Flies and infectious disease
Flies are significant vectors for over sixty-five infectious diseases affecting humans, transmitting pathogens through both biological and mechanical means. They can spread food-borne pathogens like Escherichia coli by contaminating food and can introduce bacteria into wounds or mucous membranes. While many fly-borne illnesses result in relatively mild symptoms, particularly in the Northern Hemisphere, they often go unrecognized due to lack of medical intervention. Certain fly species, such as female sandflies and black flies, are linked to specific diseases like leishmaniasis and onchocerciasis, respectively.
Flies are attracted to humans and animals through movement, warmth, moisture, and carbon dioxide, increasing bite risks, especially in warmer climates where skin is more exposed. Notably, a single fly can harbor millions of bacteria, enabling efficient disease transmission. Prevention efforts, including insecticide treatments and improved environmental hygiene, have been effective in reducing fly populations and disease rates. However, the transmission of diseases by flies is predominantly an issue in tropical and developing regions, where resources for control and treatment are limited. Environmental disasters can exacerbate risk by creating conditions favorable for fly breeding and contaminating clean water sources, leading to increased public health threats.
Flies and infectious disease
Definition
Flies can transmit more than sixty-five infectious diseases to humans and can carry more than one hundred species of pathogens. They spread disease either by biting directly into the skin (biological transmission) or by carrying disease-causing agents (pathogens) on their feet, mouths, or bodies and depositing them on humans; this process is called mechanical transmission.
Flies have been implicated in contaminating food with food-borne pathogens, such as Escherichia coli, which can be found in the mouths or in the feces of common houseflies. Flies can also leave disease-causing agents, such as bacteria, on or near areas such as wounds or mucous membranes, areas that allow the bacteria to enter the body and flourish. Most of the infectious diseases occurring from fly transmission in the Northern Hemisphere appear as relatively mild stomach upset; these diseases often go undiagnosed and rarely require medical intervention. For this reason, the causes of many fly-borne illnesses are difficult to verify or identify.
Specific types of flies have been shown to transmit certain diseases in particular geographic areas and in particular developing populations. Female sandflies are associated with the transmission of leishmaniasis, a parasitic infection, and black flies are responsible for the spread of worm larvae, which causes onchocerciasis (also known as river blindness).
Transmission
Many types of flies feed off the blood of humans and animals. The flies are able to sense the potential availability of their next meal by the movement, warmth, moisture, and carbon dioxide emission of a nearby body. In warmer climates, humans tend to expose most of their skin, thereby increasing the potential for fly bites. Flies pierce the skin with their mouths and inject a small amount of saliva. This saliva contains an anticoagulant that prevents blood from clotting, allowing a fly to consume the blood. Any bacteria or parasites, such as worm larvae, contained in the saliva of the fly are injected into the bitten body, beginning the infection process.
Other diseases, such as trachoma or strep infection, do not require an actual bite from a fly to be transmitted. A single fly can carry more than 33 million bacteria in its mouth or on its body, including its feet. The fly acts as a vector, carrying bacteria from a pathogen source or from one infected person to another. For example, after landing on the nasal or ocular discharge of a trachoma-infected child, a fly, now carrying the infectious agent, can directly deposit that agent into the eyes of other children. Young children and infants are often not able to brush flies away, which is why infection rates are so high for this age group.
Prevention and Outcomes
Research has shown that fly control programs have significantly reduced the transmission and infection rates of some diseases. Insecticide treatment of an area or community can decrease the number of flies, but environmental and hygiene improvements are also implemented to limit fly contact.
Changes to the methods a community uses to manage waste and keep livestock can reduce fly populations near living spaces. The World Health Organization (WHO) has been active in the development and support of insecticide spraying programs to reduce the breeding of flies and in education programs to teach people how to reduce fly populations. In 2021, the WHO released its sixth edition of “Global Insecticide Use for Vector-Borne Disease Control: A 10-Year Assessment (2011-2019).” This study investigated insecticides' effectiveness in fly-borne diseases such as malaria, dengue, and leishmaniasis. The WHO released "Operational Manual on Indoor Residual Spraying" in 2024 to offer guidance on how to use indoor residual spraying to prevent multiple diseases caused by mosquitoes, sandflies, and triatomine bugs.
Impact
The transmission of disease by flies is most common in tropical regions and in developing areas, where there are limited resources to control fly populations or to treat disease. Also, areas that are minimally affected by fly-transmitted diseases may find that risks increase during environmental disasters, such as flooding or tsunamis. Water increases the chance for flies to reproduce; clean water sources, following a disaster, often become contaminated with pollutants. This combination of water availability (for reproducing) and water pollution drastically increases the risk of infection in humans.
Fly population changes are strongly influenced by climate and weather patterns too. According to a study published in Scientific American in 2019, while climate change was considered likely to decrease the populations of butterflies, moths, bees, and dung beetles, it was considered likely to increase the population of houseflies because they might thrive in hotter temperatures. Further studies will forecast fly population levels and ensure the effective implementation of control measures to manage future public health risks and disease.
Bibliography
Fernando, Ranjan, et al. Tropical Infectious Diseases: Epidemiology, Investigation, Diagnosis, and Management. London: Greenwich Medical Media, 2001.
Global Insecticide Use For Vector-Borne Disease Control. World Health Organization (WHO), 2021, apps.who.int/iris/bitstream/handle/10665/345573/9789240032033-eng.pdf. Accessed 3 Feb. 2025.
Graczyk, Thaddeus, et al. “The Role of Non-biting Flies in the Epidemiology of Human Infectious Diseases.” Microbes and Infection 3 (2001): 231-235.
Intagliata, Christopher. "Warming Climate Implies More Flies—and More Diseases." Scientific American, 20 Feb. 2019, www.scientificamerican.com/podcast/episode/warming-climate-implies-more-flies-mdash-and-disease/. Accessed 3 Feb. 2025.
Marquardt, William, ed. Biology of Disease Vectors.2d ed. Burlington, Mass.: Academic Press/Elsevier, 2005.
Operational Manual on Indoor Residual Spraying: Control of Vectors of Malaria, Aedes-Borne Diseases, Chagas Disease, Leishmaniases and Lymphatic Filariasis. World Health Organization (WHO), 13 Feb. 2024, www.who.int/publications/i/item/9789240083998. Accessed 3 Feb. 2025.
Shaffer, Julie, et al. “Filthy Flies? Experiments To Test Flies as Vectors of Bacterial Disease.” American Biology Teacher 69 (2007): e28-e31.