Have 2 guineas they have bald patches

[springandbunnies]

Long haired guinea pig has quite a big bald patch on his back under all the hair
Short haired guinea has tiny bald patch by ear
Have used mite spot on for both so hoping it clears up
Guineas are 15 mnths old

There are lots of reasons for hair loss spring
Mites is a very common reasons in pigs so YY to treating them.
Mites tends to give a bald area on the back and the hair appears 'snapped'

They can get fungal infections

They can get ringworm

Female guineas can lose hair due to hormones (ovarian cysts) but yours are male from your post?

They can barber (either self or each other)

Scurvy can cause hair loss.

There are some good photos on Gorgeous Guineas.
Once the spot treatment has had a chance to work, it might be time to get some Gorgeous Guineas Shampoo and do one of those dreaded B-A-T-H things.

Good Luck.

Most likely mites is the cause. Follow the instructions and re-apply as recommended.

If it doesn't improve, pop them along to the vet. The vet will if like our vet pick up a few skin cells with sticky tape and look under a microscope to see what's crawling about.

Oh, and best to clean all their house and either steam clean or use Pet Safe spray.
Hay can carry some types of mites too.