I got a very young guinea pig yesterday and I dont know the gender so I have referred to her as a she. The person at the pet store said she was almost 4 months old. She cant seem to eat out of her bowl and when I got up this morning she was twitching around her tank. She has a bald spot above her left back hip and it looks sorta crusty or scaled over. She has been squeaking and scratching a lot. Is this something I should worry about?
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Hi!
Please have your guinea pig vet checked; only he can do a hands-on examination and make an informed diagnosis. Your girl has most likely ringworm, which is the most transmittable infection between all mammals (including humans). As the infection has been contracted and not been treated at the shop and as you have clearly been sold an ill guinea pig, you can claim back the vet cost from the shop by presenting the vet bill together with the sales receipt. This should not have happened!

Please do not treat on spec with cheap broad spectrum shop products, as they are useless and will only prolong the suffering and increase your expenses. If you come back if a ringworm infection diagnosis, please make sure that you follow our hygiene recommendations to prevent infecting yourself as well as transmitting it further on.
Alternatively, your vet may diagnose mange mties, which need different treatment with several rounds with a good quality ivermectin/selamectin product.
Ringworm: Hygiene And Pictures
Can you also please specify whether your girl was popcorning with joy or having a fit, which can happen with advanced skin infections or parasitic infestations.
Here is the best sexing guide. Please check the insides of the slit. With boars, it is going straight down while with sows there is a fleshy arrangement just below the surface. With boars you can also feel the penis shaft under the skin when you press very gently on the area above the button. Sows have nothing there.
Sexing Guinea Pigs: How to Sex a Guinea Pig
Have you made sure that boars and sows have been properly separated at the shop? If not, there is a real chance that your piggy could be pregnant if it is a sow. Boars start making babies from 3-5 weeks and sows have their first season between 4-6 weeks; they basically start as soon as they are weaned.
Please also be aware that guinea pigs are social animals and should not be kept on their own.
New Owners' Online Starter " Booklet" For Really Helpful Advice
Here is our new owners' information packet; you will find it very helpful. It includes the answers to the most often asked questions as well as some important information which helps you avoid problems in the longer term.
New Owners' Online Starter " Booklet" For Really Helpful Advice