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Brucellosis is not a major cause of febrile illness in patients at public health care facilities in Binh Thuan Province, Vietnam
Journal of Infection (2006) 53, 12–15
www.elsevierhealth.com/journals/jinf
Brucellosis is not a major cause of febrile illness in patients at public health care facilities in Binh Thuan Province, Vietnam Tran T.T. Ngaa, Peter J. de Vriesb, Theresia H. Abdoelc, Henk L. Smitsc,* a
Department of Microbiology, Cho Ray Hospital, Ho Chi Minh City, Vietnam Division of Infectious Diseases, Tropical Medicine and AIDS, Academic, Medical Center, Amsterdam, The Netherlands c Department of KIT Biomedical Research, Royal Tropical Institute/Koninklijk, Instituut voor de Tropen (KIT), Meibergdreef 39, 1105 AZ Amsterdam, The Netherlands b
Accepted 24 September 2005 Available online 2 November 2005
KEYWORDS Brucellosis; Vietnam; Epidemiology; Point-of-care; Acute undifferentiated fever
Summary Objective: To determine the presence of brucellosis among patients with acute febrile illness at health care facilities in Binh Thuan province, Vietnam. Method: A retrospective seroepidemiological study on serum samples collected at 13 not adjacent health care facilities using the Rose Bengal test as a rapid screening test and the Brucella IgM/IgG flow assay as a simple confirmatory test. Result: The seroprevalence in the Rose Bengal test among 406 patients presented with acute undifferentiated fever was 14.8%. Seven of the 64 Rose Bengal test positive samples reacted weakly (1C) positive in the Brucella IgM/IgG flow assay. No seroconversion was observed. Conclusions: Brucellosis is not a major cause of morbidity in Binh Thuan province. Q 2005 The British Infection Society. Published by Elsevier Ltd. All rights reserved.
Introduction The diagnosis of febrile patients presenting at primary health care posts and district hospitals in Vietnam is often not further differentiated than ‘acute undifferentiated fever’ (AUF) because of lacking facilities and expertise for performing or ordering laboratory confirmation of common * Corresponding author. Tel.: C31 20 5665470; fax: C31 20 6971841. E-mail address: [email protected] (H.L. Smits).
infectious diseases. Brucellosis is such an infectious disease that may be diagnosed as AUF. It may run an insidious course, may present in many atypical forms and cannot be diagnosed with certainty without laboratory testing.1,2 Fever may be the only or dominant symptom of acute brucellosis.3–6 Brucellosis is a serious veterinary and public health problem of worldwide importance. The disease is a typical zoonotic disease of livestock caused by bacteria of the genus Brucella. Humans become infected by the consumption of unpasteurized dairy and through direct contacts with
0163-4453/$30.00 Q 2005 The British Infection Society. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.jinf.2005.09.016
Seroprevalence of brucellosis in Vietnam animals.7–9 The zoonotic reservoir includes cattle, goat, sheep and pigs. Brucellosis is endemic in the Mediterranean region, Middle East and Latin America. Sporadic cases also have been reported from a number of countries in South-East Asia including Thailand,10 Indonesia11,12 and Korea.13 The disease is common in parts of China.14 In Vietnam, brucellosis was reported in the 1950s and 1960s but actual data are lacking.15,16 In this study we investigated the presence of brucellosis in patients presenting with AUF at thirteen primary health care facilities in Binh Thuan province, Vietnam. The Rose Bengal test (RB) was used to screen for the presence of Brucella specific antibodies and the Brucella IgM/IgG flow assay was used for confirmation.17 The latter was used, as it is the only confirmatory test for brucellosis that does not require training and can be used outside established laboratories.18 The reported sensitivity and specificity of the flow assay are 96 and 99%, respectively, in culture confirmed cases with acute or persistent brucellosis.
