Strangles is common down here (New Forest ) most years we get an outbreak, most of our horses have had it or been exposed as youngsters. Symptoms do range and some horses hardly show they've got it. Keep an eye out for general loss of health they look " under the weather" and a bit down, a bit of a raised temp and followed by swellings under jaw but not always, my crossbred never showed that and he was exposed as a foal when one of the horses in the field came down with it.
Keep them quiet and stress free DO NOT COMPETE this will only spread it about further and if concerned call vet. Vets here, tend to let it run it's course because we get it so often and only give covering antibiotics to horses with burst swelling likely to get infection.
Once they've had it they should be immune to it in the future.
Hope that helps