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Understanding the biological parameters of some triatomine subspecies of Meccus phyllosomus (Burmeister) is a crucial first step in estimating the epidemiological importance of this group. Biological parameters related to egg eclosion, egg-to-adult development time, number of blood meals to moult, percentage of females at the end of the cycle, number of laid eggs, and the accumulative mortality for each instar of three M. phyllosomus subspecies [Meccus phyllosomus pallidipennis (Stål), Meccus phyllosomus longipennis (Usinger), and Meccus phyllosomus picturatus (Usinger)] as well as their laboratory hybrids were evaluated and compared. No significant differences (P > 0.05) were recorded among the experimental hybrids (M. p. longipennis × M. p. pallidipennis, M. p. longipennis × M. p. picturatus, M. p. pallidipennis × M. p. picturatus) and reciprocal cohorts. In five of the six studied parameters (egg eclosion, egg-to-adult development time, number of blood meals to moult, number of laid eggs and accumulative mortality), with the exception of the non-significant percentage of females obtained among all the studied cohorts, at least one of the parental cohorts in each set of crosses exhibited better fitness results than by those of their hybrid descendants. The lack of hybrid fitness in our study indicates the maintenance of reproductive isolation of parental genotypes. Moreover, the results lead us to propose that an incipient speciation process by distance is currently developing among the three studied subspecies, increasing the differences between them that modify the transmission efficiency of Trypanosoma cruzi to human beings in Mexico.
In this paper the effect of hybrid laser arc welding on longitudinal joints for pipes of 1.27cm thick is investigated. For the investigation, an API X70 steel was welded with the HLAW process and then subjected to tensile, bending and micro hardness tests under standards for pipe manufacturing. Images of the weld seams were taken to observe the structure and size of the weld zones. Analysis was made by light microscopy to determine the phases present in the weld zones and to observe if there is a variation of grain size in the weld zones that adversely affects the mechanical properties of the API X70 steel. Results show that the mechanical properties of the joints meet the requirements for their use in pipe manufacturing; one reason is the low thickness of the weld zone that barely affects the original properties of API X70 steel. Also the presence of bainite in the microstructure of weld zones provides resistance to the joints.
Trees’ resistance to fire-induced mortality increases with bark thickness, which varies widely among species and generally increases with stem diameter. Because dry forests are more fire-prone than wetter forests, bark may be thicker in these forests. However, where disturbances such as hurricanes suppress stem diameter, trees may not obtain fire-resistant bark thickness. In two hurricane-prone Caribbean dry-forest types in Puerto Rico—deciduous forest and scrub forest—we measured bark thickness on 472 stems of 25 species to test whether tree species obtain bark thicknesses that confer fire resistance, whether bark is thicker in the fire-prone scrub forest than in the deciduous forest, and how bark thickness in Caribbean dry forest compares with other tropical ecosystems. Only 5% of stems within a deciduous-forest stand had bark thickness that would provide < 50% probability of top-kill during low-intensity fire. In contrast, thicker-barked trees dominated the scrub forest, suggesting that fires influenced it. Compared with trees of similar diameter in other regions of the tropics, bark in Caribbean dry forest was thinner than in savanna, similar to other seasonally dry forests, and thicker than moist-to-wet forests. Dry-forest species appear to invest more in fire-resistance than species from wetter forests. However, Caribbean dry forests remain highly vulnerable to fire because the trees rarely reach large enough diameters to be fire resistant.
We extrapolate from the exact master equations of epidemic dynamics on fully connected
graphs to non-fully connected by keeping the size of the state space N + 1, where
N is the
number of nodes in the graph. This gives rise to a system of approximate ODEs (ordinary
differential equations) where the challenge is to compute/approximate analytically the
transmission rates. We show that this is possible for graphs with arbitrary degree
distributions built according to the configuration model. Numerical tests confirm that:
(a) the agreement of the approximate ODEs system with simulation is excellent and (b) that
the approach remains valid for clustered graphs with the analytical calculations of the
transmission rates still pending. The marked reduction in state space gives good results,
and where the transmission rates can be analytically approximated, the model provides a
strong alternative approximate model that agrees well with simulation. Given that the
transmission rates encompass information both about the dynamics and graph properties, the
specific shape of the curve, defined by the transmission rate versus the number of
infected nodes, can provide a new and different measure of network structure, and the
model could serve as a link between inferring network structure from prevalence or
We consider the spread of an infectious disease on a heterogeneous metapopulation defined
by any (correlated or uncorrelated) network. The infection evolves under transmission,
recovery and migration mechanisms. We study some spectral properties of a connectivity
matrix arising from the continuous-time equations of the model. In particular we show that
the classical sufficient condition of instability for the disease-free equilibrium, well
known for the particular case of uncorrelated networks, works also for the general case.
We give also an alternative condition that yields a more accurate estimation of the
epidemic threshold for correlated (either assortative or dissortative) networks.