Materials and methods Study group The study was performed at 12, not adjacent, commune health posts and one clinic at the provincial malaria station between January 2001 and January 2004. The median (range) number of patients per health facility was 22 (7–119). After excluding malaria and dengue, 405 AUF patients, 249 males and 156 females with a median (range) age of 24 (4–82) years, were included in the study. AUF was defined as any febrile illness of duration less than 14 days, confirmed by an axillary temperature R38.0 8C, without any indication for either severe systemic or organ specific disease. The median (range) duration of fever in these patients was 1 (0–9) day. The main complaints of these patients were anorexia (83%), myalgia (61%), sore throat (43%), nausea (30%) and backache (30%). Malaria was diagnosed by routine microscopic examination of a thick blood smear and dengue was diagnosed by IgM capture and IgG ELISA obtained from Focus Technologies (Cypress, CA, U.S.A.). From each patient two serum samples were collected, one acute sample upon first presentation and one convalescent sample 3 weeks later. All convalescent samples were screened with RB. All positive samples were additionally tested for immunoglobulin M (IgM) and IgG antibodies in the Brucella IgM/IgG flow assay and, an approximately
13 similar number of negative samples were tested in the flow assay as controls. Of cases with positive IgM or IgG results, the flow test was also performed on the acute sample. Medical ethical clearance for the study was obtained from the Scientific Committees of Cho Ray Hospital in Ho Chi Minh City and the Binh Thuan provincial health services. Informed consent was obtained form all patients or, for children, from their parents or guardians.
Serology For RB, milk ring test antigen was diluted 1:3 by mixing one volume antigen with two volumes phosphate buffered saline (pH 7.6). RB was performed by mixing 10 ml diluted antigen with 10 ml serum on a white agglutination card and agglutination was read after 2 min. Serial two-fold serum dilutions were tested for positive samples. The Brucella IgM/IgG flow assay consists of two devices one for the detection of Brucella-specific IgM antibodies and one for the detection of Brucella-specific IgG antibodies.17 The flow assay was performed exactly according to the instructions of the manufacturer. Briefly the assay is performed by applying 5 ml serum and 130 ml running fluid to the pad in the sample well of a plastic assay device. The assay is read after 10– 15 min by visual inspection for staining of the antigen line in the test zone of the assay device. The presence of antibodies is indicated by red discoloration of the antigen line. Staining intensity is subjectively rated into weak (1C), moderately strong (2C), strong (3C) and very strong (4C).
Results Sixty-four of the 405 convalescent samples agglutinated in RB giving a seroprevalence of 15.8%. Of these 64 positive samples, 34 had a titre of 1:2, 21 a titre of 1:4, four a titre of 1:8, two a titre of 1:16 and three a titre of 1:32. Six of the RB positive patients tested (1C) positive in the IgM flow assay and one tested (1C) positive in the IgG flow assay. All other RB positive cases tested negative in the IgM/IgG flow assay. The seven positive samples had RB titres of 1:2 (nZ2, including the one IgG), 1:4 (nZ2), 1:8 (nZ1), 1:16 (nZ1) and 1:32 (nZ1). Similar results were obtained for acute samples of these seven cases. The 88 RB negative convalescent control samples were all negative in the IgM/IgG flow assay. The seroprevalence in RB ranged from 0 to 50% at the 13 different study sites but there was no significant difference of the distribution of
14 positive and negative results (general log-linear analysis). Mean age, sex and profession were similar for the RB seropositive (mean age 25.3 year, 57.8% male and 50.0% farmer) and seronegative (mean age 28.3 year, 62.2% male and 53.7% farmer) groups and no seasonal variation in seropositivity was observed.
