From https://smallpetselect.com/first-signs-of-illness-in-guinea-pigs/

​SEPTEMBER 8, 2018 BY NATALIE RIGGS

First Signs of Illness in Guinea Pigs

In the wild, guinea pigs hide signs of illness as long as possible as a survival mechanism. Guinea pigs are prey animals, and showing signs of weakness makes them an easy target for predators. According to Dr. Amy Guernsey-Youngblood, practice owner and exotic vet at University Veterinary Care Center in Topeka, KS, “They mask their illnesses for as long as they can. By the time they start exhibiting symptoms they are often very sick and have been fighting the illness for quite some time.” They’ve held on to that instinct to this day, making our job a little harder. Because they are so good at keeping symptoms a secret, the illness may be very serious by the time we catch on.
According to exotic vet Dr. Kristin Valdes at Chicago Exotics Animal Hospital, “ Guinea pigs can be prone to a number of illnesses, such as scurvy (vitamin C deficiency), respiratory infections, tumors, bladder stones or urinary irregularities, lice or mites, fungal problems, and pododermatitis (foot problems like bumblefoot).” Dr. Guernsey-Youngblood adds dental disease and fecal impaction as other common problems she sees in guinea pigs. She reminds patients that, “While some illnesses in guinea pigs are unavoidable – based on the individual’s genetics – many can be avoided or reduced with appropriate husbandry and diet.”

Early Symptoms
Guinea pigs are tight-lipped when it comes to their health. You may have to be a bit of a detective to catch signs of illness before he’s willing to admit defeat. As you get to know your guinea pig – and guinea pigs in general – you’ll become familiar with his quirks and it will be easier to recognize when his behavior isn’t normal.
One of the first hints a guinea pig isn’t feeling himself is a lack of appetite. Does his enthusiasm for dinnertime quickly subside? Is he suddenly leaving leftovers in his dish? Guinea pigs live to eat guinea pig food, so loss of interest in food is a major sign something isn’t right. Getting into the habit of weekly weigh-ins is useful to catch this early.
Dr. Valdes recommends watching for signs like “hunching, sunken eyes, loss of energy, less active and alert, and refusing to interact.” She lists symptoms that might appear shortly as sneezing, drooling, discharge from the eyes or nose, dry/itchy skin or hair loss, lumps and bumps, or abnormal bathroom habits. If you just have a feeling something is off, a vet can run tests to help connect the pieces of the puzzle.

Next Steps
Guinea pigs may need to be removed from their cagemates when being treated for and recovering from an illness. Your exotic vet will tell you whether you need to be worried about the illness being contagious. Dr. Guernsey-Youngblood recognizes that the decision of whether to separate a sick guinea pig from his cagemate depends on the individual. “If they are highly bonded, the separation may be stressful. On the other hand, a day of separation is sometimes useful to determine who is eating how much, and what kind of stools and urine they are passing. All of this is helpful information to share with your veterinarian.” As herd animals, there is also the possibility cagemates may start to bully and pick on a pig suddenly perceived as vulnerable.
Unfortunately, guinea pigs don’t get “colds” or “stomach bugs” like we do that they can get over by themselves in a week or two. They usually need medical intervention to recover from an illness. “Usually by the time you notice your guinea pig is ill, chances are he or she is much more sick than you know,” Dr. Valdes says. “For the reason, it’s important take your guinea pig to the vet the moment you become worried.”
References:
http://www.guinealynx.com/emergency.html
https://www.petfinder.com/pet-care/small-and-furry-care/guinea-pig-basics-illness-warnings/
https://vcahospitals.com/know-your-pet/guinea-pigs-problems