N uisance Health hazards 1. Diseases

Public Health Significance of Urban Pests 217 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 In a number of studies in the Middle East J. Hogsette et al., unpublished observations, 1997, large numbers of flies were produced in expected and unexpected circumstances.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