Discussion Seroprevalence rates of Brucella antibodies can be very high in particular among populations in endemic areas. For example a village survey performed in Eastern Turkey revealed a seroprevalence in RB of 26.7% with a corresponding seroprevalence in the serum agglutination test of 27.2%.19 Whereas the seroprevalence is high in specific risk groups it is usually very low or zero in groups that are not at risk because of profession, lifestyle or food habits. For instance Massenet and co-workers found a seroprevalence in RB of 14% for abattoir workers and of zero for blood donors in N’Djamena, Tchad.20 Based on confirmatory tests, the seroprevalence ranged from 1% for occupational contacts in Sudan21 to 3.2% for elderly people from Turkey,9 3.8% for nomadic pastoralists from Chad,22 6.5% for slaughterhouse workers in Djibouti23 and from 3.0 to 7.1% for different high-risk groups from Eritrea.24 In this study the seroprevalence was 15.8% in RB and 1.7% in the Brucella IgM/IgG flow assay among febrile patients seeking medical care at primary health care facilities in Binh Thuan province in Vietnam. The patients in this study are a realistic representation of febrile patients in Binh Thuan. Although private clinics are increasingly offering medical care in Binh Thuan province, it is unlikely that patients with severe acute brucellosis preferentially seek medical attention there. In this study we included patients with fever of less than 2 weeks and without localized infections. Although Brucella infections may become persistent and then often present with localized disease, it seems unlikely that this introduced a negative selection bias. In Vietnam, patients have free access to a very effective primary health care system and also brucellosis patients would have presented in the acute febrile stage.25 Although the sensitivity (96%) and specificity (99%) of the Brucella IgM/IgG flow assay are relatively high, our results do not to provide strong evidence of active brucellosis in these patients. The samples from the seven patients that reacted positive stained weakly (1C) only and
T.T.T. Nga et al. seroconversion was not observed. We previously showed that the majority of the samples from patients with confirmed brucellosis gave a moderate (2C) to very strong (4C) staining and observed seroconversion or an increase in staining intensity as evidence of acute disease in several patients with confirmed brucellosis.17,18 Even though the patients presented at a very early stage and the sensitivity of serology is less than optimal during the first weeks of illness, a strong staining in the IgM flow assay could be expected in the convalescent sera of patients with an acute infection.26 Weakly positive reactivity in the flow assay may point at previous exposure to Brucella or to a sub-clinical infection. This has been observed in veterinarians, who were exposed to the pathogen, and in asymptomatic members of an epidemic community.17,18 Despite a high specificity of 99%, the positive predictive value of a single positive test is low in low endemic areas. Further clinical investigations and repeated tests to demonstrate an increase in reactivity in the flow assay during follow-up would be needed to confirm active brucellosis in these patients. The high proportion of positive Rose Bengal test results among the population of Binh Thuan province is probably caused by reactivity with persisting specific antibodies after recovery from active brucellosis or after previous asymptomatic exposure to the pathogen. This suggests a previously high exposure rate to the pathogen. The results of the flow assay however, indicate that brucellosis is not a major medical problem in this area. A high seroprevalence of Brucella antibodies is regarded a threat to public health even when the incidence of acute disease is low. This is explained by species-specific factors. All three species may cause disease in humans, but Brucella melitensis and B. suis are more pathogenic than B. abortus.1,27,28 B. melitensis is an infection of small ruminants, B. suis is specific for swine and B. abortus for cattle. In Binh Thuan province it is common to keep buffaloes and pigs but goats and sheep have been introduced only recently. Infection often shows a seasonal fluctuation with most cases occurring in spring and early summer because of the high amounts of the bacterium in milk during this period and because farmers may be exposed to the bacterium when assisting delivery or handling abortion materials.29 In our study samples were collected throughout the year and failure to detect cases of brucellosis thus cannot be contributed to a sampling selection. In endemic areas, brucellosis is also associated with the consumption of fresh or raw animal products.7–9 The consumption of such products is uncommon in Binh Thuan province.
Seroprevalence of brucellosis in Vietnam The seroprevalence in RB varied considerably (0–50%) between the 13 study sites and this could point to regional differences in prevalence of the infection in livestock. In countries where brucellosis is endemic transmission and illness in the human population often is confined due to a limited number of active foci in livestock.22,30 Veterinary surveillance is needed to clarify the presence of brucellosis in livestock in Binh Thuan province. We conclude that the population of Binh Thuan province is probably exposed to Brucella but this does not pose a significant health problem. Transient infections causing sub-clinical or mild disease may occur.
Acknowledgements The study is part of a research program, supported by the Foundation for the Advancement of Tropical Research (WOTRO), the Netherlands. The generous gift of antigen by the Veterinary Laboratories Agency, U.K. is gratefully acknowledged.
