Risk signals of an influenza pandemic caused by highly pathogenic avian influenza subtype H5N1: spatio-temporal perspectives

Highly pathogenic avian influenza (HPAI) subtype H5N1 is a trans-boundary animal disease that has crossed the animal-human species barrier and over the past decade has had a considerable impact on the poultry industry, wild bird populations and on human health. Understanding the spatio-temporal patterns of H5N1 outbreaks can provide visual clues to the dynamics of disease spread and of areas at risk, and thus improve the cost-effectiveness of disease control and prevention. This study describes the characteristics and investigates the temporal, spatial and space-time dynamics of H5N1 outbreaks in domestic poultry between December 2003 and December 2009 using a global database. The study found that the start date of the epidemic wave was postponed, the duration of the epidemic was prolonged and its magnitude reduced over time, but the disease transmission cycle was not efficiently interrupted. Two 'hot-spot' regions of H5N1 outbreaks were identified: well-documented locations in East and Southeast Asia, as well as a novel location at the boundaries of Europe and Africa, where enhanced surveillance should be conducted. The risk of a pandemic due to H5N1 remains high.     

Geostatistical visualisation and spatial statistics for evaluation of the dispersion of epidemic highly pathogenic avian influenza subtype H5N1

The aim of this study was to evaluate a range of statistical and geostatistical methods for their usefulness in providing insights into how highly pathogenic avian influenza (HPAI) subtype H5N1 might spread through a national population of village poultry. The insights gained allow the generation of disease dispersion hypotheses. The case study data set consisted of 161 outbreaks of HPAI subtype H5N1 in village poultry reported in Romania between October 2005 and June 2006. Reports of village outbreaks (%) occurred in three waves: October-December (14%), February-March (16%), and May-June (68%). Risk mapping - based on variography and kriging - was used to visualize the evolution of the epidemic. Outbreaks first appeared in eastern and southern Romania, particularly within an area that forms part of the Danube River Delta. The largest phase of the epidemic affected villages in all parts of central, southern, and eastern Romania, but outbreaks were clustered in central Romania. Outbreaks spread in an east to west direction. By using geostatistical visualisation and spatial statistics, the evolution of the epidemic could be characterised into two parts: disease introduction, local spread, and sporadic outbreaks, and long-distance disease spread with rapid epidemic propagation. This is consistent with the hypothesis that the environment and landscape (specifically the Danube River Delta) played a critical role in the introduction and initial spread of HPAI subtype H5N1 during the autumn and winter of 2005, and that the movement of poultry might have introduced the infection into central Romania during the spring and summer of 2006. Further research focusing on the spatio-temporal interface between the two parts of the epidemic might reveal how and why it progressed from a confined, local epidemic to a large, national epidemic. Such information would assist efforts to limit the global spread of HPAI subtype H5N1.     

Controlling disease outbreaks in wildlife using limited culling: modelling classical swine fever incursions in wild pigs in Australia

ABSTRACT: Disease modelling is one approach for providing new insights into wildlife disease epidemiology. This paper describes a spatio-temporal, stochastic, susceptible- exposed-infected-recovered process model that simulates the potential spread of classical swine fever through a documented, large and free living wild pig population following a simulated incursion. The study area (300 000 km2) was in northern Australia. Published data on wild pig ecology from Australia, and international Classical Swine Fever data was used to parameterise the model. Sensitivity analyses revealed that herd density (best estimate 1-3 pigs km-2), daily herd movement distances (best estimate approximately 1 km), probability of infection transmission between herds (best estimate 0.75) and disease related herd mortality (best estimate 42%) were highly influential on epidemic size but that extraordinary movements of pigs and the yearly home range size of a pig herd were not. CSF generally established (98% of simulations) following a single point introduction. CSF spread at approximately 9 km2 per day with low incidence rates (< 2 herds per day) in an epidemic wave along contiguous habitat for several years, before dying out (when the epidemic arrived at the end of a contiguous sub-population or at a low density wild pig area). The low incidence rate indicates that surveillance for wildlife disease epidemics caused by short lived infections will be most efficient when surveillance is based on detection and investigation of clinical events, although this may not always be practical. Epidemics could be contained and eradicated with culling (aerial shooting) or vaccination when these were adequately implemented. It was apparent that the spatial structure, ecology and behaviour of wild populations must be accounted for during disease management in wildlife. An important finding was that it may only be necessary to cull or vaccinate relatively small proportions of a population to successfully contain and eradicate some wildlife disease epidemics.     

