The common house-fly has been known for some time to be an active agent in the dissemination of bacterial diseases. In intestinal disorders—such as cholera and enteric fevers, which are caused by micro-organisms, the flies convey the bacteria from the dejecta of the sick to the food of the healthy. In the recent war in South Africa they are described in the standing camps as dividing their activities ‘between the latrines and the men’s mess-tins and jam rations.’[9] In the Spanish-American War in Cuba, and in the South African War, and in several recent outbreaks of enteric fever in the British army in India, flies have been proved to be the carriers of the Bacillus typhosus. Dr. Veeder[10] writes:
‘In a very few minutes they may load themselves with dejections from a typhoid or dysenteric patient, not yet sick enough to be in hospital or under observation, and carry the poison so taken up into the very midst of the food and water ready for use at the next meal. There is no long roundabout process involved. It is very plain and direct; yet when thousands of lives are at stake in this way the danger passes unnoticed.’
Similar records come from the Boer camp at Diyatalawa in Ceylon. The bacilli are conveyed direct, just as they might be by an inoculating needle. They do not pass into the body of the fly, neither do they undergo any part of their life-history in its tissue.
Dr. Sandilands[11] has recently investigated outbreaks of epidemic diarrhœa. He points out that the prevalence of diarrhœa follows the earth’s temperature, and does not follow the temperature of the atmosphere. It is a well-known fact that this illness is more prevalent in the houses of the poor than in the mansions of the rich. As Dr. Newsholme, late Medical Officer of Health for Brighton, said:
‘The sugar used in sweetening milk is often black with flies which have come from neighbouring dust-bins or manure heaps; often from the liquid stools of diarrhœa patients in the neighbouring houses. Flies have to be picked out of the half-emptied can of condensed milk before it can be used for the next meal. When we remember the personal uncleanliness of some mothers, and that they often prepare their infants’ food with unwashed hands, the inoculation of this food with virulent colon bacilli of human origin ceases to be a matter of surprise.’
Compared with cow’s milk, which nourishes a very numerous progeny of bacteria, the bacterial content of Nestlé’s milk is very low, according to Dr. Sandilands. In certain seasons the cow’s milk is exposed to temperatures which favour an enormous multiplication of bacteria, and yet it is not then a frequent source of diarrhœa—in fact, mere numbers have little or no influence on the incidence of the illness. The greater number of cases are due to infection conveyed from some patient in the near neighbourhood and conveyed mechanically by flies.
The great attraction of the sweetened condensed milk for flies to some extent explains the greater prevalence of infantile diarrhœa among children fed on this preparation.
As was stated above, one of the most remarkable features in the prevalence of infantile diarrhœa is that it follows the rise and fall of the earth’s temperature, and not that of the air. In the same way the number of house-flies does not reach its maximum with the first burst of hot weather. The prevalence of these insects follows rather than coincides with periods of great heat. The flies, in fact, lag behind the air temperature and persist for a time after the hot weather has ceased. In other words, the meteorological conditions associated with an increase or a diminution of the prevalence of diarrhœa exercise a similar influence on the prevalence of flies.
The transference of the Filaria bancrofti, whose presence in the human body in the adult stage is associated with various diseases of the lymphatics, the most pronounced of which is the terrible elephantiasis, is due to more than one species of gnat or mosquito. It is true that no one has ever seen the actual transference of the Filaria from the biting organs of the Culex, Anopheles, Panoplites, or Stegomyia into the human body, but the circumstantial evidence is so strong that on it any jury would convict. Noè and Grassi have demonstrated a similar mode of infection for the Filaria immitis, which exists in the adult stage in such incredible numbers in the cavity of the right side of the heart of dogs, especially in tropical and in sub-tropical countries, that it is difficult to see how the circulation can be maintained at all. It is therefore interesting to note that the proboscis of our common house-fly frequently harbours a larval nematode which has been described by Carter[12] under the name of Habronema muscæ; and again (if it be the same species) by Generali[13] under the name Nematodum sp. (?), and again by Piana,[14] who is inclined to think it is the larval form of Dispharagus nasutus (Rud.). What the further history of this parasite is we do not conclusively know, but, judging by analogy—and in the case of the grosser parasites it is not always wise to do that—the nematode probably develops in some higher animal which eats the fly. Piana brings forward a good deal of evidence that this is the domestic fowl.
Another parasite which attacks flies is the fungus or mould Empusa muscæ, whose growth is fatal to the insect. The hyphæ penetrate into the body, and as they grow weaken the fly until it is unable to lift a leg, but remains glued by its viscid feet to the object upon which it rests. The fungus spreads and radiates out in all directions, covering the fly as with a velvety pile, and giving off countless minute spores, which are blown away, to alight, if they are lucky, on a further victim.