Biomedical subjects

E A Stanley

Publications and source records attributed to E A Stanley.

9 recordsLinked to original sources

PubMed2001

The initialization and sensitivity of multigroup models for the transmission of HIV.

We study two multigroup mathematical models of the spread of HIV. In the differential infectivity model, the infected population is divided into groups according to their infectiousness, and HIV is primarily spread by a small, highly infectious, group of superspreaders. In the staged-progression model, every infected individual goes through a series of infection stages and the virus is primarily spread by individuals in an initial highly infectious stage or in the late stages of the disease. We demonstrate the importance of choosing appropriate initial conditions, and define a new approach to distributing the initial population among the subgroups so as to minimize the artificial transients in the solutions due to unbalanced initial conditions. We demonstrate that the rate of removal in and out of a population is an important, yet often neglected, effect. We also illustrate the importance of distinguishing between the number of partners a person has and the number of contacts per partner. By assuming that people with many partners have fewer contacts per partner than people with few partners, we found that the epidemic is less sensitive to the partner acquisition rate than one might expect. However, because the probability of transmission of HIV per contact is low, the epidemic is very sensitive to the number of contacts per partner. Modeling this distinction is particularly important when estimating the impact of programs which encourage people to have fewer sexual partners.

Acute Disease↗

PubMed1999

The differential infectivity and staged progression models for the transmission of HIV.

Recent studies of HIV RNA in infected individuals show that viral levels vary widely between individuals and within the same individual over time. Individuals with higher viral loads during the chronic phase tend to develop AIDS more rapidly. If RNA levels are correlated with infectiousness, these variations explain puzzling results from HIV transmission studies and suggest that a small subset of infected people may be responsible for a disproportionate number of infections. We use two simple models to study the impact of variations in infectiousness. In the first model, we account for different levels of virus between individuals during the chronic phase of infection, and the increase in the average time from infection to AIDS that goes along with a decreased viral load. The second model follows the more standard hypothesis that infected individuals progress through a series of infection stages, with the infectiousness of a person depending upon his current disease stage. We derive and compare threshold conditions for the two models and find explicit formulas of their endemic equilibria. We show that formulas for both models can be put into a standard form, which allows for a clear interpretation. We define the relative impact of each group as the fraction of infections being caused by that group. We use these formulas and numerical simulations to examine the relative importance of different stages of infection and different chronic levels of virus to the spreading of the disease. The acute stage and the most infectious group both appear to have a disproportionate effect, especially on the early epidemic. Contact tracing to identify super-spreaders and alertness to the symptoms of acute HIV infection may both be needed to contain this epidemic.

Computer Simulation↗

PubMed1996

Preferential partner selection in an evolutionary study of Prisoner's Dilemma.

Partner selection is an important process in many social interactions, permitting individuals to decrease the risks associated with cooperation. In large populations, defectors may escape punishment by roving from partner to partner, but defectors in smaller populations risk social isolation. We investigate these possibilities for an evolutionary Prisoner's Dilemma in which agents use expected payoffs to choose and refuse partners. In comparison to random or round-robin partner matching, we find that the average payoffs attained with preferential partner selection tend to be more narrowly confined to a few isolated payoff regions. Most ecologies evolve to essentially full cooperative behavior, but when agents are intolerant of defections, or when the costs of refusal and social isolation are small, we also see the emergence of wallflower ecologies in which all agents are socially isolated. Between these two extremes, we see the emergence of ecologies whose agents tend to engage in a small number of defections followed by cooperation thereafter. The latter ecologies exhibit a plethora of interesting social interaction patterns.

Algorithms↗

PubMed1994

Threshold conditions for the spread of the HIV infection in age-structured populations of homosexual men.

The age-structure of a population, and the distribution of sexual behavior according to age, are significant factors determining the spread of the AIDS epidemic. The threshold conditions for age-structured models account for life-history information, and thus differ significantly from their age-independent counterparts. We examine the threshold conditions for four general age-structured models of the spread of HIV in a homosexual population: three with random partner selection and one with biased partner selection. We consider both discrete and continuous risk groups, and the duration of infection. Susceptibility and infectiousness are treated separately, and the infectivity varies with duration of infection. Through specific examples, we examine the sensitivity of the threshold conditions to the population age-structure and the shape of the infectivity profile. The effects of each are of the same order of magnitude.

