Vertical transmission of anti-ASFV antibodies as one of potential causes of seropositive results among young wild boar population in Poland.

The present study attempted to elucidate possible routes leading to the achievement of sero- positive results, among young (aged ≤1 year) wild boar population. In the years 2017-2018, the National Reference Laboratory (NRL) for African swine fever (ASF) in Poland examined nearly 27-thousand wild boar blood samples, collected during an active surveillance of ASF risk zones, for the presence of viral DNA and anti-ASFV antibodies. Out of all the examined samples, 420 were positive. However, in more than half of them (292 samples) antibodies against African swine fever virus (ASFV) were detected, while ASFV DNA was not detected in blood. Out of all 292 seropositive/PCR-negative samples, 126 belonged to young wild boars (aged ≤1 year). For this reason, the NRL in Poland has examined 10 selected seropositive wild boar carcasses to confirm or exclude post-mortem lesions for ASF as well as to investigate the presence of viral DNA in the internal organs. Neither pathological lesions for ASF nor the presence of genetic material of ASFV were found in the examined wild boars. To elucidate this outcomes, following hypotheses about possible reasons of the obtained results were drawn: the presence of convalescent animals, infection of low-virulent ASFV isolate and the vertical transmission of antibodies through the colostrum.