While there is no official record of the most commonly seen diseases that affect pet guinea pigs there is still a subjective opinion that is felt through the guinea pig community regarding common ailments of pet cavies. By knowing what is most commonly seen you can be better prepared to monitor for signs and symptoms that your guinea pig may be getting sick.
Guinea pigs should always be eating and defecating. If you see your guinea pig hasn't touched his food and you are seeing less and fewer stools being passed your guinea pig may have ileus. Ileus is when gas builds up in the gastrointestinal tract (stomach and intestines) and due to the lack of normal peristalsis and no food coming in it causes discomfort and the gas is unable to leave the body. This is actually a very life-threatening condition and your guinea pig should receive immediate medical attention to get the proper medications.
Ileus can be caused secondarily by an underlying illness or stressor which in turn causes your guinea pig to stop eating. Things as simple as moving the cage, introducing a new guinea pig, an upper respiratory infection, and even ectoparasites like lice can stress your guinea pig out enough so that he doesn't eat and develops ileus.
If your guinea pig has hair loss and is itching or scratching he may have lice or mites. While the thought of having these things in your house, much less on the guinea pig you just played with, may make you itch all over they can be easy to avoid and treat. Lice, sarcoptic mange mites (scabies), and Demodex mange mites (Demodex) can all cause itching and hair loss. Lice and their eggs are usually seen in the bald patches behind your guinea pig's ears and the mites can be seen microscopically all over the body.
Guinea pigs can give these parasites to each other and can also get them from food and bedding. By freezing your food and bedding before introducing it into the cage you can kill off any potential parasites that may have been lurking in the packages.
Uterine and Ovarian Diseases
Spaying your female guinea pig is definitely recommended for more than just population control if she is housed with a male cavy. Females often develop uterine and ovarian issues including various cancers. Sometimes the uterus and ovaries can be removed even after the problem has been discovered but other times cancer has already spread to other parts of the body rendering it untreatable. A complete ovariohysterectomy can be performed by your exotics vet on your guinea pig at about six months of age to prevent uterine and ovarian diseases just as it would in a dog or cat. While some guinea pig owners are not able to justify the cost of the procedure others see the great benefit to having their guinea pigs longer and not having to pay for emergency treatment when they notice their pig is sick.
Guinea pigs are sensitive to cold air drafts and can easily develop an upper respiratory infection or worse yet, pneumonia. They can even get Bordatella bronchiseptica from your dog, cat, or pet rabbit. Simply keeping your cavies away from drafts will help decrease the likelihood of them getting a respiratory infection as well as washing your hands after handling other animals (guinea pigs at pet stores, your dog or cat if they are coughing or sneezing, and even your rabbit).
More commonly referred to as bladder stones, uroliths often form in the bladder of pet guinea pigs. They cause pain and discomfort and the urine is often bloody due to the irritation the stone causes. Stones are often found on radiographs being taken for a diagnosis of ileus and must be surgically removed.
By keeping a close watch on your guinea pig by making sure is he eating and defecating, washing your hands before and after handling him, freezing his bedding and food before use, and keeping him away from drafts you can prevent the bulk of the most commonly seen guinea pig diseases. There are of course numerous other diseases that affect guinea pigs, therefore, an annual physical examination with your exotics vet is always recommended.