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Search Results: 1 - 10 of 4859 matches for " Luc Duchateau "
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Extending the Archimedean copula methodology to model multivariate survival data grouped in clusters of variable size
Leen Prenen,Roel Braekers,Luc Duchateau
Statistics , 2014,
Abstract: For the analysis of clustered survival data, two different types of models that take the association into account, are commonly used: frailty models and copula models. Frailty models assume that conditional on a frailty term for each cluster, the hazard functions of individuals within that cluster are independent. These unknown frailty terms with their imposed distribution are used to express the association between the different individuals in a cluster. Copula models on the other hand assume that the joint survival function of the individuals within a cluster is given by a copula function, evaluated in the marginal survival function of each individual. It is the copula function which describes the association between the lifetimes within a cluster. A major disadvantage of the present copula models over the frailty models is that the size of the different clusters must be small and equal in order to set up manageable estimation procedures for the different model parameters. We describe in this manuscript a copula model for clustered survival data where the clusters are allowed to be moderate to large and varying in size by considering the class of Archimedean copulas with completely monotone generator. We develop both one- and two-stage estimators for the different copula parameters. Furthermore we show the consistency and asymptotic normality of these estimators. Finally, we perform a simulation study to investigate the finite sample properties of the estimators. We illustrate the method on a data set containing the time to first insemination in cows, with cows clustered in herds.
Filamentation of ultrashort laser pulses in silica glass and KDP crystal: A comparative study
Jérémie Rolle,Luc Bergé,Guillaume Duchateau,Stefan Skupin
Physics , 2014, DOI: 10.1103/PhysRevA.90.023834
Abstract: Ionizing 800-nm femtosecond laser pulses propagating in silica glass and in potassium dihydrogen phosphate (KDP) crystal are investigated by means of a unidirectional pulse propagation code. Filamentation in fused silica is compared with the self-channeling of light in KDP accounting for the presence of defect states and electron-hole dynamics. In KDP, laser pulses produce intense filaments with higher clamping intensities up to 200 TW/cm$^2$ and longer plasma channels with electron densities above $10^{16}$ cm$^{-3}$. Despite these differences, the propagation dynamics in silica and KDP are almost identical at equivalent ratios of input power over the critical power for self-focusing.
Intervertebral disk width in dogs with and without clinical signs of disk associated cervical spondylomyelopathy
Steven De Decker, Ingrid MVL Gielen, Luc Duchateau, Holger A Volk, Luc ML Van Ham
BMC Veterinary Research , 2012, DOI: 10.1186/1746-6148-8-126
Abstract: Doberman pinschers with clinical signs of DA-CSM (N=17),clinically normal Doberman pinschers (N=20), and clinically normal English Foxhounds (N=17), underwent MRI of the cervical vertebral column. On sagittal T2-weighted images, intervertebral disk width was measured from C2-C3 to C6-C7. Intra –and interobserver agreement were assessed on a subset of 20 of the 54 imaging studies.Intervertebral disk width was not significantly different between Doberman pinschers with clinical signs of DA-CSM, clinically normal Doberman pinschers or clinically normal English Foxhounds (p=0.43). Intervertebral disk width was positively associated with increasing age (p=0.029). Each monthly increase in age resulted in an increase of disk width by 0.0057mm. Intervertebral disk width was not significantly affected by gender (p=0.056), but was significantly influenced by intervertebral disk location (p <0.0001). The assessed measurements were associated with a good intra –and interobserver agreement.The present study does not provide evidence that wider intervertebral disks are associated with clinical status in dogs with and without DA-CSM. Instead, it seems that cervical intervertebral disk width in dogs is positively associated with increase in age.Cervical spondylomyelopathy, also referred to as canine wobbler syndrome, is a multifactorial neurological syndrome that generally affects large and giant breed dogs [1]. Many lesions have been attributed to this syndrome and as a consequence, many synonyms can be found in the literature [2]. Over years, a few separate entities have been recognised based on clinical presentation and imaging findings [3,4]. Probably the most common of these entities is disk associated cervical spondylomyelopathy (DA-CSM) or disk associated wobbler syndrome [5]. In DA-CSM, caudal cervical spinal cord compression is mainly caused by protrusion of one or more intervertebral disks [5]. This disk associated spinal cord compression is sometimes seen in combination
