Laboratory diagnostics of chronic and asymptomatic forms of African swine fever

African swine fever (ASF) caused by African swine fever virus (ASFV) of Asfivirus genus, Asfarviridae family, can occur in peracute, acute, subacute, chronic or asymptomatic form. At early stages of epizootics, the infection usually occurs in its acute form eventually becoming chronic and/or asymptomatic. Seven to ten days post infection the survived pigs develop virus-specific antibodies which persist for a long time. An assumption is reasonable that in the near future, due to repeated passaging ASFV in wild boar populations in European countries, the ASFV isolates may appear which will cause chronic or asymptomatic rather than the acute forms of the disease. In our study we compared different tests to find those the most effective to reveal latent carriers when no apparent symptoms of the disease observed. Thus, our research was aimed at investigation of some special aspects of the laboratory diagnostics of chronic and asymptomatic forms of ASF. The chronic form of the disease was observed in a pig experimentally inoculated with an attenuated ASF virus Stavropol 01/08 A4S2/9k (at passage 33) at a dose of 106.0 HAU50. On day 5 to 7 post inoculation the signs typical of chronic forms of the infection were registered including depression and fever up to 40.5 °С. The antiviral antibody was detected in the swine blood serum from day 7. After the animal was killed on day 21, a haemadsorption assay revealed ASF virus present at low titers in spleen and mandibular lymph node samples while in liver and lung samples it was not found. Based on the results of polymerase chain reaction (PCR), the viral DNA was determined in the mandibular lymph node sample only. Furthermore, immunoblotting assay identified ASF antibody titers of 1:20 to 1:160 in all the organs examined. The asymptomatic forms of ASF were observed in a wild boar yearling which has been intramuscularly inoculated with an attenuated ASF virus strain MK-200 at a dose of 107.0 HAU50. The antiviral antibody was observed in the wild boar serum from day 8. After the animal was killed on day 25, no pathological signs typical of ASF were found, nor was ASF virus found in the organ samples examined using haemadsorption assay or its DNA was detected in PCR. In immunoblotting assay, virus-specific antibodies were identified in liver, spleen, lung and mandibular lymph node samples at dilutions of 1:40 to 1:320. The opportunity of detecting antibodies in spleen, lung and/or liver samples facilitates the monitoring for ASF to be carried out under the infection control campaigns, especially with respect to wild boars shot in game husbandries. Animals sequentially infected with an ASFV low-virulent isolate and a virulent one can survive, in which case it is quite possible to diagnose the disease using both PCR and serological methods. For making laboratory diagnosis of ASF chronic and/or asymptomatic forms, as well as carrying out monitoring studies, serological methods are recommended.