Transmission of African Swine Fever Virus via carrier (survivor) pigs does occur.

We investigated whether ASF carrier pigs that had completely recovered from an acute infection with ASFV Netherlands '86, could transmit the disease to naive pigs by direct contact transmission. For this, we used pigs that had survived an ASFV infection, had recovered from disease, and had become carriers of ASFV. These clinically healthy carriers were put together one-by-one with naive contact pigs. Two of the twelve contact pigs developed an acute ASFV infection. Using the results of the experiment we quantified the transmission parameters ßcarrier (0.039/day) and Tcarrier (25.4 days). With the survival rate of 0.3 for our ASFV isolate, these parameter values translate into the contribution of carriers to R0 in groups of pigs being 0.3. Further, we placed naive contact pigs in an ASFV contaminated environment. Here, no contact infections were observed. Our findings show that clinically healthy carriers can be a source of acute new infections, which can contribute to the persistence of ASFV in swine populations. The estimates that we provide can be used for modelling of transmission in domestic pigs and, in part, for modelling transmission in wild boar.

Estimating within-flock transmission rate parameter for H5N2 highly pathogenic avian influenza virus in Minnesota turkey flocks during the 2015 epizootic.

Better control of highly pathogenic avian influenza (HPAI) outbreaks requires deeper understanding of within-flock virus transmission dynamics. For such fatal diseases, daily mortality provides a proxy for disease incidence. We used the daily mortality data collected during the 2015 H5N2 HPAI outbreak in Minnesota turkey flocks to estimate the within-flock transmission rate parameter (ß). The number of birds in Susceptible, Exposed, Infectious and Recovered compartments was inferred from the data and used in a generalised linear mixed model (GLMM) to estimate the parameters. Novel here was the correction of these data for normal mortality before use in the fitting process. We also used mortality threshold to determine HPAI-like mortality to improve the accuracy of estimates from the back-calculation approach. The estimated ß was 3.2 (95% confidence interval (CI) 2.3-4.3) per day with a basic reproduction number of 12.8 (95% CI 9.2-17.2). Although flock-level estimates varied, the overall estimate was comparable to those from other studies. Sensitivity analyses demonstrated that the estimated ß was highly sensitive to the bird-level latent period, emphasizing the need for its precise estimation. In all, for fatal poultry diseases, the back-calculation approach provides a computationally efficient means to obtain reasonable transmission parameter estimates from mortality data.

Risk of poultry compartments for transmission of Highly Pathogenic Avian Influenza.

When outbreaks of Highly Pathogenic Avian Influenza (HPAI) occur in OIE member countries with until then disease-free status, member countries can use 'compartmentalisation'. A compartment may be defined as a subset of farms under a common management system, complying with certain stringent surveillance, control and biosecurity measures, and based on that may receive a disease-free status. Based on this disease-free status the compartment is exempted from certain transport restrictions coming into force in case of outbreaks occurring in the country. For deciding whether a candidate compartment is granted official compartment status, it is relevant to assess the additional HPAI transmission risks that would arise due to the exemptions granted. These risks consist of both additional local transmission risks as well as the additional risk of a 'jump' of HPAI infection from one poultry area, via the compartment, to another area. Here such risk assessment is carried out using a spatial mathematical model for between-farm transmission in the Netherlands, yielding insight in the roles of compartment composition and contact structure and identify relevant evaluation criteria for granting HPAI compartment status. At the core of this model are transmission probabilities associated with indirect between-farm contacts, e.g. through feed delivery, egg collection and professional visitors. These probabilities were estimated from Dutch epidemic outbreak data in earlier work. The additional risk of a jump of HPAI from one area, via the compartment, to another area is calculated relative to the direct jump risk. The results show that additional transmission risks caused by a compartment to other farms are mainly dependent on the distance of densely populated poultry areas (DPPAs) to the nearest compartment farm. Apart from conditions on these distances, we also recommend specific routing requirements for transport and other movements within the compartment.

Estimating the between-farm transmission rates for highly pathogenic avian influenza subtype H5N1 epidemics in Bangladesh between 2007 and 2013.

