Is this study evidence that a fly falling into one's drink should be dipped?

Question

About 9 years ago, a user inquired about a medical claim from an islamic text, a claim that a fly falling into your drink should be dipped in then thrown away (the claimed explanation being that dipping cancels out the harmful effects of the fly, making the liquid safe to drink again). Now, a few Muslim researchers have published papers trying to demonstrate this belief. Here's a example: Baeshin et al., 1990, "Effect of Natural Falling and Dipping of House Fly (Musca domestica) on the Microbial Contamination of Water and Milk", KAU: Science Journal, King Abdulaziz University.

In this study, the researchers measured the bacterial contamination of water and milk by flies, depending on whether the flies simply fall or were dipped (several times) in it. The reported result is that the liquids with fallen flies come to harbor more bacteria than those with several instances of dipping, which is counter-intuitive to say the least!

However, this study seems suspicious from my lay-man's perspective: it was published in the inhouse journal of a Saudi university (raising the possibility of bias) and Google Scholar indicates an egregious lack of citation (only one instance). Still, is their methodology sound, or does it contain obvious flaws? Are their results plausible?

Answer 1

The 1990 paper by Baeshin is dipping flies for 20 seconds (vs floating fly for 20 seconds) and dipping flies thrice for 20 seconds each (vs floating for 60 seconds) in some experiments using the same fly multiple times (!) in some experiments having only one no-dip statistic, ..., no variance is computed, tests are done maximally 6 times, provenance of flies is unclear, the manipulation neccessary for dipping has no clear equal on the no-dip side of things (introduction of antimicrobials via the tool?), counts fluctuate wildly over time and over media - in short, it is a hot mess.

The hypothesis is that the fly carries an antimicrobial factor that is transferred to the water in a higher concentration when dipping, thus counteracting the initially higher microbial input. The data does not bear that out, if the experimental and statistical shortcomings are taken into account.

The microbiologically sound 2019 study by Galal et al looks at bacteria and fungi found on flies by dipping. It finds bacteria and fungi on the flies, does a visual characterisation of the cultures and confirms antimicrobial and antifungal properties on some colonies. The paper has no hypothesis and is purely observational. It is never made clear why the flies are sampled by dipping, but they reference Baeshin(1990) with this: "[...]studied the effect of natural fall and dipping of M. domestica on microbial contamination of distilled water and milk" - thereby omitting any religious overtones or the hypothesis that dipping somehow is better than floating.

The funny 2014 paper by Atta even narrows it down to the wing. As is readily apparent from the picture, an assay sourced from the right wing is totally different from the assay sourced from the left wing. Science People! Sadly the author leaves out why dipping a fly would impact which wing gets more washed.

Baeshin at al resubmitted their 1990 paper with more authors but same data 2021. Why? To prove precedence. Why? Unclear.