Integrating Fasciolosis Control in the Dry Cow Management: The Effect of Closantel Treatment on Milk Production
Johannes Charlier, Miel Hostens, Jos Jacobs, Bonny Van Ranst, Luc Duchateau, Jozef Vercruysse
PLOS ONE , 2012, DOI: 10.1371/journal.pone.0043216
Abstract: The liver fluke Fasciola hepatica is a parasite of ruminants with a worldwide distribution and an apparent increasing incidence in EU member states. Effective control in dairy cattle is hampered by the lack of flukicides with a zero-withdrawal time for milk, leaving the dry period as the only time that preventive treatment can be applied. Here, we present the results of a blinded, randomized and placebo-controlled trial on 11 dairy herds (402 animals) exposed to F. hepatica to 1) assess the effect of closantel treatment at dry-off (or 80–42 days before calving in first-calving heifers) on milk production parameters and 2) evaluate if a number of easy-to-use animal parameters is related to the milk production response after treatment. Closantel treatment resulted in a noticeable decrease of anti-F. hepatica antibody levels from 3–6 months after treatment onwards, a higher peak production (1.06 kg) and a slightly higher persistence (9%) of the lactation, resulting in a 305-day milk production increase of 303 kg. No effects of anthelmintic treatment were found on the average protein and fat content of the milk. Milk production responses after treatment were poor in meagre animals and clinically relevant higher milk production responses were observed in first-lactation animals and in cows with a high (0.3–0.5 optical density ratio (ODR)), but not a very high (≥0.5 ODR) F. hepatica ELISA result on a milk sample from the previous lactation. We conclude that in dairy herds exposed to F. hepatica, flukicide treatment at dry-off is a useful strategy to reduce levels of exposure and increase milk production in the subsequent lactation. Moreover, the results suggest that treatment approaches that only target selected animals within a herd can be developed based on easy-to-use parameters.
Common mental disorders in TB/HIV co-infected patients in Ethiopia
Amare Deribew, Markos Tesfaye, Yohannes Hailmichael, Ludwig Apers, Gemeda Abebe, Luc Duchateau, Robert Colebunders
BMC Infectious Diseases , 2010, DOI: 10.1186/1471-2334-10-201
Abstract: We conducted a cross sectional study in three hospitals in Ethiopia from February to April, 2009. The study population consisted of 155 TB/HIV co-infected and 465 non-co-infected HIV patients. CMD was assessed through face to face interviews by trained clinical nurses using the Kessler 10 scale. Several risk factors for CMD were assessed using a structured questionnaire.TB/HIV co-infected patients had significantly (p = 0.001) greater risk of CMD (63.7%) than the non-co-infected patients (46.7%). When adjusted for the effect of potential confounding variables, the odds of having CMD for TB/HIV co-infected individuals was 1.7 times the odds for non-co-infected patients [OR = 1.7, (95%CI: 1.0, 2.9)]. Individuals who had no source of income [OR = 1.7, (95%CI: 1.1, 2.8)], and day labourers [OR = 2.4, 95%CI: 1.2, 5.1)] were more likely to have CMD as compared to individuals who had a source of income and government employees respectively. Patients who perceived stigma [OR = 2.2, 95%CI: 1.5, 3.2)] and who rate their general health as "poor" [OR = 10.0, 95%CI: 2.8, 35.1)] had significantly greater risk of CMD than individual who did not perceive stigma or who perceived their general health to be "good".TB/HIV control programs should develop guidelines to screen and treat CMD among TB/HIV co-infected patients. Screening programs should focus on individuals with no source of income, jobless people and day labourers.The global burden of disease report revealed that neuropsychiatric conditions accounted for up to a quarter of all the disability-adjusted life years lost[1]. In low and middle income countries (LAMIC), neuropsychiatric disorders such as depression, anxiety and somatoform disorders account for 9.8% of the global burden of diseases[2].The risk factors for mental health problems are complex [3]. Poverty, low education, social exclusion, gender disadvantage, conflict and disasters are the major social determinants of mental disorders[2]. Presence of medical illnesses
Food insecurity and age at menarche among adolescent girls in Jimma Zone Southwest Ethiopia: a longitudinal study
Tefera Belachew, Craig Hadley, David Lindstrom, Yehenew Getachew, Luc Duchateau, Patrick Kolsteren
Reproductive Biology and Endocrinology , 2011, DOI: 10.1186/1477-7827-9-125
