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.