A dynamic, optimal disease control model for foot-and-mouth-disease: II. Model results and policy implications

A dynamic optimization model was used to search for optimal strategies to control foot-and-mouth disease (FMD) in the three-county region in the Central Valley of California. The model minimized total regional epidemic cost by choosing the levels of depopulation of diagnosed herds, preemptive depopulation, and vaccination. Impacts of limited carcass disposal capacity and vaccination were also examined, and the shadow value, the implicit value of each capacity, was estimated. The model found that to control FMD in the region, (1) preemptive depopulation was not optimal, (2) vaccination, if allowed, was optimal, reducing total cost by 3–7%, (3) increased vaccination capacity reduced total cost up to US$ 119 per dose, (4) increased carcass disposal capacity reduced total cost by US$ 9000–59,400 per head with and without vaccination, respectively, and (5) dairy operations should be given preferential attention in allocating limited control resources.     

Village level risk factors for foot-and-mouth disease in Northern Thailand

A study was undertaken in northern Thailand to identify factors which put some villages at higher risk of foot-and-mouth disease (FMD) outbreaks than others. The number of FMD outbreaks experienced in the previous 5 years and data on 145 putative risk factors were obtained by interview from 60 villages during 1991–1992. Univariable analyses identified 27 factors for further investigation using logistic regression. When villages were classified into three FMD frequency groups of zero to one, two to three or four or more outbreaks in the last 5 years, the important factors explaining the differences in risk were the total number of cattle and buffaloes purchased in the previous year, the number of neighbouring villages which shared a common water source and whether agriculture was the most important source of cash income for the village. These factors were also the most important variables in explaining the difference in risk when comparing villages with zero or one outbreak with those having four or more. We concluded that the greatest impact on reducing spread of FMD among villages would be obtained through the development of strategies to reduce the likelihood of introduction through livestock purchases and for villagers to take greater care when livestock are grazed with those from neighbouring villages and when sharing common water supplies.     

A decision-tree to optimise control measures during the early stage of a foot-and-mouth disease epidemic

A decision-tree was developed to support decision making on control measures during the first days after the declaration of an outbreak of foot-and-mouth disease (FMD). The objective of the tree was to minimise direct costs and export losses of FMD epidemics under several scenarios based on livestock and herd density in the outbreak region, the possibility of airborne spread, and the time between first infection and first detection. The starting point of the tree was an epidemiological model based on a deterministic susceptible–infectious–recovered approach. The effect of four control strategies on FMD dynamics was modelled. In addition to the standard control strategy of stamping out and culling of high-risk contact herds, strategies involving ring culling within 1 km of an infected herd, ring-vaccination within 1 km of an infected herd, and ring-vaccination within 3 km of an infected herd were assessed. An economic model converted outbreak and control effects of farming and processing operations into estimates of direct costs and export losses. Ring-vaccination is the economically optimal control strategy for densely populated livestock areas whereas ring culling is the economically optimal control strategy for sparsely populated livestock areas.     

Simulation of foot-and-mouth disease spread within an integrated livestock system in Texas, USA

We used a simulation study to assess the impact of an incursion of foot-and-mouth disease (FMD) virus on the livestock industries in an 8-county area of the Panhandle region of Texas, USA. The study was conducted in a high-density livestock area, with an estimated number of cattle on-feed of approximately 1.8 million. We modified an existing stochastic, spatial simulation model to simulate 64 scenarios for planning and decision-making. Our scenarios simulated four different herd types for the index herd (company feedlot, backgrounder feedlot, large beef, backyard) and variations in three mitigation strategies (time-of-detection, vaccine availability, and surveillance during disease control). Under our assumptions about availability of resources to manage an outbreak, median epidemic lengths in the scenarios with commercial feedlot, backgrounder feedlot, large beef and backyard index herd types ranged from 28 to 52, 19 to 39, 18 to 32, and 18 to 36 days, respectively, and the average number of herds depopulated ranged from 4 to 101, 2 to 29, 1 to 15 and 1 to 18, respectively. Early detection of FMD in the index herd had the largest impact on reducing (13–21 days) the length of epidemics and the number of herds (5–34) depopulated. Although most predicted epidemics lasted only 1–2 months, and <100 herds needed to be depopulated, large outbreaks lasting 8–9 months with up to 230 herds depopulated might occur.     

Experimental studies with foot-and-mouth disease virus type Asia-1, responsible for the 2005 epidemic in China

This study was carried out to investigate the biological characteristics of the foot-and-mouth disease (FMD) virus strain Asia-1 China/2005, which is responsible for the 2005 epidemic in China. The result showed that this strain is not host restricted, and could not only cause FMD in cattle and sheep but also in pigs by either inoculation or direct contact.     

