Marmite is just yeast extract, so is vegetarian

The idea is that biting blood sucking insects like mosquitos and midgies do not like the flavour of blood that has a lot of Thiamine (Vitamin B1) in it, and they can smell this off Marmite (and Brewer's Yeast) eaters and choose not to bite them.
I don't know how much research has been done to confirm this

Also, the midgies wouldn't necessarily find a spoonful of Marmite repulsive, it needs to be metabolised by an animal first
Having said all that, I did feed my horse a tablespoon of Marmite a day last summer, and he was definitely less tail swishy than the other horses he shared with at dawn and dusk. As it happens, he likes all sorts of weird flavours and would actually quite happily lick the Marmite off my hand, but as he was already getting chaff with magnesium for his feet, it was easy to just mix it in (and I think it made plain chaff taste better...)
It's worth experimenting and seeing if it works for your horse

(BTW Kis Vihar, any research that was done was done with humans and mosquitos - so if you can bring yourself to either eat Marmite (euch) or a Brewer's Yeast tablet, it might work for you too!)
ETA - because I'm supposed to be researching something else and am procrastinating, I decided to go off and see if there was any evidence... and alas,
there's evidence that it has no effect at all 