Cattle infected with FMDV strains of different epizootiological origin developed a carrier state which persisted in the majority of animals for several months. Fluid samples taken from the oesophageal/pharyngeal region were assayed for infectivity by plaque counts on BHK monolayer cultures and by mouse inoculation. With one strain of virus, infectivity levels of up to 1000 pfu/ml. were recorded for several weeks after infection but in general the virus content of samples was below 50 pfu/ml.The sites of virus persistence and multiplication were identified by titration of suspensions of mucosae and epithelia taken post mortem. Virus was recovered from 41 of 54 cattle killed 14–196 days after infection. The chief sites of virus multiplication based on the frequency of virus recovery and infectivity titres were the dorsal surface of the soft palate and the pharynx. Virus was recovered less frequently from the ventral surface of the soft palate and the glosso-epiglottic space and only occasionally from the tonsillar sinuses, tonsils, tongue, trachea and oesophagus. No virus was detected in the turbinates or in the epithelium of the urinary bladder.

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Sheep infected with FMDV strains of different epizootiological origin developed a carrier state which persisted in the majority of animals for 1–5 months.The sites of virus persistence and multiplication in the convalescent animal were identified by titration of suspensions of mucosae and epithelia taken post mortem. Virus was recovered most frequently and in highest titre from the tonsillar area and less frequently from the pharynx and dorsal surface of the soft palate. No virus was found in samples taken from the nasal passages, the trachea or the rumen.

SUMMARYIn animals exposed to foot-and-mouth disease virus by indirect contact, virus was recovered from the blood, milk, pharynx, vagina and rectum for variable periods of time before clinical disease was apparent. Virus instilled into the mammary gland multiplied rapidly and virus concentrations greater than 107 p.f.u./ml. were recorded within 8-32 hr., depending on the virus strain and dose inoculated. Virus multiplication was accompanied by clinical signs of mastitis but the classical signs of foot-and-mouth disease did not appear for 52-117 hr. Dissemination of virus from the mammary gland occurred within 4-24 hr. and in some animals samples taken from the pharynx, mouth, nose and vagina contained virus for periods up to 97 hr. before the appearance of vesicular lesions. Virus production in the udder declined with the appearance of virus neutralizing activity in the blood and the milk but persisted in some animals for periods of 3-7 weeks. The ability of foot-and-mouth disease virus to persist in mammary tissue was confirmed by the demonstration of virus multiplication in the udders of immune animals.