Carlos M. Hernández-Suárez

Presence of Triatominae (Hemiptera, Reduviidae) and Risk of Transmission of Chagas Disease in Colima, México

With the purpose of evaluating the risk of transmission of the Chagas disease in the State of Colima, México, an entomological survey was performed to obtain triatominae and the rate of infection by Trypanosoma cruzi determined by examination of its dejections. Two hundred eighteen houses located in 16 villages were sampled. In each house the intradomestic and peridomestic habitats were examined by the man-hour-house method, sensor boxes and mouse-baited traps. Also, 12 silvatic places were explored around the same areas using the same techniques as the ones sampled. In total, 456 specimens were captured, of which 139 correspond to Triatoma phyllosoma pallidipennis; 80 to T.p. longipennis; one specimen of T. dimidiata and 236 nymphs of Triatoma sp. Two hundred ninety seven insects were captured in the intradomestic habitat, 132 in the peridomestic and 26 in the silvatic. The index of positive houses was 27%, located in the central area of the state. The rate of natural infection with T. cruzi showed 25.6%. This results confirmed the presence of two important vectors of the Chagas disease in Colima. Its preference for the domestic habitat and its high levels of natural infection with T. cruzi suggested the existence of a significant risk for its transmission in this area of the country.

Violencia doméstica y riesgo de conducta suicida en universitarios adolescentes

OBJECTIVE: To estimate the degree of association between domestic violence -physical, verbal or sexual- with suicidal behavior among university students. MATERIAL AND METHODS: A matched case-control study was done with students attending the University of Colima, Mexico. The cases were 235 teenagers who presented both suicidal ideation and suicide attempt; the controls were 470 individuals of the same age and sex. RESULTS: Sexual abuse showed the highest degree of association with suicidal behavior (OR= 27.4), followed by verbal violence (OR= 9.28), drug use (OR= 8.6), physical violence (OR= 5.5) and smoking (OR= 3.6). Multivariate logistic regression showed that verbal violence was associated with suicidal behavior independently of the other variables, while physical violence, sexual abuse, smoking and drug use seem to depend on verbal violence. CONCLUSIONS: Domestic violence, particularly verbal or sexual, is strongly associated with suicidal behavior in adolescents and should be considered in suicide prevention programs.

A basic result on the integral for birth-death Markov processes.

In this paper a regenerative argument is used to derive an expression for the expectation of the integral under the stochastic path of a birth-death Markov process up to extinction time as well as for the expected time to extinction. Some applications to classical-birth-death processes are given.

Transmisión interepidémica del dengue en la ciudad de Colima, México

OBJETIVO: Determinar la presencia de dengue interepidemico en una comunidad infestada por Aedes aegypti, y analizar sus caracteristicas epidemiologicas. MATERIAL Y METODOS: Entre 2001 y 2002 se realizo un estudio probabilistico longitudinal en 245 habitantes de la ciudad de Colima, Mexico. En cada caso se registraron: edad, sexo, nivel socioeconomico, cuadro clinico sugestivo de dengue, y se busco la presencia de IgG e IgM antidengue por inmunocromatografia rapida y por ELISA, en un seguimiento de siete meses. Los datos se analizaron mediante tablas de contingencia y regresion de Poisson univariada. RESULTADOS: Doce individuos resultaron con infeccion reciente (incidencia de 1.77%; IC 95%: 0.9-3.1%), ocho de ellos refirieron cuadro clinico reciente (ji2=19.6; p=0.0001, RM: 19.6). La regresion de Poisson no revelo correlacion de la infeccion reciente con edad, sexo, ni con nivel socioeconomico. CONCLUSIONES: En comunidades infestadas por A aegypti, como Colima, pueden ocurrir infecciones continuas por dengue sin epidemia aparente. Dichas infecciones no parecen asociarse con la edad, el sexo, ni con el nivel socioeconomico, pero si con el cuadro clinico, el cual podria considerarse como indicador precoz de posible transmision interepidemica.

Urn models and vaccine efficacy estimation.

We derive the distribution of the number of infections among unvaccinated and vaccinated individuals for model 1 (leaky) and model 2 (all/nothing) vaccines, assuming random mixing of a homogeneous population. For all/nothing vaccines, we show that the distribution of the number of infected vaccinated individuals conditioning on n observed infections follows a hypergeometric distribution, and the vaccine efficacy estimate (VE) can be derived from the usual estimate of the total population size in a capture-recapture sampling program. For leaky vaccines, we show that the number of vaccinated infected follows a distribution that was first derived by Wallenius. We found that the current point estimates of VE for each model perform very well, but the urn model construction presented here provides a strong framework for estimation and hypothesis testing on the parameters, and can be applied when the available data are a sample of the population. Since the method does not require an underlying transmission model, it can be applied to estimate the VE for non-contagious diseases.

