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G J van Oortmarssen

Publications and source records attributed to G J van Oortmarssen.

At least 19 recordsLinked to original sources
PubMed2002

Women who participate in spontaneous screening are not at higher risk for cervical cancer than women who attend programme screening.

Up to 1995, programme screening for cervical cancer in The Netherlands was targeted at women between 35 and 54 years of age at 3-yearly intervals. Spontaneous screening in addition to programme screening was common practice. Our aim was to compare the underlying risk for cervical neoplasia for women involved in both types of screening. From the national pathological database, we retrieved all primary smears (n=693318) taken in 1994 in The Netherlands. Among the smears registered for screening purposes (39%), 79% was taken within the mass screening programme and 21% was taken for spontaneous screening. The underlying risk was studied from the detection rates of histologically confirmed severe dysplasia or worse, using a multivariate loglinear model, including age and screening history. The detection rate of at least severe dysplasia, adjusted for age and screening history, was equal for women who had a spontaneous smear and for those who had a programme smear (odds ratio (OR): 0.97; 95% Confidence Interval (CI): 0.84-1.14). In our data, women participating in spontaneous screening were not at a higher risk for cervical cancer than women who used programme screening. Therefore, all asymptomatic women in the Netherlands should follow the general guidelines for age-range and screening-interval.

Adult↗
PubMed2001

Macrofilaricides and onchocerciasis control, mathematical modelling of the prospects for elimination.

BACKGROUND: In most endemic parts of the world, onchocerciasis (river blindness) control relies, or will soon rely, exclusively on mass treatment with the microfilaricide ivermectin. Worldwide eradication of the parasite by means of this drug is unlikely. Macrofilaricidal drugs are currently being developed for human use. METHODS: We used ONCHOSIM, a microsimulation mathematical model of the dynamics of onchocerciasis transmission, to explore the potentials of a hypothetical macrofilaricidal drug for the elimination of onchocerciasis under different epidemiological conditions, as characterized by previous intervention strategies, vectorial capacity and levels of coverage. RESULTS: With a high vector biting rate and poor coverage, a very effective macrofilaricide would appear to have a substantially higher potential for achieving elimination of the parasite than does ivermectin. CONCLUSIONS: Macrofilaricides have a substantially higher potential for achieving onchocerciasis elimination than ivermectin, but high coverage levels are still key. When these drugs become available, onchocerciasis elimination strategies should be reconsidered. In view of the impact of control efforts preceding the introduction of macrofilaricides on the success of elimination, it is important to sustain current control efforts.

Animals↗
PubMed2001

Impact of systematic false-negative test results on the performance of faecal occult blood screening.

The impact of systematic false-negative test results on mortality reduction and on programme sensitivity of annual faecal occult blood testing in ages 50-84 years is explored using a microsimulation model. We made calculations for test sensitivities of 80, 50 and 30%. In order to reproduce a cancer detection rate of 2.2 per 1000 at the first screening, the corresponding mean preclinical sojourn times had to be 1.42, 2.30 and 3.84 years, respectively. The fraction systematic results among the false-negative results is varied between 0 and 100%. With 80% test sensitivity, the reduction in mortality due to screening decreases from 25% without systematic results to 23% when all false-negative results are systematic and the programme sensitivity decreases from 63 to 58%. With 30% test sensitivity, mortality reduction decreases from 21 to 11% and programme sensitivity decreases from 52 to 27%. The impact of systematic false-negative test results is important if annual FOBT screening is considered.

Aged↗
PubMed2001

Patient follow up screening evaluations. Examples with regard to congenital hip dislocation and congenital heart disease.

