African Swine Fever Virus Transmission Research Articles

Reviewing the Potential Vectors and Hosts of African Swine Fever Virus Transmission in the United States

Reviewing the Potential Vectors and Hosts of African Swine Fever Virus Transmission in the United States

JOURNEY OF AFRICAN SWINE FEVER VIRUS IN EASTERN EU STATES

INTRODUCTION: In 2007, African swine fever virus (ASFv) broken its well-known boundaries. This was the reference year for the first report of African swine fever (ASF) in Georgia. Subsequently, the virus reached pigs and boars in Armenia and Russia. From the Caucasus area, ASFv jumped in all directions, between the Black Sea and the Caspian Sea, in relation to the density of backyard pigs and their trade. In the next ten years there have been notifications and registrations of ASFv outbreaks in Russia, Azerbaijan, Ukraine, Belarus, Lithuania, Poland, Estonia, Latvia, Moldova and the Czech Republic. Romania faced the first ASFv outbreak at the end of July 2017, in backyard pigs." in stead "density of backyard pigs and their trade. In the next ten years there have been notifications and registrations of ASFv outbreaks in Russia, Azerbaijan, Ukraine, Belarus, Lithuania, Poland, Estonia, Latvia, Moldova and the Czech Republic. Romania faced the first ASFv outbreak at the end of July 2017, in backyard pigs.OBJECTIVES: The aim of study is to analyse the ways ASFv spread from and into different regions recorded by Eastern European states.METHODS: The immediate notifications on ASFv to the World Organisation for Animal Health (OIE) were analysed from the Eastern-European states between 2007 and 2017. The analysis took into consideration the first occurrence of the disease under scrutiny in the country and the follow-up reports, in relation with the geospatial distribution of the outbreaks.RESULTS: The main route of ASFv introduction into local pig populations indicated by the Member States of the European Union was the trans-boundary circulation of boars. However, the spread of ASFv through both, wild and domestic pigs and also by the human alimentary customs/traditions in the affected areas shouldn’t be ignored. Three cycles of ASFv transmission have been identified and described by the epidemiologists: the domestic cycle, the sylvatic cycle and the tick-pig cycle.CONCLUSION: None of the ways to disseminate the ASFv should be excluded, and the origin of the first outbreaks remains unknown or inconclusive in Eastern EU states.

Survival and localization of African swine fever virus in stable flies (Stomoxys calcitrans) after feeding on viremic blood using a membrane feeder

Since 2014, African swine fever virus (ASFV) has been spreading within Eastern Europe. Within affected regions, the virus has infected some farms with high biosecurity and a marked seasonality of outbreaks in domestic pigs has been observed. ASFV transmission from stable flies, Stomoxys calcitrans, has previously been shown both mechanically and via ingestion of whole flies. Hence, blood-feeding flies may offer one explanation for the introductions into high biosecurity farms and for the observed seasonality. The aim of this study was to further elucidate the potential role of stable flies in ASFV transmission. Different parts of flies were analyzed for the presence of viral DNA and infectious virus at different time points following in vitro feeding of the flies on blood from an ASFV-infected pig.Using qPCR, ASFV DNA was detectable in mouth parts of flies for at least 12 h and remained in head and body samples from the flies for up to three days following feeding. Infectious virus was detected in fly body samples prepared at 3 h and 12 h after feeding.The presence of infectious ASFV in stable flies following feeding on viremic blood means that such flies are capable of transporting infectious virus. The detection of ASFV DNA in the flies for up to three days following feeding suggests that qPCR analysis of blood-feeding flies during ASFV outbreaks could be a useful method to elucidate the role of these flies in ASFV transmission under field conditions.

No evidence for long-term carrier status of pigs after African swine fever virus infection.

This study targeted the assessment of a potential African swine fever virus (ASFV) carrier state of 30 pigs in total which were allowed to recover from infection with ASFV "Netherlands'86" prior exposure to six healthy sentinel pigs for more than 2months. Throughout the whole trial, blood and swab samples were subjected to routine virological and serological investigations. At the end of the trial, necropsy of all animals was performed and viral persistence and distribution were assessed. Upon infection, a wide range of clinical and pathomorphological signs were observed. After an initial acute phase in all experimentally inoculated pigs, 66.6% recovered completely and seroconverted. However, viral genome was detectable in blood samples for up to 91days. Lethal outcomes were observed in 33.3% of the pigs with both acute and prolonged courses. No ASFV transmission occurred over the whole in-contact phase from survivors to sentinels. Similarly, infectious ASFV was not detected in any of the tissue samples from ASFV convalescent and in-contact pigs. These findings indicate that the suggested role of ASFV survivors is overestimated and has to be reconsidered thoroughly for future risk assessments.

