Relationship between prevalence and intensity of Opisthorchis viverrini infection, and clinical symptoms and signs in a rural community in north-east Thailand

Bulletin ofthe World Health Organization, 62 (3): 451-461 (1984) © World Health Organization 1984 Relationship between prevalence and intensity of Opisthorchis viverrini infection, and clinical symptoms and signs in a rural community in north-east Thailand E. S. UPATHAM,1 V. VIYANANT,1 S. KURATHONG, J. ROJBORWONWITAYA, W. Y. BROCKELMAN,' S. ARDSUNGNOEN,l P. LEE,' & S. VAJRASTHIRA4 In a large village in north-east Thailand, the overall prevalence of Opisthorchis viver- rini infection (based on Stoll's quantitative egg count) was 89.5%o in a total population of 1651 individuals. The prevalence was 32% in children under 5 years, 90% in those aged 5-9 years, and averaged 95.6%o in age groups above 10years. The meanfaecal egg output (indi- cative of intensity of infection) was highest in the 40-49-year age group and remained rela- tively constant through older ages. In all age groups the prevalence and intensity of infec- tion in both men and women were similar. A history of eating raw freshwater fish occurred more frequently in infected persons than in those uninfected. The following symptoms occurred significantly more frequently in groups with higher intensities of infection: weakness, flatulence or dyspepsia, and abdominalpain in the right upper quadrant. Nevertheless, infectedpersons did not report a reduced ability to work. Anorexia, nausea, vomiting, and diarrhoea were only weakly cor- related with the intensity of infection. A palpable liver occurred more frequently in the infected groups and was correlated with intensity of infection. Icteric conjunctivae were observed in 2.2% of infected persons but not in the uninfected. Some 5-10%o of the popu- lation had symptoms that were attributable to opisthorchiasis.. Although millions of people in north-east Thailand are infected with Opisthorchis viverrini (1-3), the frequency with which signs and symptoms of illness are found in relation to the prevalence and intensity of infection in this population remains unknown. This is because nearly all previous investigations have been epidemiological studies of selected groups of inhabi- tants in an endemic area (1, 4-6) or of hospitalized patients (7), or reviews of clinicopathological ma- terial (8, 9). In the present investigation, a cross-sectional method originally employed for studying schistoso- miasis (10, 11) was applied. With this method, the prevalence and intensity of 0. viverrini infections, as determined by quantitative egg counts, were cor- related with morbidity, as indicated by standard ' Center for Applied Malacology and Entomology, Department of Biology, Faculty of Science, Mahidol University, Rama VI Road, Bangkok 4, Thailand, Requests for reprints should be sent to Dr E. S. Upatham. 2 Department of Medicine, Faculty of Medicine, Ramathibodi Hospital, Bangkok, Thailand. 3Department of Medicine, Rajvithi Hospital, Bangkok Thailand. 4Department of Helminthology, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand. medical examinations. Compared with our prelimi- nary study in Nong Ranya (12), the present investi- gation was carried out in a larger village where 1651 (91%) of the 1812 residents living in 337 households had their stools examined and 1176 (65%) had a clinical examination as well. MATERIALS AND METHODS Population studied The district of Chonnabot is situated in Khon Kaen Province in north-east Thailand (Fig. 1). Chonnabot village, located 5 km to the west of the village of Nong Ranya, is a designated municipal area with modern facilities which smaller villages do not have. Besides easy accessibility, the village is close to shallow reser- voirs and streams where the two intermediate hosts of 0. viverrini, Bithynia snails and cyprinoid fish, abound. Chonnabot was also selected because of its large heterogeneous population. At the initiation of the project, a meeting was held in the village to explain the purpose of the survey to the villagers and to enlist their cooperation. 