Mechanical transmission of African swine fever virus by Stomoxys calcitrans: Insights from a mechanistic model

Risk and protective factors for ASF in domestic pigs and wild boar in the EU, and mitigation measures for managing the disease in wild boar

Five epidemiological aspects of ASF were evaluated using literature reviews, field studies, questionnaires and mathematical models. First, a literature review and a case-control study in commercial pig farms emphasised the importance of biosecurity and farming practices, including the spread of manure around farms and the use of bedding material as risk factors, while the use of insect nets was a protective factor. Second, although wild boar density is a relevant known factor, the statistical and mechanistic models did not show a clear and consistent effect of wild boar density on ASF epidemiology in the selected scenarios. Other factors, such as vegetation, altitude, climate and barriers affecting population connectivity, also played a role on ASF epidemiology in wild boar. Third, knowledge on Ornithodoros erraticus competence, presence and surveillance was updated concluding that this species did not play any role in the current ASF epidemic in affected areas of the EU. Available scientific evidence suggests that stable flies and horse flies are exposed to ASFV in affected areas of the EU and have the capacity to introduce ASFV into farms and transmit it to pigs. However, there is uncertainty about whether this occurs, and if so, to what extent. Fourth, research and field experience from affected countries in the EU demonstrates that the use of fences, potentially used with existing road infrastructure, coupled with other control methods such as culling and carcass removal, can effectively reduce wild boar movements contributing to ASF management in wild boar. Fences can contribute to control ASF in both scenarios, focal introductions and wave-like spread. Fifth, the use of gonadotropin-releasing hormone (GnRH) vaccines as an immune contraceptive has the potential, as a complementary tool, to reduce and control wild boar populations. However, the development of an oral GnRH vaccine for wild boar still requires substantial additional work.

Blood-feeding stable flies, Stomoxys calcitrans (Diptera: Muscidae), are attracted to, and transmit Staphylococcus aureus (Bacillales: Staphylococcaceae), a causal agent of bovine mastitis: a laboratory pilot study

Stable flies, Stomoxys calcitrans (Linnaeus) (Diptera: Muscidae), are common blood-feeding ectoparasites of cows and thus potential vectors of the skin-dwelling bacterium Staphylococcus aureus, a causal agent of bovine mastitis which inflicts udder inflammation in cows. Our objectives were to determine whether stable flies (i) are attracted to disease-causing strains of S. aureus, and (ii) transmit S. aureus from infected blood to sterile blood. In 3-chamber olfactometers, five of eight S. aureus strains grown on agar and tested versus sterile agar attracted female stable flies. When flies ingested droplets of blood inoculated with S. aureus at doses of 0 (control), 105 (low), 107 (medium), and > 109 (high) colony-forming units per milliliter and subsequently ingested sterile blood, they transmitted S. aureus to the sterile blood. The dose of S. aureus in blood droplets fed upon by flies during their first feeding bout dose-dependently affected the amount of bacteria that flies transmitted to sterile blood during their second feeding bout, but the time elapsed between feeding bouts (0 h, 1 h, 8 h, and 24 h) had no effect on the amount of microbes transmitted to sterile blood. Our data infer the existence of a positive feedback loop. First, stable flies carrying S. aureus and feeding on cows transmit S. aureus, thereby causing mastitis. As S. aureus bacteria of afflicted cows proliferate, they attract even more flies which, in turn, worsen the infection. This type of feedback loop underscores the need for effective stable fly control tactics that curtail the incidence of bovine mastitis in cows.

