Originally Posted by Ryle
As mentioned, Influenza is common in the US but other conditions can show the symptoms your are seeing. If it is influenza or equine herpes virus, then penicillin isn't effective. If it is a bacterial infection you should NOT give antibiotics without talking to a vet to get an accurate dose and treatment period. You should never ever give a single dose of antibiotics to a sick horse as you will just make it more likely that the bacteria will develop resistance to that drug. You can't know for sure what it is without an exam and generally a culture or bloodwork being sent of to determine exactly what the causative agent it.
Likely what helped was the Bute.
This horse should be quarantined and the vet called at the very least for a phone consult. He may recommend that you continue Bute to minimize fever and watch for worsening of symptoms or he may recommend a physical exam.
Very good post! I didn't know that and thanks for enlightening me! I plan on going into Equine Vetrenary Medicene, so I try to learn as much as possible, thanks!