Alternative control strategies against ASF in wild boar populations

Dynamics of the African Swine Fever Spread in Poland

Introduction

African swine fever (ASF) is a lethal haemorrhagic disease of Suidae, present in Poland since 2014. The natural reservoir of ASF in Europe is the wild boar (Sus scrofa); however, human activity facilitates long-distance introductions of the disease. In ASF control it is important to identify areas at increased risk of infection. Such identification and estimation of the disease's progress and subsequent spread will help to identify the specific preventive action needs in given zones. Serving this purpose, this study is a spatial and statistical analysis of ASF spread through noted outbreak data.

Material and Methods

The spatial-temporal analysis was conducted on the basis of data including the time and location of all ASF outbreaks both in wild boars and domestic pigs in Poland in 2014-2021.

Results

The analysis indicates possible routes and directions for further ASF spread in Poland, estimates the annual increase of the affected area (approx. 25,000 km² every year since 2017) and marks trends. The strong method-independent correlation between the year and the surface area affected by African swine fever indicated a near-linear generalised trend.

Conclusion

Given the growth trend, we can expect ASF to expand further into new territories of the country; however, it is important to realise that there is still a significant area to protect, because 60% of Poland remains ASF-free.

Estimation of the Lethality Rate, Recovery Rate, and Case Fatality Ratio of Classical Swine Fever in Japanese Wild Boar: An Analysis of the Epidemics From September 2018 to March 2019

Understanding the morbidity and lethality of diseases is necessary to evaluate the effectiveness of countermeasure against the epidemics (e.g., vaccination). To estimate them, detailed data on host population dynamics are required; however, estimating the population size for wildlife is often difficult. We aimed to elucidate the morbidity and lethality of classical swine fever (CSF) currently highly prevalent in the wild boar population in Japan. To this end, we estimated lethality rate, recovery rate, and case fatality ratio (CFR) of CSF without detailed data on the population estimates of wild boar. A mathematical model was constructed to describe the CSF dynamics and population dynamics of wild boar. We fitted the model to the (i) results of the reverse transcription polymerase chain reaction (RT-PCR) test for the CSFV gene and the (ii) results of the enzyme-linked immunosorbent assay (ELISA) test for the antibody against CSFV in sampled wild boar. In the 280 wild boar sampled from September 2018 to March 2019 in the major CSF-affected area in Japan, the lethality rate and recovery rate of CSF per week were estimated as 0.165 (95% confidence interval: 0.081–0.250) and 0.004 (0–0.009), respectively. While the estimate of lethality rate of CSF was similar with the estimates in previous studies, the recovery rate was lower than those reported previously. CFR was estimated as 0.959 (0.904–0.981) using our estimate of recovery rate. This study is the first to estimate lethality rate of CSF from the dynamics of CSF epidemics in the wild boar population. Since the value of CFR is sensitive to the value of recovery rate, the accuracy in the estimate of recovery rate is a key for the accurate estimation of CFR. A long-term transmission experiment of moderately virulent strains may lead to more accurate estimation of the recovery rate and CFR of CSF.

Epidemiological analysis of African swine fever in the European Union (September 2019 to August 2020)