4418 -451- E. S. UPATHAM ET AL. Stool examination During the survey period (March-June 1980) field workers collected one stool specimen from each of 1651 villagers and took them to a field laboratory where they were examined. Stoll's egg counts (13) were performed on all specimens to detect 0. viverrini eggs. Approximately 1 g of each stool was preserved in a vial containing Merthiolate(thiomersal)-iodine- formaldehyde; all vials were then labelled and trans- ferred to the laboratory in Bangkok where they were examined for other parasitic ova (14, 15). RESULTS The age structure of the 1651 individuals who pro- vided faecal specimens is shown in Fig. 2. The pro- portion of the population less than 20 years old con- stituted about 50070 of the total. The male: female ratio was 46:54. AGE (YEARS) FEMALES - B gL MALES E~~~~~~~~~~~~~~~~~~~ Fig. 1. Map of the study area. Medical history and physical examination A questionnaire concerning a variety of major and minor symptoms possibly attributable to 0. viverrini infection was prepared in Thai. Altogether 1176 indi- viduals who came to a local wat (monastery) were interviewed by one of two paramedical personnel using this questionnaire. After the interview, the villagers were examined by one of three physicians. Liver size was measured both in the midclavicular and midsternal lines while the examinee was recumbent. Extension of the organ below the costal margin was determined, midway between inspiration and expiration, with a tape measure. Similarly, spleen size was measured both in the anterior axillary and midaxillary lines. Neither the results of stool examination nor the findings in the questionnaire were known to the examiner in ad- vance. Data processing and analysis The data were coded and punched on cards and most of the analysis was carried out by computer using chi-square tests. I i 30 25 20 15 10 5 0 5 10 PERCENTAGE Fig. 2. Age structure of the study Chonnabot village. 15 20 25 30 population of Opisthorchis viverrini infection 0. viverrini eggs were detected in the stools of 1478 individuals or 89.5%7o of those who provided faecal specimens. The prevalence data, according to age and sex, are shown in Table 1 and in Fig. 3. The prevalence in children under 5 years, including 41 infants, was 32%7o; it rose to 90%o in those aged 5-9 years and remained relatively constant through the older age groups. Males and females showed similar per- centages of infection in all age groups, with a mean of 89%o for males and 90'70 for females. Of the 41 infants examined, 3 (7%o) were stool-positive. The population was divided according to the presence and intensity of infection as follows (Fig. 4): 10%o were uninfected, 1 Io% had light infections (< 1 egg per mg of faeces), 36%o had moderate infections (1-10 eggs per mg of faeces), 33% had heavy infec- tions (> 10-50 eggs per mg of faeces), and 9%7o had very heavy infections (> 50 eggs per mg of faeces). The mean egg counts (per mg of faeces) for the examined population were 18.82 ± 1.17 for males, 20.30± 1.16 for females, and 19.55±0.88 for both combined. The mean egg counts according to age group and sex are shown in Table 1 and Fig. 3. The intensity of infection in both males and females rose steadily in early life, peaked in the 40-49-year age group and remained relatively constant thereafter. In males, there was an unexplained surge in the 20-24- year age group. Overall, there was no consistent 452 OPISTHORCHIS VIVERRINI INFECTION IN THAILAND Table 1. Distribution 0. viverrini of the population of Chonnabot village according to age group, sex and prevalence of Egg counts/mg of faeces from infected individuals Sex Age No. Prevalence Arithmetic mean Geometric mean (years) examined Males 0-4 84 33.3 2.8±1.2 10.5±1.2 5-9 119 91.6 9.0± 1.4 2.9± 1.5 10-14 111 96.4 13.3±1.6 6.4±1.7 15-19 96 96.9 16.2± 1.7 8.5± 1.9 20-24 37 94.6 26.6±6.4 12.2±6.8 25-29 48 97.9 18.3 ± 3.7 8.3± 4.1 30-39 82 96.3 20.3± 2.8 10.2 ± 3.0 40-49 77 96.1 31.7 ± 6.1 12.8±6.5 50-59 55 98.2 30.0±6.1 