Scientific Abstract
African swine fever (ASF) is one of the most important viral disease of pigs in the world at present, and the North American swine industry is striving hard to prevent its entry to our region. Moderately and low virulent strains of ASFV pose a higher threat of entering and slowly spreading while remaining undetected in North American swine herds, particularly in breeding herds. Processing fluids (PF) are serosanguineous tissue fluid passively recovered from castration and tail docking of young piglets which is an industry practice called “piglet processing”. Currently, PF is being used to detect other viral pathogens of pigs in breeding herds The first two objectives of this study were to evaluate the utility of PF to detect ASFV genomic material, as well as ASFV specific antibodies following an ASF incursion in young piglets. The third objective of this study was to evaluate novel sample types such as superficial lymph nodes, skin biopsies, ear notches as well as muscle swabs for the early detection of ASFV genetic material in comparison to standard samples like whole blood and spleen.
A total of 80, 3 weeks old, intact male piglets were subject to two independent experiments conducted back to back. The first group of 40 piglets were infected with moderately-virulent ASFV Estonia/2014 and the second group of 40 with ASFV Malta’78 by oro-nasal route. Following infection, sampling was conducted daily up to 5 days post inoculation (dpi) and at pre-planned sampling points or humane end points thereafter. At each sampling point, four piglets were processed (castrated and tail docked), under general anaesthesia, blood samples were collected and euthanized to perform post mortem examinations and sample collection. The testicles and tails from each pig were pooled in sampling bags and processing fluids were collected on an individual basis. Following humane euthanasia of the piglets, complete post-mortem examinations were performed while collecting samples; from the most superficial ones first to the internal ones last. Total nucleic acid extraction was performed directly on PF, 10% tissue homogenates and other novel swab samples. ASFV genomic detection was performed using real time PCR. Furthermore, antibody detection in PF and corresponding serum samples was performed using two commercial indirect ELISA assays.
ASFV genome was detected in PF as early as 2-3 dpi at relatively lower levels, reached higher detection levels thereafter and continued at the same levels till the end of the study. Dilution of positive PF samples with known negative PF samples revealed that up to 100 PF samples can be pooled without loosing the detection of ASFV genomic material in the samples. Testing PF with two commercial indirect ELISA assays showed that antibodies to ASFV can be detected in processing fluids as early as 9-10 days post-infection, as observed in serum samples. ASF genome was detected in all novel sample types (superficial lymph nodes, skin biopsies, ear notches, meat swabs, abdominal and thoracic cavity swabs and nasal wipes) as early as 3-4 dpi and the detection continued thereafter with 100% specificity parallel to the detection in whole blood and spleen.
Based on the data from this study, we can suggest that PF is suitable to screen young swine herds for ASF. Unconventional, superficial samples such as superficial lymph nodes, skin biopsies, ear notches and muscle swabs can also serve as suitable alternative samples to detect the presence of ASF during large scale dying events, without having to perform complete post mortem examinations.
