In the NE, the most common, left attached, will swell up
to the size of a grape sucking blood.
As far as is known, the above does not carry lyme.
In our infected areas, it is the "deer tick" that carrys lyme, smaller than a
period, you'll never see them on any mammal or yourself unless really
lucky. The complete cycle is mice, deer, mammal. Often times, a
human bitten will develop a red circle or patchy red spots if the deer tick is
infected.
There's a lot they don't know about lyme and horses.
Have one who just completed treatment. There's apparently a lot of horses
with lyme, but never show any clinical signs. My vets informed me they
would not bother taking/measuring titers several months out, the readings are
not conclusive and can be all over the place. They will go strictly on the
clinical signs.