71 research outputs found
Maximum equilibrium prevalence of mosquito-borne microparasite infections in humans.
To determine the maximum equilibrium prevalence of mosquito-borne microparasitic infections, this paper proposes a general model for vector-borne infections which is flexible enough to comprise the dynamics of a great number of the known diseases transmitted by arthropods. From equilibrium analysis, we determined the number of infected vectors as an explicit function of the model's parameters and the prevalence of infection in the hosts. From the analysis, it is also possible to derive the basic reproduction number and the equilibrium force of infection as a function of those parameters and variables. From the force of infection, we were able to conclude that, depending on the disease's structure and the model's parameters, there is a maximum value of equilibrium prevalence for each of the mosquito-borne microparasitic infections. The analysis is exemplified by the cases of malaria and dengue fever. With the values of the parameters chosen to illustrate those calculations, the maximum equilibrium prevalence found was 31% and 0.02% for malaria and dengue, respectively. The equilibrium analysis demonstrated that there is a maximum prevalence for the mosquito-borne microparasitic infections
Equilibrium analysis of a yellow Fever dynamical model with vaccination.
We propose an equilibrium analysis of a dynamical model of yellow fever transmission in the presence of a vaccine. The model considers both human and vector populations. We found thresholds parameters that affect the development of the disease and the infectious status of the human population in the presence of a vaccine whose protection may wane over time. In particular, we derived a threshold vaccination rate, above which the disease would be eradicated from the human population. We show that if the mortality rate of the mosquitoes is greater than a given threshold, then the disease is naturally (without intervention) eradicated from the population. In contrast, if the mortality rate of the mosquitoes is less than that threshold, then the disease is eradicated from the populations only when the growing rate of humans is less than another threshold; otherwise, the disease is eradicated only if the reproduction number of the infection after vaccination is less than 1. When this reproduction number is greater than 1, the disease will be eradicated from the human population if the vaccination rate is greater than a given threshold; otherwise, the disease will establish itself among humans, reaching a stable endemic equilibrium. The analysis presented in this paper can be useful, both to the better understanding of the disease dynamics and also for the planning of vaccination strategies
Potential impact of global climate change on malaria risk.
The biological activity and geographic distribution of the malarial parasite and its vector are sensitive to climatic influences, especially temperature and precipitation. We have incorporated General Circulation Model-based scenarios of anthropogenic global climate change in an integrated linked-system model for predicting changes in malaria epidemic potential in the next century. The concept of the disability-adjusted life years is included to arrive at a single measure of the effect of anthropogenic climate change on the health impact of malaria. Assessment of the potential impact of global climate change on the incidence of malaria suggests a widespread increase of risk due to expansion of the areas suitable for malaria transmission. This predicted increase is most pronounced at the borders of endemic malaria areas and at higher altitudes within malarial areas. The incidence of infection is sensitive to climate changes in areas of Southeast Asia, South America, and parts of Africa where the disease is less endemic; in these regions the numbers of years of healthy life lost may increase significantly. However, the simulated changes in malaria risk must be interpreted on the basis of local environmental conditions, the effects of socioeconomic developments, and malaria control programs or capabilities
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Rift Valley fever in African buffalo (Syncerus caffer) : basic epidemiology and the role of bovine tuberculosis coinfection
Rift Valley fever (RVF) is a mosquito-borne zoonotic viral disease native to the African continent. Outbreaks tend to occur in the wet seasons, and can affect numerous mammalian species including African buffalo. It is debated how the virus survives the inter-epidemic period when it is not detected in mammalian populations, either in cryptic wildlife hosts or by vertical transmission in mosquito hosts. In chapters 1 and 2 of this dissertation I show that buffalo do become infected in the inter-epidemic period although that is not sufficient to maintain viral cycling in the system without additional mammalian hosts and high vertical transmission rates.
