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Jambulingam P, Subramanian S, de Vlas SJ, Vinubala C, Stolk WA. Mathematical modelling of lymphatic filariasis elimination programmes in India: required duration of mass drug administration and post-treatment level of infection indicators. Parasit Vectors 2016; 9:501. [PMID: 27624157 PMCID: PMC5022201 DOI: 10.1186/s13071-016-1768-y] [Citation(s) in RCA: 35] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/23/2015] [Accepted: 08/22/2016] [Indexed: 12/03/2022] Open
Abstract
Background India has made great progress towards the elimination of lymphatic filariasis. By 2015, most endemic districts had completed at least five annual rounds of mass drug administration (MDA). The next challenge is to determine when MDA can be stopped. We performed a simulation study with the individual-based model LYMFASIM to help clarify this. Methods We used a model-variant for Indian settings. We considered different hypotheses on detectability of antigenaemia (Ag) in relation to underlying adult worm burden, choosing the most likely hypothesis by comparing the model predicted association between community-level microfilaraemia (Mf) and antigenaemia (Ag) prevalence levels to observed data (collated from literature). Next, we estimated how long MDA must be continued in order to achieve elimination in different transmission settings and what Mf and Ag prevalence may still remain 1 year after the last required MDA round. The robustness of key-outcomes was assessed in a sensitivity analysis. Results Our model matched observed data qualitatively well when we assumed an Ag detection rate of 50 % for single worm infections, which increases with the number of adult worms (modelled by relating detection to the presence of female worms). The required duration of annual MDA increased with higher baseline endemicity and lower coverage (varying between 2 and 12 rounds), while the remaining residual infection 1 year after the last required treatment declined with transmission intensity. For low and high transmission settings, the median residual infection levels were 1.0 % and 0.4 % (Mf prevalence in the 5+ population), and 3.5 % and 2.0 % (Ag prevalence in 6–7 year-old children). Conclusion To achieve elimination in high transmission settings, MDA must be continued longer and infection levels must be reduced to lower levels than in low-endemic communities. Although our simulations were for Indian settings, qualitatively similar patterns are also expected in other areas. This should be taken into account in decision algorithms to define whether MDA can be interrupted. Transmission assessment surveys should ideally be targeted to communities with the highest pre-control transmission levels, to minimize the risk of programme failure. Electronic supplementary material The online version of this article (doi:10.1186/s13071-016-1768-y) contains supplementary material, which is available to authorized users.
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Affiliation(s)
- Purushothaman Jambulingam
- Vector Control Research Centre (Indian Council of Medical Research), Indira Nagar, Puducherry, 605006, India
| | - Swaminathan Subramanian
- Vector Control Research Centre (Indian Council of Medical Research), Indira Nagar, Puducherry, 605006, India.
| | - S J de Vlas
- Department of Public Health, Erasmus MC, University Medical Centre Rotterdam, Rotterdam, The Netherlands
| | - Chellasamy Vinubala
- Vector Control Research Centre (Indian Council of Medical Research), Indira Nagar, Puducherry, 605006, India
| | - W A Stolk
- Department of Public Health, Erasmus MC, University Medical Centre Rotterdam, Rotterdam, The Netherlands
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Cai R, Tan JG, Chen L, Richardus JH, de Vlas SJ. Prevalence and risk factors of syphilis infection among female sex workers in Shenzhen, China: an observational study (2009-2012). Trop Med Int Health 2013; 18:1531-8. [PMID: 24118535 DOI: 10.1111/tmi.12204] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
OBJECTIVE To investigate prevalence and risk factors of syphilis infection among female sex workers (FSWs) in Shenzhen, China. METHODS Observational study among (2009-2012) 1653 FSWs recruited by venue-based sampling using questionnaire-based interviews for socio-demographics, behaviours and syphilis testing results. Logistic regression was used to assess risk factors of syphilis infection. RESULTS The overall syphilis prevalence was 4.7%, showing a slightly decreasing trend. Factors significantly associated with syphilis infection were inconsistent condom use (OR = 1.87, P = 0.015), illicit drug use (OR = 5.45, P < 0.001) and older age in years (OR = 1.08, P < 0.001). Venues where FSWs were recruited and duration of commercial sex work were not significantly associated with syphilis infection (P > 0.05). CONCLUSIONS Syphilis is still common among FSWs in Shenzhen, China. Current comprehensive prevention programmes (e.g. condom promotion and peer education) should be continued to maintain and increase safe sexual practices and to reduce illicit drug use among FSWs. Expanding point-of-care syphilis screening programmes may be an important strategy for early diagnosis. We recommend timely and effective treatment programmes to be linked to such screening programmes.