Global population genetic structure and lineage differentiation of the stable fly, Stomoxys calcitrans

The bloodsucking fly, Stomoxys calcitrans (Diptera: Muscidae), is a cosmopolitan pest that transmits potential pathogens mechanically. We conduct phylogeographic analyses of S. calcitrans to resolve its global population genetic structure for establishing baseline of molecular studies. Results from mitochondrial gene suggested that the major divergence of S. calcitrans predominantly occurred 0.32-0.47 million years ago (Mya) and the subsequent diversifications took place during 0.13-0.27 Mya. The Ethiopian region was deduced as the most likely origin of S. calcitrans and the Nearctic lineages were considered to have originated from Oriental or Palaearctic regions. Our results further revealed that each biogeographic region of S. calcitrans likely maintains its genetic specialty, and yet, those non-monophyletic relationships were possibly caused by ancestral retention, dispersal with mammals, long-distance migration, and the international livestock industries. Moreover, the three highly diverged Ethiopian lineages may be putative cryptic species that require clarification of their veterinary importance. Unravelling the genetic structure of stable fly and preventing gene flow among biogeographic regions through anthropogenic activities are thus pivotal in livestock industry administration, particularly genetic exchange among differentiated lineages that might lead to the consequence of ecological trait alterations.

Three Years of African Swine Fever in South Korea (2019–2021): A Scoping Review of Epidemiological Understanding

African swine fever (ASF) is a highly contagious viral disease in domestic pigs and wild boar that causes tremendous socioeconomic damage in related industries. In 2019, the virus emerged in South Korea, which has since reported 21 outbreaks in domestic pig farms and over 2,600 cases in wild boar. In this review, we synthesize the epidemiological knowledge generated on ASF in South Korea during the first three years of the epidemic (2019–2021). We searched four international and one domestic Korean database to identify scientific articles published since 2019 and describing ASF epidemiology in South Korea. Fourteen articles met our selection criteria and were used to synthesize the origin of ASF in South Korea, the risk factors of disease occurrence, the effectiveness of the surveillance and intervention measures that were implemented, and the viral transmission dynamics. We found that timely intensive surveillance and interventions on domestic pig farms successfully blocked between-farm transmission. However, in wild boar, the ASF virus has spread massively towards the south primarily along the mountain ranges despite ongoing fence erection and intensive depopulation efforts, endangering domestic pig farms across the country. The current devastating epidemic is suspected to be the consequence of an ASF control strategy unaligned to the epidemiological context, the challenging implementation of control measures hindered by topological complexities, and inappropriate biosecurity by field workers. To improve our understanding of ASF epidemiology in South Korea and enhance disease management, future research studies should specify the ecological drivers of disease distribution and spread and devise effective control strategies, particularly in relation to Korean topography, and the latent spread of the virus in wild boar populations. Additionally, research studies should explore the psychosocial factors for ASF management, and develop tools to support evidence-based decision-making for managing ASFV in wild boar.

Stochastic modelling of African swine fever in wild boar and domestic pigs: Epidemic forecasting and comparison of disease management strategies

African swine fever (ASF), caused by the African swine fever virus (ASFV), is highly virulent in domestic pigs and wild boar (Sus scrofa), causing up to 100% mortality. The recent epidemic of ASF in Europe has had a serious economic impact and poses a threat to global food security. Unfortunately, there is no effective treatment or vaccine against ASFV, limiting the available disease management strategies. Mathematical models allow us to further our understanding of infectious disease dynamics and evaluate the efficacy of disease management strategies. The ASF Challenge, organised by the French National Research Institute for Agriculture, Food, and the Environment, aimed to expand the development of ASF transmission models to inform policy makers in a timely manner. Here, we present the model and associated projections produced by our team during the challenge. We developed a stochastic model combining transmission between wild boar and domestic pigs, which was calibrated to synthetic data corresponding to different phases describing the epidemic progression. The model was then used to produce forward projections describing the likely temporal evolution of the epidemic under various disease management scenarios. Despite the interventions implemented, long-term projections forecasted persistence of ASFV in wild boar, and hence repeated outbreaks in domestic pigs. A key finding was that it is important to consider the timescale over which different measures are evaluated: interventions that have only limited effectiveness in the short term may yield substantial long-term benefits. Our model has several limitations, partly because it was developed in real-time. Nonetheless, it can inform understanding of the likely development of ASF epidemics and the efficacy of disease management strategies, should the virus continue its spread in Europe.

