294 
Observations on Wild Rats in England 
The general findings were as follows: 
The urine of 154 brown rats was examined and leptospira-like organisms 
were seen on twenty-one occasions, i.e. in 13-6 per cent, of the cases. The urine 
of black rats proved invariably negative but it was only possible to make 
the examination in fifteen instances. 
The kidney examinations by smears and sections from brown rats yielded 
positive results in 22*6 per cent. The few black rats examined in this way were 
negative. 
At first a systematic series of inoculations of guinea pigs either with urine 
or kidney and liver emulsion was carried out. Twenty-one animals in all were 
inoculated, two with urine and nineteen with tissue emulsion. The former 
remained unaffected, of the latter three became infected and died of typical 
spirochaetal jaundice. 
Thereafter it was decided to inoculate guinea pigs only from rats in which 
leptospira had been found. Five such experiments were conducted, in three 
cases infected urine being used and in two leptospira-containing tissue emulsion. 
Of the urine cases one was positive and it is interesting to note that another 
guinea pig inoculated in a similar manner and with an equal dose of the same 
urine remained uninfected. 
Of the emulsion cases one was positive and here again two guinea pigs 
were used, one of which yielded a negative result. 
These findings tend to show that there may be a true insusceptibility to 
infection. 
Of a total of 40 rats yielding positive results by the inoculation method 
and by the examination of smears and sections from the kidney, four were 
from places outside London, two being from Maidstone, one from Colchester 
and one from Preston. In this connection Dr Stevenson’s statement in his 
paper above quoted, to the effect that a positive rat came from Cheshire, is 
incorrect. He should have stated Lancashire—the rat to which reference is 
made being that from Preston. 
The investigations of Foulerton on London rats indicated that there was 
a distinct seasonal incidence. In rats examined by the inoculation method 
between July 12th and November 18th, 1918, he obtained positive results only 
during November, a failure which may have been due to guinea pig insuscepti¬ 
bility. In our observations there is also some indication of a limitation ac¬ 
cording to season in cases where infection was proved by inoculation, the 
months in which positive results were obtained being October, November and 
December. On the other hand, if the whole series of positive cases is con¬ 
sidered there is evidence that rats harbour leptospira throughout the year. 
One record is of special interest as showing how a guinea pig may perhaps 
act as a carrier without exhibiting any visible signs of disease beyond a 
transient rise of temperature and thus coming into line with the rat itself. 
Eat 215, Rattus norvegicus, female, non-pregnant, weighing 230 grammes, from 
Cheapside, was found by dark field examination to have leptospira in the urine. 
