Guinea pigs are docile animals but they are easily alarmed by inexpert attempts at restraint. When frightened, the animals will move rapidly around their cage and will then be very difficult to catch. To avoid this problem, approach the animal rapidly and smoothly, and grasp it around the shoulders with one hand.
The guinea pig can then be lifted clear of its cage. It is essential that only minimal pressure is placed on the thorax, and that restraint is applied primarily with the thumb and first and second fingers around the shoulders. If the guinea pig weighs more than 200-300g, it is best to provide additional support to the hindquarters by cupping these with the other hand. This is essential when handling larger animals (>1000g) or pregnant females. The thumb should be positioned under the mandible, or alternatively, the forelegs can be held so that they cross beneath the chin so that the animal cannot lower its head to bite if some minor manipulation is to be undertaken.
Guinea pigs can be stomach tubed using a similar technique to the rat. An assistant restrains the animal by grasping it around the shoulders and supporting the hindquarters to prevent undue struggling. A blunted 15-16 hypodermic needle, polyethylene catheter (3-4 f.g.) or commercially manufactured dosing needle, as used for rats, is introduced into the mouth through the interdental space and advanced gently into the oesophagus. A small gag made from a solid plastic rod with a hole drilled centrally may be used to prevent the animal from biting a plastic catheter.
To enable an assistant to administer a subcutaneous injection into the scruff, the guinea pig should be restrained on a suitable surface with one hand positioned along either side of the animal. A small area of skin on the flank should be tented by the operator and the needle introduced into the raised skin, parallel to the body wall. The skin of the guinea pig is thicker than in smaller rodents and provides more resistance to needle passage; hence injection is easier if a short (1-2 cm) 21-23 gauge needle is used.
Intravenous injection can be made into the ear veins, but the animal will usually shake its head during the procedure unless the ear is first treated with local anaesthetic cream (EMLA, Astra). In anaesthetised guinea pigs, it is possible to place an over-the-needle catheter in the lateral saphenous vein (Nau and Schunk, 19931)