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Affiliation(s)
- R Cai
- Department of Public Health, Erasmus MC, University Medical Center Rotterdam, Rotterdam, The Netherlands; Shenzhen Center for Disease Control and Prevention, Shenzhen, China
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Verdier JE, de Vlas SJ, Baltussen R, Richardus JH. A systematic review of economic evaluation studies of tuberculosis control in high-income countries. Int J Tuberc Lung Dis 2011; 15:1587-98. [PMID: 21740647 DOI: 10.5588/ijtld.10.0332] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022] Open
Abstract
Tuberculosis (TB) is a leading cause of death in developing countries and an important health threat in the industrialised world. Ideally, interventions in TB control are effective, acceptable and economically attractive. This review summarises all economic evaluation studies of TB control in high-income countries over the last 20 years. We provide indications on the relative economic attractiveness of TB interventions based on the reported conclusions. A total of 118 studies using different economic evaluation methodologies on a wide range of TB interventions are included. Most studies (70%) were from North America, and about half (47%) concerned interventions among the general population. Even though the large majority of studies (85%) aimed at preventing active TB disease, 44% of these ignored the prevention of secondary infections, thereby under- estimating the benefits of the intervention. Choosing a health care instead of a societal perspective (92% vs. 8%) further underestimated the benefits. Moreover, 74 studies (62%) disregarded discounting, and for 9 of them this led to overestimated future costs. In all, 66% of the studies reported conclusions favouring the evaluated intervention, which is modest given that a publishing bias towards favourable results is to be expected. In conclusion, we demonstrate that many studies in this review have put the evaluated TB intervention at a disadvantage by the choice of methodology, i. e., underestimating benefits and overestimating costs. This may have led to an overly conservative approach to the introduction of new interventions in TB control.
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Affiliation(s)
- J E Verdier
- Department of Public Health, Erasmus MC, University Medical Center Rotterdam, Rotterdam.
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4
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Scholten JN, de Vlas SJ, Zaleskis R. Under-reporting of HIV infection among cohorts of TB patients in the WHO European Region, 2003-2004. Int J Tuberc Lung Dis 2008; 12:85-91. [PMID: 18302829] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/26/2023] Open
Abstract
SETTING The World Health Organization (WHO) European Region, which includes 52 countries. OBJECTIVE To assess an epidemiological data collection tool implemented in 2004 for recording human immunodeficiency virus (HIV) status among tuberculosis (TB) patients. DESIGN Based on WHO estimates, the reported number of TB patients with HIV was compared with the number expected. The analysis included all forms of TB and was restricted to adults, wherever possible. The numbers of TB patients detected from HIV/acquired immune-deficiency syndrome (AIDS) programmes (NAPs) were also assessed. RESULTS In 2003, 20 (38%) National TB Programmes (NTPs) reported 4602 (35%) HIV-infected TB cases of 13117 expected; in 2004, 23 (44%) NTPs reported 5902 (42%) cases of 13901 expected. In 2003, 47 (90%) NAPs reported 3575 (27%) TB cases as a new AIDS diagnosis, while in 2004, 40 (77%) NAPs reported 3901 (28%) TB cases. Those countries that did report, reported on average 60-65% of expected cases, irrespective of the kind of programme and the year. CONCLUSION Most NTPs did not report TB cases with HIV infection. Overall, the number of cases reported in countries that reported data from either NTPs or NAPs was significantly lower than expected. Improved surveillance requires concerted efforts from both NTPs and NAPs.
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Affiliation(s)
- J N Scholten
- Department of Public Health, Erasmus MC, University Medical Center, Rotterdam, The Netherlands.
