexually transmitted diseases (STD) and human immunodeficiency virus (HIV) infection, and to determine the role of sexual transmission of HBV infection in Singapore.

DESIGN

A cross sectional study of all consecutive men presenting with a new episode of STD at a government outpatient clinic and all men with HIV infection on routine follow up at a government hospital. The prevalence of various HBV markers was compared with that of healthy males aged 15 years and above (controls).

SETTING

Singapore, a city state of intermediate HBV endemicity.

SUBJECTS

These comprised 497 STD patients, 47 HIV infected patients, and 418 controls.

MAIN RESULTS

The overall seroprevalences of HBV infection in STD patients, HIV infected patients, and control subjects were 41.2%, 61.7%, and 33.3%, respectively (p < 0.001). The seroprevalences of hepatitis B surface antigen (6.2%, 8.5%, and 4.5%, respectively) were comparable in the three groups. Using stepwise logistic regression analysis, the adjusted seroprevalences of HBV infection in STD and HIV infected patients were respectively 2.4 times (95% confidence interval (CI), 1.7, 3.3) and 3.3 times (95% CI 1.7, 6.3) higher than in controls. HBV infection rates were higher among Chinese (odds ratio (OR), 1.9; 95% CI 1.6, 3.4) than non-Chinese, and among those aged 25-34 years (OR 2.4; 95% CI 1.6, 3.4), 35-44 years (OR 3.9; 95% CI 2.5, 5.9), and 45+ years (OR 6.2; 95% CI 3.8, 10.2) than in those aged 15-24 years. Sex related factors significantly associated with higher infection rates, independent of age and ethnic group, were reactive VDRL test (OR 2.4; 95% CI 1.2, 4.7), participation in anal intercourse (OR 2.3; 95% CI 1.2, 4.3), and having 10 or more lifetime sexual partners (OR 1.5; 95% CI, 1.0, 2.1).

CONCLUSION

The importance of sexual transmission of HBV in an area of intermediate HBV endemicity was confirmed. Patients attending STD clinics should be routinely screened for HBV markers and those found to be seronegative should be strongly advised to be immunised against this virus.

Long-term immunogenicity and efficacy of a reduced dose of plasma-based hepatitis B vaccine in young adults

A cohort of seronegative preclinical medical and dental students and another cohort of seronegative national service recruits who were immunized intramuscularly with a reduced dose (10 micrograms) of plasma-based hepatitis B vaccine (Merck, Sharp & Dohme) at the start of the study and at 1 month and 6 months thereafter were followed up for 5 years and 6 years, respectively. Among the medical and dental students, antibody to hepatitis B surface antigen (anti-HBs) ( > or = 10 mlU/ml) was detected in 81% of the vaccinees at the end of the 5-year follow-up and the geometric mean titre (GMT) had dropped from 412.6 mlU/ml one year after completion of vaccination to 174.9 mlU/ml after 5 years. Antibody to hepatitis B core antigen (anti-HBc) was detected in 0.4-1.0% of the vaccinees but none was positive for hepatitis B surface antigen (HBsAg) during the follow-up period. Among the national servicemen, the anti-HBs seroconversion rate and GMT were considerably lower than those of the preclinical medical and dental students. At the end of the 6-year follow-up, 55% of the vaccinees were positive for anti-HBs ( > or = 10 mlU/ml) and the GMT had dropped from 80.7 mlU/ml one year after completion of vaccination to 30.4 mlU/ml after 6 years. Anti-HBc was detected in 8 (2.7%) and transient HBs antigenaemia in 2 (0.7%) of 293 vaccinees after 4 years.(ABSTRACT TRUNCATED AT 250 WORDS)

Genome fingerprinting of Salmonella typhi by pulsed-field gel electrophoresis for subtyping common phage types

The genomic DNA of 39 strains of Salmonella typhi isolated from local residents and patients who had visited countries in the Asian region was analysed for restriction fragment length polymorphisms (RFLP). Pulsed-field gel electrophoretic (PFGE) analysis of Xba I- and Spe I-generated genomic restriction fragments established 22 PFGE types whereas phage typing differentiated the 39 isolates into 9 distinct phage types. This study showed that PFGE is more discriminatory than phage typing as it is capable of subtyping S. typhi strains of the same phage types. Genetic relatedness among the isolates was determined. Seven major clusters were identified at SABs of > 0.80 and the remaining 13 isolates were distributed into minor clusters which were related at SABs of less than 0.80. In conclusion, PFGE analysis in conjunction with distance matrix analysis served as a useful tool for delineating common S. typhi phage types of diverse origins from different geographical locales and separated in time.

