A strain of foot-and-mouth disease virus was recovered from a cow at the height of the disease, and was propagated through at least 261 passages in the guinea pig. Considerably over 2000 animals proved susceptible to the virus, and the virus could be transferred at will back to cattle and hogs, and then again returned to guinea pigs. No natural immunity was discovered in guinea pigs. Secondary lesions were easily and regularly induced, thus making this strain particularly favorable to experimental purposes. In general, the guinea pig may therefore be regarded as the animal of choice for laboratory studies.
The guinea pig could be infected by different methods of injection in different sites, but constant and regular production of primary and secondary lesions—or generalization of the disease—followed intradermal "tunneling" in the manner described, combined with subcutaneous inoculation of the posterior hairless pads of full grown animals. As we have indicated, the virus was peculiarly epitheliotropic, which in turn gives support to the opinion that its portal of entry may be limited.
The active agent could purify itself of chance concomitant bacteria in the first passages, in a susceptible animal—a character possessed by filter-passing viruses in general.
The virus was active in dilutions of 1:10,000,000. This shows not only the minuteness of the active agent, but also the necessity for a change of technique from that employed with larger sized infectious agents. Apart from this, the dilution factor is important in interpreting mere preservation of the virus rather than multiplication, when only early successive subplants in culture experiments are positive. Furthermore, some samples of virus were not so active—a factor of twenty-five existed between the weakest and strongest samples among fifteen titrated. This indicates that comparative tests, as, for example, of survival in different media, should be made with the same specimen. In any case, the rate and energy of action of the virus were proportional to its concentration, thus differing from the behavior of certain enzymes.
The incitant is not sedimented by centrifugation. Non-centrifugability, a property of some other filter-passing, infectious agents, is not an indication of the fluid character of the virus, as we have already explained. In view of the evidence presented and other tests to be reported later, failure of deposition is related to the minute size of the incitant. The method of centrifugation has also failed to remove "virucidal bodies" in the meaning of Frosch and Dahmen.