The infectivity assay of foot-and-mouth disease virus in pigs

and of resistance, either natural or acquired, possessed by an individual or a group of animals. The method of titration of foot-and-mouth disease virus (FMDV) in cattle by inoculation of the tongue (Henderson, 1949) has formed the basis for many of the standard procedures used in FMDV research involving cattle. Comparatively little work, however, has been recorded on the development of suitable methods of titration of FMDV in pigs. Brooksby (1950) inoculated individual animals with single dilutions of virus intradermally in the tongue and snout in his studies of virus strains showing natural adaptation to swine. GraJves & CunliSe (1960) carried out some comparative work on the sensitivity of pigs to different routes of inoculation and concluded that the inoculation of the coronary band region of the foot was the most sensitive £or the detection of virus. They developed a titration scheme using the simultaneous inoculation of several dilutions of virus into individual animals and showed that their results were reproducible within good statistical limits. Lucam, Dhennin, Dhennin & Fedida (1962) subsequently compared titration end-points using coronary-band and snout inoculation and confirmed the high sensitivity of the coronary-band region. Burrows (Communication to the Research Group of the Standing Technical Committee of the European Commission for the Control of Foot-and-Mouth-Disease, June 1963, summarized in F.A.O. document 15766/E) reported that the bulb of the heel of the foot was a convenient and highly sensitive site of inoculation for titration, innocuity testing and challenge of immunity procedures. Some details of the use of this method for these purposes have been recorded (Girard et al. 1963; Burrows, 1964a, b). It is desirable, when proposing an a.ssay method, to attempt to determine the precision of the method and also to compare the results with those given by established titration procedures. This paper presents the results of a number of comparative titrations of some FMDV strains in cattle, mice and pigs, with particular reference to the results obtained in pigs by the bulb of the heel inoculation method.