African swine fever: virus carriage and the role of surviving wild boar in the persistence and spread of infection (review)

The panzootic of African swine fever (ASF) in Europe and Asia caused by viruses of genotypes I and II has led to a comprehensive scientific study of the course of this infection. Researchers have noted that a certain proportion of diseased animals survive. Detection of seropositive (antibody-positive) animals in ASF-affected countries of Europe was especially noticeable in wild boar populations. The role of seropositive animals ASF survivors in the persistence and spread of infection in the population has been long and controversially debated, as they may potentially become persistently infected, acting as virus carriers. The aim of this review was to summarize the current scientific and experimental results on the chronic course of ASF in Eurasian wild boar, virus carriage and spread of infection from surviving seropositive animals. In Eurasia different forms of ASF disease are currently observed in susceptible animals: peracute, acute, subacute, less frequently chronic and asymptomatic; the latter are found to be caused by circulating ASF viruses (ASFV) of reduced virulence. Two types of survivors animals are distinguished: 1) animals that develop persistent infection with periodic viremia and signs of subacute to chronic course; 2) animals that recover completely and clear of infection. Long-term persistence and complete elimination of the virus have been observed in survivors: in the last ten years, the time of virus excretion has been experimentally determined to generally range from 35 to 99 days. Survivors of the 1st type may play a role in the spread of the ASFV due to periodic viremia. Seropositive survived animals of the 2nd type due to their small numbers are not considered by some researchers to play a significant epidemiological role in the persistence of ASFV in wild boar populations. Experimental transmission of ASFV from surviving wild boars has been studied to date to a limited extent, such research should be continued. The knowledge gained in these areas will improve the understanding of the current situation of ASF in wildlife.