September, 1957] M. R. DHANDA AND V. R. GOPALAKRISHNAN 81
A noteworthy observation in regard to the atypical strains in general is their
easy and rapid adaptability to guinea pigs. From the second passage they are adap-
ted to guinea pigs, showing the characteristic primary lesions followed by generaliza-
tion.
Cross-immunity test
All the cross-immunity tests were carried out in guinea pigs. First, guinea pigs
immune to standard type "O", " A " and " C " (Pirbright) and those against other
atypical strains were tested with the Rawalpindi atypical strain (Table II). Then
guinea pigs immune to Rawalpindi strain of virus were tested with standard types
" O ", " A " and " C " (Table III). Again, Izatnagar Dairy atypical strain was
given to immune guinea pigs of all the strains (Table IV). Similarly, the guinea
pigs immune to all the strains under experiment were tested with Izatnagar Animal
Nutrition Division atypical strain (TableV).
The observation of the tests were :
(1) Each atypical strain evidently had homologous immunity.
(2) All the atypical strains broke immunity against the three standard types,
each of which in turn broke immunity against the atypical strains.
(3) The three Izatnagar atypical strains were immunologically identical.
(4) Rawalpindi atypical strain was immunologically distinct from the Izatnagar
atypical strain.
DISCUSSION
The occurrence of the three standard types and their variants causing outbreak
of the disease has been observed in this country.
Apparently atypical strains, immunologically distinct from one another
caused repeated outbreaks of the disease at Izatnagar causing difficulties for the
control of foot-and-mouth disease by vaccination. It has been shown that variants
of the same type have different antigenic structures and the antigenic properties in
turn vary from strain to strain [Ubertini, 1951]. A recent epizootic in Europe was
characterised by the spontaneous appearance of variants of the virus [Ramon, 1952]
and vaccines failed to control the spread of infection due to the presence of
variants of type, antigenically different from one another [Lancet, 1952].
It is generally believed that variants and perhaps atypical strains are heterologous
composites derived for the standard types. When the question of control by
vaccination of the epizootic caused by variants of types is so unsatisfactory,
the control of the disease caused by atypical strains would be more complicated.
NOTE : Since this article was sent out for publication, the Rawalpindi strain, the Izatnagar 51
and 52 strains were sent to the Virus Research Institute, Pirbright (England) for confirmation of the
findings. The atypical character of the Izatnagar strains has been confirmed by the workers at Pir-
bright, whereas in respect of the Rawalpindi strain it was reported to correspond to classical type 'O'
virus. On scrutiny of the record of the Rawalpindi strain, it was found that it was typed last at this
Institute at guinea pig passage No. 58 and was atypical in character beyond any doubt. The material
despatched to Pirbright was from guinea pig passage No. 74 . Since the chances for the cross contami-
nation of the virus while being handled in the laboratory are considered very remote, it appears, that
the virus changed in its antigenic character somewhere between passage No. 58 and 74 by shedding
off its heterologous components and reverted to the type 'O' virus.
The atypical Izatnagar strain has since been designated as Asia I.