References 1. Young EJ. An overview of human brucellosis. Clin Infect Dis 1995;21:283–9. 2. Corbell MJ. Brucellosis: an overview. Emerg Infect Dis 1997; 3:213–21. 3. Memish Z, Mah MW, Al Mahmoud S, Al Shaalan M, Khan MY. Brucella bacteraemia: clinical and laboratory observations in 160 patients. J Infect 2002;40:59–63. 4. Gur A, Geyik MF, Dikici B, Nas K, Cevik R, Sarac J, et al. Complications of brucellosis in different age groups: a study of 283 cases in southeastern Anatolia of Turkey. Yonsei Med J 2003;44:33–44. 5. El-Amin EO, George L, Kutty NK, Sharma PP, Choithramani RS, Jhaveri VP, et al. Brucellosis in children of Dhofar Region, Oman. Saudi Med J 2001;22:610–5. 6. Shaalan MA, Memish ZA, Mahmoud SA, Alomari A, Khan MY, Almuneef M, et al. Brucellosis in children: clinical observations in 115 cases. Int J Infect Dis 2002;6:182–6. 7. Al-Shamahy HA, Whitty CJ, Wright SG. Risk factors for human brucellosis in Yemen: a case control study. Epidemiol Infect 2002;125:309–13. 8. Bikas C, Jelastopulu E, Leotsinidis M, Kondakis X. Epidemiology of human brucellosis in a rural area of north-western Peloponnese in Greece. Eur J Epidemiol 2003;18:267–74. 9. Sumer H, Sumer Z, Alim A, Nur N, Ozdemir L. Seroprevalence of Brucella in an elderly population in mid-Anatolia, Turkey. J Health Popul Nutr 2003;21:158–61. 10. Manosuthi W, Thummakul T, Vibhagool A, Vorachit M, Malathum K. Case report: brucellosis: a re-emerging disease in Thailand. Southeast Asian J Trop Med Public Health 2004; 35:109–12. 11. Chomel BB, Kasten R, Adams C, Lambillotte D, Theis J, Goldsmith R, et al. Serosurvey of some major zoonotic infections in children and teenagers in Bali, Indonesia. Southeast Asian J Trop Med Public Health 1993;24:321–6.
15 12. Geong M, Robertson ID. Response of Bali cattle (Bos javanicus) to vaccination with Brucella abortus strain 19 in West Timor. Prev Vet Med 2000;47:177–86. 13. Chung JS, Cho YJ, Park CK. Isolation and biotyping of Brucella abortus from dairy cattle in Kyungbuk area, Korea. Kor. J Vet Res 1988;28:339–43. 14. Deqiu S, Donglou X, Jiming Y. Epidemiology and control of brucellosis in China. Vet Microbiol 2002;90:165–82. 15. Nguyen-Van-Ai, Nguyen-Ngoc-Dai. First case of brucellosis bacteriologically confirmed in Vietnam. Bull Soc Pathol Exot Filiales 1957;50:21–3. 16. Spinu I, Vasilesco T. Brucella strains of animal origin isolated for the first time in North Vietnam. Arch Roum Pathol Exp Microbiol 1962;21:155–8. 17. Smits HL, Abdoel TH, Solera J, Clavijo E, Diaz R. Immunochromatographic Brucella-specific immunoglobulin M and G lateral flow assays for rapid serodiagnosis of human brucellosis. Clin Diagn Lab Immunol 2003;10:1141–6. 18. Irmak H, Buzgan T, Evirgen O, Akdeniz H, Demiroz AP, Abdoel TH, et al. Use of the Brucella IgM and IgG flow assays in the serodiagnosis of human brucellosis in an area endemic for brucellosis. Am J Trop Med Hyg 2004; 70:688–94. 19. Ceylan EH, Irmak T, Buzgan MK, Karahocagil O, Evirgen N, Sakarya H, et al. Van iline bagli bazi koylerde insan ve hayvan populasyonunda bruselloz seroprevalansi. Van Tip Dergisi 2003;10:1–5. 20. Massenet D, Djime O, Karifene R. Seroepidemiological survey of brucellosis in abattoir personnel in N’Djamena (Tchad). Med Trop 1993;33:253–5. 21. El-Ansary EH, Mohammed BA, Hamad AR, Karom AG. Brucellosis among animals and human contacts in eastern Sudan. Saudi Med J 2001;22:577–9. 22. Schelling E, Diguimbaye C, Daoud S, Nicolet J, Boerlin P, Tanner M, et al. Brucellosis and Q-fever seroprevalences of nomadic pastoralists and their livestock in Chad. Prev Vet Med 2003;61:279–93. 23. Chantal J, Bessiere MH, Le Guenno B, Magnaval JF, Dorchies P. Serologic screening of certain zoonoses in the abattoir personnel in Djibouti. Bull Soc Pathol Exot 1996;89: 353–7. 24. Omer MK, Assefaw T, Skjerve E, Tekleghiorghis T, Woldehiwet Z. Prevalence of antibodies to Brucella spp. and risk factors related to highrisk occupational groups in Eritrea. Epidemiol Infect 2002;129:85–91. 25. Van Nam N, de Vries PJ, Van Toi L, Nagelkerke N. Malaria control in Vietnam: the Binh Thuan experience. Trop Med Int Health 2005;10:357–65. 26. Smits HL, Basahi MA, Diaz R, Marrodan T, Douglas JT, Rocha A, et al. Development and evaluation of a rapid dipstick assay for the serodiagnosis of the early phase of brucellosis in humans. J Clin Microbiol 1999;37: 4179–82. 27. Young EJ. Brucellosis: current epidemiology, diagnosis, and management. Curr Clin Top Infect 1995;15:115–28. 28. Dokuzoguz B, Ergonul O, Baykam N, Esener H, Kilic S, Celikbas A, et al. Characteristics of B. melitensis versus B. abortus bacteraemias. J Infect 2005;50:41–5. 29. Cooper CW. The epidemiology of human brucellosis in a well defined urban population in Saudi Arabia. J Trop Med Hyg 1991;94:416–22. 30. Omer MK, Skjerve E, MacMillan AP, Woldehiwet Z. Comparison of three serological tests in the diagnosis of Brucella infection in unvaccinated cattle in Eritrea. Prev Vet Med 2001;48:215–22.
