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lesser housefly, the latrine fly and the false stable fly; the latter is commonly caused by blow flies Calliphoridae and flesh flies Sarcophagidae. Normally attracted to the rot-
ting tissue of carrion, the maggots feed primarily on necrotic tissue, but they may also invade living tissue James, 1947; Harwood James, 1979.
In the third type of myiasis, called obligatory, the species is incapable of reproducing without a living host for larvae to feed upon. In Europe, obligatory myiasis is caused by
blow flies, flesh flies and bot flies family Oestridae. Their feeding can result in dermal creeping myiasis where the path of the larvae beneath the skin can be traced, pain and
inflammation James, 1947.
7.3.3. N uisance
During its lifetime, the female housefly is capable of producing up to 1000 eggs, and the resulting larvae will develop into adults in about 7–10 days Larsen Thomsen, 1940.
The potential for a population explosion under the proper conditions is obvious. It is dif- ficult to quantify the emotional effects of large numbers of flies on people already living
under stress and expecting a fly-free environment. But people have been known to vacate their homes and apartments simply because of huge fly populations. In many parts of the
world, urban development has extended into farming areas, resulting in significant increases in housefly populations in communities adjacent to farms, even though the
source of flies may be up to 6.4 km away Winpisinger et al., 2005. Flies can cause tre- mendous problems in these situations, by restricting outdoor recreational activities, par-
ticularly those that involve cooking or consumption of food Thomas Skoda, 1993; Winpisinger et al., 2005.
In a number of studies in the Middle East J. Hogsette et al., unpublished observations, 1997, large numbers of flies were produced in expected and unexpected circumstances.
In the Gaza Strip, large numbers of houseflies were produced in the towns and cities from lack of adequate storage, collection and disposal systems for garbage. Although
public health did not seem to be adversely affected by the fly populations, reduction in fly populations would constitute an improvement in the quality of life. In many areas, flies
had access to raw sewage, so the potential for contamination and transmission of disease was high. Farmers in Israel and Jordan significantly contributed to the increase in the
numbers of flies at different times of the year by their farming practices. Housefly popu- lations were at times unbearable in both countries as a result. To reduce the populations,
one village in Israel used a large barrier of box traps. Although thousands of flies were trapped daily, results were mainly psychological. The nuisance factor was the main com-
plaint, and diseases were probably not transmitted, because the flies were not contami- nated in the harsh terrain.
In Europe and the United States, flies have long been considered to be a public health threat. This is based mainly on their past notoriety as a disease vector, as well as on their
habit of developing in and feeding on manures and other undesirable organic wastes. In the late 19
th
century, a few flies were considered to be a normal part of every house- hold life. But by the early 20
th
century, flies had been incriminated as disease vectors and
Flies
216
including poliovirus, coxsackievirus and enteroviruses Gregorio et al., 1972; Greenberg, 1973; Graczyk et al., 2001. Food exposed to flies in homes of patients with poliomyelitis
in areas where epidemics occurred acquired enough poliovirus to produce a non-paraly- tic infection or asymptomatic carrier state when consumed by chimpanzees Ward,
Melnick Horstmann, 1945. Melnick 1951 stated that sampled flies frequently tested positive for poliovirus, which was the only seasonal factor that could be correlated with
summer epidemics of poliomyelitis.
Flies, moreover, are capable of transferring the eggs and cysts of various cestodes and nematodes Olsen, 1998, particularly hookworms and ascarids. Furthermore, hamsters
have been experimentally infected with scrapie, a disease classified as a transmissible spongiform encephalopathy, after eating extracts of the larvae and pupae of the flesh fly
Sarcophaga carnaria
that fed on scrapie-infected hamster brains Post et al., 1999. Field transmission of the scrapie agent, however, has not been verified.
As stated in section 7.2, on “Biology and bionomics of filth flies in Europe and North America”, the stable fly is quite inefficient at transmitting disease under field conditions
Greenberg, 1971; Zumpt, 1973; Tarry, Bernal Edwards, 1991 and the major concern is the pain associated with its bites and the few people who are allergic to the proteins
injected by the fly. Nevertheless, there are presumptions that biting flies are involved in the transmission of Lyme disease Luger, 1990.
Most of the diseases caused by flies in urban areas are intestinal in nature, and victims may suffer a series of flu-like symptoms, including elevated temperature, diarrhoea and
vomiting. Treatment varies, depending on the causative agent and finding a physician who is knowledgeable in this area of disease management is of utmost importance. Some
bacteria, such as E. coli serotype 0157:H7, are extremely pathogenic and may cause death.
7.3.2. Myiasis
Myiasis, the infestation of living human or animal tissue with fly larvae Hall Wall, 1995, has the potential for tremendous human morbidity and mortality Sherman, 2000.
The classification of myiasis may be based on the separation of myiasis-producing Diptera into the groups that produce accidental, facultative, or obligatory myiasis. Accidental myia-
sis is most often the result of ingestion of food contaminated with maggots, the wormlike larva of any of various flies. Most ingested fly larvae are unable to complete their life
cycles in the human digestive system; however, enteric myiasis can cause malaise, vomi- ting, pain and bloody diarrhoea. The larvae of more than 50 fly species are known to
cause enteric myiasis, the most common of them are the housefly, the lesser housefly, the latrine fly and the false stable fly James, 1947.
Facultative myiasis occurs when fly species that normally develop in faeces or dead ani- mals lay their eggs or deposit their larvae in the tissues of living humans or animals.
Maggots of these flies can develop in a living host, by feeding on dead tissue, but they sometimes invade living tissue as well. Urogenital and traumatic open wound faculta-
tive myiases occur most frequently. The former is associated mainly with the housefly, the
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As one might expect, flies of certain species remain active year-round in areas between and not too far beyond the tropics of Cancer and Capricorn. Having year-round popu-
lations of flies increases the chance of a long-term fly nuisance and of the accompanying risk of fly-borne disease transmission.
As one approaches the poles, flies become seasonal, depending on their temperature tole- rance. In this region, nuisance-level populations are of short duration, thus limiting the
risk of disease transmission to a few weeks or months.
Over the past 100 years, the global average temperature has increased by about 0.6°C and this trend may show a fast rise in the future Houghton et al., 2001; Root et al., 2003. This
warming can affect the world’s biota and the functioning of ecosystems in many indirect ways Stenseth et al., 2002; Parmesan Yohe, 2003. It is also possible that warmer condi-
tions will promote the transmission of diseases by allowing a broader geographical distri- bution and an increase in the abundance of local disease-vector populations Peterson
Shaw, 2003; Brownstein, Holford Fish, 2005; Ogden et al., 2006; Poulin, 2006. When the relationship between fly numbers and weather conditions was examined, results showed
that fly population changes are driven more by climatic conditions than by biotic factors Goulson et al., 2005. With a simulated model of climate change, using recently predic-
ted values for warmer temperatures, Goulson and colleagues 2005 predicted a potential increase in fly populations of 244 by 2080, compared with current levels. If this were to
occur, concomitant increases in fly-borne diseases are expected.
7.5. Public health impact