Wing Phenotypic Variation among Stomoxys calcitrans (Diptera: Muscidae) Populations in Thailand

Stomoxys calcitrans (Linnaeus, 1758) (Diptera: Muscidae) is a cosmopolitan hematophagous ectoparasite of veterinary and medical importance. It is an important mechanical vector of several animal pathogens and can cause significant economic losses. However, the morphological variation of this species remains unknown. This study aimed to investigate the phenotypic variation in the wing size and shape of S. calcitrans populations in Thailand based on a landmark-based geometric morphometric approach. Specimens were collected from five populations in five geographical regions in Thailand. A total of 490 left wings of S. calcitrans (245 female and 245 male individuals) were used for geometric morphometric analysis. Wing size differences were detected between some populations of S. calcitrans, whereas wing shape differences were found among populations. Therefore, the phenotypic variation in S. calcitrans populations indicated that these populations are adaptive responses to local environmental pressures, suggesting the presence of phenotypic plasticity in this species.

Modelling African swine fever virus spread in pigs using time-respective network data: Scientific support for decision-makers

African Swine Fever (ASF) represents the main threat to swine production, with heavy economic consequences for both farmers and the food industry. The spread of the virus that causes ASF through Europe raises the issues of identifying transmission routes and assessing their relative contributions in order to provide insights to stakeholders for adapted surveillance and control measures. A simulation model was developed to assess ASF spread over the commercial swine network in France. The model was designed from raw movement data and actual farm characteristics. A metapopulation approach was used, with transmission processes at the herd level potentially leading to external spread to epidemiologically connected herds. Three transmission routes were considered: local transmission (e.g. fomites, material exchange), movement of animals from infected to susceptible sites, and transit of trucks without physical animal exchange. Surveillance was represented by prevalence and mortality detection thresholds at herd level, which triggered control measures through movement ban for detected herds and epidemiologically related herds. The time from infection to detection varied between 8 and 21 days, depending on the detection criteria, but was also dependent on the types of herds in which the infection was introduced. Movement restrictions effectively reduced the transmission between herds, but local transmission was nevertheless observed in higher proportions highlighting the need of global awareness of all actors of the swine industry to mitigate the risk of local spread. Raw movement data were directly used to build a dynamic network on a realistic time-scale. This approach allows for a rapid update of input data without any pre-treatment, which could be important in terms of responsiveness, should an introduction occur. This article is protected by copyright. All rights reserved.

Basic reproduction number of African swine fever in wild boars (Sus scrofa) and its spatiotemporal heterogeneity in South Korea

Background

African swine fever (ASF) is a hemorrhagic fever occurring in wild boars (Sus scrofa) and domestic pigs. The epidemic situation of ASF in South Korean wild boars has increased the risk of ASF in domestic pig farms. Although basic reproduction number (R₀) can be applied for control policies, it is challenging to estimate the R₀ for ASF in wild boars due to surveillance bias, lack of wild boar population data, and the effect of ASF-positive wild boar carcass on disease dynamics.

Objectives

This study was undertaken to estimate the R₀ of ASF in wild boars in South Korea, and subsequently analyze the spatiotemporal heterogeneity.

Methods

We detected the local transmission clusters using the spatiotemporal clustering algorithm, which was modified to incorporate the effect of ASF-positive wild boar carcass. With the assumption of exponential growth, R₀ was estimated for each cluster. The temporal change of the estimates and its association with the habitat suitability of wild boar were analyzed.

Results

Totally, 22 local transmission clusters were detected, showing seasonal patterns occurring in winter and spring. Mean value of R₀ of each cluster was 1.54. The estimates showed a temporal increasing trend and positive association with habitat suitability of wild boar.

Conclusions

The disease dynamics among wild boars seems to have worsened over time. Thus, in areas with a high elevation and suitable for wild boars, practical methods need to be contrived to ratify the control policies for wild boars.

