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In 2015, several provinces in Lao People's Democratic Republic (Lao PDR) experienced a vaccine-derived poliovirus outbreak. This survey was conducted (i) to evaluate the vaccination coverage in different settings and cohorts using the seroprevalence of anti-poliovirus (PV) antibodies as a surrogate measure, and (ii) to explore the usefulness of an ELISA in a country with limited resources and a specific epidemiological setting. IgG antibodies were assessed by ELISA in Lao children (n = 1216) and adults (n = 1228), including blood donors and health care workers. Protective antibody titers against the 3 vaccine serotypes were determined by microneutralization (VNT) in a subset of participants. More than 92% of the children had anti-poliovirus antibodies, regardless of nutritional status or access to health care, highlighting the success of the vaccination outreach activities in the country. In contrast, anti-poliovirus seroprevalence reached only 81.7% in blood donors and 71.9% in health care workers. Participants born before the introduction of poliovirus vaccination in Lao PDR were considerably less likely to be seropositive. These findings align with the epidemiology of the outbreak. Neutralizing antibodies against at least one of the 3 poliovirus serotypes were detected in all children (99/99) and 93/99 had antibodies against all serotypes. Similarly, all health care workers had neutralizing antibodies against at least one and 92/99 against all serotypes. The comparison of both assays shows an acceptable underestimation of vaccine coverage in children by ELISA, but a low sensitivity of the ELISA in the adults. We show that the ELISA is a reasonable alternative to the VNT in particular in vaccinated children, that an improved version should be serotype specific, and that negativity thresholds should be revisited for optimal sensitivity and specificity. Thus, polio-free countries with an uncertain vaccination coverage and limited laboratory capacity, that are at risk of vaccine-derived poliovirus outbreaks or of re-importation of wild poliovirus may benefit from an improved ELISA for cohort studies to evaluate their immunization program in children.

We conducted a phylogenetic analysis of 19 hepatitis B virus strains from Laos that belonged to 2 subgenotypes of a new genotype I. This emerging new genotype likely developed outside Southeast Asia and is now found in mixed infections and in recombinations with local strains in a geographically confined region.

In Lao PDR, where more than 8% of the population are chronic carriers of HBsAg, multiple genotypes and subgenotypes co-circulate and are prone to generate recombinant viruses. Phylogenetic analyses of multiple clones per donor revealed mixed infections of subgenotypes B1, B2, B4, C1, C5, I1 and I2 in almost 6% of HBsAg positive rejected blood donors. Recombination analyses and distance calculations furthermore showed that about 65% (17/26) of the mixed infected donors showed recombinations in the S-gene alone, involving the predominant genotypes B and C. These results suggest that, at least in Laos, hepatitis B virus (HBV) mixed infections lead to frequent recombinations. In many donors with recombinant strains, the recombinant fragment and a non-recombinant strain of the same genotype co-existed (127/185 analysed recombinant fragments). For a large proportion of these (60/127), the most closely related known virus was found, although not always exclusively, in the same donor. Recombinant virus strains are largely distinct. This is reflected in an unexpected diversity in recombination breakpoints and the relatively rare recombinations with identical recombination patterns of the same genotypes in different donors. Recent recombination events would explain the limited spread of each of the recombinants. Using a published mutation rate of 4.2 × 10(-5) mutations per site and year, the observed minimum genetic distances of 0-0.60% between parent strain and recombinant fragment would correspond to 0-71 years of evolution from a most recent common ancestor (MRCA). Thus several lines of evidence are suggestive of recent independent recombination events, a proportion of these even occurring within the same donors. In conclusion, our analyses revealed a high variability of mixed infections as a very probable breeding ground of multiple variable recombination events in Laos that so far have not led to new dominant strains.

