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PubMed2002

Model of white spot syndrome virus (WSSV) epidemics in Litopenaeus vannamei.

White spot syndrome virus (WSSV) is devastating shrimp aquaculture throughout the world, but despite its economic importance no work has been done on modeling epidemics of this pathogen. Therefore we developed a Reed-Frost epidemic model for WSSV in Litopenaeus vannamei. The model includes uninfected susceptible, latently infected, acutely infected, and dead infected shrimp. The source of new infections during an outbreak is considered to be dead infected shrimp. The transmission coefficient, patency coefficient, virulence coefficient, and removal coefficient (disappearance of dead infected shrimp) control the dynamics of the model. In addition, an explicit area parameter is included to help to clarify the distinction between density and absolute shrimp population size. An analysis of the model finds that as number of shrimp, initial dose, transmission coefficient, patency coefficient, virulence coefficient, or removal coefficient changes, the speed of the epidemic changes. The model predicts that a threshold density of susceptible shrimp exists below which an outbreak of WSSV will not occur. Only initial dose, transmission coefficient, removal coefficient, and area coefficient affect the predicted threshold density. Increases in the transmission coefficient reduce the threshold value, whereas increases in the other factors cause the threshold value to increase. Epidemic models may prove useful to the shrimp aquaculture industry by suggesting testable hypotheses, some of which may contribute to the eventual control of WSSV outbreaks.

Animals↗
PubMed1987

An evolutionary epidemiological mechanism, with applications to type A influenza.

In this paper I develop a model that describes an evolutionary epidemiological mechanism and apply this model to the epidemiology of type A influenza. This evolutionary epidemiological model differs from the classical nonevolutionary epidemiological model which has been applied to diseases like measles, rubella, and whooping cough in having a novel mechanism which causes susceptible individuals to be introduced into the host population. In the nonevolutionary model, susceptibles are continually introduced into the host population by demographic processes: most hosts that die are immune, while newborn hosts are susceptible. In this evolutionary model, the susceptible class is continually replenished because the pathogen changes genetically, and hence immunologically, from one epidemic to the next, causing previously immune hosts to become susceptible. I derive formulae which describe how the equilibrium number of infected hosts, the interepidemic period, and the probability that a host will become reinfected depend on the rate of amino acid substitution in the pathogen, m, a parameter describing the effect of these substitutions on host immunity, gamma, as well as the host population size, N, and the recovery rate, r. To apply the model to influenza, I show how the nondimensional parameter epsilon = m gamma N/r2 may be estimated from four types of data. The methods are applied to several data sets, and I conclude that epsilon much less than 1; sampling variation and inconsistencies between the various data sets do not permit epsilon to be estimated more precisely. The evolutionary epidemiological model has no threshold host population size, in contrast to the nonevolutionary model.

Amino Acids↗
PubMed2002

The impact of spike timing variability on the signal-encoding performance of neural spiking models.

It remains unclear whether the variability of neuronal spike trains in vivo arises due to biological noise sources or represents highly precise encoding of temporally varying synaptic input signals. Determining the variability of spike timing can provide fundamental insights into the nature of strategies used in the brain to represent and transmit information in the form of discrete spike trains. In this study, we employ a signal estimation paradigm to determine how variability in spike timing affects encoding of random time-varying signals. We assess this for two types of spiking models: an integrate-and-fire model with random threshold and a more biophysically realistic stochastic ion channel model. Using the coding fraction and mutual information as information-theoretic measures, we quantify the efficacy of optimal linear decoding of random inputs from the model outputs and study the relationship between efficacy and variability in the output spike train. Our findings suggest that variability does not necessarily hinder signal decoding for the biophysically plausible encoders examined and that the functional role of spiking variability depends intimately on the nature of the encoder and the signal processing task; variability can either enhance or impede decoding performance.

Action Potentials↗
PubMed1991

Aetiological, modifying and lethal factors in cleft lip and palate.