A dynamic model for cost-benefit analyses of foot-and-mouth disease control strategies

An integrated model for the personal computer is presented, in which a variety of preventive and control strategies with respect to foot-and-mouth disease (FMD) in Dutch cattle and pig herds were examined economically. Special attention is given to the way in which losses due to export bans are determined. Export bans would occur as a result of an outbreak of FMD. Annual costs for the Netherlands would be reduced considerably if the yearly vaccinations were stopped. This conclusion holds even if more pessimistic values are used for some major uncertain input factors. The PC model is flexible with regard to input values, making it possible to fit different conditions.     

A survey to investigate movements off sheep and cattle farms in New Zealand,with reference to the potential transmission of foot-and-mouth disease

AIM: To quantify the numbers and extent of movements off sheep and cattle farms in New Zealand, in order to construct more realistic simulation models to investigate how infectious diseases such as foot-and-mouth disease (FMD) might spread.

METHODS: Farmers from 500 randomly selected farms, comprising 100 from each of the following sectors, viz beef, dairy, grazing/dairy heifer rearing, sheep, and mixed sheep and beef, were asked to fill in diaries in which they recorded the movements of all animals, products, people, vehicles and equipment coming on to or leaving their farms during two separate 3-week periods, representing relatively ‘busy’ and ‘quiet’ times of the year with respect to livestock movements. Where possible, the destination of each movement was identified and geo-coded, to allow the distance travelled to be calculated. Each movement was then classified according to the risk of transfer of FMD virus (FMDV), should the disease have been present on the study farm at the time of the movement. The data were then analysed to establish movement frequencies and distributions of distances travelled, by the different pastoral livestock sectors.

RESULTS: Two hundred and seventeen farmers returned one or more diaries. One hundred and ninety-three farmers completed a Busy-period diary, recording a total of 12,052 movements off their farms, a crude average of 62.4 per 3-week period, or 2.97 per day. Of these, 4.0% involved the transport of livestock, equating to 0.12 livestock consignments per day. In contrast, 186 Quiet-period diaries were returned, recording a total of 10,885 movements off, representing a crude average of 58.5 during the 3-week period, or 2.78 per day. Of these, 2.1% involved livestock, equating to 0.06 livestock consignments per day. The mean and median distances travelled during the Busy periods were 30.9 km and 13.1 km, respectively (range 0–1,167 km). In comparison, the mean and median distances travelled during Quiet periods were 41.3 and 14 km, respectively (range 0.4–1,203 km).

CONCLUSIONS: People, vehicles, livestock and other items can travel off pastoral livestock farms in New Zealand to other farms either directly or via saleyards over extensive distances. This has implications for the potential spread of infectious diseases such as FMD. Movement parameters intended for use in the InterSpread Plus inter-farm simulation model of FMD were established, which will facilitate the prediction of likely spread and efficacy of controls in the unlikely event of a real-life outbreak.     

Use of heterogeneous operation-specific contact parameters changes predictions for foot-and-mouth disease outbreaks in complex simulation models

The role of contact parameters in a complex spatial simulation model of foot-and-mouth disease spread was determined by comparing predictions of number of infected premises, epidemic duration, and relative infection risk for different production sectors between a model that included the Full, heterogeneous (differing by production type) type-specific information about animal, vehicle and personnel movement between premises, and models that used partial and homogeneous (similar across production types) weighted-mean or proxy parameter sets for contacts between premises of all types. The model was run using a dataset of known premises locations in a three-county area in the Central Valley of California and categorized into 13 premises types and six production sectors.Results from models run with homogeneous contact parameters were always different from those obtained from the Full model, demonstrating that model predictions are affected by heterogeneity in contact parameters. Models simplified by using weighted-mean parameters predicted fewer infected premises. Models that were simplified by using medium dairy farm or large swine operation proxy parameters predicted longer epidemics with more infected premises, while those using small beef operation proxy parameters predicted shorter epidemics with fewer infected premises. Simplified-parameter models underestimated the impact on the economically important dairy sector, while overestimating the impact on beef and backyard operations. Results establish a need for heterogeneous, operation-specific contact parameters in complex stochastic simulation models that must be weighed against the cost of obtaining and coding premises type-specific contact information.     