Bovine tuberculosis is an emerging disease in sub-Saharan Africa, first detected in Kruger National Park buffalo populations in 1990. African buffalo are a maintenance host for BTB in the ecosystem, and there has been detailed research about pathogen provenance and diversity, effects on the host and transmission dynamics. These studies have focused on a single invasive pathogen, BTB – despite the fact that buffalo act as hosts for a multitude of pathogens. Fundamental theory in community ecology and immunology suggests that parasites within a host should interact, by sharing resources, competing for resources or by altering the immune response. In chapter 3 I show that animals with BTB are more likely to become infected with RVF, more likely to show clinical signs and that the presence of BTB increases the size of RVF epidemics in African buffalo. In chapters 4 and 5 I demonstrate that one of the mechanisms underlying this pattern may be immune-mediated whereby animals with BTB have altered susceptibility to RVF. Understanding how emerging diseases, like BTB, may affect native host-pathogen or pathogen - pathogen interactions will help us understand the full impact that emerging diseases may have on an ecosystem
Environ Health Perspect
BackgroundBecause of complex interactions of climate variables at the levels of the pathogen, vector, and host, the potential influence of climate change on vector-borne and zoonotic diseases (VBZDs) is poorly understood and difficult to predict. Climate effects on the nonvector-borne zoonotic diseases are especially obscure and have received scant treatment.ObjectiveWe described known and potential effects of climate change on VBZDs and proposed specific studies to increase our understanding of these effects. The nonvector-borne zoonotic diseases have received scant treatment and are emphasized in this paper.Data sources and synthesisWe used a review of the existing literature and extrapolations from observations of short-term climate variation to suggest potential impacts of climate change on VBZDs. Using public health priorities on climate change, published by the Centers for Disease Control and Prevention, we developed six specific goals for increasing understanding of the interaction between climate and VBZDs and for improving capacity for predicting climate change effects on incidence and distribution of VBZDs.ConclusionsClimate change may affect the incidence of VBZDs through its effect on four principal characteristics of host and vector populations that relate to pathogen transmission to humans: geographic distribution, population density, prevalence of infection by zoonotic pathogens, and the pathogen load in individual hosts and vectors. These mechanisms may interact with each other and with other factors such as anthropogenic disturbance to produce varying effects on pathogen transmission within host and vector populations and to humans. Because climate change effects on most VBZDs act through wildlife hosts and vectors, understanding these effects will require multidisciplinary teams to conduct and interpret ecosystem-based studies of VBZD pathogens in host and vector populations and to identify the hosts, vectors, and pathogens with the greatest potential to affect human populations under climate change scenarios
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Individual Host to Population Scale Dynamics of Parasite Assemblages in African Buffalo of Kruger National Park, South Africa
The last century has experienced a marked increase in emerging infectious disease (EID, hereafter) – jeopardizing human, domestic animal, and wildlife health. EIDs are commonly associated with spillover from one host species into a novel host species, with many destructive diseases, for both livestock and wildlife, emerging at the wildlife-livestock interface. As global change continues to erode the boundaries between human and wildlife systems, it will become increasingly more important to understand the key components influencing host susceptibility as well as pathogen/parasite spread and persistence.
However, understanding disease systems, especially within wildlife, is complex, as processes at multiple scales of biological organization are relevant to pathogen/parasite dynamics. At the within-host scale, pathogens interact with host cells and co-infecting pathogens, and these within-host dynamics affect host susceptibility, infectious period, and pathogen transmission potential. At the host population-level scale, heterogeneity across hosts as well as pathogen dispersal between hosts interacts with within-host processes to ultimately influence the distribution of infectious agents within-hosts, across hosts, and over time. Studying disease in natural systems enables researchers to observe the outcome of interactions of numerous multi-scale sources of variation and predict realistic parasite/pathogen dynamics. Ultimately, this work should enable the development of adaptive disease management.
