The homeopathic remedy Aconitum napellas (made from monkshood) is useful for early stages of acute illnesses such as colds, fever, cystitis or even conjunctivitis. Often the illness shows itself after exposure to cold, wind or icy temperatures.
One of the indications that Aconite is the correct remedy is suddenness. The symptoms are also intense - redness, pain, fever.
Another element of this remedy is fear - use it when a traumatic incident has happened and your animal is very fearful after. Use it for anxious animals or before a trip for those who have a fear of travel.
I usually give a low dose of 6c or 12c three or four times within one day. That should cancel out the symptoms. If not then try another remedy suitable to the symptoms.
I use this for myself and find that if I use it before an illness sets it, I clear right up.