OBJECTIVE: To discuss the merits of the patient follow up study design for the evaluation of some specific mass screening programmes. DESIGN: Theoretical evaluation illustrated by two examples. SETTING: Department of Public Health Erasmus University Rotterdam. MAIN RESULTS: The gold standard for evaluation of favourable effects of screening is the randomised controlled trial (RCT). Application of an RCT, however, is often not feasible, in which cases observational studies will have to be relied on. The case-control study design is generally considered to be second best. In some situations, however, a patient follow up study design may be applicable and may have some major advantages. The use of the patient follow up design for screening evaluation will often be very problematic or even unacceptable, particularly as far as screening for cancer is concerned. The most important objections are resulting from lead time bias, length bias, selection bias and over-treatment bias. For the evaluation of screening for congenital heart disease and congenital hip dislocation in Dutch child health care, however, these objections may relatively simply be overcome. Lead time bias will be of little importance, as the ages of onset of these disorders are fixed, namely at birth, and their ultimate outcomes may be expected within relatively short time. Length bias may largely be avoided by correction for severity of the disorder, which can be adequately assessed by modern diagnostic procedures. Selection bias is generally hard to rule out, but in these cases it probably plays a minor part. Over-treatment can be avoided by the policy of "watchful waiting", which in these disorders can be applied with little risk for fatal outcomes. In principle bias might be avoided more successfully in a case-control screening evaluation than in a patient follow up study. However, the patient follow up study is for both screening programmes discussed here the more feasible design and can provide more supplementary information. The results of two example studies suggest that both screenings probably yield considerable benefits CONCLUSION: Under a number of specific conditions a patient follow up study is an efficient alternative to more customary designs for screening evaluation.

Child↗
PubMed2001

Endocervical status is not predictive of the incidence of cervical cancer in the years after negative smears.

The clinical relevance of the lack of endocervical cells was never well established in a longitudinal study with histologically proven cervical cancer as an end point. From the Dutch Network and National Database for Pathology, results for all negative smears obtained in 1990 and 1991 in the Netherlands were retrieved, as were data for all cytologic and histologic examinations performed after the negative smears before April 1998. There were no significant differences between the proportion of preinvasive lesions (cervical intraepithelial neoplasia 1, 2, and 3) detected after negative smears without endocervical cells compared with negative smears with endocervical cells. The proportion of women in whom invasive cancer developed was the same in both groups. These data suggest there is no reason to advise women with negative smears without endocervical cells to undergo an additional smear.

Adult↗
PubMed2000

Endoscopic colorectal cancer screening: a cost-saving analysis.

BACKGROUND: Comprehensive analyses have shown that screening for cancer usually induces net costs. In this study, the possible costs and savings of endoscopic colorectal cancer screening are explored to investigate whether the induced savings may compensate for the costs of screening. METHODS: A simulation model for evaluation of colorectal cancer screening, MISCAN-COLON, is used to predict costs and savings for the U.S. population, assuming that screening is performed during a period of 30 years. Plausible baseline parameter values of epidemiology, natural history, screening test characteristics, and unit costs are based on available data and expert opinion. Important parameters are varied to extreme but plausible values. RESULTS: Given the expert opinion-based assumptions, a program based on every 5-year sigmoidoscopy screenings could result in a net savings of direct health care costs due to prevention of cancer treatment costs that compensate for the costs of screening, diagnostic follow-up, and surveillance. This result persists when costs and health effects are discounted at 3%. The "break-even" point, the time required before savings exceed costs, is 35 years for a screening program that terminates after 30 years and 44 years for a screening program that continues on indefinitely. However, net savings increase or turn into net costs when alternative assumptions about natural history of colorectal cancer, costs of screening, surveillance, and diagnostics are considered. CONCLUSIONS: Given the present, limited knowledge of the disease process of colorectal cancer, test characteristics, and costs, it may well be that the induced savings by endoscopic colorectal cancer screening completely compensate for the costs.

Adenoma↗
PubMed2000

Effectiveness of annual ivermectin treatment for Wuchereria bancrofti infection.