Adult↗

PubMed1992

Mechanisms of curve progression following sublaminar (Luque) spinal instrumentation.

Fifty-two patients with Luque instrumentation were reviewed for spinal deformities. Forty-two patients were reviewed during 1 year (longest 7.2 years) at follow-up. Two patients were included who lost correction within 1 year (both 8 months). Follow up averaged 2.9 years. Curve causes primarily were neuromuscular but included one was caused by idiopathic scoliosis, four by Scheuermann's disease, and 1 by post-laminectomy kyphosis. The Cobb angle progressed in 45% of patients postoperatively. Factors contributing to progression included progressive vertebral rotation or the crankshaft phenomenon (11), wire pull out (7), progressive pelvic obliquity (4), rod bending (3), pseudarthrosis (2), and rod migration (2). Factors correlating with progression were kyphosis, postoperative curve greater than 35 degrees, preoperative curve greater than 60 degrees, and not fusing to the pelvis in nonambulators. Crankshaft was common in patients Risser II or less but did not occur in more mature patients.

Adolescent↗

PubMed1989

Risk behavior-based model of the cubic growth of acquired immunodeficiency syndrome in the United States.

The cumulative number of cases of acquired immunodeficiency syndrome (AIDS) in the United States has grown as the cube of time rather than exponentially. We explain this by interactions involving partner choice and sexual frequency in a risk-behavior model with biased mixing. This leads to a saturation wave of infection moving from high- to low-risk groups. If this description is correct, then the decreasing growth rate of AIDS cases is not due to behavior changes; rather it is due to the intrinsic epidemiology of the disease.

Acquired Immunodeficiency Syndrome↗

PubMed1988

The need for national HIV databases.

Researchers and public health officials involved in surveying and forecasting the course of the HIV epidemic require complete and unfiltered information from many sources. Governments should respond by establishing national HIV databases.

Acquired Immunodeficiency Syndrome↗

PubMed1986

On the spatial spread of rabies among foxes.

We present a simple model for the spatial spread of rabies among foxes and use it to quantify its progress in England if rabies were introduced. The model is based on the known ecology of fox behaviour and on the assumption that the main vector for the spread of the disease is the rabid fox. Known data and facts are used to determine real parameter values involved in the model. We calculate the speed of propagation of the epizootic front, the threshold for the existence of an epidemic, the period and distance apart of the subsequent cyclical epidemics which follow the main front, and finally we quantify a means for control of the spatial spread of the disease. By way of illustration we use the model to determine the progress of rabies up through the southern part of England if it were introduced near Southampton. Estimates for the current fox density in England were used in the simulations. These suggest that the disease would reach Manchester within about 3.5 years, moving at speeds as high as 100 km per year in the central region. The model further indicates that although it might seem that the disease had disappeared after the wave had passed it would reappear in the south of England after just over 6 years and at periodic times after that. We consider the possibility of stopping the spread of the disease by creating a rabies 'break' ahead of the front through vaccination to reduce the population to a level below the threshold for an epidemic to exist. Based on parameter values relevant to England, we estimate its minimum width to be about 15 km. The model suggests that vaccination has considerable advantages over severe culling.

Animals↗

PubMed1983

The congenital undescended scapula. Surgical correction by the woodward procedure.

At The Hospital for Sick Children in Toronto, 21 undescended scapulae were corrected by the Woodward procedure in 20 patients over the past 18 years. The average age at operation was six years six months. The average follow-up period after operation was 8 years 9 months. Fifteen patients were girls and five were boys. All the patients had other associated abnormalities. Only one patient had a concomitant clavicular osteotomy. The average increase in glenohumeral abduction was 37 degrees. The average scapular lowering was two centimetres. Excellent or good cosmetic results were obtained in 80 per cent of patients. Ninety-three per cent of patients expressed satisfaction with the operative results. Complications were few. The Woodward procedure is a safe and relatively straightforward surgical means to effect a good cosmetic and functional improvement in a patient with a congenital undescended scapula. We do not recommend clavicular osteotomy as a routine, but it may be added to gain more correction with less risk of neurovascular compression.

Abnormalities, Multiple↗