11.9±6.7 > 60 56 98.2 27.9 ± 4.7 12.0 ± 5.2 Total 765 89.0 18.9 ±1.2 7.0 ±1.3 Females 0-4 72 30.6 2.9 ±1.7 0.6 ±1.8 5-9 106 87.7 9.6 ± 2.0 2.9 ± 2.1 10-14 114 94.7 8.4± 1.1 3.8± 1.2 15-19 108 96.3 18.7±3.0 8.0±3.1 20-24 78 94.9 21.2 ± 3.3 7.2 ± 3.7 25-29 63 98.4 21.2 ±4.2 9.1 ±4.6 30-39 113 97.3 24.5 ± 3.5 8.9 ± 3.8 40-49 85 97.7 31.6 ± 4.8 11.6 ± 5.3 50-59 75 96.0 28.5±4.8 14.3±5.1 ) 60 72 95.9 26.0 ± 3.5 11.8 ± 3.9 Total 886 90.0 20.0 ± 1.2 6.9 ± 1.2 Both sexes 0-4 156 32.1 2.9 ± 1.0 0.5 ± 1.0 5-9 225 39.8 9.3 ±1.2 2.9 ±1.3 10-14 225 95.6 10.8±9.4 4.9 ± 1.1 15-19 204 96.6 17.6±1.8 8.2±1.9 20-24 115 94.8 22.9 ± 3.0 8.5 ± 3.3 25-29 111 98.2 20.5±2.9 8.7±3.1 30-39 195 96.9 22.7 ± 2.3 9.4± 2.5 40-49 162 96.9 31.7±3.8 12.2±4.1 50-59 130 96.9 29.1 ±3.8 13.2±4.0 60 128 96.9 26.9±2.8 11.9±3.1 Total 1651 89.5 19.5±0.7 6.9±0.9 453 454 E. S. UPATHAM ET AL. .40 M"s FIEMALES .30 M IO O 0-4 5 -9 42 09 9 604 AGE GROPhS YEARS) Fig. 3. Intensity of Opisthorchis viverrini infection in relation to age group and sex in the infected population of Chonnabot village. 40 a- 30 ~2 D 25 0. *20 0 1515 I- INFECTION none li EGGS/MG 0 (I 1-10 )l0-50 )50 Fig. 4. Percentages of uninfected individuals and those with different intensities of Opisthorchis viverrini infec- tion in the study population of Chonnabot village. difference in intensity of infection between males and females. However, in most age groups, the mean egg count in females was slightly higher than that in males. Associated intestinal parasitic infections Of the 1651 individuals who provided stool speci- mens for examination, 726 (440Wo) were found to harbour other intestinal parasites. The numbers of persons positive for other parasites were as follows: hookworm (89), Trichuris trichiura (9), Taenia spp. (25), Echinostoma spp. (103), Enterobius ver- micularis (7), Giardia lamblia (76), and small intes- tinal flukesa (417). Personal histories Table 2 shows the prevalence of a history of eating koi-pla, a popular dish containing raw freshwater fish with viable metacercariae. In the uninfected group, only 19% recalled eating this dish, while most people (79%o) in the infected group admitted eating koi-pla. The prevalence of such histories increased with the intensity of infection, from 49%o to 9307o in the groups with light to very heavy infections, respectively. The correlation was highly significant for the sample population as a whole (P < 0.001), and for most age groups independently. Table 3 lists the prevalence of symptoms attributed to 0. viverrini. Because their reported symptoms were deemed to be relatively unreliable, the 108 children less than 5 years old were excluded from these analyses. Approximately 9701o and 9907o of the population claimed that they had not been prevented from doing their usual daily work in the preceding 24 hours and during the preceding 2 weeks, respectively, regardless of the presence or intensity of infection. In contrast, while 1607o and 2007o of the uninfected popu- lation felt weak during, respectively, the previous 24 hours and 2 weeks, these percentages rose to 3607o and 42%, respectively, in the very heavily infected group. These changes were statistically very significant (P < 0.025 and P < 0.005, respectively). Likewise, anorexia, flatulence or dyspepsia, and abdominal pain in the right upper quadrant occurred much more frequently in the infected groups, and there were cor- relations between these symptoms and the presence (and intensity) of infection (significance ranging from P < 0.025 to P < 0.005 for persons with symptoms during preceding 24 hours and 2 weeks). Vomiting occurred in approximatety 2-601o of the infected population, but not in the uninfected group; how- ever, these results were not statistically significant (P > 0.05). Diarrhoea and nausea also occurred most frequently in the heavily infected group, and this was statistically significant in cases with symptoms within the previous 24 hours or during the last 2 weeks (see Table 3). The possibility that these symptoms could be due to other coexisting intestinal