Fly Fugitives

By: Stuart Mitchell

Violating Our Health

A customer walks into a restaurant, sits down, looks at the menu, orders the soup, is served the soup, looks down, and observes a fly in his soup. This forgiving patron calls the waiter over and asks, “What is this fly doing in my soup?” The waiter looks at the fly in the soup, looks at the customer, and says, “I believe that is the backstroke.”

There was a time during the infancy stage of the evolution of our public health tolerances that both this old joke and the actual event would have been well within social acceptance. But times have changed significantly, and the type of aforementioned assault is no longer acceptable from both a tolerance and public health standpoint.

Filth flies have been indicted in both the direct and indirect mechanical transmission of several pathogens responsible for human diseases, especially pathogens that cause diarrheal illness.

Significant Pathogens of Human Diseases Known to Contaminate Filth Flies

Mechanical transmission via filth flies is the transfer of pathogens from one location to another, mostly passively or unintentionally. The mechanical transmission of disease organisms is facilitated by the adult filth flies’ habit of moving about upon, and feeding upon, materials that tend to be contaminated, then doing the same upon food consumed by humans. Secretion of salivary fluids (sometimes incorrectly referred to as vomiting) and defecating while feeding also increase the potential for transmission of pathogens by filth flies.

The unifying factor within the ecology of numerous species of filth flies is their use of decomposing organic materials as food sources for adults and developmental media for maggots (larvae). Considering that these materials are often carrion, feces and refuse (all with associated pathogens), the potential for flies becoming contaminated is quite high.

BUILT TO SPREAD THE YUCK

Filth flies have hair-like structures on their legs and exoskeleton that significantly increase their surface area and assist in harboring pathogens. Deeply channeled mouthparts and hairy feet with sticky pads can easily become contaminated when in contact with infectious surfaces. Filth flies are mechanical vectors of pathogens because organisms can be transferred from their contaminated bodies to our food, eyes, noses, mouths and open wounds.

Filth flies are attracted to a variety of rotting organic materials, feces and human foods. In addition to the impressive number of pathogens filth flies potentially carry on their body surfaces, they also transmit pathogens to our food in their saliva and feces.

Generally, filth flies have sponging mouthparts, and are capable of consuming foods only in a liquid phase. Solid foods are liquefied by regurgitating the crop contents (along with any pathogens) onto the food substance, allowing the vomit to liquefy the solid. Flies then suck the liquefied food (along with any pathogens) into their digestive tracts.

Filth flies further contaminate food by defecating on it while feeding. Fecal deposits are darker than vomit deposits. House flies, which fall into the filth fly category, can produce anywhere from 16 to 31 spots in 24 hours (mostly vomit) after just one feeding. Therefore, it is appropriate to estimate about how many spots could be produced within a food-processing facility by 10, 50, 100 or more flies having constant access to various food sources.

Kobayashi et al. (1999) demonstrated that Escherichia coli O157:H7, an extremely virulent serotype of this common bacterium, actively proliferates in the minute surfaces of house fly mouthparts, and that this proliferation leads to persistence of the bacteria in fly feces.

More than 100 pathogens that initiate human disease are known to contaminate filth flies. The role that filth flies play in transmitting pathogens to humans, and to what extent this transmission leads to disease, depends upon the pathogen and favorable or unfavorable environmental factors.

 

 

 

 

BIOLOGY AND BEHAVIOR

The term “filth fly” refers to several species of true flies (Diptera) that belong generally to the families Muscidae, Calliphoridae and Sarcophagidae. All flies undergo complete metamorphosis, with egg, larval, pupal and adult stages. Larvae (maggots) of these families complete three instars prior to pupation.

Development of immature flies to adults is largely dependent upon the temperature and moisture level of the offending substrate. Variations of developmental time within a species are usually related to these two factors.

Climatic changes, such as the onset of the rainy season, can have a significant effect on fly populations — mostly on the rate of development. Filth flies are very strong fliers, with both house flies and stable flies capable of flying about 5 miles per hour over several miles. Developmental sites can be long distances from areas where adults are causing problems (Hogsette and Farkas 2000).

CONTROL: START WITH A CENSUS

Filth fly census surveys help determine the effectiveness of sanitation practices, identify filth fly breeding sites and determine the need for management measures, such as improved structural exclusion and/or insecticide applications.

Filth fly census surveys are also necessary to determine baseline fly populations, track population trends and evaluate the effectiveness of management measures. Sanitation is the cornerstone of any effective filth fly management program. Breeding areas may be off-site, placing sanitation beyond the charge of preventive medicine and making it necessary to concentrate on adult surveillance and management.

There are five elements of effective filth fly census surveys:

1. Surveillance to identify the presence, species, size of fly populations and conditions that favor breeding.

2. Sustained monitoring of fly populations and conditions that favor breeding.

3. Evaluation of census survey results.

4. Initiation of management measures when established thresholds have been exceeded.

5. Continued surveillance to determine the success of management measures.

 

CHEMICAL CONSIDERATIONS

Integrated pest management (IPM) is paramount in filth fly suppression programs. In many events, sanitation is the key to long-term management success. Insecticidal application alone is not sustainable, being limited by both time and money. However, insecticides constitute an acute solution for reducing filth fly populations, and must be considered when a disease threat exists.

Pesticides and devices that are used in filth fly management include, but are not limited to:

1. Granular baits and bait traps. If used correctly, these can effectively reduce adult fly populations indoors and out.

2. Fly abatement strips. These products are more than the simple sticky tapes or strips of older generations.

3. Space sprays. Ultra low-volume (ULV) application is the most rapid method of indoor/outdoor adult insect management.

4. Residual insecticides. These products can be used to control adult or larval flies. In both cases, efficacy is often poor, but this approach can be useful in certain situations. Residual insecticides are generally ineffective against larvae unless the larval medium is shallow in depth, and the insecticide can penetrate to contact the larvae. Applying residual insecticides to the inside surfaces of garbage cans and other areas where maggots are seen, however, should be included in any fly management program.

5. Insect light traps (ILTs). Capable of capturing flies and augmenting sanitation inside buildings, ILTs effectively remove the small numbers of flies that enter buildings. They should be deployed for extended or permanent duration within facilities. They work by attracting flies to an ultraviolet light.

6. Air units. Air curtains stabilize internal environments and save energy by minimizing the heated or air-conditioned air loss through open doorways. Air doors repel flying insects, dust and pollutants.

As we move into the future of public health legislation, with existing laws, policies and practices, PMPs will continue to play a critical role in the success of environmental health. Locating, pursuing and arresting “fly fugitives” that violate our health is an important part of that task.

Stuart Mitchell, DO, ND, PhD, BCE, is a biologist and consulting physician for Springer Pest Solutions, Des Moines, Iowa. Contact him at docmitchell@hotmail.com.