Simulating the spatial dynamics of foot and mouth disease outbreaks in feral pigs and livestock in Queensland, Australia, using a susceptible-infected-recovered cellular automata model

We describe an approach to modelling the spatio-temporal spread of foot and mouth disease through feral animal and unfenced livestock populations. We used a susceptible-infected-recovered model, implemented in a cellular automata framework, to assess the spread of FMD across two regions of Queensland, Australia. Following a sensitivity analysis on the infectious states, scenario analyses were conducted using feral pigs only as the susceptible population, and then with the addition of livestock, and initiated in the wet season and in the dry season. The results indicate that, depending on the season the outbreak is initiated, and without the implementation of control measures, an outbreak of Foot and Mouth Disease around Winton could continue unchecked, while an outbreak around Cape York may die out naturally. The approach explicitly incorporates the spatial relationships between the populations through which the disease spreads and provides a framework by which the spread of disease outbreaks can be explored through varying the model parameters. It highlights the emergence and importance of spatio-temporal patterns, something that previous modelling of FMD in feral animal and unfenced livestock populations has lacked.     

Newcastle disease outbreaks in Italy during 2000

Among the consequences of the epidemic of highly pathogenic avian influenza which affected Italy between 1999 and 2000 was an epidemic of Newcastle disease in northern and central Italy. It affected industrially reared poultry, dealer flocks and backyard flocks, with a total of 254 outbreaks notified up to December 31, 2000. Virological investigations yielded virulent isolates of Newcastle disease virus, which produced intracerebral pathogenicity indices ranging from 1.6 to 2.0 and which, on the basis of their monoclonal antibody binding patterns, could be classified as belonging to group C1. The clinical, gross and microscopical findings were typical of Newcastle disease, and different avian species were susceptible to different degrees. Chickens and guinea fowl appeared to be the most susceptible, followed by pheasants, turkeys and ostriches. The epidemiological inquiry highlighted the crucial role of a broiler hatchery in initiating the epidemic, and of dealers in perpetuating it. The control measures imposed by Directive 92/66/EEC are discussed with reference to the outbreaks in backyard flocks.     

The effect of neighbourhood definitions on spatio-temporal models of disease outbreaks: Separation distance versus range overlap

The definition of the spatial relatedness between infectious and susceptible animal groups is a fundamental component of spatio-temporal modelling of disease outbreaks. A common neighbourhood definition for disease spread in wild and feral animal populations is the distance between the centroids of neighbouring group home ranges. This distance can be used to define neighbourhood interactions, and also to describe the probability of successful disease transmission. Key limitations of this approach are (1) that a susceptible neighbour of an infectious group with an overlapping home range - but whose centroid lies outside the home range of an infectious group - will not be considered for disease transmission, and (2) the degree of overlap between the home ranges is not taken into account for those groups with centroids inside the infectious home range. We assessed the impact of both distance-based and range overlap methods of disease transmission on model-predicted disease spread. Range overlap was calculated using home ranges modelled as circles. We used the Sirca geographic automata model, with the population data from a nine-county study area in Texas that we have previously described. For each method we applied 100 model repetitions, each of 100 time steps, to 30 index locations. The results show that the rate of disease spread for the range-overlap method is clearly less than the distance-based method, with median outbreaks modelled using the latter being 1.4-1.45 times larger. However, the two methods show similar overall trends in the area infected, and the range-overlap median (48 and 120 for cattle and pigs, respectively) falls within the 5th-95th percentile range of the distance-based method (0-96 and 0-252 for cattle and pigs, respectively). These differences can be attributed to the calculation of the interaction probabilities in the two methods, with overlap weights generally resulting in lower interaction probabilities. The definition of spatial neighbourhood has important implications for models used in decision-support systems for disease preparedness and response. This research presents a first step towards more realistic representations that could be used in spatio-temporal models of disease outbreaks.     