We conducted a phylogenetic analysis of 19 hepatitis B virus strains from Laos that belonged to 2 subgenotypes of a new genotype I. This emerging new genotype likely developed outside Southeast Asia and is now found in mixed infections and in recombinations with local strains in a geographically confined region. [ABSTRACT FROM AUTHOR]

Introduction: Hepatitis B is endemic in Lao PDR and about 9% of the adult population is chronically infected. In this study, we investigated regional, occupational, age and sex-related differences in hepatitis B epidemiology in Lao blood donors., Methods: 5017 voluntary blood donors from 8 different provinces were tested for hepatitis B markers by ELISA. Predictors for the prevalence of hepatitis B surface antigen (HBsAg) and antibodies against the core antigen (anti-HBc) were assessed by bivariate and multivariable analyses., Results: In total, 41% of the participants were positive for anti-HBc; the HBsAg prevalence was estimated at 6.9% among all participants (9.2% among first-time donors and 3.9% among repeat donors). Among first-time donors, HBsAg positivity was associated independently with being male (p<0.001), being from the North (p<0.001) and being soldier (p<0.001). Participants were more likely to be anti-HBc positive when they were male (p<0.001), from the Northern provinces (p<0.001) and older than 20 years (p<0.01)., Conclusion: In conclusion, our study confirmed an overall high HBsAg and anti-HBc prevalence in Lao PDR, albeit with considerable regional variation. The identification of a sizeable number of HBsAg positives among repeat donors warrants a thorough investigation of current blood screening, record keeping, donor identification and counselling practises., Competing Interests: The authors have declared that no competing interests exist.

In backyard farms of Lao People's Democratic Republic, mixed-species rearing of poultry is a breeding-ground for cross-species transmission. Here, the epidemiology of viruses circulating among backyard poultry in Vientiane Province was assessed to guide future control strategies. Oral/tracheal and cloacal swabs, collected from 605 poultry (308 ducks, 297 chickens) between 2011 and 2015, were screened by PCR for Newcastle disease virus (NDV), coronavirus (CoV) and chicken anaemia virus (CAV). Chicken sera were screened for anti-NDV antibodies by ELISA. Statistical and phylogenetic analyses revealed transmission patterns and relationships. Closely related strains co-circulated in chickens and ducks. While CoV RNA was detected in oral/tracheal swabs of 9.3% of the chickens and 2.4% of the ducks, rates were higher in faecal swabs of both species (27.3% and 48.2%). RNA of infectious bronchitis virus (IBV) and duck CoV was found in faecal swabs of chickens (19.7% and 7.1%) and ducks (4.1% and 44.1%). Moreover, DNA of the generally chicken-specific CAV was detected in oral/tracheal swabs of chickens (18.1%) and, sporadically, of ducks (2.4%). Despite serological evidence of NDV circulation or vaccination (86.9%), NDV RNA was not detected. We found a high prevalence and indication for cross-species transmission of different CoV strains in backyard poultry. Interestingly, ducks served as biological, or at least mechanical, carriers of viral strains closely related not only to IBV, but also to CAV. Bird containment and poultry species separation could be first steps to avoid cross-species transmission and emergence of novel strains with broad host range and enhanced pathogenicity. RESEARCH HIGHLIGHTS High rates of avian viruses were detected by PCR in backyard poultry from Lao PDR. Diverse coronavirus and chicken anemia virus strains co-circulated. Phylogenetic analyses suggested virus transmission between chickens and ducks. Serological evidence of Newcastle disease was found, but viral RNA was not detected.

In Lao People's Democratic Republic (PDR), acute respiratory infections overburden the health care system, but viral etiology, genetic diversity, and seasonality, especially in light of the introduction of influenza vaccination in the country, are poorly understood. From August 2010 to April 2011, 309 outpatients were recruited at the Luang Prabang Provincial Hospital covering highland Lao communities. Nasopharyngeal swabs were screened for the presence of 13 respiratory viruses. At least one virus was detected in 69.6% and dual/triple viral infections in 12.9%/1.9% of the patients. Influenza A and B viruses combined were the most frequently detected pathogens, followed by human adenovirus and respiratory syncytial virus (RSV). The other viruses were detected in less than 10% of the patients. Phylogenetic analyses on a representative set of RSV strains revealed that, while otherwise very rare, the RSV-B CB1/THB genotype cocirculated with other common genotypes. A single wave of influenza virus and RSV activity was observed during the rainy season, providing further support to influenza vaccination before the onset of the rains. This study provides recommendations for influenza vaccination that still needs optimization and highlights the need for revised guidelines for treatment and prevention of respiratory infections in Lao PDR, as well as for increased surveillance efforts., (© 2018 Wiley Periodicals, Inc.)

