Detection of African swine fever virus in feed dust collected from experimentally inoculated complete feed using quantitative PCR and virus titration assays

Development of a P30 protein-based indirect ELISA for detecting African swine fever antibodies utilizing the HEK293F expression system

African swine fever (ASF) is an acute, febrile, and highly lethal infectious disease in pigs caused by the African swine fever virus (ASFV). Effective detection methods and strict biosecurity measures are crucial for preventing and controlling ASF, especially since there are currently no commercially available vaccines or antiviral drugs to combat ASFV infection effectively. However, the emergence of low-virulence strains of ASFV in recent years has led to false-positive results, highlighting the importance of early-produced antibody detection methods. Therefore, detecting antibodies against ASFV produced early in the infection can facilitate the prompt identification of infected pigs. This study focused on the p30 protein, an early expressed protein during ASFV infection, to develop an indirect ELISA. This method was established using the HEK293F suspension cell expression system, which has the ability to produce large quantities of correctly folded proteins with normal functionality. In this study, we developed an indirect ELISA test utilizing the p30 recombinant protein produced by the HEK293F suspension cell expression system as the antigen coating. The concentration of the p30 protein obtained from the HEK293F suspension cell expression system was measured at 4.668 mg/mL, serving as the foundation for establishing the indirect ELISA. Our findings indicate that the indirect ELISA method exhibits a sensitivity of 1:12800. Furthermore, it demonstrates high specificity and excellent reproducibility. Comparing our results to those obtained from the commercial kit, we found a coincidence rate of 98.148 % for the indirect ELISA. In summary, we have developed a sensitive method for detecting ASFV, providing a valuable tool for monitoring ASFV infection in pig herds.

Evidence of aerosol transmission of African swine fever virus between two piggeries under field conditions: a case study

African swine fever (ASF) is a devastating and economically significant infectious disease that has caused enormous losses in the commercial pig sector in China since 2018. The primary transmission routes of the African swine fever virus (ASFV), the causative agent of ASF, are direct pig-to-pig contact or indirect contact with virus-contaminated objects. While aerosol transmission of ASFV has been previously reported under experimental conditions, no reports have described it under field conditions. In this case study, aerosol-associated samples were collected over a monitoring period of 24 days in an ASFV-positive farm. A complete and clear chain of ASFV transmission through aerosols was observed: pigs in Room A on Day 0-aerosol in Room A on Day 6-dust of air outlets in Room A on Day 9-outdoor aerosols on Day 9-dust of air inlets in Room B on Day 15-aerosols/pigs in Room B on Day 21. Furthermore, a fluorescent powder experiment confirmed the transmission of dust from Room A to Room B. This study represents the first report providing evidence of aerosol transmission of ASFV under field conditions. Further research is needed to study the laws of aerosol transmission in ASFV and develop effective strategies such as air filtration or disinfection to create a low-risk environment with fresh air for pig herds.

Stability of African swine fever virus in feed during environmental storage

African swine fever virus (ASFV) causes high case fatality in pigs and a trade-limiting disease resulting in significant economic losses to pork production. ASFV is resistant to environmental degradation and maintains infectivity in feed ingredients exposed to transoceanic shipment conditions. As ASFV is transmissible through consumption of contaminated feed, the objective of this study was to evaluate the stability of ASFV Georgia 2007 in three feed matrices (complete feed, soybean meal, ground corncobs) exposed to three environmental storage temperatures (40°F, 68°F, 95°F) for up to 365 days. ASFV DNA was highly stable and detectable by qPCR in almost all feed matrices through the conclusion of each study. Infectious ASFV was most stable in soybean meal, maintaining infectivity for at least 112 days at 40°F, at least 21 days at 68°F and at least 7 days at 95°F. These data help define risk of ASFV introduction and transmission through feed ingredients.

Non-nucleic acid extraction and ultra-sensitive detection of African swine fever virus via CRISPR/Cas12a

Early diagnosis of the African swine fever virus (ASFV) is the main preventive measure for ASFV. Here, we developed a fluorescent biosensor and lateral flow assay (LFA) strip based on direct PCR combined with CRISPR/Cas12a system for ASF. Direct PCR can simultaneously split samples and efficiently amplify without sacrificing sensitivity, which eliminated the steps of nucleic acid extraction. Furthermore, by the CRISPR/Cas12a, the biosensor addressed false positives caused by non-specific amplification and had high sensitivity with the actual limit of detection (LOD) of 7.6×10^-4 ng·μL^-1 (4 copies·μL^-1). In addition, the strategy was built on the lateral flow assay (LFA) strip to achieve visual and portable detection for point-of-care testing. Moreover, the biosensor by a fluorometer and LFA strip showed a high accuracy to rival qPCR in actual sample detection. Therefore, the biosensor is an ultra-sensitive and specific tool that can replace traditional methods. KEY POINTS: • No nucleic acid extraction, direct PCR-simplified steps, and reduced time and cost • CRISPR/Cas12a solved the false positives caused by nonspecific amplification • The combination of the LFA strip and biosensor is more convenient for POC detection.

Diagnostic sensitivity of porcine biological samples for detecting African swine fever virus infection after natural consumption in feed and liquid

African swine fever virus (ASFV) is a global threat to swine production and sustainable pork supply. Without a commercially available vaccine, prevention of ASFV entry and spread is reliant on biosecurity and early detection of infection. Although ASFV ingestion in swill or feed by naïve pigs is a likely route of initial introduction, controlled experimental studies rarely utilize natural consumption as the infection route. In the current study, we utilized biological samples collected from pigs 5 days after natural consumption of ASFV in feed and liquid to assess diagnostic sensitivity for early detection of virus infection. Biological samples (serum, spleen, lymph nodes, tonsils, and faeces) were assessed for the presence of ASFV using quantitative PCR and virus isolation. Statistical methods modelled the detection sensitivity of each sample type with each diagnostic assay in individual samples. Our results provide important information that can be incorporated into ASFV surveillance programs.