Highly Pathogenic Avian Influenza (HPAI) is classified by the World Organization for Animal Health as one of the notifiable diseases. Its occurrence is associated with severe socio-economic impacts and is also zoonotic. Bangladesh HPAI epidemic data for the period between 2007 and 2013 were obtained and split into epidemic waves based on the time lag between outbreaks. By assuming the number of newly infected farms to be binomially distributed, we fit a Generalized Linear Model to the data to estimate between-farm transmission rates (ß). These parameters are then used together with the calculated infectious periods to estimate the respective basic reproduction numbers (R0 ). The change in ß and R0 with time during the course of each epidemic wave was explored. Finally, sensitivity analyses of the effects of reducing the delay in detecting infection on a farm as well as extended infectiousness of a farm beyond the day of culling were assessed. The point estimates obtained for ß ranged from 0.08 (95% CI: 0.06-0.10) to 0.11 (95% CI: 0.08-0.20) per infectious farm per day while R0 ranged from 0.85 (95% CI: 0.77-1.02) to 0.96 (95% CI: 0.72-1.20). Sensitivity analyses reveal that the estimates are quite robust to changes in the assumptions about the day in reporting infection and extended infectiousness. In the analysis allowing for time-varying transmission parameters, the rising and declining phases observed in the epidemic data were synchronized with the moments when R0 was greater and less than one, respectively. From an epidemiological perspective, the consistency of these estimates and their magnitude (R0  ≈ 1) indicate that the effectiveness of the deployed control measures was largely invariant between epidemic waves and the trend of the time-varying R0 supports the hypothesis of sustained farm-to-farm transmission that is possibly initiated by a few unique introductions.

Modelling the Innate Immune Response against Avian Influenza Virus in Chicken.

At present there is limited understanding of the host immune response to (low pathogenic) avian influenza virus infections in poultry. Here we develop a mathematical model for the innate immune response to avian influenza virus in chicken lung, describing the dynamics of viral load, interferon-α, -ß and -γ, lung (i.e. pulmonary) cells and Natural Killer cells. We use recent results from experimentally infected chickens to validate some of the model predictions. The model includes an initial exponential increase of the viral load, which we show to be consistent with experimental data. Using this exponential growth model we show that the duration until a given viral load is reached in experiments with different inoculation doses is consistent with a model assuming a linear relationship between initial viral load and inoculation dose. Subsequent to the exponential-growth phase, the model results show a decline in viral load caused by both target-cell limitation as well as the innate immune response. The model results suggest that the temporal viral load pattern in the lungs displayed in experimental data cannot be explained by target-cell limitation alone. For biologically plausible parameter values the model is able to qualitatively match to data on viral load in chicken lungs up until approximately 4 days post infection. Comparison of model predictions with data on CD107-mediated degranulation of Natural Killer cells yields some discrepancy also for earlier days post infection.

Avian influenza transmission risks: analysis of biosecurity measures and contact structure in Dutch poultry farming.

In the 2003 epidemic of highly pathogenic avian influenza in Dutch poultry, between-farm virus transmission continued for considerable time despite control measures. Gaining more insight into the mechanisms of this spread is necessary for the possible development of better control strategies. We carried out an in-depth interview study aiming to systematically explore all the poultry production activities to identify the activities that could potentially be related to virus introduction and transmission. One of the between-farm contact risks that were identified is the movement of birds between farms during thinning with violations of on-farm biosecurity protocols. In addition, several other risky management practices, risky visitor behaviours and biosecurity breaches were identified. They include human and fomite contacts that occurred without observing biosecurity protocols, poor waste management practices, presence of other animal species on poultry farms, and poor biosecurity against risks from farm neighbourhood activities. Among the detailed practices identified, taking cell phones and jewellery into poultry houses, not observing shower-in protocols and the exchange of unclean farm equipment were common. Also, sometimes certain protocols or biosecurity facilities were lacking. We also asked the interviewed farmers about their perception of transmission risks and found that they had divergent opinions about the visitor- and neighbourhood-associated risks. We performed a qualitative assessment of contact risks (as transmission pathways) based on contact type, corresponding biosecurity practices, and contact frequency. This assessment suggests that the most risky contact types are bird movements during thinning and restocking, most human movements accessing poultry houses and proximity to other poultry farms. The overall risk posed by persons and equipment accessing storage rooms and the premises-only contacts was considered to be medium. Most of the exposure risks are considered to be similar for layer and broiler farms. Our results, including those on farmer opinions, are relevant for the communication with farmers and poultry-related businesses about practices and risks. We conclude by providing recommendations for improvement of control strategies.