In 2015, several provinces in Lao People's Democratic Republic (Lao PDR) experienced a vaccine-derived poliovirus outbreak. This survey was conducted (i) to evaluate the vaccination coverage in different settings and cohorts using the seroprevalence of anti-poliovirus (PV) antibodies as a surrogate measure, and (ii) to explore the usefulness of an ELISA in a country with limited resources and a specific epidemiological setting. IgG antibodies were assessed by ELISA in Lao children (n = 1216) and adults (n = 1228), including blood donors and health care workers. Protective antibody titers against the 3 vaccine serotypes were determined by microneutralization (VNT) in a subset of participants. More than 92% of the children had anti-poliovirus antibodies, regardless of nutritional status or access to health care, highlighting the success of the vaccination outreach activities in the country. In contrast, anti-poliovirus seroprevalence reached only 81.7% in blood donors and 71.9% in health care workers. Participants born before the introduction of poliovirus vaccination in Lao PDR were considerably less likely to be seropositive. These findings align with the epidemiology of the outbreak. Neutralizing antibodies against at least one of the 3 poliovirus serotypes were detected in all children (99/99) and 93/99 had antibodies against all serotypes. Similarly, all health care workers had neutralizing antibodies against at least one and 92/99 against all serotypes. The comparison of both assays shows an acceptable underestimation of vaccine coverage in children by ELISA, but a low sensitivity of the ELISA in the adults. We show that the ELISA is a reasonable alternative to the VNT in particular in vaccinated children, that an improved version should be serotype specific, and that negativity thresholds should be revisited for optimal sensitivity and specificity. Thus, polio-free countries with an uncertain vaccination coverage and limited laboratory capacity, that are at risk of vaccine-derived poliovirus outbreaks or of re-importation of wild poliovirus may benefit from an improved ELISA for cohort studies to evaluate their immunization program in children.

Hepatitis B virus infection is endemic in Lao People's Democratic Republic (PDR). Among 3,000 pregnant women attending an antenatal clinic at Mother and Child Hospital in Vientiane, Lao PDR, 5.8% were HBsAg positive by a rapid test. Among serum samples of 47 infants aged 9-12 months born to HBsAg-positive mothers, 38% were anti-HBs negative. Percent anti-HBs negative children is significantly higher in those born to HBeAg positive mothers than in those born to HBeAg negative mothers (60% vs 25%, p < 0.05). Out of 47 HBsAg-positive mothers, 10 had infants who were HBsAg positive. None of the infants born to HBsAg negative mothers became HBsAg positive but 10/19 (52.6%) of infants born to HBeAg positive mothers became HBsAg positive. This high rate of mother-to-child transmission of HBV in an endemic country is of concern and indicates that routine vaccination program for Lao infants needs strengthening.

Background: Despite hepatitis B vaccination at birth and at 6, 10 and 14 weeks of age, hepatitis B virus (HBV) infection continues to be endemic in the Lao People's Democratic Republic (PDR). We carried out a cross-sectional serological study in infants, pre-school children, school pupils and pregnant women to determine their burden of disease, risk of infection and vaccination status., Methods: A total of 2471 participants between 9 months and 46 years old were recruited from urban (Vientiane Capital, Luang Prabang), semi-urban (Boulhikhamxai and Savannakhet) and remote rural areas (Huaphan). All sera were tested for anti-HBs and anti-HBc. Sera testing positive for anti-HBc alone were further tested for the presence of HBsAg., Results: A low prevalence of HBsAg (0.5%) was detected among infants from Vientiane and Luang Prabang, indicating some success of the vaccination policy. However, only 65.6% had protective anti-HBs antibodies, suggesting that vaccination coverage or responses remain sub-optimal, even in these urban populations.In pre-school children from remote areas in Huaphan, 21.2% were positive for anti-HBc antibodies, and 4.6% were for HBsAg positive, showing that a significant proportion of children in these rural regions have early exposure to HBV. In pre-school children with 3 documented HBV vaccinations, only 17.0% (15/55) were serologically protected.Among school-children from semi-urban regions of Luang Prabang, Boulhikhamxai and Savannakhet provinces, those below the age of 9 who were born after HBV vaccine introduction had anti-HBc and HBsAg prevalence of 11.7% and 4.1%, respectively. The prevalence increased to 19.4% and 7.8% of 10-14 year olds and to 27% and 10.2% of 15-19 year olds.Pregnant women from Luang Prabang and Vientiane had very high anti-HBc and HBsAg prevalence (49.5% and 8.2%), indicating high exposure and risk of onward vertical transmission to the unborn infant., Conclusions: Overall, the results demonstrate a dramatic deficiency in vaccination coverage and vaccine responses and/or documentation within the regions of Lao PDR studied, which included urbanized areas with better health care access. Timely and effective hepatitis B vaccination coverage is needed in Lao PDR.