Using estimates for the anthelmintic efficacy of a single dose of ivermectin in the treatment of lymphatic filariasis patients, Anton Plaisier, Wilma Stolk, Gerrit van Oortmarssen and Dik Habbema here present and discuss model predictions of the impact of a five-year programme of annual community treatment on the intensity of infection. They show that the effectiveness of such programmes in terms of reductions in the microfilarial density depends critically on the treatment coverage and the pattern of attendance at repeated mass administrations. Improving these factors will possibly be more important than improving the efficacy of ivermectin by increasing its dosage or by adding other drugs.

Animals↗
PubMed1999

The MISCAN-COLON simulation model for the evaluation of colorectal cancer screening.

A general model for evaluation of colorectal cancer screening has been implemented in the microsimulation program MISCAN-COLON. A large number of fictitious individual life histories are simulated in each of which several colorectal lesions can emerge. Next, screening for colorectal cancer is simulated, which will change some of the life histories. The demographic characteristics, the epidemiology and natural history of the disease, and the characteristics of screening are defined in the input. All kinds of assumptions on the natural history of colorectal cancer and screening and surveillance strategies can easily be incorporated in the model. MISCAN-COLON gives detailed output of incidence, prevalence and mortality, and the results and effects of screening. It can be used to test hypotheses about the natural history of colorectal cancer, such as the duration of progressive adenomas, and screening characteristics, such as sensitivity of tests, against empirical data. In decision making about screening, the model can be used for evaluation of screening policies, and for choosing between competing policies by comparing their simulated incremental costs and effectiveness outcomes.

Colorectal Neoplasms↗
PubMed1999

Efficacy of ivermectin in the treatment of Wuchereria bancrofti infection: a model-based analysis of trial results.

Ivermectin is a promising drug for the treatment of lymphatic filariasis. A meta-analysis of trials investigating the effects of a single treatment suggested a dose-dependent effect on the production of microfilariae (mf) by adult Wuchereria bancrofti parasites. A mathematical model that describes the parasite dynamics in the human host and the impact of ivermectin treatment is presented and its outputs compared with these trials. The calculated trend in mf density after treatment appears to be particularly sensitive to the assumption about the mean life-span of mf. Adopting 0.5-2 years as a range of plausible values for this mf life-span, the model is used to estimate the impact of treatment on the parasite. It is found that irrespective of dosage, ivermectin eliminates 100% of the blood mf from a patient. Furthermore, at a dosage level of 400 micrograms/kg a single treatment irreversibly reduces the mf production of the adult parasites by at least 65%. For a dosage of 200 micrograms/kg this reduction is at least 35%. No such effect can be concluded from the results of trials using lower dosages.

Animals↗
PubMed1999

Factors associated with impairments in new leprosy patients: the AMFES cohort.

Data on the importance of the delay between onset of symptoms and registration as a risk factor for impairment are sparse. This study investigates the quantitative relationship between this delay, other risk factors and the impairment status in new leprosy patients. It reports on 592 new leprosy patients enrolled in 1988-1992 in the prospective ALERT MDT Field Evaluation Study in central Ethiopia (AMFES). The influence of the risk factors sex, age, delay, PB/MB classification in relation to BI, and prior dapsone treatment on the impairment status at intake is analysed. Estimates for the delay are based on patient recall. For the risk factors, odds ratios on impairment and on severity of impairment were calculated using both univariate and multivariate logistic regression. The registration delay was 2 years or more for 44% of new patients. The prevalence of impairment (WHO impairment grades 1 and 2 combined) increased continuously from 36% for new patients with a delay of 0-1 year to 81% for new patients with delays of 4 years or more. This prevalence also increased continuously with age; it rose from 26% in children to 80% for the age group 60 and over. In the multivariate regression, the odds ratios for new patients to be impaired were statistically significant for all delay categories (baseline 1-2 years) and age groups (baseline 15-29 years). No statistically significant differences in odds ratios were observed with respect to sex and PB/MB classification in relation to BI. Overall, 31% of new patients presented with WHO impairment grade 1 and 23% with grade 2. The risk on grade 2 also increased with the registration delay amongst the impaired new patients. Relatively few impaired males and relatively few impaired MB patients with a BI value of 3 or higher had grade 2 impairment. Registration delay and age are the main risk factors for presentation with impairment. Reduction of delay in central Ethiopia requires re-thinking of control methodologies. The search for ways to reduce delays in diagnosis and treatment should receive high priority in leprosy research and in leprosy control programmes.