An update on the African swine fever (ASF) situation in the 10 affected Member States (MS) in the EU and in two neighbouring countries from the 1 September 2019 until the 31 August 2020 is provided. The dynamics of the proportions of PCR- and ELISA-positive samples since the first ASF detection in the country were provided and seasonal patterns were investigated. The impact of the ASF epidemic on the annual numbers of hunted wild boar in each affected MS was investigated. To evaluate differences in the extent of spread of ASF in the wild boar populations, the number of notifications that could be classified as secondary cases to a single source was calculated for each affected MS and compared for the earliest and latest year of the epidemic in the country. To evaluate possible risk factors for the occurrence of ASFV in wild boar or domestic pigs, a literature review was performed. Risk factors for the occurrence of ASF in wild boar in Romanian hunting grounds in 2019 were identified with a generalised linear model. The probability to find at least one PCR-confirmed ASF case in wild boar in a hunting ground in Romania was driven by environmental factors, wild boar abundance and the density of backyard pigs in the hunting ground area, while hunting-related variables were not retained in the final model. Finally, measures implemented in white zones (ASF-free zones that are geographically adjacent to an area where ASF is present in wild boar) to prevent further spread of ASF were analysed with a spatially, explicit stochastic individual-based model. To be effective, the wild boar population in the white zone would need to be drastically reduced before ASF arrives at the zone and it must be wide enough. To achieve the necessary pre-emptive culling targets of wild boar in the white zone, at the start of the establishment, the white zone should be placed sufficiently far from the affected area, considering the speed of the natural spread of the disease. This spread is faster in denser wild boar populations. After a focal ASF introduction, the white zone is always close to the infection hence pre-emptive culling measures in the white zone must be completed in short term, i.e. in a few months.

ASF Exit Strategy: Providing cumulative evidence of the absence of African swine fever virus circulation in wild boar populations using standard surveillance measures

EFSA assessed the role of seropositive wild boar in African swine fever (ASF) persistence. Surveillance data from Estonia and Latvia investigated with a generalised equation method demonstrated a significantly slower decline in seroprevalence in adult animals compared with subadults. The seroprevalence in adults, taking more than 24 months to approach zero after the last detection of ASFV circulation, would be a poor indicator to demonstrate the absence of virus circulation. A narrative literature review updated the knowledge on the mortality rate, the duration of protective immunity and maternal antibodies and transmission parameters. In addition, parameters potentially leading to prolonged virus circulation (persistence) in wild boar populations were reviewed. A stochastic explicit model was used to evaluate the dynamics of virus prevalence, seroprevalence and the number of carcasses attributed to ASF. Secondly, the impact of four scenarios on the duration of ASF virus (ASFV) persistence was evaluated with the model, namely a: (1) prolonged, lifelong infectious period, (2) reduction in the case-fatality rate and prolonged transient infectiousness; (3) change in duration of protective immunity and (4) change in the duration of protection from maternal antibodies. Only the lifelong infectious period scenario had an important prolonging effect on the persistence of ASF. Finally, the model tested the performance of different proposed surveillance strategies to provide evidence of the absence of virus circulation (Exit Strategy). A two-phase approach (Screening Phase, Confirmation Phase) was suggested for the Exit Strategy. The accuracy of the Exit Strategy increases with increasing numbers of carcasses collected and tested. The inclusion of active surveillance based on hunting has limited impact on the performance of the Exit Strategy compared with lengthening of the monitoring period. This performance improvement should be reasonably balanced against an unnecessary prolonged 'time free' with only a marginal gain in performance. Recommendations are provided for minimum monitoring periods leading to minimal failure rates of the Exit Strategy. The proposed Exit Strategy would fail with the presence of lifelong infectious wild boar. That said, it should be emphasised that the existence of such animals is speculative, based on current knowledge.

Prediction Models in Veterinary and Human Epidemiology: Our Experience With Modeling Sars-CoV-2 Spread

The worldwide outbreak of Sars-CoV-2 resulted in modelers from diverse fields being called upon to help predict the spread of the disease, resulting in many new collaborations between different institutions. We here present our experience with bringing our skills as veterinary disease modelers to bear on the field of human epidemiology, building models as tools for decision makers, and bridging the gap between the medical and veterinary fields. We describe and compare the key steps taken in modeling the Sars-CoV-2 outbreak: criteria for model choices, model structure, contact structure between individuals, transmission parameters, data availability, model validation, and disease management. Finally, we address how to improve on the contingency infrastructure available for Sars-CoV-2.

Predicting spread and effective control measures for African swine fever-Should we blame the boars?

