Campylohacter spp

Streptococcus spp

These bacteria have been found to be the causative agents of many acute gastro-enteritis type complaints.
Often found in skin and gut infections in humans, they too can be carried by flies. Chlamydia spp
These are parasites that can cause a variety of complaints in humans and there is increasing evidence that flies can transmit them.

Escherichia colt

In recent months there have been a number of publications showing evidence for the transfer of Escherichia coil 0157:H7 by flying insects. The most recent of these is where an outbreak of E. call has been reported in a school in Japan and the pathogens that were isolated from the human sufferers were also isolated from houseflies found at the time in the kitchen. A further finding of this study was that the E. coil seemed to be contaminating the labella folds of the housefly mouthparts and, in fact, actively proliferated there, leading to the conclusion that the flies may have a higher potential to disseminate the E. coif than had been previously suspected.
Further studies recently carried out in the UK show that E. coll, when fed to flies, is taken into the flies and distributed widely within the internal structures with significant numbers of bacteria being found in the foregut, ovaries, hindgut, and abdominal haemolymph. The contamination of the ovaries is intriguing and this may imply ovariole contamination and subsequent “infection” of the egg and possibly the larvae.
There are also many fungi that cause disease such as Candida spp, Mucor spp, Aspergillus spp, etc, and research shows that flies can carry all of them.
Food poisoning outbreaks can occur from a minute dose of bacteria. In these cases, the disease has could easily have been spread by flying insects, a fact which is rarely understood or appreciated.

Feeding behaviour

All true flies, to which this group belongs, can only ingest liquid food. Should they land on a solid food source they produce large quantities of saliva together with regurgitated gut contents. The mixture, rich with digestive enzymes and undigested matter from their last meal, is vomited onto the food together with any living bacteria, viruses and protozoa that it may contain. The mass is puddied together into a sticky bolus using the forelegs before being sucked back into the gut. The process may be repeated several times, during which time the insect may defecate to reduce the overall body weight in readiness for flight.
Evidence of this feeding behaviour may be seen on surfaces visited frequently by flies. Brown and white spots indicate the remains of dried faecal smears and gut contents, both of which have been shown to be rich in viable pathogens for a considerable period of time.The gut contents and the faeces have been shown to contain the same pathogens as are found on the external surfaces, but with an important difference. The more stable and moist environment of the gut allows not only a better-protected environment for the pathogens, enhancing their survival, but also permits multiplication. This means that a larger infective dose may be transmitted. This is a critical factor as the size of the inoculum has a direct bearing on the likelihood of successful transmission.

From the turn of the century to the present day there have been dozens of studies on the correlation between the seasonal fluctuations of fly populations and the rise and fall of gastro-enteric disease. The rise of one equates to a rise in the other and vice versa. Cases where fly populations were reduced by control measures have shown an immediate decrease in the incidence of disease.

The potential of flies as mechanical vectors should never be underestimated and, as studies continue, their role in the transmission of gastro-enteric disease gains in importance.

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