Abstract: We report on 900 girls who were investigated in the first two rounds of the five year longitudinal survey. The semi-parametric frailty model was fitted to determine the effect of adolescent food insecurity on time to menarche after adjusting for socio-demographic and economic variables.Food insecure girls have menarche one year later than their food secure peer (median age of 15 years vs 14 years). The hazard of menarche showed a significant decline (P = 0.019) as severity of food insecurity level increased, the hazard ratio (HR) for mild food insecurity and moderate/severe food insecurity were 0.936 and 0.496, respectively compared to food secure girls. Stunted girls had menarche nearly one year later than their non-stunted peers (HR = 0.551, P < 0.001).Food insecurity is associated with delay of age at menarche by one year among girls in the study area. Stunted girls had menarche one year later than their non-stunted peers. Age at menarche reflects the development of girls including the timing of sexual maturation, nutritional status and trajectory of growth during the pre-pubertal periods. The findings reflect the consequence of chronic food insecurity on the development and well-being of girls in the study area.Adolescence is a period of rapid transition to adulthood marked by biological changes including sexual maturation. Puberty is a period during adolescence characterized by transformation from a stage of reproductive immaturity to a stage of full reproductive competence. The sign of puberty in girls is menarche (the first menstruation) which occurs at younger age in high income countries [1-5] compared with developing countries in Africa [6-10] with minimal decline in those in Latin America [11-13] and Asia [14]. These disparities in age at menarche are related to the improvements in childhood nutrition and health in high income countries among other environmental factors [13,15,16].There are different theories for the pathways through which environmental f
Effect of Recombinant Human Thyrotropin on the Uptake of Radioactive Iodine (123I) in Dogs with Thyroid Tumors
Miguel Campos, Kathelijne Peremans, Eva Vandermeulen, Luc Duchateau, Tim Bosmans, Ingeborgh Polis, Sylvie Daminet
PLOS ONE , 2012, DOI: 10.1371/journal.pone.0050344
Abstract: In humans, recombinant human thyrotropin (rhTSH) enhances radioactive iodine uptake (RAIU) in patients with differentiated thyroid cancer. No studies have been performed in veterinary medicine to optimize radioiodine treatment of thyroid cancer. The aim of this study was to evaluate the effect of rhTSH on the uptake of radioiodine-123 (123I) in dogs with thyroid tumors. Nine dogs with thyroid neoplasia were included in this prospective cross-over study. The dogs were divided in 2 groups. In one group, 123I was administered for a baseline RAIU determination in week 1. In week 2 (after a washout period of 2 weeks), these dogs received rhTSH (100 μg IV) 24 h before 123I injection. In the other group the order of the protocol was reversed. For each scan, the dogs received 37 MBq (1 mCi) of 123I intravenously (IV) and planar scintigraphy was performed after 8 and 24 h for tumor RAIU calculation. Overall, rhTSH administration caused no statistically significant change on thyroid tumor RAIU at 8 h (p = 0.89) or at 24 h (p = 0.98). A significant positive correlation was found between the effect of rhTSH on tumor 8h-RAIU and rhTSH serum concentrations at 6 h (τ = 0.68; p = 0.03), at 12 h (τ = 0.68; p = 0.03) and at 24 h (τ = 0.78; p = 0.02) after rhTSH injection. This study suggests that IV administration of 100 μg rhTSH 24 h before 123I has an inconsistent effect on thyroid tumor RAIU. Further studies are necessary to determine the best protocol of rhTSH administration to optimize thyroid tumor RAIU.
The Relative Biologic Effectiveness versus Linear Energy Transfer Curve as a Cell Trait  [PDF]
Quoc T. Luu, Paul DuChateau
Applied Mathematics (AM) , 2013, DOI: 10.4236/am.2013.411A3004
Abstract:

The magnitude of biological response varies with different radiation types. Using Linear Energy Transfer (LET) to differentiate types of incident radiation beam, the Relative Biologic Effectiveness (RBE) as a function of LET (RBE-LET) was found to have a characteristic shape with a peak around LET values 100 - 200 eV/nm. This general feature is believed to be a property of the incident beam. Our systems engineering model, however, suggests that the shape of the RBE-LET curve is a cell trait, a property of the cell. Like any other trait, phenotypic variations result from interactions of the genes and their context. State-space block diagram of the differential equation model suggests the genes are those in the DNA double strand break (dsb) repair pathway; and the context is cellular stress responsing to DNA damage by both external stimuli and internal redox state. At a deeper level, the block diagram suggests cell using mathematical calculations in its decision-making when facing a stress signal. The MRN protein complex, in particular, may perform addition to count the degree of DNA twisting for the homeostatic regulation of DNA supercoiling. The ATM protein may act as a feedback amplifier.