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5
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Yu JM, de Vlas SJ, Jiang QW, Gryseels B. Comparison of the Kato-Katz technique, hatching test and indirect hemagglutination assay (IHA) for the diagnosis of Schistosoma japonicum infection in China. Parasitol Int 2006; 56:45-9. [PMID: 17188018 DOI: 10.1016/j.parint.2006.11.002] [Citation(s) in RCA: 111] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/18/2006] [Revised: 11/12/2006] [Accepted: 11/17/2006] [Indexed: 11/24/2022]
Abstract
The Kato-Katz technique (duplicate 41.7 mg fecal smears), hatching test and indirect hemagglutination assay (IHA) were compared for their ability to detect human Schistosoma japonicum infection in two endemic villages (Zhonjiang and Zhuxi) in rural China. The hatching test (using a nylon bag, and based on about 30 g of feces) and IHA are conventional Chinese diagnostic methods. In both villages, the trends of prevalences with age and sex were comparable for the different methods. In Zhuxi, Kato-Katz examinations of stools from 7 different days and hatching were available, which could be used as a reliable gold standard. This resulted for IHA in a sensitivity of 80% and a specificity of 48%. The sensitivity of the Kato-Katz technique using one stool specimen was 68%, twice that of hatching (33%). In Zhonjiang, however, hatching resulted in more positive cases than Kato-Katz (prevalence 31% vs. 24%). Apparently, the result of the hatching test depends on environmental factors such as temperature and water quality. Although imperfect, Kato-Katz is recommended out of the three evaluated techniques as the method of choice for large-scale screening of S. japonicum. Hatching is much more tedious, provides inconsistent and only qualitative results, and is not much more sensitive than Kato-Katz. Its poor specificity makes IHA unsuitable for individual screening, but it may be more effective for community diagnosis.
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Affiliation(s)
- J M Yu
- Department of Preventive Medicine, Medical College, Tongji University, Shanghai 200092, PR China.
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Stolk WA, Van Oortmarssen GJ, Subramanian S, Das PK, Borsboom GJJM, Habbema JDF, de Vlas SJ. Assessing density dependence in the transmission of lymphatic filariasis: uptake and development of Wuchereria bancrofti microfilariae in the vector mosquitoes. Med Vet Entomol 2004; 18:57-60. [PMID: 15009446 DOI: 10.1111/j.0269-283x.2004.0470.x] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/24/2023]
Abstract
Understanding density dependence in the transmission of lymphatic filariasis is essential for assessing the prospects of elimination. This study seeks to quantify the relationship between microfilaria (Mf) density in human blood and the number of third stage (L3) larvae developing in the mosquito vectors Aedes polynesiensis Marks and Culex quinquefasciatus Say (Diptera: Culicidae) after blood-feeding. Two types of curves are fitted to previously published data. Fitting a linearized power curve through the data allows for correction for measurement error in human Mf counts. Ignoring measurement error leads to overestimation of the strength of density dependence; the degree of overestimation depends on the accuracy of measurement of Mf density. For use in mathematical models of transmission of lymphatic filariasis, a hyperbolic saturating function is preferable. This curve explicitly estimates the Mf uptake and development at lowest Mf densities and the average maximum number of L3 that can develop in mosquitoes. This maximum was estimated at 23 and 4 for Ae. polynesiensis and Cx. quinquefasciatus, respectively.
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Affiliation(s)
- W A Stolk
- Department of Public Health, Erasmus MC, University Medical Center Rotterdam, the Netherlands.
