Public health impact Flies

Public Health Significance of Urban Pests 219 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

The epidemiological association of flies with various diseases is well documented Olsen, 1998; Graczyk et al., 2001; Nichols, 2005. The documentation shows that certain flies are capable of contaminating food with more than one foodborne pathogen and that natural populations of flies harbour these pathogens. It also shows that E. coli serotype 0157:H7 actively proliferates in the minute spaces of the housefly mouthparts and that this proliferation leads to persistence of the bacteria in fly faeces Kobayashi et al., 1999; Sasaki, Kobayashi Agui, 2000. The authors cited used DNA techniques to implicate houseflies as the source of E. coli in an outbreak in a day-care centre in a Japanese village see section 7.4.. Also, a reduction in the transmission rate of shigellosis has been positi- vely correlated with improved fly control Watt Lindsay, 1948; Cohen et al., 1991. The relationships between houseflies and a number of other pathogens that cause gastroen- teritis can also be found in the literature Nayduch, Noblet Stutzenberger, 2002; Nichols, 2005, but the importance of this insect in causing illness in people through field transmission of these pathogens has not been verified, particularly in densely populated urban areas. Studies that show correlations between the suppression of flies and the concomitant reduction of enteric diseases can be found in the literature, but these studies were performed in rural settings. For a number of reasons, cases of fly-borne illness or disease are at times difficult to verify. For example, when examining sick patients, most physicians, despite their medical trai- Flies 218 public health laws were enacted in many countries. Typhoid was a major problem in Europe and the United States, and the housefly became known as the typhoid fly. Cities with poor sanitation had large numbers of typhoid cases while cities with good sanitation did not Greenberg, 1973. After the Second World War, sewer lines were closed and sanitation was vastly improved. As a result, flies were denied access to contamination points and fly- borne disease essentially disappeared. Despite this, flies are still considered to be an indication of unsanitary conditions in homes, hospitals and restaurants Fig. 7.2. Also, in times of disaster, proper sanita- tion levels are compromised, increasing the opportunity for flies to become contamina- ted and transmit disease. At present, a few flies in a home or restaurant may not consti- tute a serious health risk. But this may not be the case in the future.

7.4. Exposure and risk assessment