An ongoing, continually spreading, outbreak of African swine fever (ASF), following its identification in Georgia in 2007, has resulted in 17 European and 12 Asian countries reporting cases by April 2020, with cases occurring in both wild boar and domestic pigs. Curtailing further spread of ASF requires understanding of the transmission pathways of the disease. ASF is self-sustaining in the wild boar population, and they have been implicated as one of the main drivers of transmission within Europe. We developed a spatially explicit model to estimate the risk of infection with ASF in wild boar and pigs due to natural movement of wild boar that is applicable across the whole of Europe. We demonstrate the model by using it to predict the probability that early cases of ASF in Poland were caused by wild boar dispersion. The risk of infection in 2015 is computed due to wild boar cases in Poland in 2014, compared against reported cases in 2015, and then the procedure is repeated for 2015-2016. We find that long- and medium-distance spread of ASF (i.e. >30 km) is unlikely to have occurred due to wild boar dispersal, due in part to the generally short distances wild boar will travel (<20 km on average). We also predict the relative success of different control strategies in 2015, if they were implemented in 2014. Results suggest that hunting of wild boar reduces the number of new cases, but a larger region is at risk of ASF compared with no control measure. Alternatively, introducing wild boar-proof fencing reduces the size of the region at risk in 2015, but not the total number of cases. Overall, our model suggests wild boar movement is only responsible for local transmission of disease; thus, other pathways are more dominant in medium- and long-distance spread of the disease.

African swine fever in Latvian wild boar-A step closer to elimination

In 2014, African swine fever (ASF) emerged in Latvia for the first time. The majority of cases appeared in wild boar, but the presence of ASF in these animals constitutes a permanent threat to domestic pig holdings. Recent studies have shown an increase in serologically positive and a decrease in PCR-positive ASF cases in wild boar, possibly indicating a decline of ASF incidence. We aimed to investigate the course of the ASF epidemic in wild boar in Latvia, thus attaining further insights into the ASF epidemiology in this country with the goal of assessing the stage of the epidemic. Latvian ASF surveillance data of wild boar were utilized to estimate the seroprevalence and ASF virus (ASFV) prevalence in the wild boar population. Prevalence estimates were obtained for both the eastern and western part of the country and in addition for the 2014/2015 to 2018/2019 hunting seasons. Moreover, prevalence estimates for three different age classes were calculated. An increase in serologically positive yet PCR-negative wild boar samples from active surveillance was identified over time. When comparing the age groups, wild boar younger than one year displayed the ASFV prevalence to be higher than the seroprevalence, whereas older animals shared higher seroprevalence estimates. These findings support the assumption that only a small proportion of affected animals survive an infection, leading to an accumulation of their numbers over time. As a result, ASF elimination in a country with an infected wild boar population could possibly be achieved, if effective wild boar population management and surveillance is maintained and combined with the detection and removal of wild boar carcasses to reduce the viral load in the environment. In addition, the wild boar population should be kept as small as possible to break the ASFV infection cycle.

Threat to the French Swine Industry of African Swine Fever: Surveillance, Spread, and Control Perspectives

African swine fever (ASF) has one of the highest case-fatality rates among pig diseases. Europe was considered ASF-free for about two decades until 2007, when the virus was introduced into Georgia. Since then, it has been identified throughout Eastern Europe, and reached Belgium in late 2018, increasing the risk of ASF being introduced into neighboring countries-namely Germany, Luxembourg, the Netherlands, and France. French authorities have therefore reinforced surveillance measures to improve the probability of detecting ASF rapidly if it emerges in France. Predictive modeling may help to anticipate the extent of virus spread and evaluate the efficiency of these surveillance measures. A previously published and well-documented model that simulates ASF virus spread was therefore tailored to realistically represent the French situation in terms of the geographic distribution of swine production sites and the commercial trade between them on the one hand, and the implementation of surveillance protocols on the other. The outcomes confirmed the moderate spread of ASF through the swine trade network, a situation that had been previously highlighted for the case of Denmark. However, the diversity of the French pig production landscape has revealed a huge potential for the geographic dispersal of the virus, especially should the index case occur in a low-density area, with a median source-to-case distance reaching 300 km. Free-range herds, which are more likely to have interactions with wild boars, were also identified as potential entrance gate for the virus. Transmissions from conventional herds were quasi-exclusively due to swine movement on the commercial network, representing 99% of transmission events. In contrast, 81% of transmission events occurred in the neighborhood of the index herd when the virus was introduced in free-range herds. The current surveillance measures were found relatively efficient for detecting the virus in large herds, leading to detection rates of 94%. However, infections on smaller production sites-which often have free-range herds-were more difficult to detect and would require screening protocols specifically targeting these smaller herds.

