The clinical symptoms are as followed but first and for most, contact your veterinarian if you think your horse might have strangles and also quarantine your horse from others as it is a very contagious dz.
1. High temperature. So check at least once a day for a change in temp
2. Depressed and loss of appetite
3. You will note yellow discharge from the nostril.
4. Look for enlargement of glands under the jaw and in the region of the throat last.
I have copied a really good pdf that explains strangles and will answer all your questions:
It is a disease which may be called angina of the horse, but you have right, her scientific name is strangles. This disease is caused by a bacterium called "streptococcus". Young horses (less than 5 years old) catch it more easily than other horses, but they also recover more easily and quickly than the old ones.
This disease is very contagious and is characterized by inflammation of the mucosa of the first respiratory tract.
The bacterium is quite resistant in the outside environment and even more in the crusts, it can survive several days in the water. It may also remain in the organs for several months (healthy carriers).