parasitic infections was also investigated; Table 4 lists the prevalence of symptoms in 37 individuals who had neither a The small intestinal flukes covered in this study include Phaneropsolus bonnei, Prosthodendrium molenkampi, Haplorchis yokogawai, and Haplorchis taichui (16, 17). OPISTHORCHIS VIVERRINI INFECTION IN THAILAND Table 2. Number of cases classified by history of eating koi-pla and intensity of 0. viverrini infection. The last column indicates the probability at which the null hypothesis of independence between frequency of eating koi-pla and inten- sity of infection is rejected with the chi-square test. For the separate age groups the data were pooled into either 2 or 3 infection categories before testing, depending on sample sizes. No. of cases according to the Age group History of intensity of 0. viverrini infection (eggs/mg faeces) TotalP (years) eating koi-pla 0 < 1 1-10 > 10-50 > 50 0-4 No 63 23 5 1 0 92 Yes 2 2 8 2 0 14 < 0.0001 5-9 No 10 23 29 6 0 68 Yes 4 25 50 37 3 119 < 0.0001 10-14 No 2 12 38 8 0 60 Yes 6 1 2 71 47 7 143 0.0016 15-19 No 3 4 13 3 1 24 Yes 1 5 34 44 5 89 0.00046 20-29 No 5 4 8 1 2 20 Yes 1 8 39 40 13 101 < 0.0001 30-39 No 1 3 11 6 0 21 Yes 1 6 40 46 13 106 0.037 40-49 No 1 5 3 1 2 12 Yes 2 4 35 54 24 119 0.0059 50-59 No 2 0 5 0 1 8 Yes 2 2 23 48 16 91 0.031 > 60 No 1 0 2 4 1 8 Yes 2 7 22 28 15 74 All ages No 88 74 114 30 7 313 Yes 21 71 322 346 96 856 < 0.0001 0. viverrini nor any other intestinal parasitic infection and in 586 individuals who had 0. viverrini but no other intestinal parasitic infection. Approximately, 97-990o of these people claimed they were not pre- vented from carrying out their usual daily activities in the previous 24 hours and 2 weeks, regardless of the presence or the intensity of infection. Weakness, flatulence or dyspepsia, and abdominal pain in the right upper quadrant were found more frequently in the infected groups, the symptoms occurring in the previous 24 hours or during the preceding 2 weeks. The chi-square probabilities were not significant for the results given in Table 4 except in the case of abdominal pain. Nevertheless, there are correlations between symptoms (weakness, anorexia, flatulence and abdominal pain) and levels of infection; the small sample size appears to be the main reason for the lack of statistical significance. The percentage of persons with weakness increased with increasing infection over all five grades of infection in the predicted direction (P for 5 points in predicted order = 1/5! = 0.0083). In the case of anorexia, flatulence, and abdominal pain, the proportions of individuals with these symptoms at the highest intensity of infection show virtually no decline when persons infected with other parasites are eliminated. Therefore, we con- clude that these symptoms were not primarily due to other parasitic infections. Physical examination Liver enlargement was relatively common in the population (Table 5). At the midsternal line, mild to moderate hepatomegaly (arbitrarily defined as a pal- pable liver of up to 7 cm) was observed in 2207o of the uninfected group and approximately 290Vo of the 455 E. S. UPATHAM ET AL. (OCD 0 Co (Om R 0 Co N a) C- CN 0 m ai z CD Co co I-I. 0 Co 0 (C6 0) cn Co a) Coq R CD 0 Ci(N 0l)o (C) r- C) (N F D0 o al) Co (N 0 co I. I'* a r- 0 (N U: CD 0 Co (N Gn r- 0 CN CD ai (N CN (N Co (N (N N. r- 0 0o 0 ao 00 Co Co (N R cD0 C- 0 r- CD n (C D0 0 C) CD 0 Co (N r-CoN. 0 (N 0 4 r-CD co 0 CD z OR (a) (6 R(N 0 0) 0 Co aN r- () Co4 (N 0 6Co I-I co CD 06 CD tD I'* (O Co F. 0 (C) (CY 0 0 cD N. r- Cl) CsCo CD (Ci co coCD 0 (N 0 CV) N. PI Cl) 0 C- (N (N Co ,it ,it6 - -Co Co- (C)- Co f-qt00 Co CO 0 0 o g~ Co g- ,,, o C CNog DN( > ° > C > C 0O < < Z > 456 a, 0) mC)0)E Co 0)cm CD -S 0 0 CB u C .z on a) 0 C 0 0) c C 0QZ 0 0 o- C) 0) Cu 0 0 09- 0 Co A 0 Co 0I A - v 0 w-(CD0 Co (-) 0 Co 1- ur N. (C) Co (N CY) (N 't (N R LOCo0 Co 0 Co E Co SIt a) r- N I. CoO) uC )0 0)X Co a 4- s* C. cn 4-Co00 C a .-_) Co 0) CC0) c Ut ._ 0) 'naz oC Co E 9ECoO0 c 0* E O >C. 0 oCo 0 _.0 E E C0~ .o , >D C > ) ,_X . (~N 2 O a)z .- .