Pathology and Viral Distributions of the Porcinophilic Foot‐And‐Mouth Disease Virus Strain (O/Taiwan/97) in Experimentally Infected Pigs

Twenty‐four specific pathogen free pigs were inoculated intradermally at the front‐right heel bulb with 0.5 ml of viral suspension containing 10^6.0tissue culture infectious dose (TCID₅₀) with the porcinophillic strain (O/Taiwan/97) of foot‐and‐mouth disease virus (FMDV) isolated from the epizootic of FMD in Taiwan in 1997. Two pigs were euthanatized at 8 h, 1, 2, 3, 6, 8, 12, 15, 21, 26 and 63 days post‐inoculation (DPI), and two pigs remained for long‐term observation and terminated at 400 DPI. Typical symptoms of depression and inappetence appeared in the inoculated pigs at 1 DPI and subsided by 7 DPI. Vesicles developed in the epidermis over non‐inoculated metacarpals joints at 1 DPI and vesicles in the mouth and on the snout were noticed at 2 DPI. Lesions in the feet were characterized by necrosis in the stratum spinosum, intercellular oedema, and vesicle formation accompanied by neutrophilic and mononuclear cells infiltration. Baby hamster kidney‐21 cell cultures were used for virus isolation and viraemia was detected beginning at 1 DPI and persisted till 3 DPI and was no longer detectable when neutralizing antibody (NA) developed at 4 DPI. However, virus was isolated from skin samples from 1 to 12 DPI, from faeces from 2 to 8 DPI, and from 95% oesophageal‐pharyngeal (OP) fluid samples at 8 HPI. Among the samples tested in this study, skin vesicles had the highest virus titre, 10^8.63 TCID₅₀. No virus was isolated from the skin or visceral organs obtained from post‐mortem at day 15 after infection and the virus was not detectable from the OP fluid from 12 DPI till the end of this study (400 DPI). By using reverse transcriptase‐polymerase chain reaction, viral RNA was detected first from the tissues at the inoculation site at 1 DPI, and still detectable at 21 DPI. Neutralizing antibody emerged at 4 DPI and the geometric mean NA titre reached to 1:861 and 1:1097 at 21 and 301 DPI respectively. The re‐growth of hoof began at 21 DPI; however, minimal lesions including remnants of the old hoof were still presented at the end of this study. These results suggest that monitoring pig’s hooves for residual lesions should be part of the FMD diagnosis.