Modelling foot-and-mouth disease: a comparison between the UK and Denmark

Whilst the UK 2001 FMD (foot-and-mouth disease) outbreak provides an extremely rich source of spatio-temporal epidemic data, it is not clear how the models and parameters from the UK can be translated to other scenarios. Here we consider how the model framework used to capture the UK epidemic can be applied to a hypothetical FMD outbreak in Denmark. Whilst pigs played a relatively minor role in the UK epidemic (being the infected animal on just 18 farms), they dominate the Danish livestock landscape. In addition, it is not clear whether transmission parameters from the UK will transfer to Denmark where farming practices may be significantly different. We therefore explore a large volume of high-dimensional parameter space, but seek to relate final epidemic size, risk of spread to Danish islands and potential success of control measures, to early indicators of epidemic dynamics. The results of this extensive modelling exercise therefore allow us to provide timely advice on control options based on the observed behaviours of the first few generations.     

Characterization of mycoplasma gallisepticum infection in captive house finches (Carpodacus mexicanus) in 1998

Since 1995, the epidemic of mycoplasmal conjunctivitis in eastern house finches has affected the Auburn, AL, house finch population. To better characterize the current status of this host-parasite interaction, we established a captive flock of 38 seronegative, healthy finches in fall 1998. After a minimum quarantine period of 4 wk, two Mycoplasma gallisepticum (MG)-infected house finches were introduced into this flock. Over a 12-wk period, the flock was captured every 2 wk and each bird was observed for conjunctivitis. Blood and choanal swabs were collected from each bird for serologic analysis and for the detection of MG by polymerase chain reaction. The infection spread rapidly through the flock just as it had in a similar study performed in 1996 at the height of the epidemic. Unlike the earlier study in which birds remained chronically infected, most of the birds in our study recovered rapidly, and only three of the birds died during the study. Two patterns of host response to infection with MG were observed. Twenty-seven birds (73%) experienced an acute conjunctivitis that resolved, and the birds appeared to clear the infection. Ten birds (27%) suffered prolonged clinical disease, and MG could be detected in these birds intermittently throughout the experiment. These results, in conjunction with our surveys of MG in the wild population, suggest an evolving host-parasite interaction.     

Space-time interaction as an indicator of local spread during the 2001 FMD outbreak in the UK

During the 2001 FMD outbreak in the UK, decisions on the level of implementation of control measures were supported by predictive models. Models were mainly used as macro-level tools to predict the behaviour of the disease in the whole country rather than at the local level. Here we explore the use of the magnitude and characteristics of the space-time interaction as an indicator of local spread and, indirectly, of the effectiveness of control measures aimed at reducing short-range transmission during the course of a major livestock disease epidemic. The spatiotemporal evolution patterns are described in the four main clusters that were observed during the outbreak by means of the hazard rate and space-time K-function (K(s,t)). For each local outbreak, the relative measure D(0)(s,t), derived from K(s,t), which represents the excess risk attributable to the space-time interaction was calculated for consecutive 20-day temporal windows to represent the dynamics of the space-time interaction. The dynamics of the spatiotemporal interaction were very different among the four local clusters, suggesting that the intensity of local spread, and therefore the effectiveness of control measures, markedly differed between local outbreaks. The large heterogeneity observed in the relative impact of being close in time and space to an infected premises suggests that the decision making in relation to control of the outbreak would have benefited from indicators of local spread which could be used to complement global predictive modelling results. Despite its limitations, our results suggest that the real-time analysis of the space-time interaction can be a valuable decision support tool during the course of a livestock disease epidemic.     