Inferring within-herd transmission parameters for African swine fever virus using mortality data from outbreaks in the Russian Federation.

SummaryMortality data are routinely collected for many livestock and poultry species, and they are often used for epidemiological purposes, including estimating transmission parameters. In this study, we infer transmission rates for African swine fever virus (ASFV), an important transboundary disease of swine, using mortality data collected from nine pig herds in the Russian Federation with confirmed outbreaks of ASFV. Parameters in a stochastic model for the transmission of ASFV within a herd were estimated using approximate Bayesian computation. Estimates for the basic reproduction number varied amongst herds, ranging from 4.4 to 17.3. This was primarily a consequence of differences in transmission rate (range: 0.7–2.2), but also differences in the mean infectious period (range: 4.5–8.3 days). We also found differences amongst herds in the mean latent period (range: 5.8–9.7 days). Furthermore, our results suggest that ASFV could be circulating in a herd for several weeks before a substantial increase in mortality is observed in a herd, limiting the usefulness of mortality data as a means of early detection of an outbreak. However, our results also show that mortality data are a potential source of data from which to infer transmission parameters, at least for diseases which cause high mortality.

Pig traders’ networks on the Kenya-Uganda border highlight potential for mitigation of African swine fever virus transmission and improved ASF disease risk management

We applied social network analysis to pig trader networks on the Kenya-Uganda border. Social network analysis is a recently developed tool, which is useful for understanding value chains and improving disease control policies. We interviewed a sample of 33 traders about their experiences with trade and African swine fever (ASF), analyzed the networks they generated in purchasing pigs and selling pork and their potential contribution to modulating dissemination of the ASF virus (ASFV). The majority of the traders were aware of clinical signs of ASF and the risk of trade transmitting ASFV. Most said they avoided buying pigs from ASF outbreak villages or sick pigs but their experiences also indicated that inadvertent purchase was relatively common. Traders had early knowledge of outbreaks since they were contacted by farmers who had heard rumours and wanted to sell their pigs to avoid the risk of them dying. Individual traders bought pigs in up to nine villages, and up to six traders operated in a village. Although each trade typically spanned less than 5km, networks of the various traders, comprising movements of pigs from source villages to slaughter slabs/sites and retail outlets, and movement of pork to villages where it was consumed, linked up indirectly across the 100km×50km study area and revealed several trade pathways across the Kenya-Uganda border. ASF could potentially spread across this area and beyond through sequential pig and pork transactions.Regulation of the pig and pork trade was minimal in practice. The risk of ASFV being spread by traders was compounded by their use of poorly constructed slaughter slabs/sites with open drainage, ineffective or non-existent meat inspection services, lack of provision for biosecurity in the value chain, and sales of pork to customers who were unaware of the risks to their own pigs from contact with ASF infected pork. More effective regulation is warranted. However, limitations on government capacity, together with the strong self-interest that established traders have in reducing the disruption and financial losses that outbreaks cause, highlight the importance of governments and traders co-developing an approach to ASF control. Formation of trader organizations or common interest groups warrants government support as an important step in engaging traders in developing and implementing effective approaches to reduce the risk of ASF outbreaks.

African Swine Fever Virus Transmission Cycle in Nigeria: Assessment of Domestic Pig-Soft Tick Contact through Detection of Antibodies against Ornithodoros moubata Salivary Antigen TSGP1

African Swine Fever Virus Transmission Cycle in Nigeria: Assessment of Domestic Pig-Soft Tick Contact through Detection of Antibodies against Ornithodoros moubata Salivary Antigen TSGP1

Transmission routes of African swine fever virus to domestic pigs: current knowledge and future research directions

African swine fever (ASF) is a major threat to the pig industry in Europe. Since 2007, ASF outbreaks have been ongoing in the Caucasus, Eastern Europe and the Baltic countries,...

African swine fever virus transmission cycles in Central Europe: Evaluation of wild boar-soft tick contacts through detection of antibodies against Ornithodoros erraticus saliva antigen.