Vaccination against foot-and-mouth disease I: epidemiological consequences.

An epidemic of foot-and-mouth disease (FMD) can have devastating effects on animal welfare, economic revenues, the export position and society as a whole, as occurred during the 2001 FMD epidemic in the Netherlands. Following the preemptive culling of 260,000 animals during this outbreak, the Dutch government adopted emergency vaccination as preferred control policy. However, a vaccination-to-live strategy has not been applied before, posing unprecedented challenges for effectively controlling the epidemic, regaining FMD-free status and minimizing economic losses. These three topics are covered in an interdisciplinary model analysis. In this first part we evaluate whether and how emergency vaccination can be effectively applied to control FMD epidemics in the Netherlands. For this purpose we develop a stochastic individual-based model that describes FMD virus transmission between animals and between herds, taking heterogeneity between host species (cattle, sheep and pigs) into account. Our results in a densely populated livestock area with >4 farms/km(2) show that emergency ring vaccination can halt the epidemic as rapidly as preemptive ring culling, while the total number of farms to be culled is reduced by a factor of four. To achieve this reduction a larger control radius around detected farms and a corresponding adequate vaccination capacity is needed. Although sufficient for the majority of simulated epidemics with a 2 km vaccination zone, the vaccination capacity available in the Netherlands can be exhausted by pig farms that are on average ten times larger than cattle herds. Excluding pig farms from vaccination slightly increases the epidemic, but more than halves the number of animals to be vaccinated. Hobby flocks - modelled as small-sized sheep flocks - do not play a significant role in propagating the epidemic, and need not be targeted during the control phase. In a more sparsely populated livestock area in the Netherlands with about 2 farms/km(2) the minimal control strategy of culling only detected farms seems sufficient to control an epidemic.

Estimation of foot and mouth disease transmission parameters, using outbreak data and transmission experiments.

Mathematical models for the spread of foot and mouth disease (FMD) have been developed and used for a number of purposes in the recent literature. One important purpose is predicting the effect of strategies to combat between-farm epidemic spread, in support of decision-making on epidemic control. The authors briefly review the various modelling approaches, discussing the parameters used and how estimates may be obtained for these parameters. They emphasise that, in addition to the estimation of FMD transmission parameters, the choice of model structure (including the number and type of parameters used) is also crucial. Two gaps in the knowledge of FMD transmission, related to model construction and parameter quantification, are identified: transmission between different species and the way in which vaccination affects such transmission, and route-specific FMD transmission properties. In particular, the authors pay attention to the role that small-scale transmission experiments can play in bridging these gaps.

[Active surveillance for scrapie in the Netherlands: effect of a breeding programme on the prevalence of scrapie in sheep (2002-2010)?]. / Acht jaar actieve scrapiesurveillance in Nederland: het effect van het fokprogramma op de prevalentie van scrapie bij het schaap (2002-2010).

The susceptibility of sheep to scrapie is modulated by the prion protein (PrP) genotype of the animal. An ambitious voluntary scrapie control programme was started in the Netherlands in 1998, based on selection of rams with theARR/ARR genotype for breeding. This programme was followed by an obligatory programme in 2004; the programme has been voluntary since 2007. We monitored the prevalence of PrP genotype frequencies and the prevalence of scrapie in the Dutch sheep population between 2002 and June 2010. Results showed that selection for scrapie-resistant sheep resulted in an increase in the ARR allele frequency in the Dutch national flock from 37.5% in 2005 to 61.4% in 2009. Moreover, surveillance data showed that there was a significant decrease in the prevalence of scrapie a few years after the start of the obligatory breeding programme, from more than 0.2% in 2004 to 0.015% in 2009. This decrease is a consequence of the increased number of scrapie-resistant sheep in the Dutch sheep population. To date, the results and the models based on the data show that the selective breeding programme should be continued for several years in order to successfully eradicate scrapie. It will be important to monitor the PrP frequency and scrapie prevalence in the Dutch sheep population in the coming years.

Modelling control of avian influenza in poultry: the link with data.

