Abstract
A time-course study was performed to elucidate the early events of foot-and-mouth disease virus (FMDV) infection in pigs subsequent to simulated natural, intra-oropharyngeal, inoculation. The earliest detectable event was primary infection in the lingual and paraepiglottic tonsils at 6 hours post inoculation (hpi) characterized by regional localization of viral RNA, viral antigen, and infectious virus. At this time FMDV antigen was localized in cytokeratin-positive epithelial cells and CD172a-expressing leukocytes of the crypt epithelium of the paraepiglottic tonsils. De novo replication of FMDV was first detected in oropharyngeal swab samples at 12 hpi and viremia occurred at 18–24 hpi, approximately 24 hours prior to the appearance of vesicular lesions. From 12 through 78 hpi, microscopic detection of FMDV was consistently localized to cytokeratin-positive cells within morphologically characteristic segments of oropharyngeal tonsil crypt epithelium. During this period, leukocyte populations expressing CD172a, SLA-DQ class II and/or CD8 were found in close proximity to infected epithelial cells, but with little or no co-localization with viral proteins. Similarly, M-cells expressing cytokeratin-18 did not co-localize with FMDV proteins. Intra-epithelial micro-vesicles composed of acantholytic epithelial cells expressing large amounts of structural and non-structural FMDV proteins were present within crypts of the tonsil of the soft palate during peak clinical infection. These findings inculpate the paraepiglottic tonsils as the primary site of FMDV infection in pigs exposed via the gastrointestinal tract. Furthermore, the continuing replication of FMDV in the oropharyngeal tonsils during viremia and peak clinical infection with no concurrent amplification of virus occurring in the lower respiratory tract indicates that these sites are the major source of shedding of FMDV from pigs.
Highlights
Foot-and-mouth disease (FMD) is a highly contagious infection of cloven-hoofed animals, with a renowned ability of rapid transmission amongst susceptible hosts
In both IOP inoculated and contact exposed pigs, there was a consistent reduction in the activity level of the pigs observed at12 hours post exposure when the animals became less responsive to external stimuli and more reluctant to rise
Coronary band vesicles spread to include the solar epithelium of the heel bulbs which was sloughed and eventually replaced with thick, disorganized scar tissue in the animals that survived the acute phase of infection
Summary
Foot-and-mouth disease (FMD) is a highly contagious infection of cloven-hoofed animals, with a renowned ability of rapid transmission amongst susceptible hosts. Outbreaks of FMD within developed countries that are normally kept free of the disease lead to immediate and severe impact upon agricultural production, with prolonged restrictions on export of animal products. In the large regions of the world in which FMD is endemic, the disease poses a constant threat to the health and welfare of livestock, thereby compromising the livelihood of farmers and causing instability of food supplies [2]. The characteristic clinical manifestations of FMD which include blanching and vesiculation of cornified epithelium within the oral cavity and in areas of non-haired skin, can be seen across a wide range of susceptible host-species, including domestic and wild ruminants and suids [3,4,5]. Further evidence of critical host-specific differences in the molecular pathways of FMDV infection has been provided through evidence of a restricted host-range of FMDV strains that carry specific deletions within the 3A-coding region of the genome [10,11,12]
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