Results and discussion
At necropsy, the piglets had the swelling and hemorrhage of mandibular, inguinal lymph nodes, kidney petechial hemorrhage. Especially, the pigs had meninges congestion, some had hydranencephaly (Fig.1).
Moreover, all of the PCRs detecting PRRSV, PPV, CSFV, ADV and real-time PCR detecting ASFV gave negative results. However, ASFV was detected by a gel-based PCR in brain tissues but not in the other tissues. The PCR result was confỉrmed by sequencing, (GenBank accession number: MW269535), which showed that the ASFV found in this study belongs to genotype II.
Microscopic lesions were significant in brain tissues with such as severe congestion, the abundance of imflammatory spots, infiltration of lymphocytes, hyperplasia of connective tissues, profuse of vacuoles in neurons (Fig. 2). In addition, the IHC results revealed abundant viral antigen in brain tissues but few or none in other tissues (Fig. 3), which are in accordance with the gel-based PCR results.
Although clinical signs and lesions by ASFV have been recorded both in previous experimental and field studies (Schlafer and Mebus 1984, Kipanyula and Nong’ona 2017, Nga, Tran Anh Dao et al. 2020, Yoon, Hong et al. 2020), this is the first report of neurological signs in ASFV-infected neonate piglets. In these piglets, ASFV DNA and a significant amount of ASFV antigen and was found in brain tissues which showed microscopic lesions. In addition, nucleic acids of other pathogens such as classical swine fever (CSF), porcine parvovirus (PPV), porcine reproductive and respiratory syndrome (PRRSV) were not detected suggesting that ASFV might be the cause of the neurological signs. In the other hand, a trivial amount of ASFV antigen and no ASFV DNA was found in the kidney, lung, spleen, lymph nodes, suggesting that these tissues are not optimal for the diagnosis in this case. Thus, brain tissues should be considered for ASF diagnosis in piglets with neurological signs. This finding warrants for further studies of the pathogenicity as well as the pathogenesis of ASFV in piglets.
Another finding is that the OIE-recommended real-time PCR (King, Reid et al. 2003) is not optimal for detecting genotype II ASFV in this study, which is also previously reported in Viet Nam (Truong, Ly et al. 2020). Since the real-time PCR is frequently used in the diagnosis of such an important disease as ASF, it is vital to achieving more reliable ones. Meanwhile, combining several methods, such as detections of the nucleic acid, antigen, antibody, can improve the accuracy of ASF diagnosis.