My conclusion is that it is probably safe for a domestic cat to occasionally eat a fly. Domestic cats won’t eat lots of flies because they are a minor prey item for them. This helps to minimise the potential of ingesting of pathogens and toxins that may be on flies. Please read on.
The first point to make is that domestic cats will eat insects because it is in their DNA to do so. They inherited this trait from their wild cat ancestor and there’s little you can do about it. It is normal. Insects are a natural part of a domestic cat’s diet and from the standpoint of nutrition they are a perfectly adequate food because they are largely made up of protein. FYI – flies are being farmed as a source of feed.
I think there is little danger in terms of health for a cat to eat flies. However, I am hinting that there is some potential danger. My initial thought was whether a cat can acquire roundworms (a parasite) from eating flies. I can’t find any information on this on the Internet or in books. However, cats acquire roundworms by ingesting cysts containing the larvae. These cyst may be in the soil or in the faeces of a mammal that has roundworms. What if a fly rested on those faeces and a cyst became attached to the fly. The cat then eats the fly and ingests a cyst. The larvae hatch from that cyst and move around the body inside the cat’s tissue. That would seem to be possible but I am a layperson and not a scientist and therefore I can’t confirm that this theory is correct.
Flies are mechanical vectors
Flies are pretty unpleasant creatures and when a cat eats a fly they may also eat nematodes which are attached to the fly. Nematodes are small, slender worms but they vary tremendously in size and there are many species of nematodes. They are also called roundworms. Flies are good when acting as “mechanical vectors” (physically carrying parasites) for a wide range of microorganisms which become trapped on the surface of the cuticle of the fly or trapped between the hairs on the legs and other parts of the body. Could roundworm cysts be attached to a fly? And flies get a lot of the fluid and nutrients from shit and other effluvia. Flies may also carry small amounts of insecticide on their bodies because they may have come into contact with these chemicals. They may been killed by an insecticide and a cat then eats the fly.
A study found that the common housefly carries as many as 130 pathogens on their body. For example, bacteria isolated from houseflies which could cause a disease in cats include: Salmonella, Campylobacter, Staphylococcus and Listeria. Houseflies can also transmit fungi such as Microsporum and Moniliella. Parasites other than roundworms (as speculated by me), which may possibly be transmitted by flies includes: Trichuris and Giardia (and others).
You can minimise the possibility of flies being in the home by not leaving out wet cat food longer than necessary, installing screens on doors and windows and cleaning the litter tray as soon as possible. In addition garbage should be disposed of quickly or covered.
My conclusion is that when a cat eats a fly they may be ingesting stuff attached to the insect which is unhealthy despite the fact that the fly itself is nutritious. I don’t think that this makes eating flies per se unsafe but it doesn’t mean it’s a good idea either. Domestic cats, anyway, should be treated for worms on a regular basis especially if they are hunting outside.
On balance, I would tend to stop a cat eating a fly if the opportunity arises because there may be some minor health consequences such as a stomach upset if they eat lots of flies. However, I would suggest that this is unlikely. I have not read reports of cats becoming ill from eating flies. It is not something which is written about and studies have not been carried out on this as far as I know. Therefore it cannot be a major issue. It is a minor matter but perhaps consideration should be given, as mentioned, to preventing your cat eating a fly which they have caught.
Sources: various – all on the internet including Cat World and Wikipedia plus my own thoughts.