Sorry to hear about your son's dead sheep

I had this - sudden death, sometimes with and sometimes without blood from the nose - in my ewes when I first took over the moorland farm. We also had a frighteningly high number of ewes collapsing, mostly surviving after treatment for metabolic disturbances. By the second year, we had got into the swing of the place, the routine meds etc, and the 500+ sheep were used to us, and we didn't get so many of these cases. The odd one, and the odd one with metabolic disturbances (twin lamb diseases / hypocalcaemia) but not several dead sheep with no explanation.
So I can't tell you exactly what made the difference, but certainly the sheep were less stressed simply because they knew us and our dogs.
The whole flock was up-to-date with their vaccinations (which being Heptavac-P included pasteurella) when we took them on, and we continued their regime, so I am pretty sure it wasn't pasteurella with us.
The things we did that we maybe did better the second time, and may or may not have contributed, include:
- caking twin and triplet bearing ewes from 8 weeks before lambing, and all pregnant shearlings
- mineral drenching all ewes before tupping and midway through pregnancy
- ad lib Crystallyx high energy sheep buckets (red) from early in pregnancy
- twice (or more) daily checks, keep an hourly eye on any ewe that was lethargic and prompt action with subcutaneous calcium if she went down
- fluke drench (it was a very flukey area) every 6 weeks from October to March
- ensuring plenty of forage from midwinter on, and spread about enough for all ewes to be able to get at it (and the same with the cake)
- keeping our Mules seperate from our (horned) Swaledales
They also got copper mid-pregnancy, and vaccinations a couple of weeks before lambing.
One of the massive surprises for us - and my farming partner was very experienced, with 25+ years' experience of sheep farming - was just how little of a difference in what the ewes expected would cause them sufficient stress to collapse when pregnant. It actually made me feel terrible about all the times I had taken my very well-behaved dogs off-lead but under close control through flocks in autumn and winter, thinking that farmers over-reacted in insisting all dogs near sheep had to be on a lead.