Low prevalence of antibody to human parvovirus B19 in Singapore

A seroepidemiological survey was conducted in Singapore to assess the prevalence of antibody to human parvovirus B19. Sera were collected from 600 healthy individuals between 6 months and over 50 years of age and tested for IgG antibody against B19 virus by antigen capture indirect immunosorbent assay. The overall seropositivity rate was 16.2%. All the children under 5 years of age were seronegative. Antibody prevalence increased gradually from 3.5% in school children (5-14 years of age) to 7.7% in age group 15-19 years and then to 10.3% in young adults (20-24 years of age). In the age group 25-34 years the prevalence was 28% and in the age group over 35 years 65% had parvovirus B19 antibody. The results of the survey indicate that there has been very low incidence of B19 virus infection in Singapore during the last two decades.

Epidemiological investigations into an outbreak of building-associated illness in Singapore

In August 1991, the Ministry of the Environment was informed of the occurrence of illnesses suspected to be related to a modern, multi-story office building. Investigations revealed that 34 of the occupants on two floors of the building had non-specific ailments such as eye, throat or skin irritation which caused acute discomfort but resolved upon leaving the premises. Case-control study identified a number of factors significantly associated with the illness; viz. a personal history of allergy, asthma or sinusitis; a high level of work-related stress; and a lack of thermal comfort. One feature of the affected offices was numerous high partitions which tended to obstruct air movement. Adjustments of central temperature to compensate for complaints of "stuffiness" had resulted in wide fluctuations of temperature and relative humidity. Indoor air quality measurements also revealed high bacterial counts, a condition aggravated by the raised relative humidity and inadequate air movement. The problems gradually resolved following major modifications at the offices and air-handling units. This episode confirmed the need for further studies of building-associated illness in the tropics.

Vibrio cholerae 0139 'Bengal' in Singapore

Vibrio cholerae 0139 was isolated from five patients with cholera-like illness. All were imported cases. Laboratory investigations found our five isolates in show similar morphological, biochemical and serological characteristics to the V. cholerae 0139 strains causing epidemics in Bangladesh and India. Our isolates were toxin producers resistant to streptomycin and co-trimoxazole. No local transmission was known to have occurred following introduction of these imported cases.

Prevalence of hepatitis A virus infection among sewage workers in Singapore

To determine whether or not occupational exposure to sewage is associated with a higher seroprevalence of hepatitis A virus (HAV) infection, 600 sewage workers in Singapore were tested for total (IgG and IgM) antibody to HAV by enzyme immunoassay. Using logistic regression with stepwise procedure, the adjusted seroprevalence of sewage workers was 2.2 times higher than that of another non-occupationally exposed population group. Seroprevalence was significantly correlated with age and educational levels, the association being independent of the occupational association. The epidemiological data in the study show that sewage workers have an increased occupational risk of acquiring HAV infection and should be protected by active immunization.

Immunogenicity of recombinant yeast-derived hepatitis B vaccine in nonresponders to perinatal immunization

OBJECTIVE

Nonresponse to hepatitis B vaccine in the perinatal period occasionally occurs. This report documents the results of reimmunization of nonresponders to perinatal immunization.

DESIGN

From a cohort of 1154 infants immunized with plasma-based vaccine in the perinatal period and followed up for more than 8 years, 45 nonresponders were identified. These children were reimmunized at 4 years of age. Each child received a yeast-derived recombinant hepatitis B vaccine on a 0-, 1-, and 5-month schedule, 33 children with 10-micrograms and 12 with 5-micrograms doses. Blood was sampled 1 month after the third vaccination and thereafter at 1, 2, and 4 years.

SETTING

The follow-up clinic where the cohort of children was regularly seen.

PATIENTS

Forty-five 4-year-old children who had no antibody to hepatitis B despite perinatal immunization.

