Factors affecting onchocerciasis transmission: Lessons for infection control

Introduction: Onchocerca volvulus infects in excess of 15 million people. The vectors are Simulium blackflies, varieties of which differ in their ecologies, behaviour and vectorial abilities. Control of the vectors and mass administrations of ivermectin have succeeded in reducing prevalences with elimination achieved in some foci, particularly in Central and southern America. In Africa, progress towards elimination has been less successful. Areas covered: Even with community directed treatment with ivermectin (CDTI), control has been difficult in African areas with initial prevalences in excess of 55%, especially if only annual treatments are dispensed. This is partly attributable to insufficient coverage, but the appearance of incipiently resistant non-responding parasites and lack of attention to vector biology in modelling and planning outcomes of intervention programmes have also played their parts, with recrudescence now appearing in some treated areas. Expert commentary: The biology of onchocerciasis is complex involving different vectors with differing abilities to transmit parasites, diverse pathologies related to geographical and parasite variations and endosymbionts in both parasite and vector. Modelling to predict epidemiological and control outcomes is addressing this complexity but more attention needs to be given to the vectors’ roles to further understanding of where and when control measures will succeed

Do different tests of episodic memory produce consistent results in human adults?

A number of different philosophical, theoretical, and empirical perspectives on episodic memory have led to the development of very different tests with which to assess it. Although these tests putatively assess the same psychological capacity, they have rarely been directly compared. Here, a sample of undergraduates was tested on three different putative tests of episodic memory (What-Where-When, Unexpected Question/Source Memory, and Free Recall). It was predicted that to the extent to which these different tests are assessing the same psychological process, performance across the various tests should be positively correlated. It was found that not all tests were related and those relationships that did exist were not always linear. Instead, two tests showed a quadratic relationship, suggesting the contribution of multiple psychological processes. It is concluded that not all putative tests of episodic cognition are necessarily testing the same thing.We thank the research students who were involved in this project, Mathilda Hay and Stephanie Bailey. We also thank Netta Chachuma and Francesca Lewis for help on an early version of this study, and James Thom, Ljerka Ostojic, Jon Simons, and Tom Smulders for commenting on versions of the manuscript, and Daniel Booth and Anthony Cheke for proofreading. Special thanks to Chris Stephenson for creating, maintaining, and updating the computer-based task. L.G.C. was funded by an MRC doctoral training studentship, an MRC Centenary Early Career Award, and a Junior Research Fellowship at Girton College, Cambridge.This is the final version of the article. It first appeared from Cold Spring Harbor Laboratory Press via http://dx.doi.org/10.1101/lm.030502.11

Ecological characteristics of Simulium breeding sites in West Africa

Twenty-nine taxa of Simulium were identified amongst 527 collections of larvae and pupae from untreated rivers and streams in Liberia (362 collections in 1967–71 & 1989), Togo (125 in 1979–81), Benin (35 in1979–81) and Ghana (5 in 1980–81). Presence or absence of associations between different taxa were usedto group them into six clusters using Ward agglomerative hierarchical cluster analysis. Environmental data associated with the pre-imaginal habitats were then analysed in relation to the six clusters by oneway ANOVA. The results revealed significant effects in determining the clusters of maximum river width (all P < 0.001 unless stated otherwise), water temperature, dry bulb air temperature, relative humidity,altitude, type of water (on a range from trickle to large river), water level, slope, current, vegetation,light conditions, discharge, length of breeding area, environs, terrain, river bed type (P < 0.01), and the supports to which the insects were attached (P < 0.01). When four non-significant contributors (wet bulb temperature, river features, height of waterfall and depth) were excluded and the reduced data-set analysed by principal components analysis (PCA), the first two principal components (PCs) accounted for 87% of the variance, with geographical features dominant in PC1 and hydrological characteristics in PC2. The analyses also revealed the ecological characteristics of each taxon’s pre-imaginal habitats, which are discussed with particular reference to members of the Simulium damnosum species complex, whose breeding site distributions were further analysed by canonical correspondence analysis (CCA), a method also applied to the data on non-vector species