www.elsevierhealth.com/journals/jinf
Brucellosis is not a major cause of febrile illness in patients at public health care facilities in Binh Thuan Province, Vietnam Tran T.T. Ngaa, Peter J. de Vriesb, Theresia H. Abdoelc, Henk L. Smitsc,* a
Department of Microbiology, Cho Ray Hospital, Ho Chi Minh City, Vietnam Division of Infectious Diseases, Tropical Medicine and AIDS, Academic, Medical Center, Amsterdam, The Netherlands c Department of KIT Biomedical Research, Royal Tropical Institute/Koninklijk, Instituut voor de Tropen (KIT), Meibergdreef 39, 1105 AZ Amsterdam, The Netherlands b
Accepted 24 September 2005 Available online 2 November 2005
KEYWORDS Brucellosis; Vietnam; Epidemiology; Point-of-care; Acute undifferentiated fever
Summary Objective: To determine the presence of brucellosis among patients with acute febrile illness at health care facilities in Binh Thuan province, Vietnam. Method: A retrospective seroepidemiological study on serum samples collected at 13 not adjacent health care facilities using the Rose Bengal test as a rapid screening test and the Brucella IgM/IgG flow assay as a simple confirmatory test. Result: The seroprevalence in the Rose Bengal test among 406 patients presented with acute undifferentiated fever was 14.8%. Seven of the 64 Rose Bengal test positive samples reacted weakly (1C) positive in the Brucella IgM/IgG flow assay. No seroconversion was observed. Conclusions: Brucellosis is not a major cause of morbidity in Binh Thuan province. Q 2005 The British Infection Society. Published by Elsevier Ltd. All rights reserved.
Introduction The diagnosis of febrile patients presenting at primary health care posts and district hospitals in Vietnam is often not further differentiated than ‘acute undifferentiated fever’ (AUF) because of lacking facilities and expertise for performing or ordering laboratory confirmation of common * Corresponding author. Tel.: C31 20 5665470; fax: C31 20 6971841. E-mail address: [email protected] (H.L. Smits).
infectious diseases. Brucellosis is such an infectious disease that may be diagnosed as AUF. It may run an insidious course, may present in many atypical forms and cannot be diagnosed with certainty without laboratory testing.1,2 Fever may be the only or dominant symptom of acute brucellosis.3–6 Brucellosis is a serious veterinary and public health problem of worldwide importance. The disease is a typical zoonotic disease of livestock caused by bacteria of the genus Brucella. Humans become infected by the consumption of unpasteurized dairy and through direct contacts with
0163-4453/$30.00 Q 2005 The British Infection Society. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.jinf.2005.09.016
Seroprevalence of brucellosis in Vietnam animals.7–9 The zoonotic reservoir includes cattle, goat, sheep and pigs. Brucellosis is endemic in the Mediterranean region, Middle East and Latin America. Sporadic cases also have been reported from a number of countries in South-East Asia including Thailand,10 Indonesia11,12 and Korea.13 The disease is common in parts of China.14 In Vietnam, brucellosis was reported in the 1950s and 1960s but actual data are lacking.15,16 In this study we investigated the presence of brucellosis in patients presenting with AUF at thirteen primary health care facilities in Binh Thuan province, Vietnam. The Rose Bengal test (RB) was used to screen for the presence of Brucella specific antibodies and the Brucella IgM/IgG flow assay was used for confirmation.17 The latter was used, as it is the only confirmatory test for brucellosis that does not require training and can be used outside established laboratories.18 The reported sensitivity and specificity of the flow assay are 96 and 99%, respectively, in culture confirmed cases with acute or persistent brucellosis.