- m a) V = C- > E2 0)o C a ,D Vn .(C)V a, C0) ac o 0) Co0 Co 'IR LO Ri0 Co CD N N r- CV) (6 (C) (C, (6 a D 0 0 CoN. (6 4 co CD N. CD (N (N- C (CD a ai v- V- aw a C- V.: 0 0) Ca a) C CO V .C 0 -o E(D C In 0a) C)a a) C 0 .0 E C a) c _ cm C 0a-0) c0 )m E C 0 Cl c ( (N co CD (C E CD (N Co (N v) a) (D c C .E 0 .0 :3 0 CN Co ~ co a, a, 0 a) a) 0 C O C1 U. C4 N ._ a OPISTHORCHIS VIVERRINI INFECTION IN THAILAND 0 0)r~~~~~~~~r-~~~~~U) c ) C4 0 c r( 0 ) c 0 0)( ~~~~~~~~~~~~~~~~~a 0 4 '0 -c E Co M~' ac C.- 00) 0 0 0 0 0L 00 0 )0 0. (D 0 C4; C > LO ~ ~ 0 ) (0(0 (0(0 (0(0 (0(0 (0(0 (0 ) (0 CO LO) ( ) V) 0 0( a)C'CY)( ( O 's; ~~~0) 0C) C N(N4 CY)N C~ 0 L)~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~a E 0- -- c LO oa)mN coC : )6 oc ~c6c ~ v 66 c0 r CU~~~~~~~~~~~~~~~~~~~~04 C .C~~~~~~~~~~~~~~~~~~~~~~~~~ 0)4 0 O o c o o 0 C C 0 0 c o 0 0 c N C.L C r .00E C ~) E .C/) CD 457 E. S. UPATHAM ET AL. Table 5. Number of cases with liver enlargement at the midsternal line, in relation to intensity of infection by 0. viverrini. Chi-square tests were made with the 3-7 cm and 7 cm categories pooled. A. All persons sampled No. of cases according to intensity of 0. viverrini (eggs/mg faeces) Liver enlarged (midsternal line) 0 < 1 1-10 > 10-50 > 50 Total 0 79 97 314 259 70 819 1-3 cm 13 13 26 26 8 86 > 3-7 cm 9 32 87 83 22 233 >7 cm 0 0 9 10 3 22 Total 101 142 436 378 103 1160 X2(12) = 21.63; P = 0.042 B. Excluding persons with infections due to other helminth parasites No. of cases according to intensity of 0. viverrini (eggs/mg faeces) Liver enlarged (midsternal line) 0 < 1 1-10 > 10-50 > 50 Total 0 68 77 200 149 23 517 1-3cm 12 11 14 10 5 52 >3-7cm 7 24 43 45 8 127 >7cm 0 0 4 3 2 9 Total 87 112 261 207 38 705 X2(8) = 21.41; P = 0.0061 infected groups. Meanwhile, a markedly enlarged liver (at this line), arbitrarily defined as a palpable liver of more than 7 cm, was not found in the un- infected group but occurred in 207o of the infected groups. Hepatomegaly at the midclavicular line (Table 6) was observed in 2407o of the uninfected group and approximately 31 tVo of the infected groups. The relationship between degree of hepatomegaly and intensity of infection was tested with chi-square in a 3 x 5-cell table with three liver enlargement categories and five infection levels. The relationship was signifi- cant both for hepatomegaly at the midsternal line (P < 0.05) and midclavicular line (P < 0.005). Posi- tive correlations between hepatomegaly and infection level were present when the population was divided into three or more age groups, but these were not significant, mostly owing to the small sample sizes. However, hepatomegaly at neither the midsternal nor the midclavicular lines was correlated with age. Spearman rank correlation coefficients between liver size and age were + 0.032 and + 0.003, respectively (P> 0.10), so that age is not a confounding vari- able. Palpable spleen was recorded in only one of the 1166 individuals examined. There was no association between egg concentration and stool consistency. Icteric conjunctivae were not observed in the un- infected groups, but occurred in 3 persons (0.430%o) with light infections (< 1 egg per mg of faeces) and in 20 persons (4.1%o) with heavy infections (> 10-50 eggs per mg of faeces). This difference is highly sig- nificant (P < 0.0001). Percentage ofpopulation affected A rough approximation of the percentages of the total population with symptoms attributable to oplisthorchiasis may be obtained from our data. We have done this using the data in Table 4 to avoid the effects of parasites other than 0. viverrini. First, the baseline percentages of symptoms in persons not affected by 0. viverrini must be established. These were calculated from the 126 persons uninfected or lightly infected with 0. viverrini, from the question- naire data. The percentage of these "unaffected" persons showing a given symptom is subtracted from 458 OPISTHORCHIS VIVERRINI INFECTION IN THAILAND Table 6. Number of cases with liver enlargement at the midclavicular line, in relation to intensity of