We present and characterize a multi-host epidemic model of Rift Valley fever (RVF) virus
in East Africa with geographic spread on a network, rule-based mitigation measures, and
mosquito infection and population dynamics. Susceptible populations are depleted by
disease and vaccination and are replenished with the birth of new animals. We observe that
the severity of the epidemics is strongly correlated with the duration of the rainy season
and that even severe epidemics are abruptly terminated when the rain stops. Because
naturally acquired herd immunity is established, total mortality across 25 years is
relatively insensitive to many mitigation approaches. Strong reductions in cattle
mortality are expected, however, with sufficient reduction in population densities of
either vectors or susceptible (ie. unvaccinated) hosts. A better understanding of RVF
epidemiology would result from serology surveys to quantify the importance of herd
immunity in epidemic control, and sequencing of virus from representative animals to
quantify the realative importance of transportation and local reservoirs in nucleating
yearly epidemics. Our results suggest that an effective multi-layered mitigation strategy
would include vector control, movement control, and vaccination of young animals yearly,
even in the absence of expected rainfall.
We consider the spread of infectious disease through contact networks of Configuration
Model type. We assume that the disease spreads through contacts and infected individuals
recover into an immune state. We discuss a number of existing mathematical models used to
investigate this system, and show relations between the underlying assumptions of the
models. In the process we offer simplifications of some of the existing models. The
distinctions between the underlying assumptions are subtle, and in many if not most cases
this subtlety is irrelevant. Indeed, under appropriate conditions the models are
equivalent. We compare the benefits and disadvantages of the different models, and discuss
their application to other populations (e.g., clustered networks).
Finally we discuss ongoing challenges for network-based epidemic modeling.
In epidemic modeling, the Susceptible-Alert-Infected-Susceptible (SAIS) model extends the
SIS (Susceptible-Infected-Susceptible) model. In the SAIS model, “alert” individuals
observe the health status of neighbors in their contact network, and as a result, they may
adopt a set of cautious behaviors to reduce their infection rate. This alertness, when
incorporated in the mathematical model, increases the range of effective/relative
infection rates for which initial infections die out. Built upon the SAIS model, this work
investigates how information dissemination further increases this range. Information
dissemination is realized through an additional network (e.g., an online social network)
sharing the contact network nodes (individuals) with different links. These “information
links” provide the health status of one individual to all the individuals she is connected
to in the information dissemination network. We propose an optimal information
dissemination strategy with an index in quadratic form relative to the information
dissemination network adjacency matrix and the dominant eigenvector of the contact
network. Numerical tools to exactly solve steady state infection probabilities and
influential thresholds are developed, providing an evaluative baseline for our information
dissemination strategy. We show that monitoring the health status of a small but “central”
subgroup of individuals and circulating their incidence information optimally enhances the
resilience of the society against infectious diseases. Extensive numerical simulations on
a survey–based contact network for a rural community in Kansas support these findings.
When modeling the spread of infectious diseases, it is important to incorporate risk
behavior of individuals in a considered population. Not only risk behavior, but also the
network structure created by the relationships among these individuals as well as the
dynamical rules that convey the spread of the disease are the key elements in predicting
and better understanding the spread. In this work we propose the weighted random
connection model, where each individual of the population is characterized by two
parameters: its position and risk behavior. A goal is to model the effect that the
probability of transmissions among individuals increases in the individual risk factors,
and decays in their Euclidean distance. Moreover, the model incorporates a combined risk
behavior function for every pair of the individuals, through which the spread can be
directly modeled or controlled. We derive conditions for the existence of an outbreak of
infectious diseases in this model. Our main result is the almost sure existence of an
infinite component in the weighted random connection model. We use results on the random
connection model and site percolation in Z2.
In this paper, an SIS (susceptible-infected-susceptible)-type epidemic propagation is
studied on a special class of 3-regular graphs, called modified cycle graphs. The modified
cycle graph is constructed from a cycle graph with N nodes by connecting node
i to the
node i +
d in a way that every node has exactly three links.
Monte-Carlo simulations show that the propagation process depends on the value of
d in a
non-monotone way. A new theoretical model is developed to explain this phenomenon. This
reveals a new relation between the spreading process and the average path length in the
In this paper we explore the potential of the pairwise-type modelling approach to be
extended to weighted networks where nodal degree and weights are not independent. As a
baseline or null model for weighted networks, we consider undirected, heterogenous
networks where edge weights are randomly distributed. We show that the pairwise model
successfully captures the extra complexity of the network, but does this at the cost of
limited analytical tractability due the high number of equations. To circumvent this
problem, we employ the edge-based modelling approach to derive models corresponding to two
different cases, namely for degree-dependent and randomly distributed weights. These
models are more amenable to compute important epidemic descriptors, such as early growth
rate and final epidemic size, and produce similarly excellent agreement with simulation.
Using a branching process approach we compute the basic reproductive ratio for both models
and discuss the implication of random and correlated weight distributions on this as well
as on the time evolution and final outcome of epidemics. Finally, we illustrate that the
two seemingly different modelling approaches, pairwise and edge-based, operate on similar
assumptions and it is possible to formally link the two.