Materials and methods Study group The study was performed at 12, not adjacent, commune health posts and one clinic at the provincial malaria station between January 2001 and January 2004. The median (range) number of patients per health facility was 22 (7–119). After excluding malaria and dengue, 405 AUF patients, 249 males and 156 females with a median (range) age of 24 (4–82) years, were included in the study. AUF was defined as any febrile illness of duration less than 14 days, confirmed by an axillary temperature R38.0 8C, without any indication for either severe systemic or organ specific disease. The median (range) duration of fever in these patients was 1 (0–9) day. The main complaints of these patients were anorexia (83%), myalgia (61%), sore throat (43%), nausea (30%) and backache (30%). Malaria was diagnosed by routine microscopic examination of a thick blood smear and dengue was diagnosed by IgM capture and IgG ELISA obtained from Focus Technologies (Cypress, CA, U.S.A.). From each patient two serum samples were collected, one acute sample upon first presentation and one convalescent sample 3 weeks later. All convalescent samples were screened with RB. All positive samples were additionally tested for immunoglobulin M (IgM) and IgG antibodies in the Brucella IgM/IgG flow assay and, an approximately
13 similar number of negative samples were tested in the flow assay as controls. Of cases with positive IgM or IgG results, the flow test was also performed on the acute sample. Medical ethical clearance for the study was obtained from the Scientific Committees of Cho Ray Hospital in Ho Chi Minh City and the Binh Thuan provincial health services. Informed consent was obtained form all patients or, for children, from their parents or guardians.
Serology For RB, milk ring test antigen was diluted 1:3 by mixing one volume antigen with two volumes phosphate buffered saline (pH 7.6). RB was performed by mixing 10 ml diluted antigen with 10 ml serum on a white agglutination card and agglutination was read after 2 min. Serial two-fold serum dilutions were tested for positive samples. The Brucella IgM/IgG flow assay consists of two devices one for the detection of Brucella-specific IgM antibodies and one for the detection of Brucella-specific IgG antibodies.17 The flow assay was performed exactly according to the instructions of the manufacturer. Briefly the assay is performed by applying 5 ml serum and 130 ml running fluid to the pad in the sample well of a plastic assay device. The assay is read after 10– 15 min by visual inspection for staining of the antigen line in the test zone of the assay device. The presence of antibodies is indicated by red discoloration of the antigen line. Staining intensity is subjectively rated into weak (1C), moderately strong (2C), strong (3C) and very strong (4C).
Results Sixty-four of the 405 convalescent samples agglutinated in RB giving a seroprevalence of 15.8%. Of these 64 positive samples, 34 had a titre of 1:2, 21 a titre of 1:4, four a titre of 1:8, two a titre of 1:16 and three a titre of 1:32. Six of the RB positive patients tested (1C) positive in the IgM flow assay and one tested (1C) positive in the IgG flow assay. All other RB positive cases tested negative in the IgM/IgG flow assay. The seven positive samples had RB titres of 1:2 (nZ2, including the one IgG), 1:4 (nZ2), 1:8 (nZ1), 1:16 (nZ1) and 1:32 (nZ1). Similar results were obtained for acute samples of these seven cases. The 88 RB negative convalescent control samples were all negative in the IgM/IgG flow assay. The seroprevalence in RB ranged from 0 to 50% at the 13 different study sites but there was no significant difference of the distribution of
14 positive and negative results (general log-linear analysis). Mean age, sex and profession were similar for the RB seropositive (mean age 25.3 year, 57.8% male and 50.0% farmer) and seronegative (mean age 28.3 year, 62.2% male and 53.7% farmer) groups and no seasonal variation in seropositivity was observed.