Glucocorticoids and the glucocorticoid (GR) and mineralocorticoid (MR) receptors have been implicated in many processes, particularly in negative feedback regulation of the hypothalamic-pituitary-adrenal axis. Epigenetically programmed GR alternative promoter usage underlies transcriptional control of GR levels, generation of GR 3' splice variants, and the overall GC response in the brain. No detailed analysis of GR first exons or GR transcript variants throughout the human brain has been reported. Therefore we investigated post mortem tissues from 28 brain regions of 5 individuals. GR first exons were expressed throughout the healthy human brain with no region-specific usage patterns. First exon levels were highly inter-correlated suggesting that they are co-regulated. GR 3' splice variants (GRα and GR-P) were equally distributed in all regions, and GRβ expression was always low. GR/MR ratios showed significant differences between the 28 tissues with the highest ratio in the pituitary gland. Modification levels of individual CpG dinucleotides, including 5-mC and 5-hmC, in promoters 1D, 1E, 1F, and 1H were low, and diffusely clustered; despite significant heterogeneity between the donors. In agreement with this clustering, sum modification levels rather than individual CpG modifications correlated with GR expression. Two-way ANOVA showed that this sum modification was both promoter and brain region specific, but that there was however no promoter*tissue interaction. The heterogeneity between donors may however hide such an interaction. In both promoters 1F and 1H modification levels correlated with GRα expression suggesting that 5-mC and 5-hmC play an important role in fine tuning GR expression levels throughout the brain., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

In Lao PDR, where more than 8% of the population are chronic carriers of HBsAg, multiple genotypes and subgenotypes co-circulate and are prone to generate recombinant viruses. Phylogenetic analyses of multiple clones per donor revealed mixed infections of subgenotypes B1, B2, B4, C1, C5, I1 and I2 in almost 6% of HBsAg positive rejected blood donors. Recombination analyses and distance calculations furthermore showed that about 65% (17/26) of the mixed infected donors showed recombinations in the S-gene alone, involving the predominant genotypes B and C. These results suggest that, at least in Laos, hepatitis B virus (HBV) mixed infections lead to frequent recombinations. In many donors with recombinant strains, the recombinant fragment and a non-recombinant strain of the same genotype co-existed (127/185 analysed recombinant fragments). For a large proportion of these (60/127), the most closely related known virus was found, although not always exclusively, in the same donor. Recombinant virus strains are largely distinct. This is reflected in an unexpected diversity in recombination breakpoints and the relatively rare recombinations with identical recombination patterns of the same genotypes in different donors. Recent recombination events would explain the limited spread of each of the recombinants. Using a published mutation rate of 4.2 × 10(-5) mutations per site and year, the observed minimum genetic distances of 0-0.60% between parent strain and recombinant fragment would correspond to 0-71 years of evolution from a most recent common ancestor (MRCA). Thus several lines of evidence are suggestive of recent independent recombination events, a proportion of these even occurring within the same donors. In conclusion, our analyses revealed a high variability of mixed infections as a very probable breeding ground of multiple variable recombination events in Laos that so far have not led to new dominant strains.

Human papillomavirus (HPV) E6 and E7 oncoproteins are essential factors for HPV oncogenesis. These E6 and E7 gene products play a central role in the induction of malignant transformation by interacting with several cellular regulatory proteins such as p16(INK4a), p53 and nuclear factor κB (NF-κB). In the present study, conducted in northern Thailand, HPV-DNA was detected in penile cancer cases using an in situ hybridization procedure and p16(INK4a), p53 and NF-κB were detected by immunohistochemistry. Using the cell cycle regulatory proteins p16(INK4a) (61.5%) and p53 (71.8%), it was found that of the 51 cases, 39 (76.5%) were HPV-DNA-positive in penile cancer. On the other hand, 25% p16(INK4a) and 75% p53, respectively, were found in HPV-negative cases. Prevalence of HPV infection (76.5%) was shown in penile cancer cases in northern Thailand. No difference was found between HPV-positive and HPV-negative cases with respect to the presence of the cell cycle regulatory protein p53. On the other hand, p16(INK4a) was found to be different between HPV-positive and HPV-negative cases. Inactivation of tumor suppressor genes, such as p16(INK4a) and p53, to genetic instability, cell immortalization, accumulation of mutations and cancer formation, with or without HPV and irrespective of HPV infection, is therefore suggested. Of the 39 HPV-positive cases, 35 (89.7%) were NF-κB-positive in the nucleus, 29 (74.4%) in the cytoplasm and 37 (94.9%) in the nucleus and/or cytoplasm. NF-κB was detected in 4 (33.3%) of the 12 HPV-negative cases. Therefore, we propose that penile cancer cases with HPV infection are more likely to activate NF-κB than those without HPV infection.

