1,721,028 research outputs found
Epidemiologic and Economic Analyses on Highly Pathogenic Avian Influenza H5N1 in Nigeria and Egypt
No full textInfluenza A viruses have caused several devastating outbreaks in poultry with some zoonotic infections and deaths in humans. To control the viruses and their continuous circulation in poultry population, an understanding of the epidemiology of the viruses is needed. An evaluation of the situation of avian influenza A H5N1 in two African countries most affected by this virus was conducted. To that end, two aims were pursued: (i) to identify the factors that played important roles in the dissemination and circulation of the virus, and (ii) to explore the potentials of the virus from becoming endemic in the poultry populations in Africa. The analyses partly described the extent of H5N1outbreaks in Nigeria in at-risk and non-infected premises and live bird markets in the country. All samples (tracheal and cloacal swabs, parenchymatous tissues and sera) were analysed using virus isolation, reverse-transcription polymerase chain reaction and serology. Primary data on influenza A H5N1 in human as recorded by the Egyptian government was collected and analysed using different variables and risk factors for HPAI H5N1 virus infection in poultry farms in Nigeria during 2006-2007 outbreaks in Nigeria were explored using the conditional logistic regression models. Factors that supported infections in new premises included: receiving visitors on farm premises, purchase of live poultry/products, farm workers that live outside the premises. Because biosecurity is important to reduce risks of infection in poultry farms, the effects and costs associated with the implementation or neglect of biosecurity, as well as the feasibility of implementation in the household poultry in Egypt were evaluated. Risks of particular importance for the household poultry were categorised into people-related, environmental-related and other birds/animal-related risks. Biosecurity measures were compatible in the household poultry and it is 8.45 times better to implement biosecurity than to do nothing against HPAI H5N1. The financial risk analysis was robust and withstood sensitivity analysis. The change that may occur in the course of the household poultry project was accommodated profitably. The effect of road network and its contributions to the epidemics in Nigeria were evaluated to determine whether HPAI H5N1 exhibit properties of network theory. Spatial aggregation of cases (disease clusters), links among similar ‘nodes’ (assortativity), simultaneous activation of similar nodes (synchronicity), epidemic flows moving from highly to poorly connected nodes (directionality), and a Pareto analyses pattern were observed including synchronicity and directionality of spread properties. An exploratory analysis was conducted on the probability of human infection through the oral contacts with A-H5N1 contaminated meat. Detailed quantitative risk assessment using predictive microbiology-process risk model revealed that up to 15,159 humans may have contracted HPAI H5N1 in Africa, with 1,964 deaths. These figures are higher than those officially reported. The model has limitations but bias associated with official epidemiological statistics which are often prone to underreporting, lack of awareness especially in the rural communities and censoring effects may also account for this discrepancy. Finally, continuous monitoring of influenza viruses in Africa and implementation of measures that should reduce the burden of infection in Africa is necessary
Modelling volatility using a non-homogeneous martingale model for processes with constant mean on count data
Get PDFIn this article a non-homogeneous martingale model is proposed to model volatility in a stochastic time series of count data with constant mean. The approach is derived from a general non- homogeneous birth-and-death process, in which the mean and the variance of population size can vary as a function of time. This model can be important in modelling early warning signals that there is going to be a change of state in a complex system. The net reproduction ratio obtained from fitting a non-homogeneous birth–death model can be used as an additional tool to compare this model with a model where there is no change in the mean over the observation period. These models and procedures are illustrated with quarterly Methicillin resistant staphylococcus aureus prevalence data registered since 2001 from three Acute Trusts of hospitals of the National Health Service in Great Britain
The long subclinical phase of Mycobacterium avium ssp. paratuberculosis infections explained without adaptive immunity
Get PDFMycobacterium avium ssp. paratuberculosis (MAP) is an infection of the ruminant intestine. In cows, a long subclinical phase with no or low intermittent shedding precedes the clinical phase with high shedding. It is generally considered that an adaptive cell-mediated immune response controls the infection during the subclinical phase, followed by unprotective antibodies later in life. Based on recent observations, we challenge the importance of adaptive immunity and instead suggest a role of the structural organization of infected macrophages in localized granulomatous lesions. We investigated this hypothesis by mathematical modelling. Our first model describes infection in a villus, assuming a constant lesion volume. This model shows the existence of two threshold parameters, the MAP reproduction ratio R MAP determining if a lesion can develop, and the macrophage replacement ratio R MF determining if recruitment of macrophages is sufficient for unlimited growth. We show that changes in R MF during a cow's life - i.e. changes in the innate immune response - can cause intermittent shedding. Our second model describes infection in a granuloma, assuming a growing lesion volume. This model confirms the results of the villus model, and can explain early slow granuloma development: small granulomas grow slower because bacteria leave the granuloma quickly through the relatively large surface area. In conclusion, our models show that the long subclinical period of MAP infection can result from the structural organization of the infection in granulomatous lesions with an important role for innate rather than adaptive immunity. It thus provides a reasonable hypothesis that needs further investigation