For my PhD dissertation, I explored how within-host patterns and processes inform population-level patterns in African buffalo (Syncerus caffer) of Kruger National Park (KNP), South Africa. Specifically, I studied infectious agents associated with two diseases that infect cattle and buffalo at the South African wildlife-livestock interface: the bovine respiratory disease complex and theileriosis. In Chapter 2, I found that evolutionarily conserved immune responses (i.e., non-specific inflammatory response) can be used to detect disease exposure without a priori knowledge of pathogen identity – a tool than can be further developed for EID surveillance. In Chapter 3, I weighed the effect of host traits, pathogen co-occurrence and environmental variability on probability of infection by viral and bacterial pathogens within the bovine respiratory disease complex as well as characterized temporal trends in pathogen incidence. I found that the importance of each factor was inconsistent across pathogens – co-occurrence was the best indicator of virus occurrence whereas host ID was the best indicator of bacterial infection. Importantly, I found that within-host dynamics only partially elucidated seasonal cycling in population-level disease dynamics. In Chapter 4, I developed molecular methods to quantify cryptic spatio-temporal variation in vector-borne, hemoparasite (Theileria: the etiological agent of theileriosis) assemblages of African buffalo. In Chapter 5, I used the high resolution data from Chapter 4 to describe the structure of Theileria assemblages within and across hosts, in both space and time. Chapter 5 uses novel analytical approaches to distill complex Theileria assemblages into functional groups based upon their life-history patterns. This characterization enabled me to estimate the relative importance of dispersal and host heterogeneity on distribution of these parasites thereby enabling me to predict efficacy and side-effects of vector-borne disease management tools
Ecology and public health: rodents as reservoirs of zoonoses in the farmland of northwestern Spain
Zoonoses are a concerning issue for public health and the last pandemic event caused by covid-19 is a good example of it. Human activity is enhancing the transmission, intensity and emergence of practically all zoonoses. Synantropic qualities of some rodent species provide them with exceptional features as amplifiers of emerging zoonoses. Vectors are also important elements transmitting the pathogen between hosts and make it more likely for a disease to cross the species barrier and become zoonotic. Circulation of pathogens involves several reservoirs and hosts, each one with a different level of competence for vectors and transmission of pathogens. The overlap of infected hosts, competent vectors, and humans in the same habitats and at the same time increase considerably the zoonotic risk. The disease ecology based on the One-Health considers pathogens as elements interconnected with the natural environment, wildlife, domestic animals and humans. An effective monitoring, comprehensive understanding of the system functioning, and determination of the spatial-temporal patterns provide us with crucial information for disease prevention.
In this thesis, I studied the zoonotic relevance of wild populations of a sympatric small mammal community (Microtus arvalis, Apodemus sylvaticus, Mus spretus and Crocidura russula) that inhabit intensive farming in NW Spain. Here, M. arvalis is considered a host and amplifier of Francisella tularensis (the etiological agent causing tularemia) but little is known about the circulation of this and other zoonotic pathogens in the system. I focused on improving the scientific knowledge of the dynamic of zoonotic pathogens and vectors of the sympatric small mammal community inhabiting those intensive agricultural landscapes. In the first chapter, I reviewed current knowledge on the role of common vole in tularemia epidemiology and identified relevant knowledge gaps in the “Francisella tularensis–M. arvalis” system. In the second chapter, I characterized the most common arthropod vectors (fleas and ticks) parasitizing the small mammal host community. In the third and four chapters, I screened the host community for some zoonotic micropathogens and macroparasites: bacteria (F. tularensis and Bartonella), viruses (hantaviruses, arenaviruses and orthopoxviruses) and gastrointestinal helminths. Transversely, I examined variations in the parasitological parameters (prevalence, intensity and abundance) according to host species and sex, habitat (crop type), seasonality and the population dynamics of host species, with particular emphasis on the vole population cycles.
I have detected F. tularensis, nine Bartonella species, three types of viruses and eight different helminth taxa. Half of those pathogens are considered zoonotic. Results showed that the small mammals surveyed that lives in sympatry with M. arvalis seem to have no relevant role in the circulation and maintenance of F. tularensis. Fluctuating population dynamics of M. arvalis and seasonality can affect the dynamic of vectors and pathogens. High-density periods of M. arvalis (outbreak years and summer) favored the circulation of viruses and bacteria, and increased the abundance of fleas, potentially also increasing the zoonotic risk for human populations. The infestation levels by ticks and gastrointestinal helminths were higher during the crash phase of the vole cycle. These and other pathogens could contribute to the maintenance of a low vole population phase, by limiting and delaying the recovery of the vole population after a crash.
