Horses are always vulnerable to respiratory viruses, particular at season change time. There are several different types but the important factor is to recognise the symptoms quickly and stop work immediately. Keep them in an airy clean environment and warm without getting too hot.
The symptoms of a runny nose, light or thick mucus, maybe a cough, maybe runny eyes, a temperature above the norm, lethargic; can be eased by reducing dust and spores which irritate. Clean bedding, wet hay, damp feed always given on the ground, good ventilation will help. However giving an expectorant to loosen the mucus so it flows out, like Virex which also has a cough suppressant, and keeping the nasal passages open with Nasal Eze which also reduces the dust and spores going up the respiratory tract, will help improve comfort and recovery for the horse. If a virus is ignored and steps are not taken to clear the airways of mucus, the result maybe as secondary infection which will need antibiotics and a much longer recovery. Most simple viruses will go though in 10-12 days. Symptoms after that, especially if the mucus become thick and coloured, may indicate an infection and a veterinarian will be needed.
For more information on coughs and colds check out the links below.