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Landouré A, van der Werf MJ, Traoré M, de Vlas SJ. Evaluation of case management in the integrated schistosomiasis-control programme in Mali. Ann Trop Med Parasitol 2004; 97:723-36. [PMID: 14613631 DOI: 10.1179/000349803225001544] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/31/2022]
Abstract
Currently, schistosomiasis control in Mali is mainly based on treatment with praziquantel (PZQ). The policy is to ensure (1). the treatment, every 3 years, of school-age children in all areas where the prevalence of haematuria (an indicator of Schistosoma haematobium infection) in this age-group is >30%, and (2). the sound management of all patients presenting with haematuria or (bloody) diarrhoea (an indicator of S. mansoni infection) at health centres. In the present study, the application of case management was evaluated by visiting 60 healthcare facilities in four geographical areas and interviewing the health-workers at these facilities. The presence of S. haematobium and S. mansoni was reported in the coverage areas of 83% and 37%, respectively, of the healthcare facilities investigated. Health-worker knowledge of the main symptoms of schistosome infection was good and patients exhibiting symptoms attributable to schistosome infection were very likely to receive adequate treatment, particularly (at a frequency of about 80%) when they presented with haematuria. At health-centre level, patients were often directly treated with PZQ, whereas health-workers in district hospitals and private clinics requested a diagnostic test prior to any treatment. PZQ was available in most healthcare facilities but not in the private clinics. The mean cost of treatment of a patient with S. haematobium infection (euro; 2.30) was very similar to the corresponding cost for S. mansoni infection (euro; 2.37). The cost of the PZQ represented approximately 50% of the total costs borne by the patients when presenting at health centres. Patients with the symptoms of S. haematobium infection in Mali can expect adequate diagnosis and treatment in agreement with the recommendations of the World Health Organization. Patients presenting with symptoms related to S. mansoni infection are, however, less likely to be correctly diagnosed. The relatively high costs of treatment and the infrequency with which cases of schistosomiasis seek healthcare necessitate policy decisions, to ensure an affordable and more attractive, clinical system of case management.
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Affiliation(s)
- A Landouré
- Department of Public Health, Erasmus MC, University Medical Centre Rotterdam, PO Box 1738, 3000 DR Rotterdam, The Netherlands
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Sow S, de Vlas SJ, Polman K, Gryseels B. [Hygiene practices and contamination risks of surface waters by schistosome eggs: the case of an infested village in Northern Senegal]. Bull Soc Pathol Exot 2004; 97:12-4. [PMID: 15104150] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Subscribe] [Scholar Register] [Indexed: 04/29/2023]
Abstract
The transmission of intestinal schistosomiasis presumes that faecal materials containing viable schistosome eggs reach natural water bodies infested with snail intermediate host. So far there is little knowledge about the contamination dynamics of streams with schistosome eggs. We conducted a pilot study on defecating behaviour and hygienic practices in an Schistosoma mansoni endemic focus in Northern Senegal. Questionnaires were used to obtain quantitative data on hygienic practices and the use of latrines in 59 children. Although the community was well endowed with pit latrines, most of the children declared that they usually defecated somewhere else, in particular near the streams where the vegetation offers hideouts. Observations based on mapping of defecation sites showed that a considerable number of stools were left just a few meters from the riverbank, thus bearing a high risk of being washed off into the water. All these practices can easily lead to contamination of water bodies with schistosomae eggs. In order to improve hygienic practices and reduce fecal pollution of the environment, a health education model respecting local beliefs and customs would be indispensable.
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Affiliation(s)
- S Sow
- Institut de médecine tropicale Prince Léopold, Anvers, Belgique.
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Orroth KK, Korenromp EL, White RG, Changalucha J, de Vlas SJ, Gray RH, Hughes P, Kamali A, Ojwiya A, Serwadda D, Wawer MJ, Hayes RJ, Grosskurth H. Comparison of STD prevalences in the Mwanza, Rakai, and Masaka trial populations: the role of selection bias and diagnostic errors. Sex Transm Infect 2003; 79:98-105. [PMID: 12690128 PMCID: PMC1744635 DOI: 10.1136/sti.79.2.98] [Citation(s) in RCA: 38] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/04/2022] Open
Abstract
OBJECTIVES To assess bias in estimates of STD prevalence in population based surveys resulting from diagnostic error and selection bias. To evaluate the effects of such biases on STD prevalence estimates from three community randomised trials of STD treatment for HIV prevention in Masaka and Rakai, Uganda and Mwanza, Tanzania. METHODS Age and sex stratified prevalences of gonorrhoea, chlamydia, syphilis, HSV-2 infection, and trichomoniasis observed at baseline in the three trials were adjusted for sensitivity and specificity of diagnostic tests and for sample selection criteria. RESULTS STD prevalences were underestimated in all three populations because of diagnostic errors and selection bias. After adjustment, gonorrhoea prevalence was higher in men and women in Mwanza (2.8% and 2.3%) compared to Rakai (1.1% and 1.9%) and Masaka (0.9% and 1.8%). Chlamydia prevalence was higher in women in Mwanza (13.0%) compared to Rakai (3.2%) and Masaka (1.6%) but similar in men (2.3% in Mwanza, 2.7% in Rakai, and 2.2% in Masaka). Prevalence of trichomoniasis was higher in women in Mwanza compared to women in Rakai (41.9% versus 30.8%). Herpes simplex virus type 2 (HSV-2) seroprevalence and prevalence of serological syphilis (TPHA+/RPR+) were similar in the three populations but the prevalence of high titre syphilis (TPHA+/RPR >/=1:8) in men and women was higher in Mwanza (5.6% and 6.3%) than in Rakai (2.3% and 1.4%) and Masaka (1.2% and 0.7%). CONCLUSIONS Limited sensitivity of diagnostic and screening tests led to underestimation of STD prevalence in all three trials but especially in Mwanza. Adjusted prevalences of curable STD were higher in Mwanza than in Rakai and Masaka.
