First question has to be are you absolutely sure it was spinach and not some other plant that took advantage of the spinach being eaten down.

My first thought! Can you post a picture of the 'spinach', because I've never heard of spinach causing a problem like that, but . . .

Doesn't surprise me as I have mild allergy to spinach and similar dark green leaves - they bring me out in a blotchy rash. It's the chemical that makes them dark green (as opposed to just green!) and which makes them taste bitter to some people but not to others - which is a genetic thing.

Thanks for these comments. 1) It is without question spinach (the perpetual variety which I think should really be called chard?) - I simply picked the new, unravaged growth from the centre of the plants. 2) I've been growing and eating this type of spinach for decades without ill effect, and had a serving from this batch before the caterpillars did their worst, so I very much doubt it's an allergy. Nothing else I had eaten in the previous 36 hours was remotely likely to be the culprit. I guess there's only one thing for it: eat another bunch and see what happens. An experiment I'm not all that keen to try!