General and Systematic Veterinary Pharmacology

SUBCUTANEOUS AND INTRAMUSCULAR ROUTES 

Subcutaneous administration

• This route is useful when slow and continuous absorption is required. The formulation must be isotonic and at physiological pH.
• Certain drugs that are irritating can cause severe pain and necrosis. The rate of distribution of the drug is largely dependent on blood flow and hence, the rate of distribution can be slowed by including a vasoconstrictor.
• Warmth or vigorous massage will increase distribution. Addition of hyaluronidase can enhance drug dispersion. This enzyme hydrolyses the hyaluronic acid polymers that comprises the intercellular cement and thus facilitates diffusion through the tissues.
• Specialised subcutaneous preparations include dermojet and pellet implantation.
• Dermojet is a process where no needle is used. A high velocity jet of the drug solution is projected from a microfine orifice using a gun like implement. The solution passes through the superficial layer and gets deposited in the subcutaneous tissue. It is essentially painless and suitable for mass inoculation.
• Pellet implantation provides sustained release of the drug over weeks or months. The pellet impregnated with the drug is implanted in the subcutaneous tissue. Sialistic (non biodegradable) and biodegradable implants are used.
• Crystalline drug is packed in tubes made of suitable material and implanted under the skin. Constant blood levels can be maintained as the drug is released uniformly over a period of time. If non-biodegradable implant is used, it should be removed after the specified period of time.

Sites for subcutaneous injection in different species

• Cattle, sheep and goat - Fold of flank extending from the caudoventral abdominal wall to the craniomedial aspect of the thigh near stifle joint or loose skin on the lateral aspect of the neck.
• Horse - Loose skin on the lateral aspect of the neck. Normally subcutaneous injection is not preferred in horses.
• Dog - Fold of the flank

Intramuscular administration

• Drugs in aqueous solution are rapidly absorbed after intramuscular administration. However, very slow constant absorption occurs if the drug is administered in oil or suspended in other repository vehicles as depot preparations. It can be used for relatively irritant drugs and such drugs must be administered deep intramuscularly. Intramuscular injections are always painful and large volumes cannot be injected.
• A disadvantage of this route is the possibility of improper deposition in nerves, blood vessels, fat or between muscle bundles in connective tissue sheaths. Whenever drugs are administered intramuscularly, it is always advisable to confirm that the needle is not in the blood vessel. 
• Sites for intramuscular injection in different species:
• Cattle, sheep and goat – 1. Hind limb a) Gluteal region covered by gluteal muscles and b) posterior aspect of thigh between the semimembranous and semitendinous. (In sheep and goat (b) is preferred).
• 2. Neck – In the heavy muscles on the caudodorsal aspect of the neck. Needle can pierce through the following structures – skin, fascia, trapezius, rhomboideus, splenius and complexus, depending upon the length of the needle and force applied. This site is preferred only in well built animals. Care should be taken to avoid vertebral column and the dorsal branch of the XI cranial nerve.
• In indigenous cattle with hump, hump is also preferred to administer intramuscularly.
• Horse - Neck region and brisket region (pectoral muscles)
• Dog - Hind limb between the semimembranosus and semitendinosus.