ENTRANCE OF PATHOGENIC BACTERIA 51 



Many of the diseases caused by spirochaetes, an important 

 group of pathogenic organisms the place of which in the scale 

 of classification is still somewhat doubtful, are carried by biting 

 ticks. Thus African Relapsing or Tick-fever, due to the 

 Spirochaeta duttoni, is carried by the tick, Ornithodorus moubata; 

 and numerous spirochaete infections of the lower animals, for 

 example among fowls, are also carried by ticks. 



One of the best known and most important of insect-carried 

 diseases is Bubonic Plague, which is transmitted from its usual 

 victim the rat, to man by the bite of the rat-flea, which, when 

 its host the rat dies, migrates from the dead body and, if it 

 cannot find another rat on which to feed, will attack and infect 

 man. The plague bacillus can be seen microscopically in the 

 rat-flea's stomach, in which it may be found up to the number 

 of some five thousand or more. 



Anthrax may be carried from sheep to sheep by the louse ; 

 Yellow Fever, caused by a filter-passing organism, is inoculated 

 into man by the Stegomyia mosquito, and Typhus Fever, also 

 due to a filter-passer, is believed to be carried by the body- 

 louse. Many other instances of insect-borne disease might be 

 given, but it will suffice to mention, along with the above where 

 biting insects are the carriers, the case of insects, especially 

 flies, which carry disease-producing germs upon their feet or 

 wings or proboscis, or especially in their alimentary canal, 

 and so contaminate food or dishes and transmit infection to 

 man. When the breeding- and feeding-places of flies, e.g. the 

 ordinary domestic fly, are considered, the marvel is that more 

 diseases are not so carried. Summer diarrhoea of infants, 

 typhoid fever, dysentery, cholera, and similar diseases, and 

 perhaps also tuberculosis, are from recent investigation now 

 regarded as being probably extensively carried and distributed 

 by common house and other flies, and hence all food should 

 be carefully protected from them. 



Food may be contaminated with pathogenic organisms in an 

 enormous number of ways. The animal or vegetable from 

 which the food is derived may be itself diseased. For example, 

 some epidemics of meat-poisoning are due to the presence of 

 certain bacteria belonging to the Coli-Typhoid group, e.g. 

 B. enteritidis of Gaertner and its close relatives, B. paratyphosus 

 and others. Anthrax may be communicated to those working 

 with the carcases of sheep, cattle, etc., which have died from 

 the disease, butchers, tanners, dock-porters and wool-sorters 

 being among those most liable to suffer from the infection. 



By far the most important infection, however, in this country 

 which is extensively spread from diseased animals to man, is 

 Tuberculosis. In this connection the actual diseased tissues 