Prenatal factors influencing cleft lip (CL), cleft lip and palate (CLP) and isolated cleft palate (CP) may account for the development of the defect (aetiological factor) for a change in the degree or type of the defect (modifying factor), or for the death of the embryo (lethal factor). Each active factor appears to act in all three directions, all be it at different ratios. This is used for analysis of groups of cleft-effected individuals registered at the Department of Plastic Surgery, Prague, from the catchment area of Bohemia. In terms of cleft defect teratogenesis (aetiology, modification, lethality) a "protective influence" appears probable in female embryos, in blood group A (ABO system), in HLA antigens A9, A11, B35. An "embryotoxic influence" i.e., an increase in all the three influences is seen in male embryos, regional influences, in the B and AB blood groups, in HLA antigens B17, in primiparae, in multiple pregnancies, in older mothers and in cytomegalovirus infections. The Epstein-Barr virus seem to increase, in particular, CLP embryo lethality. The activity of the factors was rated by means of correlations between the cleft frequency in first-degree relatives and the frequency of the factor in six subgroups classified by the cleft defect subtypes, by means of interactions between two and more factors and by a study of the differences between male and female probands. A model of the threshold of CL and CP teratogenesis was proposed, a model in agreement with prenatal reciprocal equilibrium.

Cleft Lip↗
PubMed2005

Estimating noise-induced permanent threshold shift from audiometric shape: the ISO-1999 model.

OBJECTIVE: To describe the relation between audiometric shape and noise-induced permanent threshold shift averaged across the speech frequencies (N5123). DESIGN: Using an international standard (ISO-1999), 270 audiograms were created, representing both sexes and a variety of ages, exposure levels, and percentiles. Bulge depth (BD) was defined as the difference between pure-tone average for 2, 3, and 4 kHz and PTA for 1 and 6 kHz. RESULTS: N5123 was well-predicted by quadratic functions of BD, which accounted for 72 to 95% of the variance in N5123. CONCLUSIONS: Estimates of N5123, based on BD, can be helpful in medical-legal diagnosis and allocation of hearing loss.

Adult↗
PubMed1998

Curvature in the cancer mortality dose response in Japanese atomic bomb survivors: absence of evidence of threshold.

PURPOSES: To investigate the evidence for a threshold in the cancer dose-response curve. MATERIALS AND METHODS: Japanese atomic bomb survivor cancer mortality data, based on follow-up to 1990, was used, taking account of random errors in DS86 dose estimates. RESULTS: For all solid cancers analysed together, there is a significant positive dose response (two-sided p<0.05) if all survivors who received <0.5 Sv are considered, but the significance vanishes if doses of <0.2 Sv are considered; the same is also true for leukaemia. For solid cancer mortality there is no indication of curvilinearity in the dose response: no statistically significant improvement in fit to a linear relative risk model is provided by addition of quadratic or threshold dose terms. If a relative risk model with a threshold (the dose response is assumed linear above the threshold) is fitted to solid cancer mortality data, the best estimate of the threshold is < 0.00 Sv (95% CI <0.00-0.13). If a linear-quadratic-threshold model is used the best estimate of the threshold is < 0.00 Sv (95% CI < 0.00-0.15). For leukaemia mortality there is highly statistically significant upward curvature in the dose response. In particular, if a relative risk model with a threshold (the dose response is assumed linear above the threshold) is fitted to the leukaemia data, the best estimate of the threshold is 0.16 Sv (95% CI 0.05-0.40) (two-sided p=0.001 for test of departure of threshold from 0). However, there is no evidence for a threshold effect (two-sided p = 0.16) when a quadratic term is included in the dose response: the best estimate of threshold in this case is 0.09Sv (95% CI <0.00-0.29). Moreover, addition of a quadratic term improves the fit of a linear-threshold model at borderline levels of statistical significance (two-sided p = 0.07). Therefore, the most parsimonious description of the leukaemia dose response is provided by a linear-quadratic function of dose. CONCLUSIONS: There is no evidence of threshold-type departures from the linear-quadratic dose response either for solid tumours or for leukaemia in the Japanese atomic bomb survivor mortality data.

Dose-Response Relationship, Radiation↗
PubMed1999

Prediction of outcome in intensive care unit trauma patients: a multicenter study of Acute Physiology and Chronic Health Evaluation (APACHE), Trauma and Injury Severity Score (TRISS), and a 24-hour intensive care unit (ICU) point system.

