Small amounts of a drug can be administered directly via the oral route by using a syringe without a needle. Administration in drinking water should be avoided because it is difficult to be sure that the animal has really assumed the correct amount of drug; furthermore, if the drug has an unpleasant taste, the animal may refuse to drink.
Subcutaneous injections (up to 10 ml) are performed on the dorsum or sides of the thorax. The skin is fairly hard and difficult to perforate with a needle; furthermore, guinea pigs react to injections with intense vocalisations. Subcutaneous rehydration therapy is also poorly tolerated by guinea pigs. The maintenance dose is 100 ml/kg/day, divided into several administrations. The fluid often used is Ringer’s lactate; it is essential that all fluids are administered at body temperature to avoid problems of hypothermia.
Intramuscular injections are performed in the quadriceps; up to 0.3 ml can be injected in an adult. Intravenous administration is difficult to achieve even though, theoretically, the auricular, medial saphenous and lateral metatarsal veins could be used. Because of the difficulty in maintaining an intravenous line in place, fluids administered intravenously are usually given as a bolus. If necessary the intra-osseous route can be used, with the entrance in the proximal femur, humerus or proximal tibia. Fluids are administered by bolus injection every 2-4 hours, followed by an inoculation of diluted heparin. The technique for inserting the needle is the same as that used for the cat.
Intraperitoneal injections should be given at the centre of a caudal quadrant of the abdomen and up to 15 ml can be introduced. Before injecting the fluid, aspiration should be performed in order to ensure that the needle has not entered the bowel or bladder. This technique, commonly used in laboratories, is not advisable as routine practice in rodents because it is risky and there is a high possibility of perforating internal organs; in no case should it be used in pregnant animals.
The use of a gastric tube is very stressful and should, preferably, be avoided in conscious subjects.