R0 for vector-borne diseases: impact of the assumption for the duration of the extrinsic incubation period
Get PDFMathematical modeling and notably the basic reproduction number R0 have become popular tools for the description of vector-borne disease dynamics. We compare two widely used methods to calculate the probability of a vector to survive the extrinsic incubation period. The two methods are based on different assumptions for the duration of the extrinsic incubation period; one method assumes a fixed period and the other method assumes a fixed daily rate of becoming infectious. We conclude that the outcomes differ substantially between the methods when the average life span of the vector is short compared to the extrinsic incubation period
Risk based surveillance for early detection of low pathogenic avian influenza outbreaks in layer chickens
No full textCurrent knowledge does not allow the prediction of when low pathogenic avian influenza virus (LPAIV) of the H5 and H7 subtypes infecting poultry will mutate to their highly pathogenic phenotype (HPAIV). This mutation may already take place in the first infected flock; hence early detection of LPAIV outbreaks will reduce the likelihood of pathogenicity mutations and large epidemics. The objective of this study was the development of a model for the design and evaluation of serological-surveillance programmes, with a particular focus on early detection of LPAIV infections in layer chicken flocks. Early detection is defined as the detection of an infected flock before it infects on average more than one other flock (between-flock reproduction ratio Rf <1), hence a LPAI introduction will be detected when only one or a few other flocks are infected. We used a mathematical model that investigates the required sample size and sampling frequency for early detection by taking into account the LPAIV within- and between-flock infection dynamics as well as the diagnostic performance of the serological test used. Since layer flocks are the target of the surveillance, we also explored whether the use of eggs, is a good alternative to sera, as sample commodity. The model was used to refine the current Dutch serological-surveillance programme. LPAIV transmission-risk maps were constructed and used to target a risk-based surveillance strategy. In conclusion, we present a model that can be used to explore different sampling strategies, which combined with a cost-benefit analysis would enhance surveillance programmes for low pathogenic avian influenza
Eimeria acervulina: The influence of inoculation dose on transmission between broiler chickens
No full textThe course and clinical appearance of an Eimeria species infection in chicken flocks depend on the response of an individual bird to infection and on population-dynamics of the infection in the flock. Differences in ingested numbers of oocysts may affect oocyst load in the flock and the subsequent infectious dose for not yet infected birds. To study the link between numbers of oocysts excreted by infected birds and transmission of Eimeria acervulina, experiments were carried out with 42 pairs of broiler chickens using inoculation doses with 5, 50, 500 or 50,000 sporulated oocysts. In each pair one bird was inoculated and the other bird was contact-exposed. All contact birds became infected, which occurred on average within 34 h after exposure to an inoculated bird. Although a higher inoculation dose resulted in higher oocyst excretion in inoculated and contact-infected birds, only small non-significant differences in transmission rates between groups were foun
Controlling highly pathogenic avian influenza outbreaks: An epidemiological and economic model analysis
Get PDFOutbreaks of highly pathogenic avian influenza (HPAI) can cause large losses for the poultry sector and for animal disease controlling authorities, as well as risks for animal and human welfare. In the current simulation approach epidemiological and economic models are combined to compare different strategies to control highly pathogenic avian influenza in Dutch poultry flocks. Evaluated control strategies are the minimum EU strategy (i.e., culling of infected flocks, transport regulations, tracing and screening of contact flocks, establishment of protection and surveillance zones), and additional control strategies comprising pre-emptive culling of all susceptible poultry flocks in an area around infected flocks (1. km, 3. km and 10. km) and emergency vaccination of all flocks except broilers around infected flocks (3. km).Simulation results indicate that the EU strategy is not sufficient to eradicate an epidemic in high density poultry areas. From an epidemiological point of view, this strategy is the least effective, while pre-emptive culling in 10. km radius is the most effective of the studied strategies. But these two strategies incur the highest costs due to long duration (EU strategy) and large-scale culling (pre-emptive culling in 10. km radius). Other analysed pre-emptive culling strategies (i.e., in 1. km and 3. km radius) are more effective than the analysed emergency vaccination strategy (in 3. km radius) in terms of duration and size of the epidemics, despite the assumed optimistic vaccination capacity of 20 farms per day. However, the total costs of these strategies differ only marginally. Extending the capacity for culling substantially reduces the duration, size and costs of the epidemic.This study demonstrates the strength of combining epidemiological and economic model analysis to gain insight in a range of consequences and thus to serve as a decision support tool in the control of HPAI epidemics
Evaluating contribution of the cellular and humoral immune responses to the control of shedding of Mycobacterium avium