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Affiliation(s)
- K K Orroth
- London School of Hygiene and Tropical Medicine, London, UK.
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Scott JT, Diakhaté M, Vereecken K, Fall A, Diop M, Ly A, De Clercq D, de Vlas SJ, Berkvens D, Kestens L, Gryseels B. Human water contacts patterns in Schistosoma mansoni epidemic foci in northern Senegal change according to age, sex and place of residence, but are not related to intensity of infection. Trop Med Int Health 2003; 8:100-8. [PMID: 12581433 DOI: 10.1046/j.1365-3156.2003.00993.x] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
In an epidemic focus in northern Senegal, adults had lower intensities of infection than adolescents, a phenomenon that could not be attributed to immunity acquired over the previous 10-15 years of exposure to the parasite because all age groups had had the same number of years' experience of the worm. This article considers whether this pattern could have been because of higher levels of exposure to the parasite in younger age groups. Personal contact with infected water was recorded using a questionnaire in Schistosoma mansoni foci not more than 3 years old and in another, 10-year-old focus. Many aspects of contact (e.g. frequency, duration or time of day of contact) may contribute to the number of encounters with infective cercariae (true exposure), so various assumptions regarding the relationship between water contact and true exposure were tested resulting in a range of exposure indices. People reported a mean of 4.4 separate contacts, and spent a median of 57 min per day in water. Patterns of water contact differed depending on the exposure index used, e.g. considering duration, males spent a longer time in water than females (P < 0.001). But using frequency, females had more contacts with water than males in most villages (P < 0.001). Generally, exposure levels dropped as people become aged (P < 0.001) and residents of the older focus were more exposed than residents of other foci (P < 0.002). Intensity of (re)infection was not related to exposure either alone or in models incorporating age, sex and/or village irrespective of the index used. There is therefore evidence that age, sex and place of residence determine exposure but none to suggest that exposure had an influence on the relationship between these factors and intensity of infection. We propose therefore that in this population other factors have principal importance in determining intensity of infection.
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Affiliation(s)
- J T Scott
- Institute of Tropical Medicine, Antwerp, Belgium.
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11
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Sow S, de Vlas SJ, Engels D, Gryseels B. Water-related disease patterns before and after the construction of the Diama dam in northern Senegal. Ann Trop Med Parasitol 2002; 96:575-86. [PMID: 12396320 DOI: 10.1179/000349802125001636] [Citation(s) in RCA: 57] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Abstract
The ecological changes caused by projects for the development of water resources are known to affect the epidemiology of water-related diseases. The effects of the construction of the Diama dam (completed in 1986) in the Senegal River on the epidemiology of malaria, urinary and intestinal schistosomiasis, diarrhoea and dysentery were investigated in four districts in northern Senegal. To make allowance for any general trend in reported morbidity (caused by changes in demography or the healthcare system), the numbers of cases of these illnesses reported by the basic healthcare facilities before and after the completion of the dam were compared with those of respiratory disease. Prior to the construction of the dam, malaria was the most encountered water-related disease in the medical records of all districts, followed by diarrhoea, dysentery and urinary schistosomiasis. This order remained the same after the completion of the dam. Despite the optimism of health-assessment reports prepared prior to the construction of the Diama dam, the unexpected appearance and spread of intestinal schistosomiasis as well as an increase in the incidence of urinary schistosomiasis have aggravated public health in the Senegal River basin. It remains to be judged whether the economic benefits of the dam will counterbalance its adverse effects.
