Public Health Importance

Muscid flies affect people most frequendy as nuisances, occasionally as vectors of pathogenic organisms, and rarely as agents of human myiasis. The cosmopolitan house fly and stable fly are of greatest medical significance. Other notable examples are the bazaar fly in Africa, Asia, and Pacific islands, including Hawaii; the bush fly in Australia; and Stomoxys nigra and S. sitiens in Africa and Asia.

Filth flies pose particular risks as mechanical vectors of pathogens that cause enteric disease in humans. Among the 1.3 million cases of notifiable infectious diseases reported in 1998 to the United States Centers for Disease Control and Prevention, approximately 74,000 (6%) were enteric infections causing diarrhea or dysentery. These diseases arise from direct or indirect fecal contamination of food and water. Globally, the World Health Organization reports that diarrhea and dysentery account for more childhood deaths and morbidity than any other infectious diseases.

Enteric diseases are caused by certain bacteria, viruses, and protozoa. The bacteria include Escherichia coli> Salmonella species, and Shigella species; the viruses include Cocksackie, enterovirus 72 (hepatitis A), and enteric cytopathogenic human orphan viruses; and the protozoa include Chilomastix, Cryptosporidium, Enta-moebae, and Giardia species. Infections range in severity

Surveillance of the residents in the treated towns showed concurrent declines in the incidence of diarrhea in people of all ages and in isolates of Shigella from children under 10 years of age.

Two more recent studies have confirmed the importance of the house fly in the epidemiology of enteric diseases. Intensive trapping to remove flies at two military field bases in Israel caused declines in fly populations at mess tents, concurrent declines in frequencies of diarrhea and shigellosis among base recruits, and declines in rates of seroconversion for antibodies to Shigella and enterotoxigenic E. coli (Cohen et al, 1991). Elsewhere, village-wide spraying of six Pakistani villages during two consecutive fly seasons reduced house fly populations by 95% and lowered the incidence of childhood diarrhea by 23% (Chavasse et al, 1999).

These studies provide strong evidence that house flies can be important routes for spread of fecal-borne pathogens. Prudence dictates that the house fly and other filth flies should be controlled through sanitation in the synanthropic environment and that they should be prevented from contacting human food at ail points of production, distribution, preparation, and consumption.

from benign to fatal, being most severe among children, the elderly, and others who are infirm. Common sources of enteric pathogens are food and water contaminated with feces from infected people or animals, or indirectly via hands, utensils, and flies.

Greenberg (1971,1973) summarized the extensive literature on pathogens associated with muscid flies. Evidence is strong that filth flies in particular are mechanical vectors. Mouthparts, tarsi, and gastrointestinal tracts become contaminated when the flies feed- on contaminated substrates. Upon dispersal, the flies can inoculate new substrates with contaminated tarsi, mouthparts, fly vomit, and feces.

The medical significance of filth flies at a given time and place depends on which flies and people are involved and on circumstances in which flies and people come into contact. A substantial majority of people in the United States and Canada now live and work in urban and suburban settings where indoor and outdoor environments are essentially free of filth flies. Exceptions are rural settings lacking adequate sanitation systems or neighboring mismanaged livestock and poultry operations. Intolerance for flies is, in part, the basis for municipal health codes used to enforce proper management of organic wastes on the affected premises. Sanitary standards established by the mid-1900s have dramatically reduced the epidemiological importance of filth flies in many parts of the developed world. Too often, however, basic sanitation and filth-fly management are unsatisfactory due to poverty, famine, or war. Under these circumstances, filth flies can reach tremendous densities, breeding in and around accumulated human waste and carrion.

The following muscid flies warrant attention with regard to public health.

House fly (Musca domestica)

The house fly is the most common cause of fly annoyance in North America. Adults aggregate around garbage, compost piles, and other food sources, and they readily enter buildings. House flies are conspicuous when alighting directly on people, crawling on human food, or resting on walls, windows, and ceilings. These substrates become soiled with fly specks, dried droplets of fly vomit, and feces.

In a classic pair of experiments, Watt and Lindsay (1948) and Lindsay et al. (1953) provided strong evidence that the house fly is a significant vector of enteric pathogens. They controlled filth flies with residual insecticides in selected towns in southern Texas and southern Georgia and left neighboring towns untreated as controls. Fly surveillance in the treated and untreated towns showed that treatments greatly reduced the densities of house flies and other species.

Bazaar fly (Musca sorbens)

This nonbiting, synanthropic fly is common in Africa, Asia, and many Pacific islands. Adults feed persistently at the eyes, noses, and mouths of people (Fig. 14.15) and other large mammals. The flies are also conspicuous wherever human food is exposed outdoors. Fortunately, the species is strongly exophilic. Greenberg

FIGURE 14.15 Aggregating bazaar flies (Musca sorbens) on human hosts. (Photo by B.. Lewis and D. Dawnway, with permission, © The Natural History Museum, London.)

(1971, 1973) summarized the extensive literature that associates the bazaar fly and its close relatives with human pathogens. Most notably, these flies are strongly suspected of mechanically transmitting enteric pathogens and the causal agents of acute bacterial conjunctivitis and trachoma. A recent study involving paired villages in The Gambia (Emerson et al., 1999) showed that community spraying, which reduced bazaar fly populations by around 75%, lowered incidence of trachoma eye disease (caused by Chlamydia trachomatis) by 75% and the incidence of childhood diarrhea by 22%.

