Studies on low virulence African swine fever virus strain of Brazilian isolates to approach the surveillance program

African swine fever (ASF) disease was eradicated from domestic swine herds in Brazil in 1984, after six years of hard work, determination and with highly cost due to the occurrence in pig small farms. The ASF virus (ASFV) belongs to the family Asfarviridae being the only member of Asfivirus genus. Our previous studies on ASFV epizootiology and virulence demonstrated that virus has dispersed from the first ASF outbreak in pig herds in Paracambi city of state of Rio de Janeiro to other regions especially to Southeastern and Southern where the first industrial pig farms were being established. And, pointed to the possibility of Brazilian ASF outbreaks were caused by low virulence ASFV strain. The high mortality rate in the first ASF outbreak was associated to the poor sanitary conditions and other concomitant infections. In Plum Island Animal Disease Center – PIADC, US the Brazilian isolates were inoculated healthy pigs where the ASF low virulence was reproduced. In order to understand, clarify and improve the prevention against virus entry in the country, this work was conducted to survey of pig herd field epidemiological surveillance records (FORMS) of pig herds in better sanitary conditions from the same period and which ASFV was isolated at least in one pig.

From 559 epidemiological surveillance records (FORMS) investigated, 283 filled out in emergency phase were analyzed. Number of passages needed for ASFV isolation by haemadsorption (HAD), description of ASF pathogenic signs, and mortality rates were compared. In PR, of 58 samples, 25 were ASFV-positive in the first, second or third passage of the leukocyte cultures, eighteen FORMS corresponding to 72 % of the positive results could be investigated and compared with the results of experimental Brazilian ASFV isolates inoculations held in PIADC.

Relevant data about ASFV-positive pig herds such as the number of infected animals (1,060), animals with disease signs (91) and number of dead animals (58). The main ASF clinical sign descriptors in the FORMS were fever, anorexia, recumbence, flushing and/or cyanotic skin, diarrhea, hemorrhages in the skin, tremors, with hind legs appearing weak. The pathology was quite similar to found in PIADC. The mortality rates in the ASF outbreaks in PR from June (10%) to July (0.16%) indicated a less virulent ASF infection.

The sub-clinical course of ASF caused by low virulence virus strains increases the threat; because the virus can enter a country insidiously highlighted the importance of establishing and maintaining secure measures to prevent ASFV entrance into ASF-free countries.

Disclosure of Interest: None Declared.