BackgroundAfrican swine fever (ASF) is one of the most complex viral diseases affecting both domestic and wild pigs. It is caused by ASF virus (ASFV), the only DNA virus which can be efficiently transmitted by an arthropod vector, soft ticks of the genus Ornithodoros. These ticks can be part of ASFV-transmission cycles, and in Europe, O. erraticus was shown to be responsible for long-term maintenance of ASFV in Spain and Portugal. In 2014, the disease has been reintroduced into the European Union, affecting domestic pigs and, importantly, also the Eurasian wild boar population. In a first attempt to assess the risk of a tick-wild boar transmission cycle in Central Europe that would further complicate eradication of the disease, over 700 pre-existing serum samples from wild boar hunted in four representative German Federal States were investigated for the presence of antibodies directed against salivary antigen of Ornithodoros erraticus ticks using an indirect ELISA format.ResultsOut of these samples, 16 reacted with moderate to high optical densities that could be indicative of tick bites in sampled wild boar. However, these samples did not show a spatial clustering (they were collected from distant geographical regions) and were of bad quality (hemolysis/impurities). Furthermore, all positive samples came from areas with suboptimal climate for soft ticks. For this reason, false positive reactions are likely.ConclusionIn conclusion, the study did not provide stringent evidence for soft tick-wild boar contact in the investigated German Federal States and thus, a relevant involvement in the epidemiology of ASF in German wild boar is unlikely. This fact would facilitate the eradication of ASF in the area, although other complex relations (wild boar biology and interactions with domestic pigs) need to be considered.

Small-scale pig farmers' behavior, silent release of African swine fever virus and consequences for disease spread.

The expanding distribution of African swine fever (ASF) is threatening the pig industry worldwide. Most outbreaks occur in backyard and small-scale herds, where poor farmers often attempt to limit the disease’s economic consequences by the emergency sale of their pigs. The risk of African swine fever virus (ASFV) release via this emergency sale was investigated. Simulation modeling was used to study ASFV transmission in backyard and small-scale farms as well as the emergency sale of pigs, and the potential impact of improving farmers and traders’ clinical diagnosis ability–its timeliness and/or accuracy–was assessed. The risk of ASFV release was shown to be high, and improving farmers’ clinical diagnosis ability does not appear sufficient to effectively reduce this risk. Estimates obtained also showed that the distribution of herd size within the backyard and small-scale sectors influences the relative contribution of these farms to the risk of release of infected pigs. These findings can inform surveillance and control programs.

Survival of African Swine Fever Virus in Excretions from Pigs Experimentally Infected with the Georgia 2007/1 Isolate.

SummaryAfrican swine fever virus (ASFV) causes a lethal haemorrhagic disease of swine which can be transmitted through direct contact with infected animals and their excretions or indirect contact with contaminated fomites. The shedding of ASFV by infected pigs and the stability of ASFV in the environment will determine the extent of environmental contamination. The recent outbreaks of ASF in Europe make it essential to develop disease transmission models in order to design effective control strategies to prevent further spread of ASF. In this study, we assessed the shedding and stability of ASFV in faeces, urine and oral fluid from pigs infected with the Georgia 2007/1 ASFV isolate. The half‐life of infectious ASFV in faeces was found to range from 0.65 days when stored at 4°C to 0.29 days when stored at 37°C, while in urine it was found to range from 2.19 days (4°C) to 0.41 days (37°C). Based on these half‐lives and the estimated dose required for infection, faeces and urine would be estimated to remain infectious for 8.48 and 15.33 days at 4°C and 3.71 and 2.88 days at 37°C, respectively. The half‐life of ASFV DNA was 8 to 9 days in faeces and 2 to 3 days in oral fluid at all temperatures. In urine, the half‐life of ASFV DNA was found to be 32.54 days at 4°C decreasing to 19.48 days at 37°C. These results indicate that ASFV in excretions may be an important route of ASFV transmission.

