Handbook of Medical Entomology

by William A. Riley; O. A. Johannsen

A type of purulent opthalmia which is very prevalent in Egypt is often said to be carried by flies. Nuttall and Jepson (1909) consider that the evidence regarding the spread of this disease by flies is conclusive and that the possibility of gonorrhœal secretions being likewise conveyed cannot be denied.

Many studies have been published, showing a marked agreement between the occurrence of typhoid and other intestinal diseases and the prevalence of house-flies. The most clear-cut of these are the studies of the Army Commission appointed to investigate the cause of epidemics of enteric fever in the volunteer camps in the Southern United States during the Spanish-American War. Though their findings as presented by Vaughan (1909), have been quoted very many times, they are so germane to our discussion that they will bear repetition:

"Flies swarmed over infected fecal matter in the pits and fed upon the food prepared for the soldiers in the mess tents. In some instances where lime had recently been sprinkled over the contents of the pits, flies with their feet whitened with lime were seen walking over the food." Under such conditions it is no wonder that "These pests had inflicted greater loss upon American soldiers than the arms of Spain."

Similar conditions prevailed in South Africa during the Boer War. Seamon believes that very much of the success of the Japanese in their fight against Russia was due to the rigid precautions taken to prevent the spread of disease by these insects and other means.

Veeder has pointed out that the characteristics of a typical fly-borne epidemic of typhoid are that it occurs in little neighborhood epidemics, extending by short leaps from house to house, without regard to water supply or anything else in common. It tends to follow the direction of prevailing winds (cf. the conclusions of Hindle and Merriman). It occurs during warm weather. Of course, when the epidemic is once well under way, other factors enter into its spread.

In general, flies may be said to be the chief agency in the spread of typhoid in villages and camps. In cities with modern sewer systems they are less important, though even under the best of such conditions, they are important factors. Howard has emphasized that in such cities there are still many uncared-for box privies and that, in addition, the deposition of feces overnight in uncared-for waste lots and alleys is common.

Not only unicellular organisms, such as bacteria and protozoa, but also the eggs, embryos and larvæ of parasitic worms have been found to be transported by house-flies. Ransom (1911) has found that Habronema muscæ, a nematode worm often found in adult flies, is the immature stage of a parasite occurring in the stomach of the horse. The eggs or embryos passing out with the feces of the horse, are taken up by fly larvæ and carried over to the imago stage.

Grassi (1883), Stiles (1889), Calandruccio (1906), and especially Nicoll (1911), have been the chief investigators of the ability of house-flies to carry the ova and embryos of human intestinal parasites. Graham-Smith (1913) summarizes the work along this line as follows:

"It is evident from the investigations that have been quoted that house-flies and other species are greatly attracted to the ova of parasitic worms contained in feces and other materials, and make great efforts to ingest them. Unless the ova are too large they often succeed, and the eggs are deposited uninjured in their feces, in some cases up to the third day at least. The eggs may also be carried on their legs or bodies. Under suitable conditions, food and fluids may be contaminated with the eggs of various parasitic worms by flies, and in one case infection of the human subject has been observed. Feces containing tape-worm segments may continue to be a source of infection for as long as a fortnight. Up to the present, however, there is no evidence to show what part flies play in the dissemination of parasitic worms under natural conditions."

Enough has been said to show that the house-fly must be dealt with as a direct menace to public health. Control measures are not merely matters of convenience but are of vital importance.