The impact of smoking on orthopaedic patients

Smoking has devastating effects on general health, including the outcome after surgery. There were three main objectives in this thesis. Firstly, to investigate the effect of smoking on complications after orthopaedic surgery. Secondly, to evaluate whether the negative effect of smoking on the complication rate after acute orthopaedic surgery was reversible after smoking cessation therapy and thirdly, to evaluate whether the administered smoking cessation therapy had any long-lasting effect on the smoking abstinence rate. Study I included 906 patients with surgically treated ankle fractures. Background data were collected from patient charts and the outcome regarding postoperative complications was recorded prospectively in a clinical audit. Studies II and III were based on the same population from a single-blinded, randomized, controlled, clinical, multicenter trial at three hospitals in Stockholm, Sweden. We randomized 105 daily smokers with an acute fracture of a lower or upper extremity requiring acute surgical procedures into an intervention group (smoking cessation therapy) or into a control group. The primary outcome in Study II was any complication occurring, as predefined in the study protocol, within 6–12 weeks. The outcome in Study III was medium- and long-term successful smoking cessation. In Study IV the background data were taken from the SALT cohort in the Swedish Twin Registry. The SALT data were then linked to the Swedish Inpatient Registry, identifying 8773 individuals who had had orthopaedic surgery and who also had had a complication from that surgery. In Study I it was shown that 30.1% of the smokers had a postoperative complication compared to 20.3% of the non-smokers (OR 1.9, CI: 1.3–2.8, p=0.005). In study II the administered smoking cessation therapy significantly reduced the number of postoperative complications (p=0.048). Study III showed that the administered smoking cessation therapy had a significant effect during the first 6–12 weeks, but not after one year. Study IV demonstrated that smokers had a significantly increased risk of developing complications requiring inpatient care; among the smokers, 14.9%, compared to 11.4% of the non-smokers, had such a complication (HR 1.27, CI: 1.10–1.48, p=0.002). Smoking is a strong and significant factor associated with development of postoperative complications. Smoking cessation intervention program during the first six weeks after acute fracture surgery decreases the risk of postoperative complications. Smoking patients in need of both acute and elective orthopaedic surgery should be offered an intensive smoking cessation programme

Circular Stochastic Fluctuations in SIS Epidemics with Heterogeneous Contacts Among Sub-populations

The conceptual difference between equilibrium and non-equilibrium steady state (NESS) is well established in physics and chemistry. This distinction, however, is not widely appreciated in dynamical descriptions of biological populations in terms of differential equations in which fixed point, steady state, and equilibrium are all synonymous. We study NESS in a stochastic SIS (susceptible-infectious-susceptible) system with heterogeneous individuals in their contact behavior represented in terms of subgroups. In the infinite population limit, the stochastic dynamics yields a system of deterministic evolution equations for population densities; and for very large but finite system a diffusion process is obtained. We report the emergence of a circular dynamics in the diffusion process, with an intrinsic frequency, near the endemic steady state. The endemic steady state is represented by a stable node in the deterministic dynamics; As a NESS phenomenon, the circular motion is caused by the intrinsic heterogeneity within the subgroups, leading to a broken symmetry and time irreversibility.Comment: 29 pages, 5 figure

The time to extinction for an SIS-household-epidemic model

We analyse a stochastic SIS epidemic amongst a finite population partitioned into households. Since the population is finite, the epidemic will eventually go extinct, i.e., have no more infectives in the population. We study the effects of population size and within household transmission upon the time to extinction. This is done through two approximations. The first approximation is suitable for all levels of within household transmission and is based upon an Ornstein-Uhlenbeck process approximation for the diseases fluctuations about an endemic level relying on a large population. The second approximation is suitable for high levels of within household transmission and approximates the number of infectious households by a simple homogeneously mixing SIS model with the households replaced by individuals. The analysis, supported by a simulation study, shows that the mean time to extinction is minimized by moderate levels of within household transmission

