Foot-and-mouth disease virus (FMDV) can be spread by a variety of mechanisms. The objective of the current study was to obtain aerosol excretion data for the O UK 2001 isolate in sheep and in cattle and to measure the time course of virus load (infectivity and viral RNA) in nasal swabs, rectal swabs and serum to formulate a viral load framework for assessment of transmission risks. Oesophageal-pharyngeal (probang) samples were collected from the sheep at 28 days after exposure to establish

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... er any of them became persistently infected. Virus replicated rapidly in inoculated sheep from which a peak infectivity of airborne virus of 10 4.3 TCID 50 per sheep per 24 hours was recovered. Around 24 hours later contact-infected sheep excreted airborne virus maximally, also around 10 4.3 TCID 50 per sheep. The peaks of airborne excretion of the inoculated and contact sheep were 24 hours apart, lasted for 24 hours and then fell to below detection limits. Similar peak amounts of airborne virus were recovered from cattle, however, they maintained this level of excretion for about 3 days. The excretion of virus by the sheep fell into four phases. Firstly, a period of high excretion of airborne virus as described above. Secondly, a highly infectious period of 5-7 days, when excretions (nasal swabs and rectal swabs as well as serum) had significant levels of infectivity. Thirdly, a period of a few days (1-3 days) just after the infectious period when low amounts of viral RNA were recovered in nasal and rectal swabs as well as in serum. Fourthly, at 4 weeks when oesophagealpharyngeal samples showed that 50% of the sheep were carriers. This data provides a basis for developing a more comprehensive picture of the various transmission risks from livestock, especially sheep, at various stages of the infectious process.