Endpoint titers of virus suspension and blind passage
In general, endpoint titers of the virus suspensions remained stable over a five-week storage period at 4°C. At room temperature, virus titers declined from 6.0 to 4.13 log10HAD50/ml over time (Table S3). Dee et al. (2018) also observed a comparable titer reduction (from 5.0 to 3.0 log10 TCID50) in a virus stock during 30 days of storage mimicking transatlantic shipment conditions. Overall, the virus suspension remained infectious over the entire observation period, irrespective of the storage temperature.
Regarding blind passage samples of contaminated SDPP stored at 4°C, virus titers remained constant over the entire study period (between 7.75 and 6.50 log10 HAD50/ml, Figure 2B). Thus, contaminated SDPP stored at 4°C was infectious for at least five weeks. In contrast, blind passage samples of contaminated SDPP stored at room temperature displayed only residual ASFV infectivity (one out of three replicates HAT positive) and a distinct titer reduction after one week (Figure 2B). After two weeks, complete ASFV inactivation was observed. This finding is in accordance with the corresponding real-time PCR results (Figure 2A). In comparison to PEDV, which is fully inactivated on spray dried bovine plasma after three weeks at 4°C or after one week at room temperature (Pujols and Segalés, 2014), ASFV displayed a higher stability on SDPP.
When judging these results, the sensitivity of the HAT detection system should be taken into consideration. Compared with a bioassay, i.e. the inoculation of pigs, the HAT is less sensitive (Heuschele, 1967). However, this discrepancy in sensitivity should not be a concern in this case, since Blázquez et al. (2020) demonstrated that the minimum infectious dose in feed mixed with ASFV contaminated liquid porcine plasma was higher than 5.0 log10TCID50/pig (50% tissue culture infectious dose per pig).
SDPP as a risk factor for ASFV transmission
It has been shown that the spray drying process itself has an inactivating effect on various viruses affecting pigs, like swine vesicular disease virus (SVD), pseudorabies virus (PRV), porcine reproductive and respiratory syndrome virus (PRRSV) or ASFV (Blázquez et al., 2018, Polo et al., 2005, Pujols and Segalés, 2014, Pujols et al., 2007). Hence, we investigated the re-contamination of SDPP after processing as well as the possibility of residual ASFV infectivity after an initial titer reduction during production. Thereby this study provides valuable data for evaluating the risk of ASFV transmission via contaminated SDPP. Based on the obtained results, we conclude that the residual risk of re-contamination of SDPP that is produced to European standards can be mitigated by room temperature storage for at least 14 days.