Adolescent↗
PubMed1999

SIMLEP: a simulation model for leprosy transmission and control.

SIMLEP is a computer program for modeling the transmission and control of leprosy which can be used to project epidemiologic trends over time, producing output on indicators such as prevalence, incidence and case-detection rates of leprosy. In SIMLEP, health states have been defined that represent immunologic conditions and stages of leprosy infection and disease. Three types of interventions are incorporated: vaccination, case detection and chemotherapy treatment. Uncertainties about leprosy have led to a flexible design in which the user chooses which of many aspects should be included in the model. These aspects include natural immunity, asymptomatic infection, type distribution of new cases, delay between onset of disease and start of chemotherapy, and mechanisms for leprosy transmission. An example run illustrates input and output of the program. The output produced by SIMLEP can be readily compared with observed data, which allows for validation studies. The support that SIMLEP can give to health policy research and actual decision making will depend upon the extent of validation that has been achieved. SIMLEP can be used to improve the understanding of observed leprosy trends, for example, in relation to early detection campaigns and the use of multidrug therapy, by exploring which combinations of assumptions can explain these trends. In addition, SIMLEP allows for scenario analysis in which the effects of control strategies combining different interventions can be simulated and evaluated.

BCG Vaccine↗
PubMed1998

Screening for congenital heart malformation in child health centres.

BACKGROUND: Although screening for congenital heart malformations is part of the child health care programme in several countries, there are very few published evaluations of these activities. This report is concerned with the evaluation of this screening at the Dutch Child Health Centres (CHC). METHODS: All consecutive patients, aged between 32 days and 4 years, presented at the Sophia Children's Hospital Rotterdam throughout a period of 2 years, with a congenital heart malformation were included in this study. Paediatric cardiologists established whether or not these patients were diagnosed after haemodynamic complications had already developed (diagnosed 'too late'). Parents and CHC-physicians were interviewed in order to establish the screening and detection history. Test properties were established for all patients with a congenital heart malformation (n = 290), intended effects of screening were established in patients with clinically significant malformations (n = 82). RESULTS: The sensitivity of the actual screening programme was 0.57 (95% CI : 0.51-0.62), the specificity 0.985 (95% CI : 0.981-0.990) and the predictive value of a positive test result 0.13 (95% CI: 0.10-0.19). Sensitivity in a subpopulation of patients adequately screened was 0.89 (95% CI: 0.74-0.96). Adequately screened patients were less likely to be diagnosed 'too late' than inadequately screened patients (odds ratio [OR] = 0.20, 95% CI: 0.04-1.05). The actual risk of being diagnosed 'too late' in the study-population (48%) was only slightly less than the estimated risk for patients not exposed to CHC-screening (58%, 95% CI: 43%-72%). Adequately screened patients however were at considerably less risk (17%, 95% CI: 4%-48%). CONCLUSION: Screening for congenital heart malformations in CHC contributes to the timely detection of these disorders. The actual yield, however, is far from optimal, and the screening programme should be improved.

Child Health Services↗
PubMed1998

The LYMFASIM simulation program for modeling lymphatic filariasis and its control.

The LYMFASIM modeling framework for the transmission and control of the tropical parasitic disease lymphatic filariasis is described and its use in the context of an endemic community in north-eastern Brazil is illustrated. Lymphatic filariasis is a disease with a complex natural history with many unknowns. This complicates decision making with respect to control strategies. With LYMFASIM, a variety of hypotheses can be tested about the life history of the parasite Wuchereria bancrofti, its transmission from man to man through mosquitoes, the role of the immune system in regulating parasite numbers, the development of disease symptoms, and the effects of control measures (drug treatment or mosquito control). The implications of alternative assumptions and uncertainty about the quantification of parameters for the effectiveness of control strategies can be investigated. Thanks to the use of stochastic microsimulation, LYMFASIM is highly flexible and can be adapted and extended as new knowledge emerges.

