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Again, mosquitoes of the Culex variety have been examined for plague bacilli but always with a negative result.
Pediculi of several species also gave negative results.
Bugs, on the other hand, I have found, according to the methods already described, to harbour plague bacilli on the surface of their bodies. Cockroaches (Blatta orientalis) also preserve plague bacilli on their body surfaces.
This again is important, as the disgusting insect is found in large numbers, especially in store rooms and cupboards. "About the same time as I was carrying on my experi- ments. similar observations were being made in the Health Department of Bris- bane by Mr. C. J. POUND. Extremely strong evidence is brought forward in favour of the role played by cockroaches in the transmission of plague. The following was noted by Mr. POUND. "In a room specially set apart for keeping all inoculated animals were two large stands with wide shelves on which long lead lined trays about two inches deep, and containing carbolic acid, were placed. Standing in these trays and surrounded by the carbolic acid solution were various strong glass jars containing experimental animals. Each jar contained only one animal-a Guinea pig, rat or mouse. The jars were covered with a mosquito proof fine wire gauze lid. Qu'one occasion a healthy Guinea pig, that was being kept as à control for certain experiments, suddenly became sick and after three days it died, Post-mortem and bacteriological examination proved that this Guinea pig had died from a generalised form of plague, but no lesion was found to indicate that it had been infected through the skin. A careful examination revealed the fact that in the zinc binding of the wire cover there were several very young cockroaches. These were promptly destroyed. On examination of the covers of the other adjoining jars, more young cockroaches were discovered. These cockroaches had become hidden in the zinc lining of the covers when the jars were not in use, and standing on the shelf alone, unprotected by the tray of carbolic solution.
As soon as a jar was occupied by an experimental animal the cockroaches that had been hidden from view in the zinc lining during the day time would, after dark, crawl down the inside of the jar and feed on the animals' food. Apparently, before their presence was discovered some of these cockroaches had fallen from a jar contain- ing a plague infected animal into the carbolic solution, and then swam either to the jar containing the healthy Guinea pig or to the side of the tray, and then hid away in the cover of an empty jar. In any case it was more than probable that the food had become contaminated with plague bacilli.
In order to ascertain whether the cockroaches had anything to do with the transmission of plague, a healthy Guinea pig was placed in a sterilised jar covered with the usual wire lid, but whose zinc lining was free from cockroaches. The jar was placed on the shelf, but not on the tray. In the course of a few days young cockroaches made their appearance and, as usual. lived during the day time in the zine lining. Eventually the Guinea pig sickened and died of plague.
After this experience, the whole of the building and everything such as shelves. benches. jar, etc., were subjected to thorough and repeated disinfection and all holes and crevices carefully closed. The result is that no cockroaches have been seen since, and although every day during the past eight months numbers of plague and healthy Guinea pigs and rats have been kept in the same jars and standing in the same trays, no symptom of the disease has appeared in any animal unless specially infected.”
In my opinion the lesson taught us by this experiment is of great importance. My own result showed the presence of plague bacilli on the body surfaces of this insect, and also in its alimentary canal. Coupling these two results together, therefore, we have strong evidence in favour of the fact that cockroaches dis- seminate plague bacilli. As is well known these insects are most frequently found in houses, and particularly in places where food stuffs are conserved. A conclusion which appears justified from these results, is that in all probability cockroaches play an important role in plaque infected districts, and that extreme danger exists when these insects can gain access to plague infected places, and further that in all probability food stuffs are frequently plague infected by contact with cockroaches on or in which plague bacilli are present.