Please do not use the topical spot-on this weekend because
a) it doesn;t work on mange mite infection - it works only on lice and hay mites (static lice)
b) if you do, the vet won;t be able to give an ivermectin injection on monday and will have to wait at least a week (possibly two) until they can.
This does sound like mange mites from your description of the wound area and the fact she is continuing to enlarge it.She will need 3 injections 7-14 days apart - either if ivermectin or preferably doramectin (better penetrating power in the tissues - this is what my vets use as routine)
A short-acting dose of steroid given together with the mite treatment may also help
You may want to ask for some buprecare painkiller (we found this helped relieve the pain associated with scratching - metacam was useless)
For the existing wound - bathe or spray with dilute F10 disinfectant to keep it clean and keep secondary fungal at bay
BOTH PIGS NEED TO BE TREATED....and it will get worse before it gets better. You also need to follow good hygiene practice
You have caught it right at the early stages so it shouldn;t be a problem to clear up. There is lots of information/tips on how I have been treating some very serious cases of mange can in the thread below
https://www.theguineapigforum.co.uk/threads/2-more-dying-piggies.122408/
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