Every spring, my assistant, since March, coughs and weakens. From the nostrils, white and yellow discharge is observed. Such symptoms are noted until mid-June. Every fall, at the beginning of the stall period, I give drugs against worms. Two times, the interval is ten days. In the spring I also give drugs. There is a slight and short-lived improvement. Then the symptoms return.
I’m certainly not a veterinarian, but I’ll say that worms have nothing to do with discharge from the nostrils. If she had worms, you would have noticed it, your horse has some kind of disease related to the respiratory system, moreover, bacterial etiology. Type of tracheitis or bacterial bronchitis, it is necessary to treat with antibiotics during the period of exacerbation, when there are discharge. Antibiotics can also be injected into the muscle, the back will do. Moreover, the dose matters, if human preparations are used, consider how many times the horse is heavier than a person, and the dose should be so much. But the question arises: how old is your horse?