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Affiliation(s)
- S Sow
- Helminthology Unit, Department of Parasitology, Prince Leopold Institute of Tropical Medicine, Nationalestraat 155, B-2000 Antwerp, Belgium.
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12
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Nagelkerke NJD, de Vlas SJ, Alley ES. Validation of parasite transmission models: the example of onchocerciasis. Neth J Med 2002; 60:44-9; discussion 50-1. [PMID: 12430590] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/27/2023]
Abstract
Most parasites have complex life cycles; and mathematical models can help in targeting interventions and predicting disease-control efforts. For actual applications, quantification and validation of models is a key issue. We illustrate the process of validation by presenting a (re)analysis of fly-feeding experiments carried out by the Onchocerciasis (river blindness) Control Programme (OCP/WHO) in West Africa, with the objective to validate ONCHOSIM, an onchocerciasis transmission model. In these experiments flies were fed on human patients and dissected to count the number of microfilariae they had ingested. To assess microfilarial skin densities, skin snips (biopsies) were taken and examined. Originally, the resulting curve was interpreted as showing saturation and considered the main regulating (density-dependent) mechanism of onchocerciasis transmission in the model. Taking into account measurement errors in the skin microfilarial density of human subjects (on whom the flies were fed) we now conclude that the relationship is essentially linear. This prompts us to requantify ONCHOSIM. Possible alternative density-dependent mechanisms are discussed.
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Affiliation(s)
- N J D Nagelkerke
- Erasmus University, Department of Public Health, Rotterdam, The Netherlands
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13
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Abstract
BACKGROUND If sexually transmitted disease (STD) cofactor effects are strong and STDs are highly prevalent, STD control can be a strategy for HIV prevention. OBJECTIVE To review possibilities for estimating cofactor effects of STDs on HIV transmission based on observational studies. STUDY DESIGN This study consisted of an analysis of factors influencing associations between HIV and STDs, which can bias STD cofactor studies, from a sexual network perspective. Study designs that reduce distortions and methods to improve estimates in the presence of confounding are discussed. RESULTS Standard statistical adjustments of cofactor estimates are insufficient because they ignore clustering between HIV and STDs in partners of study subjects, resulting from population heterogeneity in risk factors and assortative mixing. Reverse causation due to HIV-related immunosuppression may further inflate cofactor estimates. Misclassification of STDs and clustering between STDs can bias estimates in either direction. This study demonstrates quantitatively that ignorance of sexual network effects may result in considerable overestimation of cofactor magnitudes. CONCLUSION The limitations of observational studies complicate quantitative inferences on the role of STDs in HIV transmission.
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Affiliation(s)
- E L Korenromp
- Department of Public Health, Erasmus University, Rotterdam, The Netherlands.
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Polman K, de Vlas SJ, Gryseels B, Deelder AM. Relating serum circulating anodic antigens to faecal egg counts in Schistosoma mansoni infections: a modelling approach. Parasitology 2000; 121 Pt 6:601-10. [PMID: 11155931 DOI: 10.1017/s0031182000006843] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022]
Abstract
Circulating anodic antigen (CAA) levels in serum and faecal egg counts are both quantitative measures of Schistosoma mansoni worm burdens. In this study, we have tested whether circulating anodic antigens can be included into an established egg count model. A data set with 3 repeated faecal egg count and serum CAA measurements of 50 individuals from a community in Burundi with moderate endemicity was used. By means of Monte Carlo simulation, both antigens and egg counts were related to an underlying worm pair distribution, taking into account the variation in repeated measurements (within individuals) and the variation in worm burdens (between individuals). Models with various assumptions (e.g. presence or absence of density-dependent egg production) were tested. Whereas observed and predicted egg counts agreed fairly well, the circulating antigen data could not be described satisfactorily. In particular, the predicted number of negative antigen concentrations was much lower than observed, while the number of light positives was overestimated. There seems to be a mechanism that causes a shift of expected (low) positive CAA concentrations towards zeros, which the proposed models do not provide for. Possible biological as well as assay-related mechanisms that may account for this shift are discussed. The assumption that serum CAA concentrations are a simple direct reflection of worm (pair) burdens could not be corroborated by this modelling exercise. Apparently, the relationship between (measured) CAA concentrations, egg counts and worm burdens in human S. mansoni infections is more complex than assumed.
