Handbook of Medical Entomology

by William A. Riley; O. A. Johannsen

The above cited work is of a general nature, but, especially in recent years, many attempts have been made to determine more specifically the ability of flies to transmit pathogenic organisms. The critical reviews of Nuttall and Jepson (1909), Howard (1911), and Graham-Smith (1913) should be consulted by the student of the subject. We can only cite here a few of the more striking experiments.

Celli (1888) fed flies on pure cultures of Bacillus typhosus and declared that he was able to recover these organisms from the intestinal contents and excrement.

Firth and Horrocks (1902), cited by Nuttall and Jepson, "kept Musca domestica (also bluebottles) in a large box measuring 4 × 3 × 3 feet, with one side made of glass. They were fed on material contaminated with cultures of B. typhosus. Agar plates, litmus, glucose broth and a sheet of clean paper were at the same time exposed in the box. After a few days the plates and broth were removed and incubated with a positive result." Graham-Smith (1910) "carried out experiments with large numbers of flies kept in gauze cages and fed for eight hours on emulsions of B. typhosus in syrup. After that time the infested syrup was removed and the flies were fed on plain syrup. B. typhosus was isolated up to 48 hours (but not later) from emulsions of their feces and from plates over which they walked."

Several other workers, notably Hamilton (1903), Ficker (1903), Bertarelli (1910) Faichnie (1909), and Cochrane (1912), have isolated B. typhosus from "wild" flies, naturally infected. The papers of Faichnie and of Cochrane we have not seen, but they are quoted in extenso by Graham-Smith (1913).

On the whole, the evidence is conclusive that typhoid germs not only may be accidentally carried on the bodies of house-flies but may pass through their bodies and be scattered in a viable condition in the feces of the fly for at least two days after feeding. Similar, results have been reached in experiments with cholera, tuberculosis and yaws, the last-mentioned being a spirochæte disease. Darling (1913) has shown that murrina, a trypanosome disease of horses and mules in the Canal zone is transmitted by house-flies which feed upon excoriated patches of diseased animals and then pass to cuts and galls of healthy animals.

Since it is clear that flies are abundantly able to disseminate viable pathogenic bacteria, it is important to consider whether they have access to such organisms in nature. A consideration of the method of spread of typhoid will serve to illustrate the way in which flies may play an important rôle.

Typhoid fever is a specific disease caused by Bacillus typhosus, and by it alone. The causative organism is to be found in the excrement and urine of patients suffering from the disease. More than that, it is often present in the dejecta for days, weeks, or even months and years, after the individual has recovered from the disease. Individuals so infested are known as "typhoid carriers" and they, together with those suffering from mild cases, or "walking typhoid," are a constant menace to the health of the community in which they are found.

Human excrement is greedily visited by flies, both for feeding and for ovipositing. The discharges of typhoid patients, or of chronic "carriers," when passed in the open, in box privies, or camp latrines, or the like, serve to contaminate myriads of the insects which may then spread the germ to human food and drink. Other intestinal diseases may be similarly spread. There is abundant epidæmiological evidence that infantile diarrhœa, dysentery, and cholera may be so spread.

Stiles and Keister (1913) have shown that spores of Lamblia intestinalis, a flagellate protozoan living in the human intestine, may be carried by house-flies. Though this species is not normally pathogenic, one or more species of Entamœba are the cause of a type of a highly fatal tropical dysentery. Concerning it, and another protozoan parasite of man, they say, "If flies can carry Lamblia spores measuring 10 to 7µ, and bacteria that are much smaller, and particles of lime that are much larger, there is no ground to assume that flies may not carry Entamœba and Trichomonas spores."

Tuberculosis is one of the diseases which it is quite conceivable may be carried occasionally. The sputum of tubercular patients is very attractive to flies, and various workers, notably Graham-Smith, have found that Musca domestica may distribute the bacillus for several days after feeding on infected material.