The scope of this work is to discuss the necessity
for the quantitative study of a filterable virus, the
methods available for such a study and those applicable
to foot-and-mouth disease virus; to discuss the use of
the guinea -pig and its limitations, with the description
of the evolution of a method of study using cattle, with
a discussion of the effectiveness of the method as at
resent devised; and to present the information gained
as a result of these quantitative studies.
A review is given of the methods available for the
quantitative study of filterable viruses and those
methods applicable to the virus of foot-and-mouth disease.
The quantitative study of foot-and-mouth disease virus depends on the detection of the infective agent in progressive dilutions of the virus suspension by inoculation of susceptible animals.
Since the discovery of the susceptibility of the guinea-pig to the virus, this animal has been largely used in tests for its detection.
The usefulness of the guinea-pig for this work is
limited by the fact that strains of foot-and-mouth
disease virus possess a strong species adaptation.
This adaptation to a particular species may usually be modified by serial passage in another species.
Until a virus strain has become adapted to the
guinea-pig, this animal cannot be used for the detection of small amounts of the virus of that particular strain. For example, the limiting infective dilution of bovine strains is from ten to a thousandfold lower when determined by using guinea-pigs compared with cattle.
For the quantitative study of bovine strains of
foot-and-mouth disease virus methods of titration using cattle have been evolved.
The most successful of these methods consists of the simultaneous inoculation of the bovine tongue with different dilutions of the virus suspension. The routine method of titration eventually adopted consists of the simultaneous inoculation of the tongues of two cattle with four dilutions using five inoculation sites on each tongue for each dilution, thus providing ten observations from two animals for each of four dilutions.
The end-point of such a titration is expressed as
the theoretical dilution that should give an equal
umber of positive and negative results when inoculated into the same host species under the same conditions, the fifty per cent. positive end-point.
Using two Devon steers, 1.5 to 2 years old, the estimated error of such a test is ± 0.34 of the logarithmic scale of the dilutions.
A method is described of the graphical representation of the result of a titration experiment by plotting the dilutions on a logarithmic scale against the units of probability, or "probits".
A brief report is given of unsuccessful attempts
to improve the technique used in quantitative studies of foot-and-mouth disease virus.
The performing of these methods of titration using cattle is discussed in connection with the difficulty of obtaining susceptible animals in countries where the disease is endemic.
The mean fifty per cent. positive end-point of
vesicle lymph of well-adapted guinea-pig strains titrated in guinea-pigs was found to be 10⁻⁶.²⁶.
The mean fifty per cent. positive end-points of
bovine defibrinated blood, bovine vesicle epithelium and bovine vesicle lymph titrated in cattle were found to be 10⁻³.⁵, 10⁻⁶.⁷ and 10⁻⁷.⁸. respectively.