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Observations on Wild Rats in England 
Rat parasites were studied by Dr A. C. Stevenson and Mr Gerald Dudgeon. 
Dr Stevenson made the majority of the examinations for intestinal protozoa 
and determined the incidence of Leptospira infections. Mr Dudgeon carried 
out a study of the helminths and ecto-parasites. Dr Wenyon, with a view 
to determining if the rat harboured cultural flagellates other than trypano¬ 
somes, examined by culture in NNN medium the blood of a hundred rats, 
but found the only organism present was Trypanosoma lewisi. 
Throughout, our Senior Laboratory Assistant, Mr W. J. Muggleton, 
rendered valuable aid, arranging for the supply of material and assisting in 
all branches of the enquiry. 
Mr H. A. Baylis of the British Museum was good enough to aid Mr Dudgeon in 
the identification of worms and to advise on certain points connected with them. 
In all 478 wild rats were examined, of which 34 were Rattus rattus and the 
remainder Rattus norvegicus. No conclusion can be drawn from these figures 
as to the relative proportion of the two species, as the dealers only sold black 
rats when these were specially wanted, the skins of black rats being of greater 
value than those of grey rats. It seems also that the black rat is rarely captured 
save on board ships or at the docks, and this although a higher price is paid 
the rat-catchers for black rats. 
The rodents were for the most part obtained from dealers in London and 
were London rats, but specimens were also secured from such towns as 
Bedford, Brighton, Chester, Eastleigh (Hants), Horsham, Liverpool, Luton, 
Maidstone, Preston, Reading and Sandwich. Occasionally rats from country 
districts were secured and a few v r ere captured from ships at the London 
Docks. We were much indebted to Mr G. L. Moore of Maidstone for the kind 
help he gave us in obtaining country rats. 
The period of work covered more than a year. The investigation was started 
on August 16th, 1920, and, with a month’s intermission in August, 1921, ended 
on December 14th, 1921. 
Generally speaking the condition of the rodents was good. As a rule they 
were well nourished and, beyond showing recent and sometimes septic wounds 
and the scars of old lesions, rarely exhibited any external sign of injury or 
deformity. 
Every rat was weighed and this gave some idea regarding maturity. So 
far as female rats went, the almost invariable presence of corpora lutea . 
indicated that we were dealing with adults and it would seem that a minority 
of immature animals were examined. The weights of male Rattus rattus varied 
from 75 to 180 grammes and of females from 75 to 225 grammes, no pregnant 
black rats being found. The heaviest female black rat not showing corpora 
lutea weighed 118 grammes. 
The weight of male Rattus norvegicus ranged from 100 to 510 grammes and 
of females from 85 to 470 grammes, excluding pregnant animals. 
The heaviest female brown rat not showing corpora lutea turned the scale 
at 170 grammes. 
