Where do horse lice come from?

have all of my horses at home, never had lice, gelding just got lice in his mane only where would they have come from? I do ride him out on trails and in my personal arena but have been pretty home bound with him since the ehv1 herpes outbreak so we have been riding alone. havent bought any new horses or had any strange ones on my land lately ideas where this came from? he is quarantined from the others but I have only been riding him since my mares are hormonal. they have no lice he does. what carries them? birds or leftover from other horses on trail? thrown out his fly sheets and all horses brushes since I am paranoid about this. 20yrs of horse ownership and first lice!