The severe measures routinely enacted for control of foot-and-mouth disease (FMD) outbreaks are based upon extreme contagiousness of the disease, rapidity of spread, and fear of residual contagion associated with long-term asymptomatic virus carriers. It is well established that many ruminant species are capable of this prolonged carrier state; however, it is largely believed that pigs are incapable of becoming FMD virus (FMDV) carriers. The purpose of the work described herein was to investigate the potential for pigs to become long-term carriers of FMDV with the ultimate goal of using this information to design rational approaches to species-specific outbreak response planning.

A large proportion of FMD research has focused on pathogenesis and prevention in cattle. The work performed within this project contributes detailed knowledge of the early and late stages of FMDV infection in pigs, which is critical to development of improved FMDV countermeasures for use in pigs.

The FMDV carrier state in ruminants (cattle, sheep, etc) is a period subsequent to clinical disease in which animals carry and shed infectious FMDV for long periods without showing any symptoms of the disease. Because of this, an animal that has recovered from the disease, or that has been protected from developing disease by vaccination, is treated as if it may still be capable of spreading the virus. This means that large numbers of healthy animals are routinely destroyed during FMD outbreaks in countries that are normally free of the disease (USA, UK, France, etc). Based on limited data, it has become generally accepted as ‘conventional wisdom’ that pigs are not capable of becoming long term carriers of FMDV. However, during recent years, there have been a limited number of scientific publications that have suggested otherwise. The main purpose of the work performed within this project has been to thoroughly investigate if pigs are capable of harboring infectious FMDV for extended time after recovery from the disease.

To achieve these goals, pigs were experimentally inoculated with one of five distinct strains of FMDV. Infection dynamics were characterized for periods ranging from 6 hours to 100 days after which pigs were euthanized for tissue collection. These investigations demonstrated that infectious virus could not be detected in blood or secretions from pigs once the clinical signs of disease had disappeared. Furthermore, live virus was not detected in tissue samples obtained at 35-100 days after infection from pigs that had recovered from the disease. Although live virus was not recovered, viral degradation products (RNA and structural proteins) could be detected in select lymph nodes harvested at 35 days after infection. This detection of viral remnants declined markedly by 60 days after infection, and was completely absent at 100 days post infection.

The data described herein provides the most extensive investigation of potential FMDV persistence in pigs. The overall conclusion is that domestic pigs are unlikely to carry infectious FMDV for prolonged periods after recovery from clinical disease. The significance of detection of viral degradation products in lymph nodes is debatable and has not been thoroughly investigated in the current studies. Overall, this work provides a basis for considering species-specific control plans. However, further research and validation would be required to enact such policy.