I agree with most of that, especially loading up into a dirty trailer compared to a shiny one, but like belly clipping, health and safety doesn't seem to matter to those that make the rules when it suits them but they don't mind adding the injuries to the statistics when they want to emphasise our poor safety record.There is no evidence that a bit of muck in an animal transporter has any effect on spread of diseases. In fact, there is evidence that ''phages'' in muck and slurry engulf and destroy a number of bacteria and viruses. (who can remember amoeba from school biology?)
There is plenty of evidence that a bit of muck makes it far easier to load cattle. Conversely, we all know that a shiny clean aluminium container is a definite deterrent to easy calm loading.
There is every reason to believe that spraying water and disinfectant everywhere is wasteful and harmful to the environment.
I have watched Lorries wash out at auction marts, and thought that the vapour from the spray will carry whatever pathogens are present onto all the adjacent vehicles.