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FREQUENCY OF STRAINS OF FOOT-AND-MOUTH DISEASE
[Vol. XXI, Part IV
variant nature of the strain is only a temporary phase and thus cannot be classified
as a fixed type. Again according to Daubney [Hutyra and Marek], we are coming
to the stage regarding the virus of foot-and-mouth disease as an extremely liable
virus liable to mutations or changes which may completely alter the antigenic pro-
perties of any given strain.
METHODS AND MATERIALS
Under a Scheme financed by the Indian Council of Agricultural Research,
investigations were started in May 1943 to evolve a suitable vaccine for protecting
the Indian cattle against foot-and-mouth disease. To find out first the type or
types of the virus responsible for the spontaneous outbreaks of the disease in this
country, arrangements were made with the State authorities to collect numbers of
field strains of the virus. Tubes containing glycerine phosphate buffer pH 7.6 were
distributed to the Veterinary Investigation Officers for collecting the virus in the
form of unruptured vesicles or scrapings from the lesions of affected animals. The
three standard strains obtained from the British foot-and-mouth Research Station,
Pirbright, in 1943 have since been maintained in guinea-pigs. The purity of these
strains was tested from time to time and they were found to have remained distinct
from one another over 62, 69 and 61 passages respectively [Seetharaman, 1949].
Eversince Waldmann and Pape [1920] reported the successful transmission of the
disease to guinea-pigs, work on type determination and even quantitative studies on
foot-and-mouth disease virus [Henderson, 1949] has been conducted using guinea-
pigs because of the ease, convenience and economy with which they can be liberally
used. Since these animals do not contract the disease by cohabitation many strains
of the virus can be handled in one place without any fear of their getting mixed up.
Further, because of the wide prevalence of the disease in cattle, it is difficult to
procure clean animals for typing work. It is generally accepted that the type
determination carried out using guinea-pigs is as good as with cattle. In all the type
determination experiments described in this note, guinea-pigs were used although
sometimes it was found necessary to passage the virus received from the field in
cattle before it could be successfully transmitted to guinea-pigs. A total of 210
specimens received from the field were examined out of which only 62 proved viable
on receipt at Mukteswar.
The materials on receipt were emulsified with phosphate buffer solution pH 7.6
and inoculated intradermally in the hind foot pads of guinea-pigs. After 24-96
hours depending upon the virulence of the virus the guinea.pigs developed blisters
which also contained lymph. The foot pads were stripped, emulsified as above and
inoculated in another set of guinea-pigs. By repeating this process four to six times
the field strains were adapted to guinea-pigs. Subsequently, guinea-pigs (immunised
against unknown field strains) were tested against Vallee 'A' and 'O' and Wald-
mann 'C' strains of the virus. By means of the immunological reactions produced
in the guinea-pigs the types of the field strains were determined.
OBSERVATIONS
During type determination it was observed that besides the occurrence of the
typical 'O', 'A' and 'C' strains, variants of these strains also occurred. There was