OBJECTIVE: To conduct a multicenter study to validate the accuracy of the Acute Physiology and Chronic Health Evaluation (APACHE) II system, APACHE III system, Trauma and Injury Severity Score (TRISS) methodology, and a 24-hour intensive care unit (ICU) point system for prediction of mortality in ICU trauma patient admissions. METHODS: The study population consisted of retrospectively identified, consecutive ICU trauma admissions (n = 2,414) from six Level I trauma centers. Probabilities of death were calculated by using logistic regression analysis. The predictive power of each system was evaluated by using decision matrix analysis to compare observed and predicted outcomes with a decision criterion of 0.50 for risk of hospital death. The Youden Index (YI) was used to compare the proportion of patients correctly classified by each system. Measures of model calibration were based on goodness-of-fit testing (Hosmer-Lemeshow statistic less than 15.5) and model discrimination were based on the area under the receiver operating characteristic curve (AUC). RESULTS: Overall, APACHE II (sensitivity, 38%; specificity, 99%; YI, 37%; H-L statistic, 92.6; AUC, 0.87) and TRISS (sensitivity, 52%; specificity, 94%; YI, 46%; H-L statistic, 228.1; AUC, 0.82) were poor predictors of aggregate mortality, because they did not meet the acceptable thresholds for both model calibration and discrimination. APACHE III (sensitivity, 60%; specificity, 98%; YI, 58%; H-L statistic, 7.0; AUC, 0.89) was comparable to the 24-hour ICU point system (sensitivity, 51%; specificity, 98%; YI, 50%; H-L statistic, 14.7; AUC, 0.89) with both systems showing strong agreement between the observed and predicted outcomes based on acceptable thresholds for both model calibration and discrimination. The APACHE III system significantly improved upon APACHE II for estimating risk of death in ICU trauma patients (p < 0.001). Compared with the overall performance, for the subset of patients with nonoperative head trauma, the percentage correctly classified was decreased to 46% for APACHE II; increased to 71% for APACHE III (p < 0.001 vs. APACHE II); increased to 59% for TRISS; and increased to 62% for 24-hour ICU points. For operative head trauma, the percentage correctly classified was increased to 60% for APACHE II; increased to 61% for APACHE III; decreased to 43% for TRISS (p < 0.004 vs. APACHE III); and increased to 54% for 24-hour ICU points. For patients without head injuries, all of the systems were unreliable and considerably underestimated the risk of death. The percentage of nonoperative and operative patients without head trauma who were correctly classified was decreased, respectively, to 26% and 30% for APACHE II; 33% and 29% for APACHE III; 33% and 19% for TRISS; 20% and 23% for 24-hour ICU points. CONCLUSION: For the overall estimation of aggregate ICU mortality, the APACHE III system was the most reliable; however, performance was most accurate for subsets of patients with head trauma. The 24-hour ICU point system also demonstrated acceptable overall performance with improved performance for patients with head trauma. Overall, APACHE II and TRISS did not meet acceptable thresholds of performance. When estimating ICU mortality for subsets of patients without head trauma, none of these systems had an acceptable level of performance. Further multicenter studies aimed at developing better outcome prediction models for patients without head injuries are warranted, which would allow trauma care providers to set uniform standards for judging institutional performance.

APACHE↗
PubMed1988

Relations between liver cadmium, cumulative exposure, and renal function in cadmium alloy workers.

Detailed biochemical investigations of renal function were made on 75 male workers exposed to cadmium and an equal number of referents matched for age, sex, and employment status. The exposed group consisted of current and retired workers who had been employed in the manufacture of copper-cadmium alloy at a single factory in the United Kingdom for periods of up to 39 years and for whom cumulative cadmium exposure indices could be calculated. In vivo measurements of liver and kidney cadmium burden were made on exposed and referent workers using a transportable neutron activation analysis facility. Significant increases in the urinary excretion of albumin, retinol binding protein, beta 2 microglobulin, N-acetylglucosaminidase (NAG), alkaline phosphatase, gamma-glutamyl transferase and significant decreases in the renal reabsorption of calcium, urate, and phosphate were found in the exposed group compared with the referent group. Measures of glomerular filtration rate (GFR) (creatinine clearance, serum creatinine, and beta 2 microglobulin) indicated a reduction in GFR in the exposed population. Many of these tubular and glomerular function indicators were significantly correlated with both cumulative exposure index and liver cadmium burden. Using cumulative exposure index and liver cadmium as estimates of dose, a two phase linear regression model was applied to identify an inflection point signifying a threshold level above which changes in renal function occur. Many biochemical variables fitted this model; urinary total protein, retinol binding protein, albumin, and beta 2 microglobulin gave similar inflection points at cumulative exposure levels of about 1100 y.micrograms/m3 whereas changes in the tubular reabsorption of urate and phosphate occurred at higher cumulative exposure indices. Measures of GFR, although fitting the threshold model did not give well defined inflection points. Fewer variables fitted the two phase model using liver cadmium; those that did gave threshold levels in the range 20.3-55.1 ppm. When cadmium workers with cumulative exposure indices of less than 1100 y.micrograms/m3 were compared with their respective referents only serum beta 2 microglobulin and urinary NAG were significantly increased in the exposed group and these differences were not related to the degree of cadmium exposure.(ABSTRACT TRUNCATED AT 400 WORDS)