Bush fly (Musca vetustissima)

The earliest European travelers in Australia recorded the annoying presence of the bush fly. This nonbiting dung fly, like the closely related bazaar fly, is strongly exophilic and is a probable irritant to humans almost anywhere in Australia. Flies that are attracted to people swarm around the head, feed at eyes and nostrils, and settle on the head, back, and shoulders. Once on hosts, the flies are peculiarly sedentary. More than a casual brush of the face with the hand is required to dislodge them, leading to a hand gesture that is humorously called an "Aussie salute." Larvae are known to occur in human and animal feces, so adults are a potential mechanical vector of enteric pathogens. Furthermore, the propensity of the adults to feed at a host's eyes makes the bush fly a prime suspect in transmission of eye pathogens (Greenberg, 1971).

Face fly (Musca autumnalis) and cluster fly (Pollenia rudis)

The face fly and the cluster fly are two of several species of flies that can be a nuisance in households during winter and early spring. Other species include various blow flies (Calliphoridae), Muscina species, and Ceroxys latiusculus (Otitidae). Overwintering flies in buildings can be activated by heaters or warm weather and become attracted by light to inhabited rooms. Often people first take notice when live and dead specimens occur at sunny windows. Dead flies that have accumulated over years in an infested building can attract dermestid beetles. Although the flies and beetles can be a source of allergens, these insects do not pose any other known medical threat.

Stable fly (Stomoxys calcitrans)

The stable fly is an important nuisance in outdoor environments throughout North America. This fly will readily attack people, usually on the lower part of the legs, causing a searing pain with each probe of its bayonet-like proboscis (Fig. 14.12A). It does not take many stable flies to disrupt activities of sunbathers, anglers, and others seeking outdoor leisure. Outbreaks have been recorded in the United States at tourist spots in the Great Lakes area, the Atlantic seaboard, and the Gulf Coast. Annoyance by stable flies is not confined to resorts and beaches; the flies can occur wherever people, fly breeding sites, and favorable weather coincide.

Adoption of the United States Declaration of Independence on July 4, 1776, by delegates to the Continental Congress may have been hastened by stable flies. According to Fuller (1913), debate on the Declaration drafted by Thomas Jefferson and his committee might have lasted much longer were it not for torment from stable flies. Jefferson noted the weather was oppressively warm that day in Philadelphia, and the meeting room was next to "...a stable, whence the hungry flies swarmed thick and fierce, alighting on their legs and biting hard through their thin silk stockings. Treason was preferable to discomfort." Clearly Jefferson had a wit, but he also knew enough entomology to infer that the nearby stable was the source of the flies.

As with any blood-feeding arthropod, stable flies provide an opportunity for transmission of blood-borne human pathogens. Experimental evidence suggests that the fly can acquire animal pathogens as mouthpart contaminants. Ingested particles can remain viable in the lumen of a fly's gut and diverticulum for hours to several days. However, none of these pathogens infects the fly, so only mechanical transmission would be possible. Experimental evidence using animal disease models in realistic settings suggests that the stable fly is not a vector of any consequence to human health.

False stable fly (Muscina stabulans) and its relatives The false stable fly and its relatives are common around filthy habitats, including latrines, household wastes, and accumulations of animal manure. The adults have feeding habits similar to those of the house fly and present similar risks for mechanical transport of food-borne pathogens. These flies remain outdoors and rarely feed on human food. However, they do feed and defecate on fruit and serve as potential vectors wherever breeding sites are near open-air markets and roadside fruit stands. Larvae of the false stable fly and of M. lev-ida have been involved in rare cases of intestinal and urinary myiasis.

Little house fly (Fannia canicularis) and its relatives These nonbiting filth flies can become nuisances when swarms occur inside inhabited buildings. Hovering Fannia species often occur at head height indoors, where they can be particularly distracting and bothersome.

Adults of both sexes can be contaminated with pathogenic microbes from filthy larval breeding sites such as latrines, rotting garbage, and poultry litter. It is important, therefore, to exclude Fannia species from areas where human food is prepared or consumed. Nonetheless, Fannin species generally pose less of a health hazard than house flies because Fannin species rarely land and feed on human food.

In the western United States, Fannia species in the benjamini group are commonly attracted to human sweat and mucus. One species in this group, F. the-laziae, is a developmental vector for the mammalian eyeworm> Thelazia californimsis. Definitive hosts of this nematode include deer, canids, horse, rabbit, sheep, and black bear; people are rare, accidental hosts. Females of T. californiensis live in their host's lachrymal ducts, where they cause mild irritation and ophthalmia (Soulsby, 1965). Eggs are shed and hatch in eye fluids. First-stage larvae are ingested by eye-feeding flies, penetrate the midgut, and develop further in the fly's haemocoel. After 2—4 weeks' extrinsic incubation, infectious third-stage nematodes exit the fly's mouth-parts when the vector feeds on another host.

The little house fly and the latrine fly have been involved in cases of intestinal, aural, and urinary myiasis of people. Most of the cases are thought to have arisen from eggs laid on clothing or bedding soiled with human feces.

Garbage flies (Hydrotaea spp.J

Garbage flies and their larvae are common around municipal garbage dumps, compost sites, poultry houses, hog barns, and dairies. As occurs with other flies from these kinds of environments, garbage flies can be contaminated with microbial pathogens. However, garbage flies are more sedentary than house flies and are far less inclined to enter buildings and contaminate human food.

Sweat flies (Hydrotaea spp.) Very little is known about the medical importance of sweat flies in North America. Females of six North American species, including H. meteorica and H. scam-bus, are persistent in their attempts to obtain perspiration and secretions from the eyes, nostrils, lips, and other parts of their hosts. Because sweat flies are ex-ophilic and occur most frequentiy in wooded areas, they are encountered by people in wooded parks, golf courses, and similar outdoor habitats. Except for their annoyance, sweat flies are not regarded as medically significant.

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Responses

  • niklas
    What is the public importance of blackfly?
    3 months ago
  • aili
    Is black fly one of pest of public health importance?
    1 month ago

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