Panton-Valentine Leukocidin Does Play a Role in the Early Stage of Staphylococcus aureus Skin Infections: A Rabbit Model
Urszula Lipinska,Katleen Hermans,Lieve Meulemans,Oana Dumitrescu,Cedric Badiou,Luc Duchateau,Freddy Haesebrouck,Jerome Etienne,Gerard Lina
PLOS ONE , 2012, DOI: 10.1371/journal.pone.0022864
Abstract: Despite epidemiological data linking necrotizing skin infections with the production of Panton-Valentine leukocidin (PVL), the contribution of this toxin to the virulence of S. aureus has been highly discussed as a result of inconclusive results of in vivo studies. However, the majority of these results originate from experiments using mice, an animal species which neutrophils - the major target cells for PVL - are highly insensitive to the action of this leukocidin. In contrast, the rabbit neutrophils have been shown to be as sensitive to PVL action as human cells, making the rabbit a better experimental animal to explore the PVL role. In this study we examined whether PVL contributes to S. aureus pathogenicity by means of a rabbit skin infection model. The rabbits were injected intradermally with 108 cfu of either a PVL positive community-associated methicillin-resistant S. aureus isolate, its isogenic PVL knockout or a PVL complemented knockout strain, and the development of skin lesions was observed. While all strains induced skin infection, the wild type strain produced larger lesions and a higher degree of skin necrosis compared to the PVL knockout strain in the first week after the infection. The PVL expression in the rabbits was indirectly confirmed by a raise in the serum titer of anti-LukS-PV antibodies observed only in the rabbits infected with PVL positive strains. These results indicate that the rabbit model is more suitable for studying the role of PVL in staphylococcal diseases than other animal models. Further, they support the epidemiological link between PVL producing S. aureus strains and necrotizing skin infections.
The influence of the Gilgel-Gibe hydroelectric dam in Ethiopia on caregivers' knowledge, perceptions and health-seeking behaviour towards childhood malaria
Delenasaw Yewhalaw, Wondwossen Kassahun, Kifle Woldemichael, Kora Tushune, Morankar Sudaker, Daniel Kaba, Luc Duchateau, Wim Van Bortel, Niko Speybroeck
Malaria Journal , 2010, DOI: 10.1186/1475-2875-9-47
Abstract: A total of 1,003 caregivers in 'at-risk' villages in close proximity to the Gilgel-Gibe hydroelectric dam in south-western Ethiopia, and 953 caregivers in 'control' villages further away from the dam were surveyed using structured questionnaires to assess their knowledge, perceptions and health seeking behaviour about childhood malaria.Malaria (busa) was ranked as the most serious health problem. Caregivers perceived childhood malaria as a preventable ('at-risk' 96%, 'control' 86%) and treatable ('at-risk' 98% and 'control' 96%) disease. Most caregivers correctly associated the typical clinical manifestations with malaria attacks. The use of insecticide-treated nets (ITNs) was mentioned as a personal protective measure, whereas the role of indoor residual spraying (IRS) in malaria prevention and control was under-recognized. Most of the caregivers would prefer to seek treatment in health-care services in the event of malaria and reported the use of recommended anti-malarials.Health education to improve knowledge, perceptions and health-seeking behaviour related to malaria is equally important for caregivers in 'at risk' villages and caregivers in 'control' villages as minimal differences seen between both groups. Concluding, there may be a need of more than one generation after the introduction of the dam before differences can be noticed. Secondly, differences in prevalence between 'control' and 'at-risk' villages may not be sufficient to influence knowledge and behaviour.Malaria remains a serious public health problem, causing 1.2 million deaths [1] and 300 to 660 million clinical cases in tropical and subtropical areas each year [2]. More than 90% of the lethal cases occur in children under five years of age in Africa [3]. In Ethiopia, malaria is one of the most important health problems with nearly 52 million (68%) of the populations being at risk to malaria infection [4]. It is the leading cause of morbidity and mortality, accounting for 10-20% of hospital admi
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