infection by 0. viverrini. Chi-square tests made with the 3-7 cm and > 7 cm categories pooled. A. All persons sampled No. of cases according to intensity of 0. viverrini (eggs/mg faeces) Liver enlarged (midclavicular line) 0 < 1 1-10 > 10-50 > 50 Total 0 78 96 322 260 79 835 1-3cm 17 31 51 43 12 154 >3-7 cm 8 15 62 75 12 172 >7cm 0 0 1 0 0 1 Total 103 142 436 378 103 1162 X2(s) = 24.53; P = 0.00 1 9 B. Excluding persons with infections due to other helminth parasites No. of cases according to intensity of 0. viverrini (eggs/mg faeces) Liver enlarged (midclavicular line) 0 < 1 1-10 > 10-50 > 50 Total 0 66 78 202 146 28 520 1-3 cm 16 23 31 22 5 97 > 3-7 cm 5 11 27 39 5 87 >7cm 0 0 1 0 0 1 Total 87 112 261 207 38 705 x2(8) = 1 9.48; P = 0.012 Table 7. Percentages of total sample population with symptoms attributable to opisthorchiasis, calculated from the percentages of persons in the moderate, heavy, and very heavy infection categories with symptoms in excess of the baseline levels. A negative value indicates an insignificant deficit in the percentage showing symptoms in relation to the baseline level. % of population with symptoms attributable to opisthorchiasis during previous: Symptoms 24 hours 2 weeks Inability to work - 0.6 0.4 Weakness 3.9 5.7 Anorexia 0 1 .2 Nausea - 1.1 3.3 Vomiting - 0.1 2.6 Flatulence 4.4 6.6 Abdominal pain 2.8 8.0 Diarrhoea 2.1 2.1 Hepatomegaly (midsternal line) > 3 cm 4.3 (midclavicular line) > 3 cm 5.0 the percentages of persons showing the symptom in the moderate, heavy and very heavy infection grades to yield the percentages of persons with symptoms attributable to 0. viverrini in these grades. When these differences are multiplied by the proportions of the sample population in these grades and summed, we obtain an estimate of the total population with measurable symptoms attributable to opisthorchiasis in excess of the baseline levels. Table 7 summarizes these percentages for symptoms within the 24-hour and 2-week periods reported in the questionnaire, and for hepatomegaly of more than 3 cm below the costal margin. DISCUSSION Previous studies to evaluate the morbidity of opis- thorchiasis due to 0. viverrini have either been hampered by other concomitant intestinal parasitoses or failed to include an adequate uninfected control group. In an earlier study, the signs and symptoms were found to occur with equal frequency in the infected and uninfected groups (5). But as in this study the intensity of infection was not measured, the infected group was likely to contain a large 459 E. S. UPATHAM ET AL. proportion of lightly infected individuals whose chances of showing symptoms were small. The present study of morbidity of opisthorchiasis in Chonnabot is the first in which (1) an attempt was made to examine an entire community, (2) the infected subjects were separated according to the intensity of infection (based on quantitative egg ex- cretion data), (3) an uninfected control group from the same community was included, and (4) in order to exclude other associated intestinal parasitoses, a separate analysis was carried out on persons who were not infected by other parasites. Furthermore, the paramedical personnel who completed the question- naires and the physicians who examined the subjects had no prior knowledge of their infection status, and thus observer bias was eliminated. A comparison of the present study with the prelimi- nary study in Nong Ranya (12) shows that, in both, the populations were Thai of north-eastern origin and both areas were not endemic for malaria. Unlike the situation in Nong Ranya, not all the inhabitants in Chonnabot were farmers; many were shopkeepers or labourers. Furthermore, the population in Chon- nabot was slightly more heavily infected than that in Nong Ranya (12). Males were more heavily infected in Nong Ranya, while in Chonnabot the infection in males and females was similar overall. The age struc- ture and sex distribution of the two populations were similar. Hepatomegaly, both at the midsternal and midclavicular lines, occurred more frequently