Research finds flies carry bacteria — some harmful to people and animals

They come every summer without fail, but flies are more than an aesthetic problem.

Animals stamp, swish and shake to rid themselves of these pesky insects. In addition to restlessness, painful fly bites and irritation cause livestock to lose condition.

Other effects of fly infestations include poor feed conversion rates, lower milk production and skin sores. Some types of flies make tiny cuts to the skin and then feast on the oozing blood. In severe instances, this blood loss can be substantial.

Other flies consume tears, mucous and skin cells. Larval stages of flies act like parasites, such as with nose bots in sheep and stomach bots in horses. Perhaps most arresting is when flies deposit eggs in open wounds. Left untreated, these fester wounds can become rife with maggots. Fly strike leading to maggots is a major issue in sheep production in many parts of the world, but flies also have the ability to pick up and spread harmful pathogens.

We already have evidence that flies are important for transmitting equine infectious anemia and the equine encephalitis diseases in horses. In cattle, flies may have an important role in spreading pink eye. Yet, there is much to learn about the role of flies in spreading infectious agents.

A study just published in the journal Scientific Reports applied modern genetic tests to understand the bacteria carried by house and blow flies. The international group of researchers, headed by Ana Carolina M. Junqueira of the Nanyang Technological University in Singapore, analyzed 116 flies from sites within cities, rural and natural areas in the United States, Singapore and Brazil.

In total, they identified a remarkable 431 bacterial species carried by the two types of flies. Many of these are common bacteria found in decaying organic material and soil. However, they also found several types of bacteria that are capable of causing disease in people and animals. Among the identified bacteria were those that cause furunculosis in fish, erysipelas in several animal species and clostridial bacteria, salmonella and listeria.

In another line of investigation, the researchers also determined that the fly legs and wings harbour a large number of bacteria. Most previous studies have examined what flies carry in their guts rather than the outsides of their bodies.

These results show that flies may transmit bacteria via contact in addition to vomiting and pooping.

Of particular interest to human health, they found many flies carry Helicobactor pylori, the stomach bacteria that is associated with stomach ulcers and gastric cancer. The authors suggest that flies may be an important method of transmitting these bacteria among people. The role of this bacterium in animal diseases remains uncertain.

Collectively, the results of this study confirm that our squeamishness around flies is warranted. They have a tremendous ability to pick up bacteria from their environment and move it around.

The techniques of this study may be useful to the study of animal diseases. For instance, it may be helpful to examine the suite of bacteria carried by flies in cattle feedlots and hog barns to understand their role in pathogen maintenance between animal groups and transmission among animals.

This research also suggests that flies could be useful for detecting pathogenic bacteria in the environment. Germ-free flies released into an area could be later collected to see what bacteria they pick up. A study of this type could examine the role of flies in the spread of antibiotic-resistant bacteria.

Although there is much to learn about the bacterial communities of flies in agricultural settings, studies like this enforce the importance of fly management on farms.

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