In this paper the authors discuss the use of modelling in the evaluation of strategies designed to control epidemics of highly pathogenic avian influenza (HPAI) in poultry. Referring to a number of published models for HPAI transmission in poultry, the authors describe the different ways that modellers use quantitative information. Quantitative information can be used for model building, parameter estimation, and model validation. The authors emphasise that in the case of HPAI transmission in poultry there are important gaps in our understanding. Due to these gaps the models for the effects of certain control strategies, especially those involving vaccination of poultry, need to be based on provisional assumptions. Hence, it is necessary to validate these models and to do research to improve our understanding of the underlying processes in order to better parameterise the models and better estimate the parameters.

Epidemiological analysis of data for scrapie in Great Britain.

In recent years, the control or eradication of scrapie and any other transmissible spongiform encephalopathies (TSEs) possibly circulating in the sheep population has become a priority in Britain and elsewhere in Europe. A better understanding of the epidemiology of scrapie would greatly aid the development and evaluation of control and eradication strategies. Here we bound the range of key epidemiological parameters using a combination of relatively detailed pathogenesis and demography data, more limited data on susceptibility and incubation times, and recent survey data on scrapie incidence in Great Britain. These data are simultaneously analysed using mathematical models describing scrapie transmission between sheep and between flocks. Our analysis suggests that occurrence of scrapie in a flock typically provokes changes in flock management that promote termination of the outbreak, such as the adoption of selective breeding, and that a large fraction of cases (possibly over 80%) goes undetected. We show that the data analysed are consistent with the within-flock reproduction number of scrapie lying in the range 1.5-6, consistent with previous epidemiological studies.

Spatial heterogeneity and the persistence of infectious diseases.

The endemic persistence of infectious diseases can often not be understood without taking into account the relevant heterogeneities of host mixing. Here, we consider spatial heterogeneity, defined as 'patchiness' of the host population. After briefly reviewing how disease persistence is influenced by population size, reproduction number and infectious period, we explore its dependence on the level of spatial heterogeneity. Analysis and simulation of disease transmission in a symmetric meta-population suggest that disease persistence typically becomes worse as spatial heterogeneity increases, although local persistence optima can occur for infections with oscillatory population dynamics. We obtain insight into the dynamics that underlie the observed persistence patterns by studying the infection prevalence correlation between patches and by comparing full-model simulations to results obtained using simplified patch-level descriptions of the interplay between local extinctions and between-patch transmissions. The observed patterns are interpreted in terms of rescue effects for strong spatial heterogeneity and in terms of between-patch coherence and synchronization effects at intermediate and weak levels of heterogeneity.

Dynamics of a scrapie outbreak in a flock of Romanov sheep--estimation of transmission parameters.

Knowledge of epidemiological mechanisms and parameters underlying scrapie transmission in sheep flocks remains very limited at present. Here we introduce a method for fitting stochastic transmission models to outbreak data to estimate bounds on key transmission parameters. We apply this method to data describing an outbreak of scrapie in a closed flock of Romanov sheep. The main findings are that the relative infectiousness of infected animals in this outbreak becomes appreciable early into disease incubation and that the mean incubation period is less than 1.5 years. We also find that the data are consistent with a broad range of values for the basic reproduction number R0 and describe how the boundaries of this range depend on assumptions about the mean incubation period and the contribution to transmission of a long-lived environmental reservoir of infectivity.

A mathematical model of Staphylococcus aureus control in dairy herds.

An ordinary differential equation model was developed to simulate dynamics of Staphylococcus aureus mastitis. Data to estimate model parameters were obtained from an 18-month observational study in three commercial dairy herds. A deterministic simulation model was constructed to estimate values of the basic (R0) and effective (Rt) reproductive number in each herd, and to examine the effect of management on mastitis control. In all herds R0 was below the threshold value 1, indicating control of contagious transmission. Rt was higher than R0 because recovered individuals were more susceptible to infection than individuals without prior infection history. Disease dynamics in two herds were well described by the model. Treatment of subclinical mastitis and prevention of influx of infected individuals contributed to decrease of S. aureus prevalence. For one herd, the model failed to mimic field observations. Explanations for the discrepancy are given in a discussion of current knowledge and model assumptions.

Estimating the human health risk from possible BSE infection of the British sheep flock.