Quantification of airborne African swine fever virus after experimental infection

Knowledge on African Swine Fever (ASF) transmission routes can be useful when designing control measures against the spread of ASF virus (ASFV). Few studies have focused on the airborne transmission route, and until now no data has been available on quantities of ASF virus (ASFV) in the air. Our aim was to validate an air sampling technique for ASF virus (ASFV) that could be used to detect and quantify virus excreted in the air after experimental infection of pigs. In an animal experiment with the Brazil'78, the Malta'78 and Netherlands'86 isolates, air samples were collected at several time points. For validation of the air sampling technique, ASFV was aerosolised in an isolator, and air samples were obtained using the MD8 air scan device, which was shown to be suitable to detect ASFV. The half-life of ASFV in the air was on average 19 min when analysed by PCR, and on average 14 min when analysed by virus titration. In rooms with infected pigs, viral DNA with titres up to 10(3.2) median tissue culture infective dose equivalents (TCID50eq.)/m(3) could be detected in air samples from day 4 post-inoculation (dpi 4) until the end of the experiments, at dpi 70. In conclusion, this study shows that pigs infected with ASFV will excrete virus in the air, particularly during acute disease. This study provides the first available parameters to model airborne transmission of ASFV.

Assessment of interactions between African swine fever virus, bushpigs (Potamochoerus larvatus), Ornithodoros ticks and domestic pigs in north-western Madagascar

Since its introduction in Madagascar in 1998, African swine fever (ASF) has severely affected national pig production and persists as a common disease in that country. Two of its natural hosts in the African continent, the bushpig (Potamochoerus larvatus) and tick vectors of the Ornithodoros moubata complex, are reported in west and central regions of the island. However, their role in the maintenance and transmission of the virus has been insufficiently studied. In this work, we tried to assess their potential role in the epidemiology of the disease in Madagascar, by assessing the levels of interaction between (i) ASF virus (ASFV) and bushpigs and (ii) between soft ticks and domestic and wild suids in north-western Madagascar. Twenty-seven sera and 35 tissue samples from bushpigs were collected and analysed for the presence of anti-ASF antibodies and viral DNA. In addition, the sera from 27 bushpigs and 126 domestic pigs were analysed with an ELISA test for the detection of antibodies against salivary antigens from Ornithodoros ticks. No circulation of ASFV or anti-ASFV antibodies nor anti-tick antibodies were detected in bushpigs. However, seven of the domestic pig sera (5.6% of the total sample population) were antibody positive for O.moubata antigens. The probability of freedom from ASFV in the bushpig population using Bayesian statistical methods ranged between 73% and 84%. The probabilities of absence of anti-tick antibodies in domestic and wild pigs were estimated at 63% and 71%, respectively. These preliminary results suggest that bushpigs are unlikely to play a significant role in the maintenance and transmission of ASFV in Madagascar. Nevertheless, further ASFV surveys are needed on that species to confirm this assumption. In addition, the presence of antibodies against O.moubata in domestic pigs suggests that soft ticks may be able to maintain ASFV within a domestic pig cycle in areas of Madagascar where they remain present.

African swine fever virus infection in the argasid host, Ornithodoros porcinus porcinus.

The pathogenesis of African swine fever virus (ASFV) infection in Ornithodoros porcinus porcinus was examined in nymphal ticks infected with the ASFV isolate Chiredzi/83/1. At times postinfection (p.i.) ranging from 6 h to 290 days, ticks or dissected tick tissues were titrated for virus and examined ultrastructurally for evidence of virus replication. The ASFV infection rate in ticks was 100% in these experiments, and virus infection was not associated with a significant increase in tick mortality. Initial ASFV replication occurred in phagocytic digestive cells of the midgut epithelium. Subsequent infection and replication of ASFV in undifferentiated midgut cells was observed at 15 days p.i. Generalization of virus infection from midgut to other tick tissues required 2 to 3 weeks and most likely involved virus movement across the basal lamina of the midgut into the hemocoel. Secondary sites of virus replication included hemocytes (type I and II), connective tissue, coxal gland, salivary gland, and reproductive tissue. Virus replication was not observed in the nervous tissue of the synganglion, Malpighian tubules, and muscle. Persistent infection, characterized by active virus replication, was observed for all involved tick tissues. After 91 days p.i., viral titers in salivary gland and reproductive tissue were consistently the highest detected. Successful tick-to-pig transmission of ASFV at 48 days p.i. correlated with high viral titers in salivary and coxal gland tissue and their secretions. A similar pattern of virus infection and persistence in O. porcinus porcinus was observed for three additional ASFV tick isolates in their associated ticks.