The purpose of this study was to determine the prevalence and distribution of HCV genotypes among voluntary blood donors in northern Thailand. From 1998 to 2000, 167 serum samples which tested positive for anti-HCV antibodies in the screening of voluntary blood donors from 5 provinces in northern Thailand were selected for genotyping. Viral RNA was extracted from the sera. The core-E1 region of the HCV-RNA genome was amplified using a OneStep RT-PCR kit. The core-E1 amplicon was sequenced and the HCV genotype was assigned by comparing with the reference sequences available in the GenBank database. Of 167 anti-HCV positive serum samples, 126 (75.4%) contained HCV RNA as detected by PCR. HCV genotype 3 was the most predominant genotype (39.6%), of which 33.3% belonged to genotype 3a and 6.3% to 3b. Genotype 6 was detected in 31%, and genotype 1 was detected in 27.8%. Of the genotype 1 isolates, 14.3% were la, 12.7% were 1b, and 0.8% were 1c. Two HCV isolates detected in the present study were untypeable. About 75% of anti-HCV positive blood donors had chronic HCV infection. In northern Thailand, genotype 3a was the most predominant genotype, while genotype 6, 1a and 1b were also commonly found. The genotypic distribution of HCV isolates from various regions of Thailand were more or less similar. Nevertheless, in this study, the prevalence of HCV genotype 6 (31%) was higher than previously reported by others (8-18%). Phylogenetic analysis of the HCV isolates detected in the present study was also performed.

In sub-Saharan Africa, the Pacific, and particularly Asia, hepatitis B virus (HBV) infection is highly endemic, the most common route of transmission is perinatal. To minimize the number of horizontal transmissions, we determined the prevalence of HBV genotypes among children in northern Thailand. From a survey of 1,231 schoolchildren in Chiang Mai during 1998 to 2000, 55 (4.5%) were found positive for HBsAg. Fifty-three HBsAg-positive samples were available for this study. These came from 28 girls (52.8%) and 25 boys (47.2%), age 5-16 years, with a mean age of 12.8 (+/-2.6) years. The laboratory method was based on a multiplex-PCR for the detection of 6 HBV genotypes (A-F). Among 53 HBsAg positive cases, 48 (90.6%) were genotype C, followed by 4 cases of genotype B (7.5%), and 1 case (1.9%) with mixed infection with genotypes B and C. The high prevalence of HBV genotype C follow by genotype B is similar to that found among blood donors in northern Thailand and the nationwide epidemiological survey conducted in 2004. Perinatal transmission may play an important role in the spread of the virus in this area, as in other Asian countries, where genotypes C and B are highly prevalent.

There have been no previous reports of the prevalence of hepatatis B virus (HBV) and hepatitis C virus (HCV) infections in Lao PDR. From 2003 to 2005, 13,897 first-time blood donors were screened for the presence of hepatitis B surface antigen (HBsAg) and hepatitis C virus antibody (anti-HCV). The seroprevalence of HBsAg positive blood donors was 8.7%. The prevalence among males (9.7%) was higher than in females (6.2%). The prevalence of anti-HCV positive blood donors was 1.1%, with no significant differences between males (1.1%) and females (1.0%). Annual positive rates for HBsAg and anti-HCV during the years 2003 to 2005 did not differ significantly. Lao PDR has a high endemicity of HBV carriers (8.7%). Dual infection with HBV and HCV was 0.12%. For preventing HBV infection, the country introduced DPT-Hepatitis B vaccines into the National Immunization Program in 2001. The large reservoir of HBV and HCV infections will cause an enormous burden of patients with cirrhosis and hepatocellular carcinoma in the future.