A threshold policy to interrupt transmission of West Nile Virus to birds

This paper proposes a model of West Nile Virus (WNV) with a Filippov-type control strategy of culling mosquitoes implemented once the number of infected birds exceeds a threshold level. The long-term dynamical behaviour of the proposed non-smooth system is investigated. It is shown hat as the threshold value varies, model solutions ultimately approach either one of two endemic equilibria for two subsystems or a pseudo-equilibrium on the switching surface, which is a novel steady state. The results indicate that a previously chosen level of infected birds can be maintained when the threshold policy and other parameters are chosen properly. Numerical studies show that under the threshold policy, strengthening mosquito culling together with protecting bird population is beneficial to curbing the spread of WNV

Models of impulsive culling of mosquitoes to interrupt transmission of West Nile Virus to birds

A mathematical model describing the transmission of West Nile virus (WNV) between vector mosquitoes and birds, incorporating a control strategy of culling mosquitoes and defined by impulsive differential equations is presented and its properties investigated. First, we consider a strategy of periodic impulsive culling of the mosquitoes. Theoretical results indicate that if the threshold R 0 is greater than unity the disease uniformly persists, but, if not, the disease does not necessarily become extinct. The explicit conditions determining the backward or forward bifurcation were obtained. The culling rate has a major effect on the occurrence of backward bifurcation. Analysis shows that the disease is most sensitive to mosquito-bird contacts, mosquito-culling rate and intervals between culls. The dependence of the outcomes of the culling strategy on mosquito biting rate is discussed. When the complete elimination of disease is impossible, mosquito culls are implemented once the infected birds reach a predefined but adjustable threshold value. Numerical analysis shows that the period of mosquito culling finally stabilizes at a fixed value. In addition, variations of mean prevalence of \{WNV\} in birds and the culling period are simulated

Nonlinear pulse vaccination in an SIR epidemic model with resource limitation

Mathematical models can assist in the design and understanding of vaccination strategies when resources are limited. Here we propose and analyse an SIR epidemic modelwith a nonlinear pulse vaccination to examine how a limited vaccine resource affects the transmission and control of infectious diseases, in particular emerging infectious diseases. The threshold condition for the stability of the disease free steady state is given. Latin Hypercube Sampling/Partial Rank Correlation Coefficient uncertainty and sensitivity analysis techniques were employed to determine the key factors which are most significantly related to the threshold value. Comparing this threshold value with that without resource limitation, our results indicate that if resources become limited pulse vaccination should be carried out more frequently than when sufficient resources are available to eradicate an infectious disease. Once the threshold value exceeds a critical level, both susceptible and infected populations can oscillate periodically. Furthermore, when the pulse vaccination period is chosen as a bifurcation parameter, the SIR model with nonlinear pulse vaccination reveals complex dynamics including period doubling, chaotic solutions, and coexistence of multiple attractors. The implications of our findings with respect to disease control are discussed

A locust phase change model with multiple switching states and random perturbation

Insects such as locusts and some moths can transform from a solitarious phase when they remain in loose populations and a gregarious phase, when they may swarm. Therefore, the key to effective management of outbreaks of species such as the desert locust Schistocercagregaria is early detection of when they are in the threshold state between the two phases, followed by timely control of their hopper stages before they fledge because the control of flying adult swarms is costly and often ineffective. Definitions of gregarization thresholds should assist preventive control measures and avoid treatment of areas that might not lead to gregarization. In order to better understand the effects of the threshold density which represents the gregarization threshold on the outbreak of a locust population, we developed a model of a discrete switching system. The proposed model allows us to address: (1) How frequently switching occurs from solitarious to gregarious phases and vice versa; (2) When do stable switching transients occur, the existence of which indicate that solutions with larger amplitudes can switch to a stable attractor with a value less than the switching threshold density?; and (3) How does random perturbation influence the switching pattern? Our results show that both subsystems have refuge equilibrium points, outbreak equilibrium points and bistable equilibria. Further, the outbreak equilibrium points and bistable equilibria can coexist for a wide range of parameters and can switch from one to another. This type of switching is sensitive to the intrinsic growth rate and the initial values of the locust population, and may result in locust population outbreaks and phase switching once a small perturbation occurs. Moreover, the simulation results indicate that the switching transient patterns become identical after some generations, suggesting that the evolving process of the perturbation system is not related to the initial value after some fixed number of generations for the same stochastic processes. However, the switching frequency and outbreak patterns can be significantly affected by the intensity of noise and the intrinsic growth rate of the locust population