Alloys↗
PubMedDate not supplied

Effects of quarantine in six endemic models for infectious diseases.

Thresholds, equilibria, and their stability are found for SIQS and SIQR epidemiology models with three forms of the incidence. For most of these models, the endemic equilibrium is asymptotically stable, but for the SIQR model with the quarantine-adjusted incidence, the endemic equilibrium is an unstable spiral for some parameter values and periodic solutions arise by Hopf bifurcation. The Hopf bifurcation surface and stable periodic solutions are found numerically.

Communicable Disease Control↗
PubMed1995

Significance of the spinal cord position in spinal cord stimulation.

The effects of the antero-posterior and medio-lateral positions of the spinal cord in the dural sac on the perception threshold and paresthesia coverage in spinal cord stimulation were analyzed. The distributions of the dorsal cerebrospinal fluid (CSF) layer thickness, measured from transverse MR scans of normal subjects at various spinal levels, were used to calculate the distributions of threshold voltages for the stimulation of spinal nerve fibers by a computer model. These theoretical threshold distributions were shown to fit well to the corresponding distributions of perception threshold measured in patients. It is concluded that the thickness of the dorsal csf layer is the main factor determining the perception threshold and paresthesia coverage in spinal cord stimulation: an increasing thickness raises the threshold and reduces the coverage, and vice versa. The effects of an asymmetrical electrode position with respect to the spinal cord midline were also analyzed by computer modeling. It is concluded that a lateral asymmetry of less than 1 mm gives a significant reduction of perception threshold and may result in unilateral paresthesiae.

Adult↗
PubMed2006

New critical frontiers for the potts and percolation models.

We obtain the critical threshold for a host of Potts and percolation models on lattices having a structure which permits a duality consideration. The consideration generalizes the recently obtained thresholds of Scullard and Ziff for bond and site percolation on the martini and related lattices to the Potts model and to other lattices.

Journal Article↗
PubMed2001

Bayesian analysis of logistic regression with an unknown change point and covariate measurement error.

We discuss Bayesian estimation of a logistic regression model with an unknown threshold limiting value (TLV). In these models it is assumed that there is no effect of a covariate on the response under a certain unknown TLV. The estimation of these models in a Bayesian context by Markov chain Monte Carlo (MCMC) methods is considered with focus on the TLV. We extend the model by accounting for measurement error in the covariate. The Bayesian solution is compared with the likelihood solution proposed by Küchenhoff and Carroll using a data set concerning the relationship between dust concentration in the working place and the occurrence of chronic bronchitis.

Bayes Theorem↗
PubMed2006

Prediction of absolute thresholds and equal-loudness contours using a modified loudness model.

The loudness model described by Moore et al. [J. Audio Eng. Soc. 45, 224-240 (1997)] forms the basis for a recent ANSI standard for the calculation of the loudness of steady sounds. However, the model does not give accurate predictions of the absolute thresholds published in a recent ISO standard. Here it is described how the assumed middle-ear transfer function in the model can be modified to give more accurate absolute threshold predictions. The modified model also gives reasonably accurate predictions of the equal-loudness contours published in a recent ISO standard.

Auditory Threshold↗
PubMed1988

Percolation thresholds in square-lattice Kauffman model.

