September, 1954]
ABSTRACTS
179
around the 70th hour, a fall was noticed, when these were destroyed for harvesting
the virus. On autopsy the lesions were found to resemble those described by other
workers. The virus was found 'fixed' for the local breed of the rabbits and during
the 21 passages at Kabeta no change in virulence for rabbits or attenuation of the
antigenic properties for cattle was observed. Rabbits under 4½ months of age and
pregnant does were not found to be suitable subjects for virus propagation. Freeze-
dried virus stored in vacuo in the refrigerator at between minus 20°C. and minus
30°C. was seen to remain viable for 25 months, but when kept at room temperature
(Maximum 24°C, Minimum 18°C.) it maintained sufficiently high titre for seven
clays. Alternate passage of the virus in cattle and rabbits appeared to alter the
character of the virus, which showed a decreased virulence for rabbits with an
irregular and somewhat increased virulence for cattle. No evidence was found
to indicate that the virus would pass from infected rabbits to susceptible rabbits,
or from infected cattle to susceptible cattle.
A series of titration experiments in cattle and rabbits with fresh as well as
dried virus were carried out; It was observed that 0.0000005 gm. of fresh virus
in 0.5 c.c. of saline caused the typical syndrome in rabbits when injected intravenously
and that the minimum quantity required to immunise an ox was 0.001 gm. in one c.c.
of saline. With the freeze-dried virus the minimum infective dose for a rabbit
and the minimum protective dose for an ox were found to be 0.00001 gm. and 0.001
respectively. However, by improving the technique and using more efficient
methods of freeze drying, the minimum protective dose of the dried virus for an
ox was lowered from 0.001 gm. to 0.0001 gm. In contra-distinction to goat-adapted
virus, lapinised virus was found to provoke a very mild reaction, in high and medium
grade cattle at Kabete which could not be readily recognised. The development
of an immunity in cattle so inoculated could not be determined until challenged
with the virulent bovine rinderpest virus. The immunity was found to be sufficient
to prevent clinical symptoms of rinderpest after challenge with virulent virus.
It was further observed that lapinised virus induced a serviceable protection in
cattle, when challenged 84 and 108 hours respectively after inoculation.
II. Indigenous Zebu cattle, high and low grade crosses of European breeds
with the Zebu cattle, pure bred European cattle and hypersusceptible Ankole cattle
were successfully immunised against rinderpest with freeze-dried lapinised virus
vaccine. Of these, only the Ankole cattle were found to exhibit a marked clinical
reaction. When vaccinated cattle were challenged by the subcutaneous inoculation
of virulent rinderpest virus, the duration of immunity was shown to last for between
8 and 15 months and when tested by exposure to cattle infected with rinderpest
the immunity lasted for 13 months.
The injection of lapinised virus was found to have no adverse effect on pregnant
cattle, and did not reduce the yield of cows in milk. Many thousands of cattle
had been vaccinated with the lapinised virus but so far comparatively few had been
tested for immunity. However, when lapinised virus had been used in Kenya
in areas where natural rinderpest was present, it had not even once failed to protect
the cattle. On one occasion in a herd of 1886 grade cattle, some of which were
infected with rinderpest vaccination curtailed the extension of the disease and
considerably reduced the fatalities. (R.N.S.)