e.Coli is quite rare. Dairy farmers are more likely to see mastitis in the form of clots in the milk. Also, if you have milked cows for a while, you can spot the signs of health problems such as mastitis, blood clots, e.Coli or injuries to teats, and treat them appropriately. Infected milk does not contain pus; it’s more likely to be mastitic clots, which are treated with antibiotics. Antibiotic milk does not go in the main tank, as you would have to pay a heavy fine for this, and your milk would be subjected to rigorous testing for some time as a result. You may have to dispose of your milk down the drain, or you could lose your milk contract. Antibiotic milk goes into a small dump bucket and is then discarded.
The only time that muck is ‘flying around’ the parlour is when cows cough and poo at the same time. That’s known as projectile poo! This is why waterproofs and hats come in handy for milking staff. Normal poos do splash, as yours would if they were liquid and you dropped them from a great height. As parlours are equipped with hosepipes, muck can be washed away very quickly.
Cryptosporidium is more common in calves. It comes from ingesting contaminated faeces. As there is a range of zoonotic diseases which can pass from cattle to humans, most farmers either wear gloves, or do an excellent job of washing their hands before eating or going into the main house!
Dairy farmers sometimes choose to clip the tails of the cattle (i.e. trim the hair from the end of the tail) to prevent it from becoming full of muck and urine, and to make it more hygienic for milking.
Cows are generally cleaner at this time of year, when they start going outside.
Some cows do get muck on their udders. If it’s very bad, they are hosed down with a high-volume wash (obviously, not on full-blast) to get the bulk of it off. Each teat is then dipped individually, left to soak while the cows on the same side of the parlour are dipped, and then the teats of every cow on that side are dried with cloths or paper towels. These cloths or paper towels are usually either used for one cow each, or one between two (depending on the size of the towel). You’d get quite quick at cleaning them if you had to milk several hundreds twice a day, every day, for years on end, especially if you work alone. Then the unit goes on, they’re milked, and then either dipped or sprayed (depending on the farm) to encourage the teat duct to close itself quickly to prevent entry routes for bacteria.
On slightly more modern farms, vacuum sucks the milk from the parlour, through a filter and then into a bulk tank, where the milk is cooled. It has to be below 5 degrees C for tankers to collect it. Farm tanks usually cool the milk to 2.9-3.5 degrees C. The entrance point to the tank is washed down at every use, to avoid a build-up of milky scum and bacteria. Once cooled, the milk is then sucked up into the milk tanker (which makes milk splashes impossible, because the tube from the tanker is screwed tightly onto the farm’s bulk tank).
Dairy farms are regularly visited by Red Tractor (if they are a member), Food Standards, National Milk Records, vets (for TB testing, routine visits and other emergency cases), companies that their milk contract is with (Arla, Muller, etc.) and a few other organisations that I can’t remember the names of. They’re tightly regulated.