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Affiliation(s)
- K Polman
- Department of Parasitology, Leiden University Medical Centre, The Netherlands.
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Giver H, de Vlas SJ, Johansen MV, Christensen NO, Nansen P. Schistosoma japonicum: day to day variation in excretion and hatchability of parasite eggs in the domestic pig, Suis suis. Exp Parasitol 2000; 95:8-18. [PMID: 10864513 DOI: 10.1006/expr.2000.4506] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The aim of the present study was to describe the course of an Schistosoma japonicum infection in individual pigs over time, with special regards to fecal egg counts and egg hatchability, emphasizing the extent of variation during and between days. Five specific pathogen free Danish Landrace/Yorkshire/Duroc crossbred male pigs were each given 3500 cercariae intramuscularly. From day 36 to 62 post infection, fecal samples were collected from each pig, morning and evening. Pigs were perfused 62 days post infection. Actual fecal egg counts, miracidial counts, and worm burdens were determined. The trend of fecal egg excretion was adequately described with a third order polynomial and logarithmic link function. Miracidial counts were related directly to morning fecal egg counts through a simple linear function. The study revealed a fairly similar overall pattern of egg excretion for all pigs, showing an increase in egg excretion until week 8, followed by a marked reduction to almost zero. In general, large fluctuations around this trend were seen for all pigs, as revealed in variations in egg counts within the same day as well as between days. However, as revealed by the estimated model lines, these fluctuations are due to random variation in egg density in stools rather than being caused by biologically determined cycles. An exact time for both first occurrence of eggs in feces (days 35-38 post infection) and for peak egg counts (days 44-48 post infection) was predicted for each individual pig. Furthermore, the model revealed that miracidial counts can be related directly to the expected fecal egg counts of the same day through a parameter, which represents hatchability. This hatchability parameter was found to be independent of time post infection for each pig, but large individual differences were seen in hatchability between the pigs.
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Affiliation(s)
- H Giver
- Danish Bilharziasis Laboratory, Jaegersborg Allé 1D, Charlottenlund, DK-2920, Denmark
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Abstract
Variations in fecal Schistosoma japonicum egg counts were studied in ZhuXi administrative village, JiangXi Province, China. Population stool examinations were collected with duplicate, standard, 41.5-mg Kato-Katz thick smears on seven consecutive days for 570 individuals from two natural (individual) villages: village I with high endemicity and village II with low endemicity. The proportion of individuals with at least one positive count increased from 42.4% after a single measurement to 68.3% after seven measurements in village I (n = 356), and from 17.0% to 36.0% in village II (n = 214), respectively. This demonstrates a very high variation in repeated S. japonicum egg counts and a considerable lack of sensitivity of the Kato-Katz technique; light and moderate infections are especially missed with a single or a few measurements. The observed day-to-day variation in individual egg counts is highly aggregated (variance higher than the mean) and suggestive of a negative binomial distribution. For five individuals on three days, repeated sampling from different locations of a stool specimen shows a clear trend with egg counts decreasing from the beginning of the stool to the end and from the outside layer to the center. Ten multiple samples from a particular subsection (10-30 g) of a stool specimen for 44 positive individuals still showed aggregation in egg counts, particularly for high intensities of infection. This means that the aggregation in repeated daily S. japonicum egg counts cannot be explained alone by a specific day-to-day component and variation in the concentration of eggs at different locations in the stool. There also exists clustering of eggs within parts of the stool.