Analysis of Estonian surveillance in wild boar suggests a decline in the incidence of African swine fever

African swine fever (ASF) in wild boar populations is difficult to control. In affected areas, samples from all wild boar shot and found dead are investigated. The use of laboratory tests allows estimating the duration of the infection in affected animals. The study aimed to test the hypothesis that the stage of the epidemic in different areas of Estonia can be assessed on the basis of prevalence estimates. ASF surveillance data of Estonian wild boar were used to estimate prevalences and compare them between the East and West of Estonia. The temporal trend of the estimated prevalence of ASF virus positive animals and of the estimated seroprevalence of wild boar showing antibodies against ASFV was analyzed. Due to the potential influence of population density on the course of ASF in wild boar, also population density data (number of wild boar/km2) were used to investigate the relationship with laboratory test results. In areas, where the epidemic had already lasted for a long time, a small number of new cases emerged recently. The prevalence of samples that were only seropositive was significantly higher in these regions as compared to areas, where the epidemic is in full progress. The observed course of the disease could be the beginning of an ASF endemicity in this region. However, the results may also indicate that ASF has started to subside in the areas that were first affected in Estonia.

Epidemiological evaluation of Latvian control measures for African swine fever in wild boar on the basis of surveillance data

A wild boar population infected with African Swine Fever (ASF) constitutes a constant threat to commercial pig farms and therefore to the economy of the affected country. Currently, ASF is still spreading in several countries and the implementation of intensive measures such as reducing wild boar population densities seems not to be able to stop the further spread of the disease. In addition, there are still substantial knowledge gaps regarding the epidemiology of the disease. To identify risk factors for a higher probability of a wild boar sample being virological or serological positive, comprehensive statistical analyses were performed based on Latvian surveillance data. Using a multivariable Bayesian regression model, the effects of implemented control measures on the proportion of hunted or found dead wild boar or on the estimated virus prevalence were evaluated. None of the control measures applied in Latvia showed a significant effect on the relevant target figure. Also, the estimated periodic prevalence of wild boar that had tested ASF positive by PCR appeared to remain unaffected over time. Therefore, there is an urgent need to reconsider the implemented control measures. The results of this study and the course of ASF in other affected countries, raise the question, whether an endemic situation of ASF in wild boar is reversible.

Mapping Disease Transmission Risk of Nipah Virus in South and Southeast Asia

Since 1998, Nipah virus (NiV) (genus: Henipavirus; family: Paramyxoviridae), an often-fatal and highly virulent zoonotic pathogen, has caused sporadic outbreak events. Fruit bats from the genus Pteropus are the wildlife reservoirs and have a broad distribution throughout South and Southeast Asia, and East Africa. Understanding the disease biogeography of NiV is critical to comprehending the potential geographic distribution of this dangerous zoonosis. This study implemented the R packages ENMeval and BIOMOD2 as a means of modeling regional disease transmission risk and additionally measured niche similarity between the reservoir Pteropus and the ecological characteristics of outbreak localities with the Schoener's D index and I statistic. Results indicate a relatively high degree of niche overlap between models in geographic and environmental space (D statistic, 0.64; and I statistic, 0.89), and a potential geographic distribution encompassing 19% (2,963,178 km²) of South and Southeast Asia. This study should contribute to current and future efforts to understand the critical ecological contributors and geography of NiV. Furthermore, this study can be used as a geospatial guide to identify areas of high disease transmission risk and to inform national public health surveillance programs.