MAIN OUTCOME MEASURE

Antibody levels to hepatitis B surface antigen.

RESULTS

Seroconversion with titers higher than 10 mIU/mL occurred in all children. More than 70% still had titers higher than 10 mIU/mL 4 years after vaccination.

CONCLUSION

Nonresponders to perinatal hepatitis B vaccination respond well to subsequent vaccination.

Seroepidemiology of hepatitis A and hepatitis B virus infection in a Gurkha community in Singapore

The overall prevalence of hepatitis A virus (HAV) infection among the Gurkhas and their families stationed in Singapore was 83.2%, with antibody to HAV detected in 67 (38.1%) of 176 children under 5 years of age, 121 (76.6%) of 158 children between 5 and 14 years, and 600 (97.9%) of 613 older children and adults over age 15 years. Children born in Singapore (102/223 or 45.7%) had a significantly lower crude prevalence than those born in Nepal (85/110 or 77.3%) (P < 0.000001). The age-adjusted difference in prevalence was also statistically significant (P < 0.001). Susceptible children had a high risk of acquiring HAV infection when they travelled to Nepal. As for hepatitis B virus (HBV) infection, the prevalence of hepatitis B surface antigen (HBsAg) was 2.8%, with a higher rate in males (20/560 or 3.6%) than in females (6/384 or 1.6%). The highest HBsAg prevalence (4/26 or 15.4%) was in young adult Gurkhas 15-19 years of age. No significant difference in the crude prevalence of HBV markers was noted between children born in Singapore (45/224 or 20.1%) and those born in Nepal (19/107 or 17.8%). Even after age adjustment, the difference remained insignificant. The prevalence of HBV markers of children of seropositive parents (27/156 or 17.3%) was not significantly higher than that of seronegative parents (19/131 or 14.5%). Married men had a significantly higher age-adjusted prevalence of infection than unmarried men (45.4% and 32.4%, respectively; P < 0.05). Seropositivity of one spouse was significantly correlated with the seropositivity of the other (P < 0.05), suggesting a role for heterosexual transmission of HBV infection.

A recent epidemic of acute hemorrhagic conjunctivitis

The coxsackievirus A24 variant was implicated in four of six major acute hemorrhagic conjunctivitis outbreaks in Singapore since its discovery in 1970. Hela cell suspension in 24-well flat-bottom tissue culture plates was a satisfactory alternative to monolayer cells grown in test tubes for virus isolation. Respiratory illness occurred in 20 of 98 patients with acute hemorrhagic conjunctivitis. Apart from conjunctival secretions, respiratory and oral transmission of coxsackievirus A24 variant would explain the rapid and extensive spread of acute hemorrhagic conjunctivitis during an outbreak.

Epidemiology of sudden unexpected death syndrome among Thai migrant workers in Singapore

A total of 235 cases of sudden unexpected death syndrome (SUDS) among apparently healthy male Thai migrant workers in Singapore were reported between 1982 and 1990. Most of the deaths occurred during sleep and 13% were not sleep-related. The median age at the time of death was 33 years and the median interval between arrival and death was 8 months. These deaths occurred singly and sporadically throughout the year. Post-mortem examination revealed few abnormal findings except for haemorrhagic congestion or oedema of the lungs. There were moderate to severe intra-alveolar haemorrhages with some evidence of myocarditis or pneumonitis. Preliminary findings of serial sections of the hearts indicate evidence of anomalies in the cardiac conduction system. Epidemiological investigations showed that a family history of similar deaths and serological evidence of current or recent infection with Pseudomonas pseudomallei were significantly associated with SUDS. Extensive biochemical and toxicological investigations were inconclusive. There was no evidence of chronic deficiency in thiamine or potassium among the healthy Thai workers living and working in the same conditions as the cases, and no significant abnormalities were detected on electrocardiographic examination. As these migrant workers experienced various psychosocial problems which could stem from maladjustment to an urban environment, separation from the family, burden of debts and long hours of work, stress could be a precipitating factor for SUDS.

