Antigenic characteristics of African swine fever virus in artificial and natural mixed populations

Heterogeneity of African swine fever (ASF) virus is manifested through differences in its physico-chemical, genetic, virulent, haemadsorption and/or antigenic properties. Here some results of studies of ASF virus experimental mixtures and natural isolates are represented based on their haemadsorbing activity and antigen specificity levels which were determined from the data of haemadsorption-inhibition tests (HAIT) and immunological bioassays in pigs. For the investigations, three mixtures of ASFV virulent isolates were prepared: 1) isolates of seroimmunotypes II and IV (Kikassa 74 + Brazil 80), 2) those of seroimmunotypes III and IV (Mozambique 78 + Sao Tome and Principe), and 3) of serotypes I and IV (Diamang + Dnopa Luanda). The mixed in vivo infections were studied in an artificially organized microepizootic outbreak. We found that at infecting animals with mixtures of ASF virus isolates of two different seroimmunotypes, one isolate steadily dominated over the other one for its haemadsorption level, so the typing data for blood samples of pigs contact-infected with mixtures of isolates in HAIT do not correspond to those observed in the immunological bioassays in pigs. The above phenomenon is apparently due to the fact that the biological materials under study contain ASF viruses of seroimmunotypes which are present in the initial mixtures but lost their haemadsorbing properties in the course of the mixed infection of the animals. A principal opportunity of an in vitro recovery of the haemadsorbing potentiality of the non-haemadsorbing variant of the virus present in the mixtures using a method of passage in a porcine bone marrow (PBM) cell culture supplemented with haemadsorption-inhibiting specific sera to isolates that dominated in the mixtures for haemadsoption is shown. At the same time, another approach, namely using alternating passages both in an insensitive cell system (Siberian mountain goat kidney culture) and PBM cells gave no results. Furthermore, we determined that ASF virus can be present in the environment in the form of a population heterogenous with respect to its seroimmunotype. Also, the carried out research work showed that there was a principal opportunity to provide protection against isolate T-67 when immunizing animals with attenuated virus variants prepared from its virulent variants belonging to four seroimmunotypes.