春季养鸡疾病防治与养殖管理

自2018年8月受到非洲猪瘟疫情影响,生猪养殖产业受到巨大打击,直接促进家禽养殖产业的发展。随国家对畜禽养殖结构的调整力度不断加大,规模化鸡养殖场的建设数量不断增加。依托规模化鸡养殖产业,带来更多经济收入的同时,也面临较高的疫情防范风险。初春季节外界温度忽高忽低,昼夜温差较大,该时期的鸡身体免疫功能出现下降的现象,造成各种传染性疾病频发。因此做好春季传染性疾病的防治工作,成为需要重点攻克的难题。该文主要结合地区的鸡养殖现状,探讨春季鸡疾病的发生情况,并提出相应的养殖管理措施,更好地控制春季传染性疾病的发生蔓延。     

Legislation for the control of avian influenza in the European union

In the light of experience gained with avian influenza (AI) outbreaks in Europe and elsewhere in the world, the European Union (EU) legislation has recently been updated. The strategy to control the introduction and spread of AI relies on rapid disease detection, killing of infected birds, movement restrictions for live birds and their products, cleaning and disinfection and vaccination. Measures are not only to be implemented in case of outbreaks of highly pathogenic AI (HPAI), but are now also directed against occurrence of low pathogenic AI of H5 and H7 (LPAI) subtypes in poultry, albeit in a modified manner proportionate to the risk posed by these pathotypes. Enhanced surveillance in poultry holdings and wild birds, as well as preventive vaccination, has also been introduced. EU Measures are flexible and largely based on risk assessment of the local epidemiological situation. The occurrence of HPAI H5N1 of the Asian lineage in the EU and its unprecedented spread by wild migratory birds necessitated the adoption of additional control measures. Although HPAI H5N1 has affected wild birds and poultry holdings in several EU Member States, EU legislation and its implementation in Member States has so far successfully limited the impact of the disease on animal and human health.     

Confirmation of spatial patterns and temperature effects in Bluetongue virus serotype-8 transmission in NW-Europe from the 2007 reported case data

Two separate analyses were carried out to understand the epidemiology of Bluetongue virus serotype 8 (BTV-8) in 2007 in North West Europe: First, the temporal change in transmission rates was compared to the evolution of temperature during that season. Second, we evaluated the spatio-temporal dynamics of newly reported outbreaks, to estimate a spatial transmission kernel. For both analyses, the approach as used before in analysing the 2006 BTV-8 epidemic had to be adapted in order to take into account the fact that the 2007 epidemic was not a newly arising epidemic, but one advancing from whereto it had already spread in 2006. We found that within the area already affected by the 2006 outbreak, the pattern of newly infected farms in 2007 cannot be explained by between-farm transmission, but rather by local re-emergence of the virus throughout that region. This indicates that persistence through winter was ubiquitous for BTV-8. Just like in 2006, we also found that the temperature at which the infection starts to spread lies close to 15 °C. Finally, we found that the shape of the transmission kernel is in line with the one from the 2006 epidemic. In conclusion, despite the substantial differences between 2006 and 2007 in temperature patterns (2006 featured a heat wave in July, whereas 2007 was more regular) and spatial epidemic extent, both the minimum temperature required for transmission and the transmission kernel were similar to those estimated for the 2006 outbreak, indicating that they are robust properties, suitable for extrapolation to other years and similar regions.     