Epidemiological analyses of African swine fever in the Baltic States and Poland: (Update September 2016-September 2017)

EFSA assisted four countries in the analysis of epidemiological data on African swine fever (ASF), collected until September 2017. The temporal analysis demonstrated that the average proportions of PCR and antibody‐ELISA positive samples from the hunted wild boar remained below 3.9 and 6.6, respectively. A peak in the ASF incidence was observed 6 months after the first observed case, followed by a significant reduction of the number of cases and low levels of African swine fever virus (ASFV) circulation at the end of 38 months follow‐up period at different spatial resolutions. The spatial analysis concluded that human‐mediated spread of ASFV continues to play a critical role in the ASF epidemiology, despite all measures currently taken. ‘Wild boar density’, ‘total road length’ (as proxy for human activity) and ‘average suitable wild boar habitat availability’ were identified as predictors for the occurrence of ASF in Estonia by a Bayesian hierarchical model, whereas ‘wild boar density’ and ‘density of pig farms’ were predictors according to a generalised additive model. To evaluate the preventive strategies proposed in EFSA's Scientific Opinion (2015) to stop the spread of ASFV in the wild boar population, a simulation model, building on expert knowledge and literature was used. It was concluded that reduction of wild boar population and carcass removal to stop the spread of ASFV in the wild boar population are more effective when applied preventively in the infected area. Drastic depopulation, targeted hunting of female wild boar and carcass removal solely implemented as measures to control ASF in the wild boar population need to be implemented in a highly effective manner (at or beyond the limit of reported effectivity in wild boar management) to sustainably halt the spread of ASF.

This publication is linked to the following EFSA Supporting Publications article: http://onlinelibrary.wiley.com/doi/10.2903/sp.efsa.2017.EN-1312/full

Behaviour of free ranging wild boar towards their dead fellows: potential implications for the transmission of African swine fever

The behaviour of free ranging wild boar (Sus scrofa) towards carcasses of their conspecifics potentially infected with African swine fever (ASF) may significantly influence the course of an ASF epidemic. This study aims to better understand the behaviour of wild boar towards their dead fellows. Thirty-two wild boar carcasses on nine study sites in northeast Germany were monitored under field conditions by photo-trapping from October 2015 until October 2016. During this period, a total of 122 160 pictures were taken, thereof 16 111 pictures of wild boar. In both winter and summer, wild boar seemed to be particularly interested in the soil next to and underneath the carcasses. About one third of the visits of wild boar led to direct contact with dead conspecifics. The contacts consisted mostly in sniffing and poking on the carcass. Under the given ecological and climatic conditions, there was no evidence for intra-species scavenging. However, piglets were observed several times chewing bare bones once skeletonization of the carcasses was complete. It must be assumed that all these types of contact may represent a risk of transmission. Both the high tenacity of ASF virus and the long time wild boar carcasses can remain in the environment, allow the persistence of the virus for several months or even years. We therefore consider the rapid detection and removal (or destruction on the spot) of contaminated carcasses as an important control measure against ASF in wild boar.

Preventive measures aimed at minimizing the risk of African swine fever virus spread in pig farming systems

African swine fever (ASF) is one of the most severe diseases of pigs; it has a drastic impact on the pig industry, causing serious socio-economic consequences to pig farmers and pork producers. In Europe, there are currently two main clusters of infection; one in Sardinia caused by strains of African swine fever virus (ASFV) belonging to genotype I and another in Eastern Europe caused by strains of ASFV belonging to genotype II. The latter is inducing an acute form of ASF and it represents a serious threat to the pig sector. ASF is a disease for which there is no effective vaccine; therefore, prevention has a pivotal role in the control strategy of the disease. This review describes the main preventive measures to adopt to mitigate the risk of ASF spread in pig farming systems.