Growth of foot-and-mouth disease virus in the upper respiratory tract of non-immunized, vaccinated, and recovered cattle after intranasal inoculation.

Non-immunized, vaccinated, and recovered cattle were inoculated intranasally with various doses of foot-and-mouth disease virus. Samples of oesophageal-pharyngeal (OP) fluid were taken periodically for up to 7 days after inoculation and virus titres of these samples were plotted as pharyngeal virus growth curves. In non-immunized cattle, the length of the lag period and of the growth period were inversely proportional to the dose of virus given. Maximum titres were observed when clinical signs were first detected. Three of the 10 cattle studied had virus growth rates that were lower than rates of others given the same dose of virus, and clinical signs appeared later than expected in these three cattle. Cattle vaccinated with an inactivated virus oil-adjuvant vaccine had pharyngeal virus growth curves that were similar to those obtained from non-immunized cattle for 30 h. after inoculation. Titres of virus in OP fluid samples taken 2-7 days after inoculation were substantially lower in cattle with a high pre-exposure serum mouse protection index than titres from partly-immunized or non-immunized cattle. Nine of 14 cattle had detectable but reduced virus growth after intranasal inoculation with homologous virus. Five recovered cattle inoculated with heterologous virus reacted similarly to non-immunized animals.