In Kauffman's random Boolean network model for genetics, each gene is either on or off, depending in a fixed random way on whether K neighbor genes are on or off. Our computer simulation puts these genes on the sites of a square lattice and asks if the "off" genes, the "on" genes, the "oscillating" genes and the non-oscillating "stable" genes are percolating, i.e. form one connected network of neighboring sites. The percolation thresholds for stable and for oscillating genes are found to coincide numerically with the transition to chaos at p = 0.29. Up to one million sweeps through the lattice were made to find that agreement.

Animals↗
PubMed1983

Drug effects on afterdischarge and seizure threshold in lissencephalic ferrets: an epilepsy model for drug evaluation.

In normal and lissencephalic ferrets with chronically implanted electrodes, two antiepileptic drugs, (E)-2-[(amino)phenylmethylen]-benzo [b] thiophen-3(2H)-on (AF-CX 921 XX) and carbamazepine (CBZ), were compared. The variables included afterdischarges (AD) and seizures induced by cortical electrical stimulations (ES). Both drugs were given orally, 100 mg/kg of pure substance. ES was applied before and 1, 2, 3, 4, 5, and 24 h after drug administration. Lissencephaly was produced by a single intraperitoneal injection of 15 mg methylazoxymethanol acetate to pregnant animals. The administration of both drugs resulted in increases of the AD threshold current to 240% in the normal and to 170% in the lissencephalic ferrets, in comparison with control stimulations (the difference significant at p less than 0.001). Moreover, duration of the AD was shorter (p less than 0.01) than before the drugs. Seizure threshold also increased 170% after AF-CX 921 XX and 175% after CBZ in normal and 153% and 138% in lissencephalic ferrets, respectively. The difference between the two drugs was significant. However, in contrast to the threshold, duration of seizures during AF-CX 921 XX administration was significantly shorter (p less than 0.05) than during CBZ. In general, lissencephalic ferrets responded less than normal ferrets to both drugs, but AF-CX 921 XX had a greater inhibitory effect on the duration of seizures. The lissencephalic ferret is proposed as an animal model of epilepsy with diffuse developmental defects.

Animals↗
PubMed2004

Phase locking in integrate-and-fire models with refractory periods and modulation.

It is known [8, 11, 16, 26] that phase locking can entrain frequency information when the leaky integrate-and-fire (IF) model of a neuron is forced by a periodic function. We show that this is still the case when the IF model is made more biologically realistic. We incorporate into our model spike dependent threshold modulation and refractory periods. Consecutive firing times from this model and their respective interspike intervals are related by an annulus map. We prove a general theorem concerning orientation reversing annulus twist homeomorphisms, which shows that our map admits a unique rotation number. This implies, in particular, that chaotic behaviour is not possible in our model and phase locking is predicted.

Action Potentials↗
PubMed2005

P-glycoprotein induction and tumor cell-kill dynamics in response to differential doxorubicin dosing strategies: a theoretical pharmacodynamic model.

PURPOSE: The objectives of this work were 1) to develop a theoretical pharmacodynamic model that captures dynamic changes resulting from drug/therapy mediated P-glycoprotein (P-gp) induction and 2) to compare the pharmacodynamic outcomes of several doxorubicin (DOX) dosing schemes through simulations. METHODS: We developed a theoretical model that included a pharmacokinetic (PK) model for intracellular DOX-mediated P-gp induction and a pharmacodynamic (PD) model using a threshold trigger function for tumor cell-kill. In this model, both the level of P-gp induction and rate of tumor cell death were modulated by intracellular DOX concentration. Most model parameters were obtained from literature sources, and a few were either fixed or reasonably estimated. RESULTS: Comparative dosing simulations showed that a 10-week constant infusion in which a tumor cell population was continuously exposed to the drug did not produce the best PD profile. On the other hand, dosing schemes where the cell population was initially challenged with a high dose, followed by intermittent dosing, generated the best PD profile. The favorable outcome of the latter dosing schemes was correlated with the lowest expression of P-gp in terms of area under the curve (AUC) during treatment period. CONCLUSIONS: The simulations led us to conclude that drug resistance, particularly resistance caused by P-gp overexpression, induced during chemotherapy may, in part, be circumvented by designing optimal dosing strategies that minimize P-gp induction.

ATP Binding Cassette Transporter, Subfamily B, Mem↗