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Affiliation(s)
- J M Yu
- Department of Epidemiology, Shanghai Medical University, People's Republic of China
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Jusot JF, de Vlas SJ, van Oortmarssen GJ, De Muynck A. [Contribution of a mathematical model in the control of a parasitosis: the case of human African trypanosomiasis due to Trypanosoma brucei gambiense]. Ann Soc Belg Med Trop 1995; 75:257-72. [PMID: 8669973] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Abstract
Trypanosoma brucei gambiense sleeping sickness transmitted by tsetse flies (Glossina spp.) is lethal if not treated adequately. The endemicity was generally well under control in the sixties. However, since the seventies the disease is returning in most of its old foci, with alarming endemic levels in several areas. Mathematical modelling provides a rational basis for finding the optimal strategies to control these recrudescences. We present a deterministic model of the basic transmission of trypanosomiasis between human and vector hosts in natural situations. The parameters were quantified on the basis of available evidence from the literature. The model predicts a stable equilibrium state with very high prevalences: approximately 95% of humans and 27% of flies being infected. The model further shows that the build-up of an epidemic is initially very slow, and it takes several months before the equilibrium state is reached. Consequently communities have enough time to avoid catastrophic situations by migrating to safer areas. If is therefore unlikely that such high equilibrium situations will occur in practice. The expression of the basic reproductive rate R0, the number of new infections during the lifetime of an infected subject with high values of R0 implies that efforts to diminish transmission to levels where the disease cannot maintain itself in the population, have to be substantial. The necessary reduction of fly numbers in order to enable eradication, has been calculated. In almost all situations a reduction of at least 90% is necessary, which is in accordance with the field experiences of vector control programmes. The present model can be considered as a starting point in the further development of a complete simulation model, which could be applied in supporting decision making in trypanosomiasis control.
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Affiliation(s)
- J F Jusot
- Institute de Médecine Tropicale (IMTA), Service d'Epidémiologie, Antwerpen, Begique
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Dik J, Habbema F, de Vlas SJ. Impact of improved treatment of sexually transmitted disease on HIV infection. Lancet 1995; 346:1157-8; author reply 1159-60. [PMID: 7475615] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [MESH Headings] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
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de Vlas SJ, Gryseels B, van Oortmarssen GJ, Polderman AM, Habbema JD. A pocket chart to estimate true Schistosoma mansoni prevalences. Parasitol Today 1993; 9:305-7. [PMID: 15463790] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Subscribe] [Scholar Register] [Indexed: 04/30/2023]
Abstract
In a recent paper, Sake de Vlas and Bruno Gryseels have explored the extent and implications of the underestimation of Schistosoma mansoni prevalences, as measured by faecal screening. Together with Gernt van Oortmorssen, Anton Polderman and Dik Habbema, they now present a practical and generally applicable chart to infer 'true' prevalences from simple egg count surveys.
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Affiliation(s)
- S J de Vlas
- Centre for Decision Sciences in Tropical Disease Control, Department of Public Health, Erasmus University Rotterdam, PO Box 1738, 3000 DR Rotterdam, The Netherlands
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Abstract
The estimation of prevalence and incidence of parasitic infections is considered. As the detectability of such infections is not 100% and may furthermore depend on their intensity, statistical methods are often required to arrive at meaningful results. It appears to be essential to distinguish between parasites that multiply within the (human) host and those that do not. An overview of some models discussed in the literature is presented. These models can indeed be used in assessing detectability of infection, and they indicate that observations may lead to considerable misinterpretation of 'true' prevalences and incidences.
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Affiliation(s)
- S J de Vlas
- Department of Public Health and Social Medicine, Erasmus University Rotterdam
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Affiliation(s)
- S J de Vlas
- Department of Public Health and Social Medicine, Erasmus University Rotterdam, The Netherlands
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Abstract
Field methods used for detecting Schistosoma mansoni infection miss a certain proportion of the infections. Prevalences of infection appear to be far under-estimated by faecal screening, with important consequences for control and research. Sake de Vlos and Bruno Gryseels investigate how the number of undetected infections can be statistically inferred from population surveys.
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Affiliation(s)
- S J de Vlas
- Centre for Decision Sciences in Tropical Disease Control (CDTDC), Department of Public Health and Social Medicine, Erasmus University Rotterdam, PO Box 1738, 3000 DR Rotterdam, The Netherlands
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