STILL got a snotty nose!

[Morrigan_Lady]

Mums horse has now had two lots of anti-biotics and a course of Ventipulmin which has cleared his snotty nose up each time, but its back again! Mum is going to speak to the vet again today, but I just wondered if anyone here had any ideas?
He's on good quality haylage and straw, so other put him on shavings Im out of ideas. Hes been on straw for years though and even been on some rather crappy straw in the past and hes been ok.

 

[Miss L Toe]

The offending bacteria can be tested for antibiotic resistance [nasal swab], not sure if vets do this much as the nasal group are usually similar.
I'd certainly change bedding to eliminate things also soak forage.