Following the controversial failure of a recent study and the small numbers of animals yet screened for infection, it remains uncertain whether bovine spongiform encephalopathy (BSE) was transmitted to sheep in the past via feed supplements and whether it is still present. Well grounded mathematical and statistical models are therefore essential to integrate the limited and disparate data, to explore uncertainty, and to define data-collection priorities. We analysed the implications of different scenarios of BSE spread in sheep for relative human exposure levels and variant Creutzfeldt-Jakob disease (vCJD) incidence. Here we show that, if BSE entered the sheep population and a degree of transmission occurred, then ongoing public health risks from ovine BSE are likely to be greater than those from cattle, but that any such risk could be reduced by up to 90% through additional restrictions on sheep products entering the food supply. Extending the analysis to consider absolute risk, we estimate the 95% confidence interval for future vCJD mortality to be 50 to 50,000 human deaths considering exposure to bovine BSE alone, with the upper bound increasing to 150,000 once we include exposure from the worst-case ovine BSE scenario examined.

Persistence patterns of scrapie in a sheep flock.

The epidemiology and transmission dynamics of sheep scrapie is as yet poorly understood. Here we present a theoretical analysis of the transmission dynamics within a sheep flock, concentrating on how persistence properties depend on transmission scenario and flock size. Patterns of disease persistence and extinction are studied analytically using branching-process approximations and numerically using stochastic model simulations. For a given basic reproduction number, disease extinction is most likely when late-stage infected animals are responsible for most of the transmission. This effect can be understood in terms of aggregation in the distribution of the number of secondary infections arising from a single primary infection. The presence of an environmental reservoir reduces the probability of extinction.

The transmission dynamics of the aetiological agent of scrapie in a sheep flock.

We formulate and investigate the properties of a model framework to mimic the transmission dynamics of the aetiological agent of scrapie in a sheep flock. We derive expressions for summary parameters that characterize transmission scenarios, notably the basic reproduction number R(0) and the mean generation time T(g). The timescale of epidemic outbreaks is expressed in terms of R(0) and cumulants of the generation time distribution. We discuss the relative contributions to the overall rate of transmission of horizontal and vertical routes during invasion and in endemicity. Simplified models are used to obtain analytical insight into the characteristics of the endemic state.

Feed-borne transmission and case clustering of BSE.

An unresolved issue in the epidemiology of bovine spongiform encephalopathy (BSE) in the UK is what precisely determines the degree to which cases of disease in cattle are clustered within herds throughout the course of the epidemic. This paper presents an analysis of feed-borne transmission at the herd level and tests various models of case-clustering mechanisms, associated with heterogeneity in exposure to infectious feed, against observed epidemic pattern. We use an age-structured metapopulation framework in which the recycling of animal tissue between herds via feed producers is explicitly described. We explore two alternative assumptions for the scaling with herd size of the within-herd risk of exposure of an animal to infectious material. We find that whereas exposure heterogeneity caused by variation in feed and offal processing methods and by variation in per-animal feed uptake can explain the pattern of case clustering seen in the BSE epidemic, exposure heterogeneity due to the aggregation of infectivity within feed cannot.

Epidemiological determinants of the pattern and magnitude of the vCJD epidemic in Great Britain.

Understanding the epidemiology and aetiology of new-variant Creutzfeldt-Jakob (vCJD) disease in humans has become increasingly important given the scientific evidence linking it to bovine spongiform encephalopathy (BSE) in cattle and hence the wide exposure of the population of Great Britain (GB) to potentially infectious tissue. The recent analysis undertaken to determine the risk to the population from dorsal route ganglia illustrated the danger in presenting point estimates rather than ranges of scenarios in the face of uncertainty. We present a mathematical template that relates the past pattern of the BSE epidemic in cattle to the future course of any vCJD epidemic in humans, and use extensive scenario analysis to explore the wide range of possible outcomes given the uncertainty in epidemiological determinants. We demonstrate that the average number of humans infected by one infectious bovine and the incubation period distribution are the two epidemiological factors that have the greatest impact on epidemic size and duration. Using the time-series of the BSE epidemic and the cases seen to date, we show that the minimum length of the incubation period is approximately nine years, and that at least 20% of the cases diagnosed to date were exposed prior to 1986. We also demonstrate that the current age distribution of vCJD cases can only arise if younger people were either exposed to a greater extent, more susceptible to infection, or have shorter incubation periods. Extensive scenario analyses show that given the information currently available, the very high degree of uncertainty in the future size of the epidemic will remain for the next 3-5 years. Furthermore, we demonstrate that this uncertainty is unlikely to be reduced by mass screening for late-stage infection.