See .... From the OIE website - an important new scientific review of FMD:

Unapparent foot and mouth disease infection (sub-clinical infections and carriers): implications for control

P. Sutmoller & R. Casas Olascoaga

Rev. sci. tech. Off. int. Epiz., 2002, 21 (3), 519-529 http://www.oie.int/eng/publicat/rt/2103/A_r21312.htm

Summary

Unlike animals which are carriers of foot and mouth disease (FMD), sub-clinically infected animals may be highly contagious. The implications of sub-clinical infections for the control of FMD are serious because such animals are likely to disseminate the disease when in contact with susceptible livestock. Recent dissemination of FMD virus (FMDV) in Europe shows that sub-clinically infected animals render trade in animals or animal products a potential risk for importing countries. This clearly demonstrates that the paradigm free of FMD without vaccination is not synonymous with risk-free. The risk of introduction of sub-clinical FMD into FMD-free countries may increase significantly, with the occurrence of large susceptible animal populations, changed agricultural practices, expansion of trade in live animals and animal movements, increased trade in animal products and greater mobility of people. Such changes in circumstances require that national and international authorities remain continuously vigilant to determine any altered risk for importation of FMD.

A few historical reports and some recent observations in southern Africa indicate the possibility of dissemination of FMD by bovine carriers into herds of susceptible cattle. These reports have greatly influenced FMD trade policies and thus, FMD control and eradication strategies. However, other field evidence does not support this claim and several controlled experiments were unable to show that carriers are able to initiate disease. When millions of cattle were systematically vaccinated with good quality vaccines, FMD disappeared in spite of a large sentinel population in the form of calves and unvaccinated sheep and pigs. A low number of carriers most likely persisted, but they did not hamper the eradication of the disease.

Vaccination policies and trade regulation must be based on risk assessments taking these factors into consideration.