Potential impact of species and livestock density on the epidemic size and effectiveness of control measures for foot-and-mouth disease in Japan

The characteristics of a livestock area, including farm density and animal species, influence the spread of foot-and-mouth disease (FMD). In this study, the impact of livestock area on FMD epidemics was examined using an FMD transmission model. For this simulation, three major livestock areas were selected: the 2010 FMD epidemic area in Japan as the baseline area (BS), a cattle and pig mixed production area (CP) and a cattle production area (C). Simulation results demonstrated that under the 24-hr culling policy, only 12% of epidemics among 1,000 simulations were abated within 100 days in the CP area, whereas 90% of the epidemics ceased in the BS area. In the C area, all epidemics were successfully contained within 100 days. Evaluation of additional control measures in the CP area showed that the 0.5-km pre-emptive culling, even when only targeting pig farms, raised the potential for successful containment to 94%. A 10-km vaccination on day 7 or 14 after initial detection was also effective in halting the epidemics (80%), but accompanied a large number of culled or vaccinated farms. The combined strategy of 10-km vaccination and 0.5-km pre-emptive culling targeting pig farms succeeded in containing all epidemics within 100 days. The present study suggests the importance of preparedness for the 24-hr culling policy and additional control measures when an FMD outbreak occurs in a densely populated area. Considering the characteristics of the livestock area is important in planning FMD control strategies.     

RT-PCR evaluation for identification and sequence analysis of foot-and-mouth disease serotype O from 2006 to 2007 in Punjab, Pakistan

FMD clinically positive 250 tissue samples (mouth and hoof epithelium and vesicle swabs, tongue tissue) and 175 secretion samples (milk, saliva, serum, plasma) were evaluated by RT-PCR for the diagnosis of FMD with different pair of universal and serotype-specific primers from 2006 to 2007 in Punjab, Pakistan. Universal primer pair P1/P2 from VP1 gene detected FMD in 182 out of 250 (72.8%) tissues and 92 out of 175 (52.6%) secretion samples, while universal primer 1F/1R from 5′UTR region detected FMD in 218 out of 250 (87.2%) tissues and 142 out of 175 (81.1%) secretion samples. 1F/1R proved better than the P1/P2 primer pair for primary diagnosis of FMD, direct from the clinical positive samples. Direct sequencing of the universal primer pair P1/P2 revealed that O serotype of FMD was circulating in this region. O serotype of FMD was detected with O-1C(ARS4)/PNK 61, AU(O)/AU(rev), AU(O)/PNK61 primer pairs, these primer pairs also compared with each other. AU(O)/AU(rev) and AU(O)/PNK61 detected O serotype of FMD in 88.9% tissue and swab (mouth and hoof vesicle swabs) samples and 71.9% different secretion (milk, saliva, serum, plasma) samples, while O-1C(ARS4)/PNK 61 detected 48.1% tissue and swab (mouth and hoof vesicle swabs) samples and 37.5% different secretion (milk, saliva, serum, plasma) samples. AU(O)/AU(rev), AU(O)/PNK61 primer pairs detected 40.8% more tissue and swab samples, while these pairs detected 34.4% more secretion samples. Cloning of PCR product of AU(O)/AU(rev) VP1 gene and sequencing for phylogenetic studies revealed that O serotype of FMD circulating in Punjab, Pakistan was genetically very diverse from the ‘O’ serotype in Middle East and Europe. The dendrogram showed that Pakistan ‘O’ serotype was very much similar genetically to its neighbor countries (Sri Lanka, India, Iran, Iraq, and China) and PanAsia 1 lineage which caused 2001-outbreak in UK and 1994-outbreak in Saudi Arabia, etc.     

Epidemiological basis useful for the control of foot-and-mouth disease

Although known for many years, foot-and-mouth disease is still able to represent a real threat to many farming economies in the world. The recent 2001 Western European epizootics linked to O PanAsia virus strain can illustrate the fact that many questions are still unanswered in the field of foot-and-mouth epidemiology. It also demonstrates that the increase in international trade, including livestock, animal products and animal food, means an increase in the probability of transmitting, through the same way, some animal diseases, foot-and-mouth included. In our economies, a rapid identification of the virus and a fast elimination of infected, contaminated and even some contact animals are still the key factors to react in front of such a disease.

Résumé

Bien que connue depuis des années, la fièvre aphteuse représente toujours une menace réelle pour beaucoup d'économies agricoles de la planète. L'épisode récent de 2001 en Europe Occidentale, lié au virus O PanAsia, illustre le fait que de nombreuses questions sont toujours sans réponse au niveau de l'épidémiologie de la maladie. Cet épisode démontre aussi que le développement du commerce international des animaux et des produits animaux se traduit par une augmentation de la probabilité de transmettre, de la même façon, diverses maladies animales dont la fièvre aphteuse. Dans nos économies, une identification rapide du virus, suivie d'une élimination précoce des animaux malades, contaminés, voire contact, restent les clefs d'une maîtrise de la fièvre aphteuse.     