Adolescent↗
PubMed1997

Non-progression of cervical intraepithelial neoplasia estimated from population-screening data.

Non-progression and duration of preclinical neoplastic lesions of the cervix uteri were studied using screening data from a previously unscreened population, Maribo County, Denmark (1966-82). To estimate regression rates, the incidence of clinical cancer before the screening programme was related to the prevalence and incidence of preclinical lesions estimated from the detection rates of first smear and third and subsequent smears respectively. Duration was estimated from the time lag between the cumulative incidence of preclinical lesions and the combined cumulative incidence of clinical cancer and the estimated 'incidence of regression'. Of all preclinical lesions in women aged 25-50, 24% progressed, 39% regressed and 38% remained. Even if we assume no onset of preclinical lesions above age 50, we estimated that 48% of the preclinical lesions would not progress to clinical cancer in the women's lifetime. The estimated mean duration of preclinical lesions was 16 years. In Maribo County during the 1970s, the positive rate (1.6%) was low compared with current rates in several countries. We conclude that the detection of non-progressive lesions was outweighed by the prevention of clinical cancer.

Adult↗
PubMed1997

Required duration of combined annual ivermectin treatment and vector control in the Onchocerciasis Control Programme in west Africa.

In the extension areas of the Onchocerciasis Control Programme in West Africa, aerial larviciding is supplemented with annual ivermectin treatment, mainly to achieve better control of morbidity. The purpose of this study is to determine whether and to what extent the addition of annual ivermectin treatment permits earlier cessation of vector control than originally recommended. The effectiveness of combined ivermectin distribution and vector control was assessed using an epidemiological model. Model predictions suggest that, dependent on the pre-control endemicity of the area and the proportion of persons treated during each ivermectin round, large-scale annual treatment permits a considerable reduction in the duration of vector control. Taking into account uncertainty about the efficacy of ivermectin, our results indicate that, provided treatment coverage is at least 65% and there is no importation of infection from elsewhere, 12 years of combined control will be sufficient to reduce the risk of recrudescence to below 1% in even the most afflicted areas.

Adolescent↗
PubMed1997

Trends in leprosy case detection rates.

BACKGROUND: A systematic review of the trends in leprosy incidence is lacking. The question of whether leprosy transmission has declined remains, therefore, unanswered. This study investigates trends in new case detection rates (NCDRs) in selected leprosy-endemic areas from different continents. METHODS: A literature search using specific inclusion criteria was performed. Average annual rates of change in NCDRs were obtained by exponential curve fitting. The variation in trends within individual areas was investigated using direct and indirect information on leprosy control activities. RESULTS: This review covers 16 areas in the Pacific, Asia, Africa and Latin America. For 10 out of the 16 areas, the trend was seen to be declining consistently over the last 10 years or longer. Near stabilization or stabilization after decline was observed for two areas. For three areas, interpretation of recent NCDRs was difficult due to changes in control, but two of them showed a decline over the study period. A consistently increasing trend was observed over the last 20 years in the one remaining area. The observed downward trends could not be attributed to reduced control activities or changed diagnostic criteria. A general acceleration of downward trends in the NCDR after the introduction of multidrug therapy (MDT) has not so far occurred. CONCLUSION: Our main conclusion is that despite many differences between the studies and study areas, the review demonstrates a considerable tendency of downward NCDR trends. Lack of information and changing control conditions necessitate caution in interpreting NCDR trends in individual areas. A general impact of MDT on NCDR trends is so far not visible. The coming years will be crucial for MDT-based control to prove its ability to reduce leprosy incidence.

Adult↗