Histopathologic evaluation of abnormal meat induced by foot-and-mouth disease vaccination

In 2010, three outbreaks of foot-and-mouth disease (FMD) occurred in South Korea. Among these outbreaks, the largest occurred from November 2010 to April 2011 and induced 153 total cases occurring in both cattle and pigs. During this outbreak, a nationwide vaccination policy was enforced and carried out in susceptible animals, including pigs. However, local reactions at the injection site were present after vaccination. Even now, these local reactions are an important cause of economic loss for the Korean pork industry. The aim of this study was to evaluate the histopathologic features of injection sites after FMD vaccination at multiple time periods.

60-day-old 156 domestic pigs were injected intramuscularly with 2mL of an oil-adjuvanted FMD vaccine. The animals were processed at 7, 30, 60, 90, or 135 days in a slaughterhouse. After gross examination, tissues of the injection site were fixed in 10% buffered formalin. The fixed samples were embedded in paraffin and stained with hematoxylin and eosin (H&E) stain for histopathologic analysis.

Histopathologically, the injection sites showed varying degrees of an inflammatory myopathy with intralesional, clear, round vacuoles. At 7 days after vaccination, all animals showed severe local inflammatory reactions with a mixed population of inflammatory cells containing polymorphonuclear and mononuclear cells. Muscle fibers in the region of the injection were atrophied or degenerated. At 30 to 60 days after vaccination, typical granulomatous and pyogranulomatous inflammatory lesions were observed in most of the samples. Many clear vacuoles were phagocytosed by multinucleated giant cells. Focal intralesional mineralization and abscessation were also found. At 90 days after vaccination, granulomatous lesions decreased slightly. At 135 days after vaccination, partial replacement of adipose tissue was observed within the muscle of the injection site.

Formation of abnormal meat at the injection site was the result of various inflammatory reactions that also lead to muscular degeneration. Clear vacuoles within the lesion probably represented the oil adjuvant used in the FMD vaccine. Most granulomatous inflammatory reactions were observed within the periphery of vacuoles. Although the degree of typical granulomatous lesions decreased at 90 days after vaccination, focal microscopic lesions persisted at the injection site until 135 days post-injection. Based on this analysis, we confirmed that the increased occurrence of abnormal meats was associated with FMD vaccination in pigs.

Disclosure of Interest: None Declared.