Biological Control of the Chagas Disease Vector Triatoma infestans with the Entomopathogenic Fungus Beauveria bassiana Combined with an Aggregation Cue: Field, Laboratory and Mathematical Modeling Assessment

Background Current Chagas disease vector control strategies, based on chemical insecticide spraying, are growingly threatened by the emergence of pyrethroid-resistant Triatoma infestans populations in the Gran Chaco region of South America. Methodology and findings We have already shown that the entomopathogenic fungus Beauveria bassiana has the ability to breach the insect cuticle and is effective both against pyrethroid-susceptible and pyrethroid-resistant T. infestans, in laboratory as well as field assays. It is also known that T. infestans cuticle lipids play a major role as contact aggregation pheromones. We estimated the effectiveness of pheromone-based infection boxes containing B. bassiana spores to kill indoor bugs, and its effect on the vector population dynamics. Laboratory assays were performed to estimate the effect of fungal infection on female reproductive parameters. The effect of insect exuviae as an aggregation signal in the performance of the infection boxes was estimated both in the laboratory and in the field. We developed a stage-specific matrix model of T. infestans to describe the fungal infection effects on insect population dynamics, and to analyze the performance of the biopesticide device in vector biological control. Conclusions The pheromone-containing infective box is a promising new tool against indoor populations of this Chagas disease vector, with the number of boxes per house being the main driver of the reduction of the total domestic bug population. This ecologically safe approach is the first proven alternative to chemical insecticides in the control of T. infestans. The advantageous reduction in vector population by delayed-action fungal biopesticides in a contained environment is here shown supported by mathematical modeling.

The long-run distribution of births across environments under environmental stochasticity and its use in the calculation of unconditional life-history parameters

Matrix population models assume individuals develop in time along different stages that may include age, size or degree of maturity to name a few. Once in a given stage, an individuals ability to survive, reproduce or move to another stage are fixed for that stage. Some demographic models consider that environmental conditions may change, and thus the chances of reproducing, dying or developing to another stage depend on the current stage and environmental conditions. That is, models have evolved from a single transition matrix to a set of several transition matrices, each accounting for the properties of a given environment. These models require information on the transition between environments, which is in general assumed to be Markovian. Although great progress has been made in the analysis of these models, they present new challenges and some new parameters need to be calculated, mainly the ones related to how births are distributed among environments. These parameters may help in population management and to calculate unconditional life history parameters. We derive for the first time an expression for the long-run distribution of births across environments, and show that it does not depend only on the long-range frequency of different environments, but also on the set of all transition and fertility matrices. We also derive the long-run distribution of deaths across environments. We provide an example using a real data set of the dynamics of Saiga antelope. Theoretical values closely match the observed values obtained in a large set of stochastic simulations.

An application of queuing theory to SIS and SEIS epidemic models

In this work we consider every individual of a population to be a server whose state can be either busy (infected) or idle (susceptible). This server approach allows to consider a general distribution for the duration of the infectious state, instead of being restricted to exponential distributions. In order to achieve this we first derive new approximations to quasistationary distribution (QSD) of SIS (Susceptible- Infected- Susceptible) and SEIS (Susceptible- Latent- Infected- Susceptible) stochastic epidemic models. We give an expression that relates the basic reproductive number, R0 and the server utilization,p.

Empirical evidence of the effect of school gathering on the dynamics of dengue epidemics

Introduction Dengue fever is an important vector-transmitted disease that affects more than 100 countries worldwide. Locations where individuals tend to gather may play an important role in disease transmission in the presence of the vector. By controlling mosquitoes’ breeding places, this study aims to analyze the effect of reducing transmission in elementary schools (grades 1–9) on the dynamics of the epidemic at a regional level. Materials and methods In 2007, we implemented a massive campaign in a region of México (Colima state, 5,191 km2, population 568,000) focused on training janitors to locate and avoid mosquitoes’ breeding places, the objective being to maintain elementary schools free of mosquitoes. Results We observed 45% reduction in dengue incidence compared to the previous year. In contrast, the rest of Mexico observed an 81% increase in incidence on average. Discussion Costs associated with campaigns focusing on cleaning schools are very low and results seem to be promising. Nevertheless, more controlled studies are needed.Introduction Dengue fever is an important vector-transmitted disease that affects more than 100 countries worldwide. Locations where individuals tend to gather may play an important role in disease transmission in the presence of the vector. By controlling mosquitoes’ breeding places, this study aims to analyze the effect of reducing transmission in elementary schools (grades 1–9) on the dynamics of the epidemic at a regional level. Materials and methods In 2007, we implemented a massive campaign in a region of México (Colima state, 5,191 km2, population 568,000) focused on training janitors to locate and avoid mosquitoes’ breeding places, the objective being to maintain elementary schools free of mosquitoes. Results We observed 45% reduction in dengue incidence compared to the previous year. In contrast, the rest of Mexico observed an 81% increase in incidence on average. Discussion Costs associated with campaigns focusing on cleaning schools are very low and results seem to be promising. Nevertheless, more controlled studies are needed.

Aggregation is the key to succeed in random walks.

In a random walk (RW) in Z an individual starts at 0 and moves at discrete unitary steps to the right or left with respective probabilities p and 1-p. Assuming p > 1/2 and finite a, a > 1, the probability that state a will be reached before -a is Q(a, p) where Q(a, p) > p. Here we introduce the cooperative random walk (CRW) involving two individuals that move independently according to a RW each but dedicate a fraction of time θ to approach the other one unit. This simple strategy seems to be effective in increasing the expected number of individuals arriving to a first. We conjecture that this is a possible underlying mechanism for efficient animal migration under noisy conditions.