In Vitro Cultivation of the Mexican Strain (Vallée “A” Type) of Foot-and-Mouth Disease Virus

A method is described for the successful propagation of a Mexican strain (MP-VI) of the Vallee "A" type of foot-and-mouth disease virus. This virus was carried through 40 tissue culture serial passages. The cultures routinely showed virus titers, in susceptible cattle, in a range of$10^{5.0}$to$10^{6.5}$. Eight additional culture series were carried successfully through 8 to 12 passages under identical conditions. A 10th culture series failed to grow virus beyond the 3rd serial passage. The method that proved successful for the propagation of the virus in vitro employed Roux flasks as culture containers. To 100 cc of culture medium were added from 4 to 25 g of equal parts of upper and lower layers of minced bovine tongue epithelium. The flasks were inoculated with from 1 to 15 of seed virus and then laid on their sides. in an incubator maintained at 37 C for 24 hours. During this period the cultures were aerated by a slow flow of equal parts of oxygen and nitrogen that replenished the atmosphere above the surface of the culture. Although 10 to 15 cc of seed virus were used for routine inoculation in culture passages, 1 cc of seed virus was later found to be sufficient. Cultures prepared with 4 g of equal parts of upper and lower layers of bovine tongue epithelium showed virus titers as high as in cultures prepared in amounts up to 25 g. When a modification of Baker's fluid was tried as the medium for propagation of the virus in tissue cultures the virus failed to grow on serial passage. When 20% ox serum ultrafiltrate was added to the modified medium it then contained the essential ingredients for virus growth. The addition of nicotinic acid, pantothenic acid, riboflavin, thiamine chloride, pyridoxine, arginine and lysine to the medium failed to enhance the growth of virus. There was no evidence as determined by cross immunity and complement fixation tests that the serial propagation of the MP-VI strain of virus in tissue cultures changed its antigenic structure or produced aberrant forms. Cattle recovered from induced infection with MP-VI virus, that had been serially passed through tissue cultures, were solidly immune when challenged with native MP-VI virus and the homologous tissue culture virus. Virus propagated in tissue culture was used to produce an aluminum absorbate vaccine similar to that made with virus propagated in the tongues of live cattle. It was found on limited trialsto produce satisfactory immunity in cattle exposed to the native MP- VI