Global dynamics of a state-dependent feedback control system

The main purpose is to develop novel analytical techniques and provide a comprehensive qualitative analysis of global dynamics for a state-dependent feedback control system arising from biological applications including integrated pest management. The model considered consists of a planar system of differential equations with state-dependent impulsive control. We characterize the impulsive and phase sets, using the phase portraits of the planar system and the Lambert W function to define the Poincaré map for impulsive point series defined in the phase set. The existence, local and global stability of an order-1 limit cycle and sharp sufficient conditions for the global stability of the boundary order-1 limit cycle have been provided. We further examine the flip bifurcation related to the existence of an order-2 limit cycle. We show that the existence of an order-2 limit cycle implies the existence of an order-1 limit cycle. We derive sufficient conditions under which any trajectory initiating from a phase set will be free from impulsive effects after finite state-dependent feedback control actions, and we also prove that order-k (k ≥ 3) limit cycles do not exist, providing a solution to an open problem in the integrated pest management community. We then investigate multiple attractors and their basins of attraction, as well as the interior structure of a horseshoe-like attractor. We also discuss implications of the global dynamics for integrated pest management strategy. The analytical techniques and qualitative methods developed in the present paper could be widely used in many fields concerning state-dependent feedback control

Exploring the relative contributions of reward-history and functionality information to children's acquisition of the Aesop's fable task.

Investigation of tool-using behaviours has long been a means by which to explore causal reasoning in children and nonhuman animals. Much of the recent research has focused on the "Aesop's Fable" paradigm, in which objects must be dropped into water to bring a floating reward within reach. An underlying problem with these, as with many causal reasoning studies, is that functionality information and reward history are confounded: a tool that is functionally useful is also rewarded, while a tool that is not functionally useful is not rewarded. It is therefore not possible to distinguish between behaviours motivated by functional understanding of the properties of the objects involved, and those influenced by reward-history. Here, we devised an adapted version of the Aesop's Fable paradigm which decouples functionality information and reward history by making use of situations in which the use of a particular tool should have enabled a subject to obtain (or not obtain) a reward, but the outcome was affected by the context. Children aged 4-11 were given experience of a range of tools that varied independently in whether they were functional or non-functional and rewarded or non-rewarded. They were then given the opportunity to choose which tools they would like to use in a test trial, thereby providing an assessment of whether they relied on information about functionality or the reward history associated with the object or a combination of the two. Children never significantly used reward history to drive their choices of tools, while the influence of functionality information increased with age, becoming dominant by age 7. However, not all children behaved in a consistent manner, and even by 10 years of age, only around a third exclusively used functionality as a basis for their decision-making. These findings suggest that from around the age of 7-years, children begin to emphasize functionality information when learning in novel situations, even if competing reward information is available, but that even in the oldest age-group, most children did not exclusively use functionality information

How do children solve Aesop's Fable?

Studies on members of the crow family using the "Aesop's Fable" paradigm have revealed remarkable abilities in these birds, and suggested a mechanism by which associative learning and folk physics may interact when learning new problems. In the present study, children between 4 and 10 years of age were tested on the same tasks as the birds. Overall the performance of the children between 5-7-years was similar to that of the birds, while children from 8-years were able to succeed in all tasks from the first trial. However the pattern of performance across tasks suggested that different learning mechanisms might be being employed by children than by adult birds. Specifically, it is possible that in children, unlike corvids, performance is not affected by counter-intuitive mechanism cues.This is the final version of the article. It first appeared from PLOS via http://dx.doi.org/10.1371/journal.pone.004057