Discussion Seroprevalence rates of Brucella antibodies can be very high in particular among populations in endemic areas. For example a village survey performed in Eastern Turkey revealed a seroprevalence in RB of 26.7% with a corresponding seroprevalence in the serum agglutination test of 27.2%.19 Whereas the seroprevalence is high in specific risk groups it is usually very low or zero in groups that are not at risk because of profession, lifestyle or food habits. For instance Massenet and co-workers found a seroprevalence in RB of 14% for abattoir workers and of zero for blood donors in N’Djamena, Tchad.20 Based on confirmatory tests, the seroprevalence ranged from 1% for occupational contacts in Sudan21 to 3.2% for elderly people from Turkey,9 3.8% for nomadic pastoralists from Chad,22 6.5% for slaughterhouse workers in Djibouti23 and from 3.0 to 7.1% for different high-risk groups from Eritrea.24 In this study the seroprevalence was 15.8% in RB and 1.7% in the Brucella IgM/IgG flow assay among febrile patients seeking medical care at primary health care facilities in Binh Thuan province in Vietnam. The patients in this study are a realistic representation of febrile patients in Binh Thuan. Although private clinics are increasingly offering medical care in Binh Thuan province, it is unlikely that patients with severe acute brucellosis preferentially seek medical attention there. In this study we included patients with fever of less than 2 weeks and without localized infections. Although Brucella infections may become persistent and then often present with localized disease, it seems unlikely that this introduced a negative selection bias. In Vietnam, patients have free access to a very effective primary health care system and also brucellosis patients would have presented in the acute febrile stage.25 Although the sensitivity (96%) and specificity (99%) of the Brucella IgM/IgG flow assay are relatively high, our results do not to provide strong evidence of active brucellosis in these patients. The samples from the seven patients that reacted positive stained weakly (1C) only and
T.T.T. Nga et al. seroconversion was not observed. We previously showed that the majority of the samples from patients with confirmed brucellosis gave a moderate (2C) to very strong (4C) staining and observed seroconversion or an increase in staining intensity as evidence of acute disease in several patients with confirmed brucellosis.17,18 Even though the patients presented at a very early stage and the sensitivity of serology is less than optimal during the first weeks of illness, a strong staining in the IgM flow assay could be expected in the convalescent sera of patients with an acute infection.26 Weakly positive reactivity in the flow assay may point at previous exposure to Brucella or to a sub-clinical infection. This has been observed in veterinarians, who were exposed to the pathogen, and in asymptomatic members of an epidemic community.17,18 Despite a high specificity of 99%, the positive predictive value of a single positive test is low in low endemic areas. Further clinical investigations and repeated tests to demonstrate an increase in reactivity in the flow assay during follow-up would be needed to confirm active brucellosis in these patients. The high proportion of positive Rose Bengal test results among the population of Binh Thuan province is probably caused by reactivity with persisting specific antibodies after recovery from active brucellosis or after previous asymptomatic exposure to the pathogen. This suggests a previously high exposure rate to the pathogen. The results of the flow assay however, indicate that brucellosis is not a major medical problem in this area. A high seroprevalence of Brucella antibodies is regarded a threat to public health even when the incidence of acute disease is low. This is explained by species-specific factors. All three species may cause disease in humans, but Brucella melitensis and B. suis are more pathogenic than B. abortus.1,27,28 B. melitensis is an infection of small ruminants, B. suis is specific for swine and B. abortus for cattle. In Binh Thuan province it is common to keep buffaloes and pigs but goats and sheep have been introduced only recently. Infection often shows a seasonal fluctuation with most cases occurring in spring and early summer because of the high amounts of the bacterium in milk during this period and because farmers may be exposed to the bacterium when assisting delivery or handling abortion materials.29 In our study samples were collected throughout the year and failure to detect cases of brucellosis thus cannot be contributed to a sampling selection. In endemic areas, brucellosis is also associated with the consumption of fresh or raw animal products.7–9 The consumption of such products is uncommon in Binh Thuan province.
Seroprevalence of brucellosis in Vietnam The seroprevalence in RB varied considerably (0–50%) between the 13 study sites and this could point to regional differences in prevalence of the infection in livestock. In countries where brucellosis is endemic transmission and illness in the human population often is confined due to a limited number of active foci in livestock.22,30 Veterinary surveillance is needed to clarify the presence of brucellosis in livestock in Binh Thuan province. We conclude that the population of Binh Thuan province is probably exposed to Brucella but this does not pose a significant health problem. Transient infections causing sub-clinical or mild disease may occur.
Acknowledgements The study is part of a research program, supported by the Foundation for the Advancement of Tropical Research (WOTRO), the Netherlands. The generous gift of antigen by the Veterinary Laboratories Agency, U.K. is gratefully acknowledged.
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