Assembling evidence for identifying reservoirs of infection

Many pathogens persist in multihost systems, making the identification of infection reservoirs crucial for devising effective interventions. Here, we present a conceptual framework for classifying patterns of incidence and prevalence, and review recent scientific advances that allow us to study and manage reservoirs simultaneously. We argue that interventions can have a crucial role in enriching our mechanistic understanding of how reservoirs function and should be embedded as quasi-experimental studies in adaptive management frameworks. Single approaches to the study of reservoirs are unlikely to generate conclusive insights whereas the formal integration of data and methodologies, involving interventions, pathogen genetics, and contemporary surveillance techniques, promises to open up new opportunities to advance understanding of complex multihost systems

Predation effects on mean time to extinction under demographic stochasticity

Methods for predicting the probability and timing of a species' extinction are typically based on a combination of theoretical models and empirical data, and focus on single species population dynamics. Of course, species also interact with each other, forming more or less complex networks of interactions. Models to assess extinction risk often lack explicit incorporation of these interspecific interactions. We study a birth and death process in which the death rate includes an effect from predation. This predation rate is included via a general nonlinear expression for the functional response of predation to prey density. We investigate the effects of the foraging parameters (e.g. attack rate and handling time) on the mean time to extinction. Mean time to extinction varies by orders of magnitude when we alter the foraging parameters, even when we exclude the effects of these parameters on the equilibrium population size. In particular we observe an exponential dependence of the mean time to extinction on handling time. These findings clearly show that accounting for the nature of interspecific interactions is likely to be critically important when estimating extinction risk.Comment: 11 pages, 4 figures; Typos removed. For further discussion about the paper go to http://purl.org/net/extinctio

Extinction times in the subcritical stochastic SIS logistic epidemic

Many real epidemics of an infectious disease are not straightforwardly super- or sub-critical, and the understanding of epidemic models that exhibit such complexity has been identified as a priority for theoretical work. We provide insights into the near-critical regime by considering the stochastic SIS logistic epidemic, a well-known birth-and-death chain used to model the spread of an epidemic within a population of a given size $N$. We study the behaviour of the process as the population size $N$ tends to infinity. Our results cover the entire subcritical regime, including the "barely subcritical" regime, where the recovery rate exceeds the infection rate by an amount that tends to 0 as $N \to \infty$ but more slowly than $N^{-1/2}$. We derive precise asymptotics for the distribution of the extinction time and the total number of cases throughout the subcritical regime, give a detailed description of the course of the epidemic, and compare to numerical results for a range of parameter values. We hypothesise that features of the course of the epidemic will be seen in a wide class of other epidemic models, and we use real data to provide some tentative and preliminary support for this theory.Comment: Revised; 34 pages; 6 figure

Estimation of changes in the force of infection for intestinal and urogenital schistosomiasis in countries with Schistosomiasis Control Initiative-assisted programmes

The last decade has seen an expansion of national schistosomiasis control programmes in Africa based on large-scale preventative chemotherapy. In many areas this has resulted in considerable reductions in infection and morbidity levels in treated individuals. In this paper, we quantify changes in the force of infection (FOI), defined here as the per (human) host parasite establishment rate, to ascertain the impact on transmission of some of these programmes under the umbrella of the Schistosomiasis Control Initiative (SCI)

Moment Closure - A Brief Review

Moment closure methods appear in myriad scientific disciplines in the modelling of complex systems. The goal is to achieve a closed form of a large, usually even infinite, set of coupled differential (or difference) equations. Each equation describes the evolution of one "moment", a suitable coarse-grained quantity computable from the full state space. If the system is too large for analytical and/or numerical methods, then one aims to reduce it by finding a moment closure relation expressing "higher-order moments" in terms of "lower-order moments". In this brief review, we focus on highlighting how moment closure methods occur in different contexts. We also conjecture via a geometric explanation why it has been difficult to rigorously justify many moment closure approximations although they work very well in practice.Comment: short survey paper (max 20 pages) for a broad audience in mathematics, physics, chemistry and quantitative biolog