Epidemiology of typhoid fever in Singapore

A total of 1452 cases of typhoid fever was notified in Singapore from 1980-9. The morbidity rates of indigenous cases showed a steady decline from 5.9 per 100,000 population in 1980 to 1.2 per 100,000 population in 1989. The mean case fatality was 0.8%. Children, adolescents and young adults were most susceptible to typhoid fever. There was no significant difference in morbidity rates between the major ethnic groups. The vast majority of indigenous cases were sporadic while outbreaks accounted for almost one third of them. Food was the main vehicle of transmission. The commonest indigenous phage types were B1, D1 and A. Antimicrobial resistance was infrequently seen. The proportion of imported cases rose from 32% in 1980 to 72% in 1989. Almost half (48.5%) of all imported cases were local residents who contracted typhoid fever while travelling in endemic countries. As imported cases assume greater importance in the epidemiology of typhoid fever in Singapore, further drop in typhoid fever incidence would require reduction of travel-related cases through greater awareness of food hygiene and effective vaccination.

Epidemiology and control of an outbreak of typhoid in a psychiatric institution

An outbreak of typhoid caused by Salmonella typhi of the same Vi-phage type (D1) and of the same antibiogram was reported in a large psychiatric institution in Singapore. A total of 95 (4.8%) of the 1965 inmates were infected, 47 with symptoms and 48 asymptomatic. Transmission was through close person-to-person contact and not through contaminated food or water. The source of infection could not be established. The outbreak was brought under control by maintaining a high standard of environmental sanitation, active search for fever and diarrhoeal cases, identification of asymptomatic cases by rectal swabbing, and isolation of those found to be infected. Mass immunization with two doses of heat-phenol inactivated typhoid vaccine was also carried out concurrently. The vaccine was found to have an efficacy of 65.8% in preventing clinical illness.

Prevalence of varicella-zoster virus infection in Singapore

The incidence of chickenpox in Singapore has been increasing since 1984. A seroepidemiological survey was conducted in 1989-90 to determine the prevalence of varicella-zoster virus-specific IgG antibody in the healthy population in Singapore aged from 6 months to over 45 years. Overall, 42.8% of 500 samples assayed qualitatively by the ELISA method were positive, with less than one fifth of those aged under 15 years and less than half of the 15-24 year olds possessing antibody to the virus. However, antibody prevalence increased markedly to between 86% and 98% in those over 25 years of age. No significant sex or ethnic difference was noted. The survey confirmed the low herd immunity and the higher susceptibility of the young adult population to varicella infection in a tropical city compared to that of temperate countries.

Comparison of the immune response of four different dosages of a yeast-recombinant hepatitis B vaccine in Singapore children: a four-year follow-up study

The immunogenicity of four different dosages of yeast-derived hepatitis B vaccine (Merck, Sharp & Dohme: 0.6 micrograms, 1.25 micrograms, 2.5 micrograms and 5.0 micrograms), administered at 0, 1 and 6 months (0-1-6 schedule) intramuscularly, was evaluated in 122 seronegative healthy children 1-12 years of age. Three months after the first dose, 83.9-100% of the vaccinees seroconverted. Peak geometric mean titres (GMT) of between 1088 mlU/ml and 1699 mlU/ml were attained 3 months after completion of the vaccination schedule. After 24 months, anti-HBs (antibody to hepatitis B surface antigen) was detected in 93.1-100% of the vaccinees, but the GMT dropped to between 214.3 mlU/ml and 303.5 mlU/ml. After 48 months, 88.8-100% of the vaccinees continued to possess anti-HBs and 70.3-87% had titres above 10 mlU/ml. As expected, the GMT declined further to between 72.6 mlU/ml and 118.8 mlU/ml. There were no significant differences in seroconversion rates and GMT among the different dosage groups. All the vaccinees remained asymptomatic and free from hepatitis B virus infection. The study showed that reduced dosages of the vaccine (0.6 micrograms, 1.25 micrograms and 2.5 micrograms) were as immunogenic as the standard dose (5 micrograms); the 2.5-micrograms dose was recommended for the national childhood immunization programme in Singapore. No booster is necessary for at least four years after vaccination.

Sudden unexplained death syndrome--a new manifestation in melioidosis?