in the infected individuals in Chonnabot, which can be explained by the higher average intensity of infection there. The results of the present study confirm that, with or without any other associated intestinal parasitosis, the symptoms and signs of weakness, flatulence or dyspepsia, abdominal pain in the right upper quad- rant, icteric conjunctivae, and hepatomegaly (at both midsternal and midclavicular lines) occurred signifi- cantly more frequently in the infected groups and were related to the intensity of infection. The ability to work and signs of nausea, vomiting, diarrhoea, and splenomegaly were unrelated to the intensity of opisthorchiasis. When other associated intestinal parasitoses were excluded, flatulence and abdominal pain remained just as prevalent in the highly infected individuals but the proportion of persons with weak- ness declined somewhat. We conclude that of all the symptoms and signs previously attributed to Opis- thorchis viverrini infection, only abdominal pain, flatulence or dyspepsia, weakness or malaise, and hepatomegaly are associated with this infection. The percentages of the population with symptoms attributable to opisthorchiasis are in the order of 5-10% for weakness, flatulence, and abdominal pain, and 5% for hepatomegaly. These proportions are high enough to indicate that opisthorchiasis is a disease of public health significance in heavily in- fected villages, and is not one that produces medical complications in only rare cases. The percentages have been corrected for baseline levels of symptoms in uninfected or lightly infected groups, and are likely to be underestimates owing to limitations in the ques- tionnaire that was used. Our studies in Nong Ranya and Chonnabot reveal a very high prevalence of opisthorchiasis in these com- munities, living near water-beds and streams where the two intermediate hosts of 0. viverrini are com- monly present, and where people are fond of eating koi-pla (18). Whether these cases occur in villages farther from these sources of infection remains to be investigated. ACKNOWLEDGEMENTS We thank the District Officer of Chonnabot, Khon Kaen Province, for permission to work in the area and Dr Kenneth S. Warren, Director of Health Sciences Division, the Rockefeller Foundation, for his advice and encouragement. This study was supported by a grant from the Rockefeller Foundation. RESUME RELATION ENTRE LA PREVALENCE ET L'INTENSITE DE L'INFESTATION PAR OPISTHORCHIS VIVERRINI ET LES SIGNES ET SYMPTOMES CLINIQUES DANS UNE COLLECTIVITE RURALE DU NORD-EST DE LA THAILANDE A Chonnabot, gros village de la province de Khon Kaen, dans le nord-est de la Thailande, la prevalence globale de la distomatose a Opisthorchis viverrini (&valuee par la methode de denombrement des aeufs de Stoll) s'est revele egale a 89,5% sur une population totale de 1651 habitants. La pr&valence etait de 32% chez les moins de 5 ans, de 90% chez les enfants de 5 a 9 ans, s'etablissant en moyenne A 95,6%7o dans les groupes d'age au-dela de 10 ans. L'evalua- tion moyenne d'ceufs dans les selles (revelatrice du degre d'infestation) etait maximale dans le groupe d'age 40-49 ans et relativement constante au-dela. Dans tous les groupes d'age, la prevalence et l'intensite de l'infestation etaient 460 OPISTHORCHIS VIVERRINI INFECTION IN THAILAND 461 comparables chez les hommes et les femmes. Chez les sujets parasites plus frequemment que chez les autres, l'interrogatoire a montre qu'ils avaient consomme anterieurement du koi pla (un plat a base de poisson d'eau douce mange cru qui contient des metacercaires viables). Dans les groupes oui l'infestation etait plus massive, on a observe beaucoup plus souvent les sympt6mes suivants: asthenie, flatulence ou dyspepsie et douleur abdominale au niveau du quadrant superieur droit. Toutefois, les sujets parasites n'ont pas declare etre moins aptes au travail que les sujets non infestes. Anorexie, nausees, vomissements et diarrhee n'etaient que faiblement correles avec le degre d'infestation. 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