Conjunctivitis, which is basically inflamation of the white of the eye can be cause by bacteria or anything which can cause an allergic reaction. It can also be caused by a blocked tear duct.
As guinea pigs are so low to the ground it is important that whatever bedding they are on it is dust free. This is why sawdust or shavings are a no no, and in saying this I am going against what I stated in my 'Proper care of guinea pigs.' I can only say that eight years on I am older and wiser!.
The symptoms can be redness, discharge, and nearly always irritation which is usually made worseby the guinea pig scratching.
First check to make sure there is no foreign body, see Eye injury, the hay husk is the most likely, then treat as follows.
Towel wrap the guinea pig and lay it on it's side. Flush out the eye with Optrex eye wash a couple of times. I always use the small dropperbottles which make it easier to administer the wash. Follow this up by puttinga small dab of Brolene eye oinment in. This usually stops the irritation and consequently, the guinea pig from continuing to scratch.
Any ailment of the eye has to be carefully monitored and if an improvement is not seen within twenty four hours, or it becomes more inflamed then it is almost certainly to be a bacterial infection and a visit to a vet will be necessary for it will need an antibiotic, something like Chlormycetin. This seldom fails to succeed.
If the problem is caused by a blocked tear duct what looks like a discharge is nothing but the saline fluid which continually cleans the eyes. With the tear duct blocked, the fluid cannot drain away, internally, and overflows the bottom eyelid. This also sometimes causes the hair to fall out under the eye where the skin is constantly wet.
There is seldom the redness and the irritation is more likely to be in the area which has the hair loss.
Towel wrap, put a couple of drops of Optrex in the eye then massage just in front where the tear duct drains down into the nasal passage. Sometimes the guinea pig will snuffle or sneeze asthe blockage is cleared and the Optrex flushes through. Repeat the procedure a couple of times to make sure that any debris is properly cleared.