Nik
Bathing your guinea pig should be done if it is really necessary because they find it very stressful. If you have to bath your cavy, its important to use specific shampoo because human shampoo can trigger and allergic reaction. You should only wash their body being careful not to get any water or shampoo in their ears and eyes. Rinse off your pet very thoroughly before towel drying them and keeping them in a nice warm environment until they are completely dry to avoid them getting a chill.
