Why isn't myiasis more common?

[Lambda Tau]

This is a general question but after reading about this: https://en.wikipedia.org/wiki/Lucilia_bufonivora I wonder why it doesn't happen to other animals that often. L. bufonivora is just a green bottle fly but lays its eggs in a toad's nostrils without the toad being injured/compromised in any way beforehand. Essentially, a fly kills a toad outright (since the hatching maggots eat the toad's brain). Now, given that toads can eat flies very well yet can still suffer this, why don't these flies (or others like them) do the same to other animals, large mammals for example, that would have no defense whatsoever?

 

[Greasylake]

Bot flies lay eggs in pretty much anything. I've seen lots and lots of pictures and videos of mice and other small mammals with bot fly larvae growing under their skin. I can't really answer why the blow fly doesn't do the same thing though.

 

[Salmonsaladsandwich]

Note something it says in the wikipedia article about L. bufonivora... "It often preferentially chooses sick or injured individuals."

This is true of most if not all vertebrates that can come under attack by blowflies. The immune system not only fights off microbes, but is hostile to maggots and other potential parasites that attempt to invade living tissue. There's a reason why many blowflies (including L. bufonivora's relative, Lucilia sericata) are not normally dangerous and can be used medically to remove necrotic flesh: the maggots don't eat living tissue, because if they did your body would do everything in its power to stop them. Only when the immune system is weakened (as happens when an animal is sick or really in any prolonged state of distress) or the animal has a vulnerability such as a wound do fly species that do eat living flesh attack. Also, weakened animals may be less able to avoid flies and perform grooming behaviors to prevent maggots from becoming established.

When basically any large animal does have an injury, or is starving and weak, or otherwise provides an easy target, some fly species go to town.

https://en.wikipedia.org/wiki/Cochliomyia_hominivorax

The screwworm's specific name is hominivorax, meaning "human- eating" just as bufonivora means toad- eating, has been reported to feed on and even kill humans. It mainly preys on ungulates, not only livestock such as sheep, but also wild deer. It lays eggs on individuals with wounds, usually, but the larvae are also capable of burrowing through skin and they are major killers of newborn lambs and fawns where present. They've been mostly eradicated from the US through the use of sterile male programs however.

Basically, miyiasis is common, just not in healthy animals. Animals that allow themselves to be eaten by blowfly maggots when there's nothing else wrong don't tend to survive, and they have adapted so that they are not defenseless to flies.

The exception is botflies, which are common even in otherwise healthy animals. The reason botflies survive this way is that they don't eat occur in huge numbers and eat flesh the way blowflies do, they merely burrow into the skin or attach themselves to internal organs and feed relatively harmlessly on bodily fluids. As long as the infestation isn't extremely heavy, botflies can complete their life cycle while leaving their host in relatively good shape, so there isn't a strong selective pressure to evolve defenses against botflies like there is for blowflies that will gladly devour their host entirely given the chance.