The indirect haemagglutination (IHA) test using sensitized turkey erythrocytes and the indirect immunofluorescence assay (IgM-IFA) was confirmed to be sensitive in the detection of a recent or current Pseudomonas pseudomallei infection in 19 culture-confirmed Singapore melioidosis patients. All were found to have antibody titres from 4 to 32768 in the IHA test and 10 to 320 in the IgM-IFA test. When these tests were employed on sera from 16 immigrant Thai construction workers who died of sudden unexplained death syndrome (SUDS) and 73 healthy Thai fellow workers, 93.8% and 68.8% of SUDS cases had IHA titre of greater than or equal to 4 and IgM-IFA titre of greater than or equal to 10 respectively, in contrast to 39.7% and 12.3% found among healthy Thai workers. These data indicate that at the time of death, most of the SUDS patients had an active infection with P. pseudomallei, possibly resulting from reactivation of a latent infection. The aetiological role of P. pseudomallei as the major cause of SUDS is discussed.

Typhoid fever in Singapore: a review of 370 cases

The medical records of 370 patients treated for typhoid fever between 1986 and 1988 at the Communicable Disease Centre, Singapore, were reviewed. The disease was generally mild. There was no mortality. Fever was found in 98.4% of patients on admission and diarrhoea in 21%. Cough was predominantly a symptom of children and occurred in 7.1% of patients aged below 15 years. Other symptoms were uncommon. Hepatomegaly was found in 71% and splenomegaly in 47%. Leucopenia was not a helpful diagnostic marker. Chloramphenicol was the drug of choice. The relapse rate was 5.4% and the convalescent and temporary carrier rates 11.6%. The risk of developing the carrier state was significantly higher among patients who were afebrile on admission compared with those who were febrile (P less than 0.001); it was also higher in patients treated with ampicillin as compared to those treated with chloramphenicol (P less than 0.001, chi 2 = 22.7, odds ratio = 5.25, 95% confidence limits: 2.46 and 11.29). The role of ampicillin as a first line treatment for acute typhoid fever may need further re-evaluation.

Tussockosis: an outbreak of dermatitis caused by tussock moths in Singapore

An outbreak of acute pruritic rash occurred in March 1990 among 141 residents of a high-rise public housing estate in Bukit Panjang, Singapore. The typical rash consisted of urticarial lesions distributed over the limbs and trunk. The outbreak was associated with a transient increase in tussock moths in the residential estate following an unusual, short dry spell. The aetiology was established when patch tests with crude moth material produced similar eruptions in 5 out of 7 adult volunteers between 40 min and 12 h. Pharmacological experiments with an aqueous extract of moth hairs in isolated guinea pig ileum elicited a response similar to that induced by histamine.

Clearance of neonatal antiHBc following hepatitis B vaccination: relationship to antiHBs levels

The persistence of antiHBc following hepatitis B virus (HBV) infection is sometimes regarded as evidence of a past infection, and the antiHBc inhibition of T-cell cytotoxic killing of HBV may be one way of tolerance leading to the carrier state. A model for studying the possibility of an immune tolerance occurring in the human situation was to observe the natural clearance rate of total antiHBc in babies who have acquired naturally maternal transferred antibodies, and to compare antiHBcIgG clearance and development of hepatitis B surface antigenaemia with the level of antibody to HBsAg (antiHBs), which is normally known to be virucidal. Two hundred and ninety-one normal infants who received hepatitis B vaccination in 1984 were followed and studied at six months, one, two and four years. AntiHBc was not found in any control infants born of HBsAg negative mothers. One child became antiHBc positive at four years reflecting acquired infection when antiHBs level had fallen to 36 mIU per ml. AntiHBc cleared in 66% of HBsAg pos./'e' neg. children at one year and completely by two years. In contrast, antiHBc clearance in children born of HBsAg pos./'e' pos. was inversely related to the antiHBs levels. Those with antiHBs levels over 50 mIU per ml cleared antiHBc more rapidly and were HBsAg negative, whereas in 28/49 (57%) HBs antigenaemia were associated more often with antiHBs under 10 mIU per ml and in 2/43 (5%) at levels between 11 to 50 mIU per ml. Two children who had later developed HBs antigenaemia in the presence of antiHBs after responding to earlier passive-active immunisation, suggest either defective antiHBs and/or the development of mutants or variants.(ABSTRACT TRUNCATED AT 250 WORDS)