The basic reproduction number, R0, is often defined as the average
number of infections generated by a newly infected individual in a fully susceptible
population. The interpretation, meaning, and derivation of R0 are
controversial. However, in the context of mean field models, R0 demarcates
the epidemic threshold below which the infected population approaches zero in the limit of
time. In this manner, R0 has been proposed as a method for
understanding the relative impact of public health interventions with respect to disease
eliminations from a theoretical perspective. The use of R0 is made more
complex by both the strong dependency of R0 on the model form and the stochastic
nature of transmission. A common assumption in models of HIV transmission that have closed
form expressions for R0 is that a single individual’s
behavior is constant over time. In this paper we derive expressions for both
R0 and probability of an epidemic in a
finite population under the assumption that people periodically change their sexual
behavior over time. We illustrate the use of generating functions as a general framework
to model the effects of potentially complex assumptions on the number of transmissions
generated by a newly infected person in a susceptible population. We find that the
relationship between the probability of an epidemic and R0 is not
straightforward, but, that as the rate of change in sexual behavior increases both
R0 and the probability of an epidemic also
A feature often observed in epidemiological networks is significant heterogeneity in
degree. A popular modelling approach to this has been to consider large populations with
highly heterogeneous discrete contact rates. This paper defines an individual-level
non-Markovian stochastic process that converges on standard ODE models of such populations
in the appropriate asymptotic limit. A generalised Sellke construction is derived for this
model, and this is then used to consider final outcomes in the case where heterogeneity
follows a truncated Zipf distribution.
The aims of this study are to assess L1 and L2 variables that influence the reading acquisition of students of Moroccan origin in the South of Spain and compare their reading ability with native Spanish-speaking children. Participants were 38 students of Moroccan origin and 37 native Spanish-speaking students from the same classes. We used an oral vocabulary test and a reading comprehension test, which taps lexical, semantic, and syntactic reading processes, and reading fluency. The results indicated that immigrant students differed from native Spanish-speaking students in word reading, reading fluency, and the use of punctuation marks, but there were no significant differences in reading comprehension. In native Spanish-speaking students, reading comprehension correlated significantly with oral vocabulary and the other reading processes, but in the students of Moroccan origin, only receptive oral vocabulary in L2 correlated with the use of punctuation marks. Being in schools with educational resources specifically aimed at helping the Moroccan pupils was associated with a higher level of word reading in immigrant students.
Bismuth ferrite (BiFeO3) is a magnetoelectric, multiferroic material with coexisting ferroelectric and magnetic orderings. It is considered as a candidate for the next generation of ferroelectric random-access memory devices because BiFeO3, in contrast to industrial ferroelectrics used today, does not contain the toxic element lead. Furthermore, its polarization values are higher than those of lead-based ferroelectrics. The magnitude of the polarization of a BiFeO3 film is dependent on its orientation and is related to the domain structure. This contribution presents and discusses the preparation of epitaxial BiFeO3 (BFO) thin films grown on SrRuO3/SrTiO3 substrates by pulsed laser deposition (PLD) and their characterization, especially by piezo force microscopy (PFM) and atomic force acoustic microscopy (AFAM). The thickness of an individual BFO film varies between 100 and 200 nm. The epitaxial nature of films in the crystallographic (100), (110), and (111) directions was confirmed by x-ray diffraction (XRD). Thin SrRuO3 layers, also prepared by PLD, were used as bottom electrode for the ferroelectric hysteresis measurements. Low frequency PFM measurements showed a monodomain structure for the as-grown (110) and (111) oriented samples. In BFO (100) films, different polarization variants were observed by ultrasonic piezo force microscopy (UPFM). The domain structure is reproduced from minimization of the electrostatic and elastic energies. Switching experiments using standard PFM as well as UPFM were carried out on the three samples with the objective of testing the coercive field and domain stability. The AFAM technique was used to map the elastic properties of the BFO thin-films at the micro- and nanoscale.
Bismuth iron oxide BFO films were produced by the pulsed laser deposition technique. These films are a mixture of BiFeO3 ferroelectrical and Bi25FeO40 piezoelectrical phases. The ferroelectrical domain structure of these films was studied via contact resonance piezoresponse force microscopy (CR-PFM) and resonance tracking PFM (RT-PFM). The proportions of area of these BFO phases were derived from the PFM images. The ferroelectrical domain size corresponds to the size of the BiFeO3 crystals. The CR-PFM and RT-PFM techniques allowed us to be able to distinguish between the ferroelectric domains and the piezoelectric regions existing in the polycrystalline films.
An outbreak of gastroenteritis affected at least 240 persons who had eaten at a gourmet restaurant over a period of 7 weeks in 2009 in England. Epidemiological, microbiological, and environmental studies were conducted. The case-control study demonstrated increased risk of illness in those who ate from a special ‘tasting menu’ and in particular an oyster, passion fruit jelly and lavender dish (odds ratio 7·0, 95% confidence interval 1·1–45·2). Ten diners and six staff members had laboratory-confirmed norovirus infection. Diners were infected with multiple norovirus strains belonging to genogroups I and II, a pattern characteristic of molluscan shellfish-associated outbreaks. The ongoing risk from dining at the restaurant may have been due to persistent contamination of the oyster supply alone or in combination with further spread via infected food handlers or the restaurant environment. Delayed notification of the outbreak to public health authorities may have contributed to outbreak size and duration.