基于家禽产业链视角分析广东珠三角地区H7N9禽流感传播途径

珠三角地区H7N9禽流感传播途径具有复杂性和特殊性。为进一步明确传播途径,基于家禽产业链视角,在H7N9禽流感最为严重的广州市、深圳市、佛山市,采用分层抽样法选取有代表性且能反映整体情况的养殖场、批发市场、屠宰场、农贸市场,调查H7N9禽流感的动物防疫和个人防护情况。结果显示:养殖场的生物安全隔离仍不完善,存在活禽接触候鸟感染禽流感的风险;批发市场和屠宰场防疫水平高,人感染风险较低;农贸市场的动物防疫条件和个人防护不充分,易扩散病毒;最有可能的传播途径是与候鸟接触后携带病毒的活禽,通过"养殖—批发—零售"产业链蔓延。该结论在明确"禽传人"、"活禽市场环境暴露"观点上深化了产业链各环节间的传播路径。因此,珠三角地区H7N9禽流感的防控重点要加强养殖环节的生物安全隔离,并做好零售环节中活禽与人之间的防控。     

An analysis of the spatial and temporal patterns of highly pathogenic avian influenza occurrence in Vietnam using national surveillance data

The objectives of this study were to describe the spatio-temporal pattern of an epidemic of highly pathogenic avian influenza (HPAI) in Vietnam and to identify potential risk factors for the introduction and maintenance of infection within the poultry population. The results indicate that during the time period 2004-early 2006 a sequence of three epidemic waves occurred in Vietnam as distinct spatial and temporal clusters. The risk of outbreak occurrence increased with a greater percentage of rice paddy fields, increasing domestic water bird and chicken density. It increased with reducing distance to higher population density aggregations, and in the third epidemic wave with increasing percentage of aquaculture. The findings indicate that agri-livestock farming systems involving domestic water birds and rice production in river delta areas are important for the maintenance and spread of infection. While the government's control measures appear to have been effective in the South and Central parts of Vietnam, it is likely that in the North of Vietnam the vaccination campaign led to transmission of infection which was subsequently brought under control.     

The role of spatial mixing in the spread of foot-and-mouth disease

A model of epidemic dispersal (based on the assumption that susceptible cattle were homogeneously mixed over space, or non-spatial model) was compared to a partially spatially explicit and discrete model (the spatial model), which was composed of differential equations and used geo-coded data (Euclidean distances between county centroids). While the spatial model accounted for intra- and inter-county epidemic spread, the non-spatial model did not assess regional differences. A geo-coded dataset that resembled conditions favouring homogeneous mixing assumptions (based on the 2001 Uruguayan foot-and-mouth disease epidemic), was used for testing. Significant differences between models were observed in the average transmission rate between farms, both before and after a control policy (animal movement ban) was imposed. They also differed in terms of daily number of infected farms: the non-spatial model revealed a single epidemic peak (at, approximately, 25 epidemic days); while the spatial model revealed two epidemic peaks (at, approximately, 12 and 28 days, respectively). While the spatial model fitted well with the observed cumulative number of infected farms, the non-spatial model did not (P<0.01). In addition, the spatial model: (a) indicated an early intra-county reproductive number R of approximately 87 (falling to <1 within 25 days), and an inter-county R<1; (b) predicted that, if animal movement restrictions had begun 3 days before/after the estimated initiation of such policy, cases would have decreased/increased by 23 or 26%, respectively. Spatial factors (such as inter-farm distance and coverage of vaccination campaigns, absent in non-spatial models) may explain why partially explicit spatial models describe epidemic spread more accurately than non-spatial models even at early epidemic phases. Integration of geo-coded data into mathematical models is recommended.     

包虫病传播的数学模型及控制策略研究

通过对包虫病流行现状和传播机理的分析，选择适当的参数，利用动力学方法构建包虫病在人畜之间传播的数学模型，分析影响疾病传播和控制的关键因素。结果得到了决定疾病传播与否的基本再生数，证明了当基本再生数小于1时疾病将会消除，当基本再生数大于1时疾病将变成一种地方病。利用收集和估计的参数对模型进行了数值仿真，分析比较了感染系数的敏感性，对两种不同的控制措施进行了比较，得到了将两者结合是更好的防控措施。本研究构建的模型能够很好的反映影响包虫病传播的关键因素，为进一步制定控制策略提供参考依据。