Mechanistic modelling of African swine fever: A systematic review

The spread of African swine fever (ASF) poses a grave threat to the global swine industry. Without an available vaccine, understanding transmission dynamics is essential for designing effective prevention, surveillance, and intervention strategies. These dynamics can often be unraveled through mechanistic modelling. To examine the assumptions on transmission and objectives of the mechanistic models of ASF, a systematic review of the scientific literature was conducted. Articles were examined across multiple epidemiological and model characteristics, with filiation between models determined through the creation of a neighbor-joined tree using phylogenetic software. Thirty-four articles qualified for inclusion, with four main modelling objectives identified: estimating transmission parameters (11 studies), assessing determinants of transmission (7), examining consequences of hypothetical outbreaks (5), assessing alternative control strategies (11). Population-based (17), metapopulation (5), and individual-based (12) model frameworks were represented, with population-based and metapopulation models predominantly used among domestic pigs, and individual-based models predominantly represented among wild boar. The majority of models (25) were parameterized to the genotype II isolates currently circulating in Europe and Asia. Estimated transmission parameters varied widely among ASFV strains, locations, and transmission scale. Similarly, parameter assumptions between models varied extensively. Uncertainties on epidemiological and ecological parameters were usually accounted for to assess the impact of parameter values on the modelled infection trajectory. To date, almost all models are host specific, being developed for either domestic pigs or wild boar despite the fact that spillover events between domestic pigs and wild boar are evidenced to play an important role in ASF outbreaks. Consequently, the development of more models incorporating such transmission routes is crucial. A variety of codified and hypothetical control strategies were compared however they were all a priori defined interventions. Future models, built to identify the optimal contributions across many control methods for achieving specific outcomes should provide more useful information for policy-makers. Further, control strategies were examined in competition with each other, which is opposed to how they would actually be synergistically implemented. While comparing strategies is beneficial for identifying a rank-order efficacy of control methods, this structure does not necessarily determine the most effective combination of all available strategies. In order for ASFV models to effectively support decision-making in controlling ASFV globally, these modelling limitations need to be addressed.

Semi-quantitative risk assessment by expert elicitation of potential introduction routes of African swine fever from wild reservoir to domestic pig industry and subsequent spread during the Belgian outbreak (2018-2019)

Since the introduction in Georgia in 2007 of an African swine fever (ASF) genotype 2 virus strain, the virus has rapidly spread to both Western European and Asian countries. It now constitutes a major threat for the global swine industry. The ongoing European transmission cycle has been related to the 'wild boar habitat' with closed transmission events between wild boar populations and incidental spillovers to commercial and non-commercial (backyard) pig holdings. During the epidemic in Belgium, only wild boar were infected and although the introduction route has not yet been elucidated, the 'human factor' is highly suspected. While ASF was successfully contained in a small region in the Southern part of Belgium without affecting domestic pigs, the risk of spillover at the wild/domestic interface remains poorly assessed. In this study, we used a semi-quantitative method, involving national and international experts, to assess the risk associated with different transmission routes for ASF introduction from wild boar to domestic pig holdings and subsequent dissemination between holdings in the Belgian epidemiological context. Qualitative responses obtained by our questionnaire were numerically transformed and statistically processed to provide a semi-quantitative assessment of the occurrence of the hazard and a ranking of all transmission routes. 'Farmer', 'bedding material', 'veterinarian' and 'professionals from the pig sector' were considered as the most important transmission routes for ASF introduction from the wild reservoir to pig holdings. 'Animal movements', 'farmer', 'veterinarian', 'iatrogenic', 'animal transport truck' and 'animal care equipment' were considered as the most important transmission routes posing a risk of ASF spread between pig holdings. Combined with specific biosecurity checks in the holdings, this assessment helps in prioritizing risk mitigation measures against ASF introduction and further spread in the domestic pig industry, particularly while the ASF situation in Western Europe is worsening.