[header] How to know if a pet guinea pig is ill [title] Check its eating patterns. [step] It is not far from the truth to say that healthy guinea pigs eat constantly, so any changes in eating patterns or frequency should always be cause for concern. Sometimes, not eating or eating less than usual may be the only observable sign of a serious condition. 
[substeps] Guinea pigs cannot go more than a few hours without eating before causing potential damage. If your pet has not eaten at all (or very little) in the past 16-20 hours, seek immediate veterinary attention.