ABSTRACTS
71
Of all the existing lines, susceptible and resistant to natural B. enteritidis infection,
only susceptible lines 1 and 2, which showed a relatively stable mortality rate of ap-
proximately 85 per cent from the outset and resistant lines 1 and 2, which showed a
fluctuating mortality rate of about 15 to 20 per cent were retained for further selective
breeding and the rest were discarded. Unselected controls showed a relatively stable
mortality rate of 37 per cent. Selections for breeding were made as previously, from
litters unexposed to infection, thereby insuring against introduction or persistence of the
infection in the stock. The method has been to remove the first and often the second
litter from the breeding room at 4 to 6 weeks of age and test each with B. enteritidis or
virus. If the mortality was maximum or minimum as required, an additional sibling
litter was selected and mated brother to sister without testing. First and second litters of
the succeeding generations were then tested in the same manner and a third litter chosen
for mating. The mouse colony was subjected to continued search for infectious agents.
The occasional sickly mouse was sacrificed and tested for the presence of pathogenic
bacteria or virus. Faeces from breeders were tested 6 to 12 times a year for mouse
typhoid.
Selections of mice for breeding had been made entirely on basis of maximum sus-
ceptibility or resistance of progeny to B. enteritidis. The susceptible and resistant lines
which developed were later found to be susceptible and resistant respectively to Pasteurel¬
la avicida, B. friedlanderi and pneumococci administered intranasally. Accordingly the
former has been designated as bacteria-susceptible (BS) lines and the latter as bacteria-
resistant (BR). But, in 1932 and 1933, when it was shown that BS lines were resistant to
louping ill virus and BR. lines susceptible, it became necessary to complicate the termi-
nology further and to identify the one as bacteria-susceptible-virus-resistant (BSVR)
lines and the other as bacteria-resistant-virus-susceptible (BRVS) lines. Finding the
mortalities of BSVR lines to louping ill virus averaging about 50 per cent and often
concentrated only in certain litters led the author to attempt to develop sub-lines highly
resistant to virus (BSVR) and highly susceptible to virus (BSVS) respectively. Parents
whose progeny had shown least and greatest mortality following nasal instillation of
louping ill virus were selected for further breeding ; all others in these bacteria-susceptible
lines were discarded. Subsequent tests were made with St. Louis encephalitis virus.
Rigid selection, testing and discarding procedures on approximately 13,200 mice
for twelve generations from 1930 to November 1934, resulted in three inbred lines which
have shown percentage mortalities to B. enteritidis and virus averaging as follows:
BSVS, 95.2 and 86.3 per cent; BSVR, 96.2 and 6.7 per cent; BRVS, 17.3 and 96.6
per cent respectively. High susceptibility proved relatively stable throughout, while
high resistance appeared to fluctuate from generation to generation. The susceptible
lines showed high mortalities following doses as little as 1/100th of the standard and the
resistant lines showed low mortalities following doses 100 times the standard. Conse-
quently, the resistant lines, both bacterial and virus, were considered to withstand 1,000
to 10,000 times the dose fatal for susceptibles.
The results of the cross-breeding tests support the selective breeding data in indi-
cating that resistance to these infections is controlled basically by inherited factors
and confirm the previous findings that:
(1) the factors are not sex-linked,
(2) resistance is dominant over susceptibility,