Characterisation of a SAT-1 outbreak of foot-and-mouth disease in captive African buffalo (Syncerus caffer): clinical symptoms, genetic characterisation and phylogenetic comparison of outbreak isolates

African buffalo (Syncerus caffer) play an important role in the maintenance of the SAT types of foot-and-mouth disease (FMD) in southern Africa. These long-term carriers mostly become sub-clinically infected, maintaining the disease and posing a threat to other susceptible wildlife and domestic species. During an unrelated bovine tuberculosis experiment using captive buffalo in the Kruger National Park (KNP), an outbreak of SAT-1 occurred and was further investigated. The clinical signs were recorded and all animals demonstrated significant weight loss and lymphopenia that lasted 100 days. In addition, the mean cell volume and mean cell haemoglobin values were significantly higher than before the outbreak started. Virus was isolated from several buffalo over a period of 167 days post infection and the molecular clock estimated to be 3 × 10⁻⁵ nucleotide substitutions per site per day. Seven amino acid changes occurred of which four occurred in hypervariable regions previously described for SAT-1. The genetic relationship of the outbreak virus was compared to buffalo viruses previously obtained from the KNP but the phylogeny was largely unresolved, therefore the relationship of this outbreak strain to others isolated from the KNP remains unclear.     

Evaluation of cross-protection against three topotypes of serotype O foot-and-mouth disease virus in pigs vaccinated with multi-epitope protein vaccine incorporated with poly(I:C)

Epitope-based vaccines are always questioned for their cross-protection against the antigenically variable foot-and-mouth disease virus (FMDV). In this study, we proved the cross-protection effect of a multi-epitope vaccine incorporated with poly(I:C) against three topotypes of O type FMDV. A total of 45 naïve pigs were vaccinated with different doses of multi-epitope protein vaccine incorporated with poly(I:C). At 28 days post-vaccination, 45 vaccinated and 6 unvaccinated control pigs (two pigs for each group) were challenged with three topotypes of virulent O type FMDV, namely, O/Mya/98 (Southeast Asia topotype), O/HN/CHA/93 (Cathay topotype) and O/Tibet/CHA/99 (PanAsia topotype) strains. All unvaccinated pigs developed generalised FMD clinical signs. Results showed that all pigs (n = 15) conferred complete protection against the O/Mya/98 and O/HN/CHA/93 FMDV strains, 11 of which were protected against the O/Tibet/CHA/99 FMDV strain. The 50% protective dose values of the vaccine against the O/Mya/98, O/HN/CHA/93 and O/Tibet/CHA/99 FMDV strains were 15.59, 15.59 and 7.05, respectively. Contact challenge experiment showed that transmission occurred from the donors to the unvaccinated but not to vaccinated pigs. These results showed that vaccination with multi-epitope protein vaccine incorporated with poly(I:C) can efficiently prevent FMD in pigs.     

Brucellosis in sub-Saharan Africa: epidemiology,control and impact

Brucellosis is an important disease among livestock and people in sub-Saharan Africa. In general, the incidence is the highest in pastoral production systems and decreases as herd size and size of landholding decreases. The prevalence of risk factors for infections are best understood for bovine brucellosis and to a lesser extent for ovine and caprine brucellosis. The occurrence and epidemiology of brucellosis in pigs is poorly understood. This species bias is also reflected in control activities. As with other public-sector animal health services, the surveillance and control of brucellosis in sub-Saharan Africa is rarely implemented outside southern Africa. Brucellosis is even more ignored in humans and most cases go undiagnosed and untreated, leading to considerable suffering for those affected. Decision-making to determine the importance of brucellosis control relative to other public concerns and what brucellosis control strategies should be applied is urgently required. A strategy for how brucellosis decision-making might be considered and applied in future is outlined.     