Get PDFMycobacterium avium spp. paratuberculosis (MAP) causes a persistent infection and chronic inflammation of the gut in ruminants leading to bacterial shedding in feces in many infected animals. Although there are often strong MAP-specific immune responses in infected animals, immunological correlates of protection against progression to disease remain poorly defined. Analysis of cross-sectional data has suggested that the cellular immune response observed early in infection is effective at containing bacterial growth and shedding, in contrast to humoral immune responses. In this study, 20 MAP-infected calves were followed for nearly 5 years during which MAP shedding, antigen-specific cellular (LPT) and humoral (ELISA) immune responses were measured. We found that MAP-specific cellular immune response developed slowly, with the peak of the immune response occurring one year post infection. MAP-specific humoral immunity expanded only in a subset of animals. Only in a subset of animals there was a statistically significant negative correlation between the amount of MAP shedding and magnitude of the MAP-specific cellular immune response. Direct fitting of simple mechanistic mathematical models to the shedding data suggested that MAP-specific immune responses contributed significantly to the kinetics of MAP shedding in most animals. However, whereas the MAP-specific cellular immune response was predicted to suppress shedding in some animals, in other animals it was predicted to increase shedding. In contrast, MAP-specific humoral response was always predicted to increase shedding. Our results illustrate the use of mathematical methods to understand relationships between mycobacteria and immunity in vivo but also highlight problems with establishing cause-effect links from observational data
Differences in intermittent and continuous fecal shedding patterns between natural and experimental Mycobacterium avium subspecies paratuberculosis infections in cattle
Get PDFThe objective of this paper is to study shedding patterns of cows infected with Mycobacterium avium subsp. paratuberculosis (MAP). While multiple single farm studies of MAP dynamics were reported, there is not large scale meta-analysis of both natural and experimental infections. Large difference in shedding patterns between experimentally and naturally infected cows were observed. Experimental infections are thus probably driven by different pathological mechanisms. For further evaluations of shedding patterns only natural infections were used. Within such infections, the transition to high shedding was studied as a proxy to the development of a clinical disease. The majority of studied cows never developed high shedding levels. Those that do, typically never reduced their shedding level to low or no shedding. Cows that eventually became high shedders showed a pattern of continuous shedding. In contrast, cows with an intermittent shedding pattern had a low probability to ever become high shedders. In addition, cows that start shedding at a younger age (less than three years of age) have a lower hazard of becoming high shedders compared to cows starting to shed at an older age. These data suggest the presence of three categories of immune control. Cows that are intermittent shedders have the infection process under control (no progressive infection). Cows that start shedding persistently at a young age partially control the infection, but eventually will be high shedders (slow progressive infection), while cows that start shedding persistently at an older age cannot effectively control the infection and become high shedders rapidly
Modelling the potential spatial distribution of mosquito species using three different techniques
Get PDFBACKGROUND: Models for the spatial distribution of vector species are important tools in the assessment of the risk of establishment and subsequent spread of vector-borne diseases. The aims of this study are to define the environmental conditions suitable for several mosquito species through species distribution modelling techniques, and to compare the results produced with the different techniques. METHODS: Three different modelling techniques, i.e., non-linear discriminant analysis, random forest and generalised linear model, were used to investigate the environmental suitability in the Netherlands for three indigenous mosquito species (Culiseta annulata, Anopheles claviger and Ochlerotatus punctor). Results obtained with the three statistical models were compared with regard to: (i) environmental suitability maps, (ii) environmental variables associated with occurrence, (iii) model evaluation. RESULTS: The models indicated that precipitation, temperature and population density were associated with the occurrence of Cs. annulata and An. claviger, whereas land surface temperature and vegetation indices were associated with the presence of Oc. punctor. The maps produced with the three different modelling techniques showed consistent spatial patterns for each species, but differences in the ranges of the predictions. Non-linear discriminant analysis had lower predictions than other methods. The model with the best classification skills for all the species was the random forest model, with specificity values ranging from 0.89 to 0.91, and sensitivity values ranging from 0.64 to 0.95. CONCLUSIONS: We mapped the environmental suitability for three mosquito species with three different modelling techniques. For each species, the maps showed consistent spatial patterns, but the level of predicted environmental suitability differed; NLDA gave lower predicted probabilities of presence than the other two methods. The variables selected as important in the models were in agreement with the existing knowledge about these species. All model predictions had a satisfactory to excellent accuracy; best accuracy was obtained with random forest. The insights obtained can be used to gain more knowledge on vector and non-vector mosquito species. The output of this type of distribution modelling methods can, for example, be used as input for epidemiological models of vector-borne diseases
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