This thesis contributed new knowledge of the circulation of zoonotic pathogens and vectors in this farming system, with public health implications. Of particular importance are the roles that vole outbreaks play as an amplifier and spill-over agent of zoonotic diseases; the need to consider new viruses (in particular, hantavirus) in public health surveillance; and the usefulness of community-based monitoring of pathogen circulation, maintenance and transmission to improve prevention.Las zoonosis son un tema preocupante para la salud pública y el último evento pandémico causado
por la covid-19 es un buen ejemplo de ello. La actividad humana está potenciando la transmisión,
intensidad y emergencia de prácticamente todas las zoonosis. Las cualidades sinantrópicas de
algunas especies de roedores les confieren características excepcionales como amplificadores de
zoonosis emergentes. Los vectores también son elementos importantes que transmiten patógenos
entre hospedadores y hacen más probable que una enfermedad salte la barrera de especie y se
convierta en zoonótica. En la circulación de patógenos intervienen varios reservorios y hospedadores,
cada uno con diferente nivel de idoneidad para la supervivencia de los vectores y de transmisión de
cada patógeno. El solapamiento en lugar y tiempo de huéspedes infectados, vectores competentes y
seres humanos aumenta considerablemente el riesgo zoonótico. La ecología de las enfermedades
basada en el concepto de One-Health considera a los patógenos como elementos interconectados
con el entorno natural, la fauna salvaje, los animales domésticos y los seres humanos. Un seguimiento
eficaz, la comprensión exhaustiva del funcionamiento del sistema y la determinación de los patrones
espacio-temporales proporcionan información crucial para la prevención de enfermedades.
En esta tesis, he estudiado la relevancia zoonótica de las poblaciones silvestres de una
comunidad de pequeños mamíferos simpátricos (Microtus arvalis, Apodemus sylvaticus, Mus spretus
y Crocidura russula) que habitan zonas agrarias intensificadas del noroeste de España. En este caso,
M. arvalis se considera un hospedador y amplificador de Francisella tularensis (el agente etiológico
causante de la tularemia), pero poco se conoce sobre la circulación de éste y otros patógenos
zoonóticos en el sistema. Me centré en mejorar el conocimiento científico de la dinámica de los
patógenos zoonóticos y los vectores en la comunidad de pequeños mamíferos simpátricos que
habitan esos paisajes agrícolas intensivos. En el primer capítulo, revisé los conocimientos actuales
sobre el papel del topillo común en la epidemiología de la tularemia e identifiqué algunas lagunas de
conocimiento relevantes en el sistema "Francisella tularensis-M. arvalis". En el segundo capítulo,
caractericé los vectores artrópodos más comunes (pulgas y garrapatas) que parasitan a la comunidad
de pequeños mamíferos. En los capítulos tercero y cuarto, examiné la comunidad de hospedadores
en busca de algunos micropatógenos y macroparásitos zoonóticos: bacterias (F. tularensis y Bartonella), virus (hantavirus, arenavirus y ortopoxvirus) y helmintos gastrointestinales. De forma transversal, he examinado las variaciones de los parámetros parasitológicos (prevalencia, intensidad y abundancia) en función de la especie y el sexo del hospedador, el hábitat (tipo de cultivo), la estacionalidad y la dinámica poblacional de las especies hospedadoras, con especial atención a los
ciclos poblacionales de los topillos.
He detectado F. tularensis, nueve especies de Bartonella, tres tipos de virus y ocho taxones
de helmintos diferentes. La mitad de esos patógenos se consideran zoonóticos. Los resultados mostraron que los pequeños mamíferos estudiados que co-habitan con M. arvalis no parecen tener un papel relevante en la circulación y el mantenimiento de F. tularensis. La dinámica poblacional fluctuante de M. arvalis y la estacionalidad pueden afectar a la dinámica de vectores y patógenos. Los periodos de alta densidad de M. arvalis (años de brotes y verano) favorecieron la circulación de virus y bacterias, y aumentaron la abundancia de pulgas, incrementando también potencialmente el riesgo zoonótico para las poblaciones humanas. Los niveles de infestación por garrapatas y helmintos gastrointestinales fueron mayores durante la fase de colapso poblacional del topillo campesino. Estos
y otros patógenos podrían contribuir al mantenimiento de la población de topillos en una fase de baja
densidad, limitando y retrasando la recuperación de la población de topillos después del colapso
poblacional.
Esta tesis aportó nuevos conocimientos sobre la circulación de patógenos y vectores zoonóticos en este sistema agrícola, con implicaciones para la salud pública. Son especialmente relevantes el papel de los brotes de topillos como amplificadores y propagadores de enfermedades zoonóticas; la necesidad de tener en cuenta nuevos virus que pueden estar circulando en el sistema (en particular, hantavirus) de cara a la vigilancia con motivos de la salud pública; y la utilidad de la
monitorización de zoonoses basada en la comunidad para conocer la circulación, persistencia y
transmisión de patógenos de cara a mejorar las estrategias de prevención.Escuela de DoctoradoDoctorado en Conservación y Uso Sostenible de Sistemas Forestale
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