A Conserved African Swine Fever Virus IκB Homolog, 5EL, Is Nonessential for Growthin Vitroand Virulence in Domestic Swine

An African swine fever virus (ASFV) gene with similarity to the cellular inhibitor of NFκB (IκB) was described in the pathogenic African isolate Malawi Lil-20/1 (ORF 5EL) and a cell-culture-adapted European virus, BA71V (ORF A238L). Recently, this gene was shown to be a functional IκB homolog capable of downregulating NFκB-regulated gene expression. This observation suggests the gene may be of significance to aspects of ASFV pathogenesis and virulence in domestic swine by interfering with a normal antiviral host response. Here we show, using nucleotide sequence analysis, that5ELis highly conserved among various African and European pathogenic field isolates and that in all cases its similarity to IκB genes is limited to the presence of four low complexity ankyrin repeats in the ASFV gene. The5ELgene of Malawi Lil-20/1 encodes a 28-kDa protein which was expressed early in virus-infected macrophage cell cultures with maximum levels observed at 3 to 5 hr postinfection. To study gene function, a Malawi Lil-20/15ELgene deletion mutant (Δ5EL) was constructed. Growth characteristics of Δ5EL in porcine macrophage cell cultures were indistinguishable from those of the parental virus. And, Δ5ELexhibited an unaltered parental Malawi Lil-20/1 disease and virulence phenotype in domestic swine. Thus, although highly conserved among ASFV isolates,5ELis nonessential for growth in porcine macrophagesin vitroand for viral virulence in domestic swine. A possible role for this gene in transmission of ASFV in nature, a setting which involves the cycling of ASFV between two highly adapted hosts,Ornithodorosticks and warthogs or bush pigs, in sub-Saharan Africa is discussed.

Attempted transovarial and venereal transmission of African swine fever virus by the Iberian soft tick Ornithodoros (Pavlovskyella) marocanus (Acari: Ixodoidea: Argasidae).

Transovarial transmission experiments were conducted with three groups of Ornithodoros (Pavlovskyella) marocanus Velu; one group consisted of 27 pairs of adults that had been fed as larvae on a pig with a viremia of 10(7.4) HAd50/ml of African swine fever virus (ASFV). The second and third groups each consisted of 100 pairs of adults fed on a viremic pig (10(4.5) HAd50/ml) as adults. The first group underwent five gonotrophic cycles over a 554-d period. The second and third groups underwent three and two gonotrophic cycles, respectively. All larvae were fed in individual cohorts on naive pigs and the resulting nymphs were assayed by cohort for ASFV. None of the larvae transmitted ASFV to naive pigs by bite and ASFV was not isolated in swine buffy coat cultures from any cohort of nymphs. Therefore, O. marocanus does not exhibit transovarial transmission of ASFV. Venereal transmission experiments were conducted with pairs of O. marocanus in which either the female (100 pairs) or the male (100 pairs), respectively, had fed on a viremic pig (10(4.5) HAd50/ml). Both groups underwent at least two gonotrophic cycles over a 470-d period, were sampled periodically for the presence of ASFV, and the progeny were tested for the presence of ASFV. Venereal transmission from male to female occurred in 10% (1/10) of O. marocanus after the first gonotrophic cycle, but not after the second or third gonotrophic cycle, and transovarian transmission in these groups was not observed. Venereal transmission from infected females to uninfected males did not occur. ASFV persisted through five gonotrophic cycles over a 554-d period in 30% of adults fed on a viremic pig as larvae. ASFV was cleared during three gonotrophic cycles within a year from nearly all ticks fed on a viremic pig as adults. Virus-induced mortality rats of 12-80% occurred among ticks fed on viremic animals, whereas, no mortality was seen in ticks fed on uninfected animals. ASFV infection in ticks did not effect feeding frequency, egg-hatch rate, or the oviposition rate among females fed on a viremic pig as adults. The oviposition rate for females fed on a viremic pig as larvae was reduced by 63.4%. Parthenogenesis was not observed among O. marocanus. The mean gonotrophic cycle duration for pig-fed O. marocanus at 27 degree C was 19.8 d and the mean fecundity was 88.3 +/- 17.9 eggs/female/gonotrophic cycle.

Experimental transmission of African swine fever virus by Ornithodoros savignyi (Audouin)

The 'sand tampan', Ornithodoros savignyi, is susceptible to oral infection with African swine fever (ASF) virus in the laboratory. Infected ticks can transmit the virus transstadially and are able to maintain it for at least 106 days. Transmission of ASF virus by infected ticks to healthy pigs was achieved on five separate occasions between 50 and 106 days after infection. Pigs infected in this way developed typical acute African swine fever. The distribution of O savignyi in Africa suggests that this tick could be a natural field vector of ASF.