Asia1型FMDV非结构蛋白3A基因的克隆、表达与抗原性鉴定

为了研究Asia1型口蹄疫病毒(FMDV)非结构蛋白3A的抗原性,试验对Asia1型口蹄疫病毒非结构蛋白3A基因进行扩增、亚克隆及测序,将3A克隆至表达载体pET-32a(+)中,选取阳性克隆转化Rosetta(DE3)pLysS大肠杆菌感受态细胞,用IPTG诱导表达和纯化3A蛋白,并进行SDS-PAGE鉴定与Western-blot分析。结果表明:在大肠杆菌中成功地表达了3A蛋白,表达的目的蛋白能与Asia1型FMDV阳性血清发生特异性反应。说明非结构蛋白3A具有较好的抗原活性。     

The North American Animal Disease Spread Model: a simulation model to assist decision making in evaluating animal disease incursions

The North American Animal Disease Spread Model is a stochastic, spatial, state-transition simulation model for the spread of highly contagious diseases of animals. It was developed with broad international support to assist policy development and decision making involving disease incursions. User-established parameters define model behavior in terms of disease progression; disease spread by animal-to-animal contact, contact with contaminated personnel or equipment, and airborne dissemination; and the implementation of control measures such as destruction and vaccination. Resources available to implement disease control strategies, as well as the direct costs associated with these strategies, are taken into consideration. The model records a wide variety of measures of the extent of simulated outbreaks and other characteristics. The graphical interface and output visualization features also make it a useful tool for training and preparedness exercises. This model is now being used to evaluate outbreak scenarios and potential control strategies for several economically important exotic animal diseases in the United States, Canada, and elsewhere. NAADSM is freely available via the Internet at http://www.naadsm.org.     

综合防控口蹄疫的关键技术——疫苗免疫

口蹄疫的发生和流行严重危害畜牧业的发展,造成惨重的经济损失,因此防控和消灭口蹄疫成为许多国家共同关注的问题。疫苗免疫是特异性预防和控制该病的有效手段。本文从免疫方式、动物种类、地域因素、生物模型4个方面对疫苗防控措施的建立加以论述,提出了常规免疫策略以及非疫苗预防免疫策略,并对紧急疫苗接种与感染动物扑杀进行了时效性分析,旨在面临疫病挑战时制订具有针对性的疫病防控措施,从而使疫苗免疫在控制、消灭和防止口蹄疫传播等方面更能有效地发挥作用。     

Pig farmers’ perceptions,attitudes, influences and management of information in the decision-making process for disease control

The objectives of this study were (1) to explore the factors involved in the decision-making process used by pig farmers for disease control and (2) to investigate pig farmers’ attitudes and perceptions about different information sources relating to disease control.     

Adapting a scenario tree model for freedom from disease as surveillance progresses: The Canadian notifiable avian influenza model

Scenario tree models with temporal discounting have been applied in four continents to support claims of freedom from animal disease. Recently, a second (new) model was developed for the same population and disease. This is a natural development because surveillance is a dynamic process that needs to adapt to changing circumstances – the difficulty is the justification for, documentation of, presentation of and the acceptance of the changes.     

Spatial and stochastic simulation to evaluate the impact of events and control measures on the 1997-1998 classical swine fever epidemic in The Netherlands. I. Description of simulation model

The simulation model InterCSF was developed to simulate the Dutch Classical Swine Fever (CSF) epidemic of 1997–98 as closely as possible. InterCSF is a spatial, temporal and stochastic simulation model. The outcomes of the various replications give an estimate of the variation in size and duration of possible CSF-epidemics. InterCSF simulates disease spread from an infected farm to other farms through three contact types (animals, vehicles, persons) and through local spread up to a specified distance. The main disease-control mechanisms that influence the disease spread in InterCSF are diagnosis of the infected farms, depopulation of infected farms, movement-control areas, tracing, and pre-emptive slaughter. InterCSF was developed using InterSpread as the basis. InterSpread was developed for foot-and-mouth disease (FMD). This paper describes the process of modifying InterSpread into InterCSF. This involved changing the assumptions and mechanisms for disease spread from FMD to CSF. In addition, CSF-specific control measures based on the standard European Union (EU) regulations were included, as well as additional control measures that were applied during the Dutch epidemic. To adapt InterCSF as closely as possible to the Dutch 1997/98 epidemic, data from the real epidemic were analysed. Both disease spread and disease-control parameters were thus specifically based on the real epidemic. In general, InterSpread turned out to be a flexible tool that could be adapted to simulate another disease with relative ease. The most difficult were the modifications necessary to mimic the real epidemic as closely as possible. The model was well able to simulate an epidemic with a similar pattern over time for number of detected farms as the real outbreak; but the absolute numbers were (despite many relevant modifications) not exactly the same — but were within an acceptable range. Furthermore, the development of InterCSF provided the researchers with a better insight into the existing knowledge gaps. In part II (see the final paper in this issue), InterCSF was used to compare various control strategies as applied to this epidemic.