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(b.) Introduction of Viri into the Intestine of Insects.-Just as in the case of germs on the surface of the body of insects, so also do we find bacteria of the most varied species in the alimentary canal of these animals. The majority of such micro-organisms are harmless and non-pathogenic for man.
In certain cases, however, bacteria may be present in the digestive tract and excrement of insects which are capable of setting up disease in man. By their presence in the fœces of these animals, such pathogenic micro-organisms may become widely scattered in nature, and the role played by such insects of the greatest importance in regard to the spread of certain diseases. The evidence is very strong that infectious material may be widely disseminated in nature through the excrement of insects. Research and experimental study have pointed to the great possibility of such an
occurrence in nature.
SPILLMANN and HAUSHALTER (BAUMGARTEN'S Jahresberichte, Bd. III, 1887) have found the Tubercle Bacillus in the forces of flies fed on tubercular sputum and a searching investigation by CELLI (Bull. del. Soc. Lancis. deg. osped d. Roma. Fasc. I, 1888) showed that by feeding experiments, the tubercle bacillus, vibrio choler, bacillus typhosus, bacillus anthracis, and staphylococcus pyogenes aureus, could be recovered from the dejecta of the artificially infected flies. SAWTCHENKI (Revue d'Hygiène, Tome XV, 1872) succeeded in cultivating the vibrio choleræ from the faces of flies fed on cholera bouillon. So far my own experiments have been limited to the plague bacillus. After numerous ex-
periments, the B. pestis has been found in the alimentary canal and fœces of flies. Such plague bacilli were found to be virulent, and set up typical plague in artifi- cially inoculated animals, e.g., the rat. Large numbers of the flies caught in the Public Mortuary were found to contain plague bacilli in their fœces. Of those caught in the Infectious Diseases Hospital, a much less number were found infected in this way. These facts alone, prove how widely such insects as flies may disseminate the specific germs, and show us that in plague infected areas, every precaution possible ought to be taken to prevent access of flies to infected material.
Again, I have obtained similar positive results with bugs, spiders, and a few cockroaches. These experiments show us the possibility of micro-organisms finding a refuge in the body of an insect where it may multiply indefinitely and by way of its excrement distribute its progeny over wide areas.
That is to say, given such an occurrence we have before us a means whereby any particular species of pathogenic micro-organism may become widely diffused in nature and give rise to widespread disease. In Hongkong. where insects abound and become in themselves a jest, ample opportunity is afforded for the spread of plague by such means. Foods, articles of diet, cooking utensils, fruit, etc., are constantly being attacked by such insects, and should the latter be infected or their infected dejecta be deposited on such articles, the conditions necessary for the spread of the disease are complete. Many of the foods consumed are cooked previous to their being used, but fruits and many articles of diet used by the Chinese which are swallowed in the uncooked or cold condition are liable to the grossest contamination.
(c.) The Virulence of the Virus infecting the Insect.-In connection with this point, it is important to bear in mind that although the infection of the insect is natural and the bacteria in question are living and virulent, there is a vast difference so far as the possibility of conveyance or infection is concerned between micro- organisms which are deposited on the surface of the body and those which are introduced into the alimentary canal. HEIM (Compt. Rend. No. 3, 1894) has found living and virulent anthrax bacilli on the surface of the body of various insects. PROUST (Bullet. de l'Aca émie de Médicine, 1894), YERSIN (Annal. d'Hygiène, 1899) and others have made similar observations. SIMMOND (Annal. Pasteur, l'ome 12, 1898) found the vibrio choleræ deposited on the surface of the bodies of flies. The effects produced by drying, killed these vibrios in 11⁄2 hours.
In regard to the intestine, the reports of different investigators vary. CELLI (Bulletin Lancisiana, 1888) and ALESSI have found living anthrax bacilli in the dejections of flies. SAWTCHENKI, (Rev. d'Hyg. T. 15, 1892), HOFMANN (Korres- pondenz-blatt, Sachsen, 1888, No. 12) and others found cholera vibriò in the intestines of insects.