I'm sorry to hear that Bree is sick. You can use phenylbutazone or banamine with caution in the nursing mare to help control any fever that is leading to the lethargy and decreased appetite that you are noticing. (Normal temperature is 98.0F - 100.8F).
That being said, I would use it at the lowest dose possible and for as short a period as possible. Either should control any fever for about 12 hours. I prefer banamine for control of fever over bute personally, and the paste formulation allows for a weight
appropriate dosage (most mini's are 150 - 250 pounds generally). You can also talk to your veterinarian about using dipyrone if the fever isn't responding to banamine or bute.
With Bute, I would recommend a max dose of 1mg per pound (mini's can be very sensitive to bute). So most mini's it would only be about 150 - 250mg. Most bute is delivered in 1 gram (1000mg) tablets or 1 gram per scoop in powder form. So a small mini would only get like 1/4th tablet or 1/4th scoop powder at most. Smaller mini's will take considerably less.
Both Bute (phenylbutazone) and Banamine (flunixamine) can be passed on to the nursing foal via milk. So you must also monitor baby for any signs of colic suggestive of gastric ulcers
. This is why we use it with caution in nursing mares. But 1 or 2 weight appropriate doses should be fine. We really don't like using it for prolonged periods of time...
You might also consider a 5-7 day course of SMZ/TMS (Trimethoprim-Sulfamethoxazole) as a broad spectrum anti-biotic to aid in the cough and nasal discharge. The rough average dose is 1 tablet (800mg Sulfamethoxazole and 160mg Trimethoprim) per 100 pounds body weight twice daily.
A single dose of penicillin will only last about 12 hours. Generally with penicillin I recommend a twice daily dosage for 5 days. They can get pretty sore at the injection sites with it. Be sure to shake it very, very well before administering in the muscle. There are some potential adverse reactions with it. The procaine penicillin we use in horses typically has 300,000 IU per ml. The standard IM dosage for horses is 22000 IU per kg. of body weight.
As you know, it is important to keep mom feeling well enough to continue eating and drinking normally as baby is pulling a lot of energy and fluid off her. She may begin to produce less and less milk for the foal and then the foal can run into some issues as well.
If she is not improveing or continuing to get worse, I would recommend having her seen by your veterinarian sooner rateher than later as sometimes a nasal swab is needed to submit for bacterial culture and sensitivity to determine if the antibiotics Bree is on are appropriate, or if the bacteria are resistant to then and you need to try a different type.
Also keep an eye on the foal as it is very possible that baby can pick up the viral/bacterial infection as well.
I hope this has fully answered your question. If you have any further questions, please do not hesitate to ask. Also, please remember to rate my response as it helps me determine the quality of my responses, and allow me to continue improving my help going forward.