There are distinct ethnogeographic variations for the distribution of various hepatitis B virus (HBV) genotypes, and pathogenic and therapeutic differences are also observed. In general, genotype B infection has a relatively better prognosis than genotype C. In Thailand, genotypes C and B were reported as the major genotypes; however, there were no previous reports of HBV genotyping in the north of the country. From 1998 to 2000, 216 HBsAg-positive serum samples (164 males and 52 females, aged 16-52 years), were screened and collected from voluntary blood donors in four provinces of northern Thailand. The method of Naito et al. was employed in this study, with the multiplex-PCR approach and genotype-specific primers to identify genotypes A-F. We found that the HBV genotype C was highly predominant in northern Thailand (89.3%), when compared with the previous reports of genotype C distribution among voluntary blood donors from other areas in the country (50-65%), followed by genotype B (7.4%), mixed infection of genotype B and C (1.9%) and genotype A (0.5%). Four samples (1.9%) were unclassifiable. There was no significant difference of genotype distribution among four northern Thai provinces or each age group.

Chiang Mai is a province in northern Thailand that started a vaccination program for hepatitis B virus (HBV) infection in 1989. In this paper, we report the long-term efficacy of this program. Of children aged 4-9 years, 65.7% had a complete course and 3.8% had an incomplete vaccination course. Urban schoolchildren had higher percentage of HB vaccination than rural schoolchildren (89.1% vs 46.9% for the complete course, p < 0.001). The overall prevalence rate of HBsAg in Chiang Mai schoolchildren was 1.2%, with no significant differences between gender (p = 0.496) and school areas (p = 0.477). Anti-HBc antibodies were detected in 6.9% of children. Overall, 26.2% of children had protective levels of anti-HBs antibodies (> 10.0 mlU/ml), and 11.2% had low levels of these antibodies (1.0-9.9 mlU/ml). Compared to previous reports, our results show a lower percentage of anti-HBs antibodies, 33.8% of children age 4 years had protective anti-HBs antibodies, dropping to 18.4% by age 9 years. Among those anti-HBs seropositive, 9.1% were anti-HBc positive, indicating a natural infection with HBV. We found a small number of children, despite adequate immunization, developed HBV infection.

Context: Penicillium marneffei, an opportunistic fungus, is endemic in Southeast Asia, especially in human immunodeficiency virus-infected individuals living in northern Thailand., Objective: We present the results of a clinicopathologic study of hepatic penicilliosis among human immunodeficiency virus/acquired immunodeficiency syndrome patients., Design: A search of liver biopsies in one institution from 1998 to 1999 identified 30 cases of penicilliosis., Results: Histologically, hepatic lesions could be classified into 1 of 3 patterns: diffuse, granulomatous, and mixed. The diffuse pattern showed a diffuse infiltration of foamy macrophages that contained numerous P marneffei. The granulomatous pattern showed a formation of multiple granulomata with various degrees of inflammatory cell infiltration. The mixed pattern showed features intermediate between the diffuse and granulomatous patterns. Liver function tests of the 3 pathologic pattern groups were evaluated, but there were no statistically significant differences in aspartate aminotransferase, alanine aminotransferase, or alkaline phosphatase levels among the various histologic groups., Conclusion: To our knowledge, this is the largest series to date that documents the liver pathology that results from this pathogen. We hypothesize that the histologic patterns seen on biopsy reflect the level of the host's immunity. Hence, in addition to a diagnosis of penicilliosis, a liver biopsy may also provide an assessment of the host's immune status, whereas liver function tests do not.

Hepatitis A virus (HAV) infection is common in Southeast Asia, and most of the inhabitants acquire a lifelong immunity as a result of natural infection during childhood. However, the age-specific seroprevalence is changing with development of socioeconomic and hygiene status in this area and the infection is predicted to shift to adulthood with more severe clinical manifestations in the future. In this study, we report the present epidemiological pattern of antibody to HAV (anti-HAV) among schoolchildren in Chiang Mai, Northern Thailand. The overall prevalence rate of anti-HAV was 9.6% (11.4% in female and 7.5% in male children, and 10.8% in urban and 8.9% in rural schoolchildren, respectively). Our study, comparing with previous reports from other parts in Thailand, indicates a steady decline of anti-HAV prevalence among schoolchildren in Chiang Mai area, and discussed a possibility of an outbreak of HAV infection among urban schoolchildren.