House flies (Diptera: Muscidae) are scavengers that feed on human and animal feces, garbage, and food waste. A house fly feeds by regurgitating the liquid contents of its crop onto the surface of its food. While feeding, disease-causing bacteria such as Shigella or E. coli may be transferred from the fly’s crop to food in a process termed “vomit droplet deposition.” Because fly regurgitation is an important cause of food contamination, researchers sought to determine whether vomit droplets can transfer bacteria from contaminated surfaces to non-contaminated ones. The investigators found that a single feeding event can result in many vomit droplets deposited over a large area on the surface of a food item.
House flies are attracted to food waste, garbage, and other filth. They can be found on the ground near outdoor restaurants and picnic areas. They are not only a nuisance but they can spread disease by contaminating food with their saliva or feces which they deposit when they land on fresh food. Flies carry germs on their feet, mouth parts, and bodies that can cause diarrhea, dysentery, eye infections, and even death in humans if swallowed.
House fly vomit has also been shown to be toxic to bacteria such as E. coli O157:H7 the strain linked to outbreaks of foodborne illness. A study published in Applied Environmental Microbiology showed that house fly excreta (vomit) was able to reduce levels of pathogenic bacteria such as Salmonella typhimurium DT104 by 99% after 24 hours of exposure.
These pesky insects feed by regurgitating liquid from their crop. This is a form of digestion, as the house fly’s saliva mixes with food particles in its crop and turns into a semi-digested paste. When this paste comes out of the fly’s mouth, it can contain bacteria from the inside of its body.
While feeding, disease-causing bacteria such as Shigella or E. coli may be transferred from the fly’s crop to food in a process termed “vomit droplet deposition.”
As they feed, house flies regurgitate some of the saliva-rich food (containing nutrients and microorganisms) into the crop. They then swallow it again and expel it through their mouthparts onto objects nearby. This newly deposited saliva contains feeding-related microbes that can contaminate surfaces with which flies come into contact.
Because house flies regurgitate liquid food onto food and then deposit vomit droplets on top of it, pathogens from the vomit are transferred to the food. Vomit droplets are deposited on food in a wide range of environments, including kitchens, restaurant, and grocery store surfaces, farm fields, and livestock farms.
Flies have been found to lay eggs in fresh manure and decaying organic matter such as animal carcasses or roadkill (dead animals), they can easily contaminate these areas with bacteria like Salmonella enterica or Campylobacter jejuni — both common causes of foodborne illness.
In the study, the investigators found that a single feeding event can result in many vomit droplets deposited over a large area on the surface of a food item. In addition, they also discovered that vomit droplets can last for several hours and are not easily removed from surfaces. This finding is important because it means that even if you thoroughly clean your food preparation surfaces and utensils, you cannot be sure that they are completely free of vomit if flies have been nearby.
The investigators also examined whether adult flies could transmit disease from contaminated surfaces to other flies under laboratory conditions and found evidence for transmission between sick and healthy flies at three times/week for six weeks (12 total transfers). All these results suggest that adult house flies may be able to spread infectious diseases through their digestive tract contents as well as through contact with infectious body fluids from people or infected animals (e.g., ticks).
If a house fly lands on food, its vomit can become a vector for disease. This suggests that using a fly swatter will do little good against clusters of droplets deposited on your food.
While the study did not find any pathogens in the barf, it’s still best to avoid contamination by thoroughly washing your hands before and after handling raw meat, or other items that could be contaminated with vomit from house flies or other insects.
Written by Sheena Robertson
Science Alert: Why House Fly Barf Is an Overlooked Potential Vector of Disease
Top and Featured Image Courtesy of Lesley Wilson’s Flickr Page – Creative Commons License
Inset Image Courtesy of rushil chopra‘s Flickr Page -Creative Commons License