Guinea pig, loosing weight

Question
Hello I have lost 2 weeks ago an 8+year old female guinea pig! she detiorated over 3 weeks, loosing use of her back legs, but eventually gave up after a determined fight. I have 4 other guineas, one is a 3 year old neutered male who was soul mates with the deceased female.
I noticed he was loosing weight also 5 weeks ago so started weighing him also whilst I was caring for the old one.
He had a dose of antibiotics as I thought he had a bit of a wheezy noise, and vet thought it was best to try this. That was 4 weeks ago.
So over the last 4 weeks he has gone from 810g to 660g.
In the middle of this time the old female died.
Last week I took him to vet to get some rabbit rescue food to syringe feed him. But his weight has not really gained at all.
I also noticed he has a large soft lump in his right abdomen, and also a small hard pea sized lump in left abdomen, but he doesn't't seem in pain from feeling these.
He is still eating and always drinks a lot of water.
But obviously he is not eating a lot.
The vet thought that removing lumps is probably best avoided at the moment.
To keep an eye on situation.
I don't know how long to leave him, with loosing weight, and how much of that could be stress or sadness.
Also how dangerous to remove lumps in that area and how related to his weight loss. I know he is still passing small solid droppings, which look normal.
Do you have any suggestions on any action, he just seems quiet, and so thin, other general appearance is still ok.
Thank you.

Answer
Your vet is right, you don't want to put any animal under the stress of surgery when they are not in to shape.

I am not familiar with the rabbit rescue food but you need to be careful with any rabbit food or supplement that they don't have any antibiotics in them. Generally speaking guinea pigs don't tolerate antibiotics. If antibiotics are indicated they should be a sulfa based med such as Bactrim or Baytril. They absolutely cannot tolerate any form of Penicillin.

What happens with the penicillin is a destruction of the natural flora in the gut that is necessary for digestion. The result is a severe belly ache and a secondary infection that depresses the appetite, thus causing weight loss. I am assuming your vet is aware of this and did not prescribe any pcn.

8 years is a very old pig comparable to a 98 yr old human. The sow simply died of old age. At 3 we wouldn't consider age related issues. Weight loss is often caused b a dental malocclusion aka overgrown teeth. Cavy teeth never stop growing so they need something to chew on to help keep them properly worn, otherwise they begin to decline from starvation. This would not have anything to do with the lumps that has now has.

Guinea pigs are prone to certain cancers. That's possibly what it going on with him. In this deteriorate state he would not survive surgery to remove them. If the vet is able to do it an ultrasound might be more diagnostic. He may be able to see the nature of these masses.

Unfortunately guinea pigs do not show us they are I'll until it's too late. At this point his care is paliative, or just for the purpose of keeping him alive in the hopes that he will turn around and stabilize.

If it turns out that the rabbit rescue contains penicillin you an use a product called Critical Care which is designed for guinea pigs. If that is not available I have used baby formula with a soy base.

I know this isn't terribly encouraging, but unfortunately your little guy is very I'll and may not turn around.. Please don't feel that it is anything you have done. These little animals are difficult at best to treat when ill, typically because they are sicker than we think by the time we realize their is a problem.