PRACTICE TIP

Choosing the site and frequency of hormone injections

Roy Kirkwood DVM, PhD

Royal Veterinary College

The usual routes of administration for hormones are intramuscular and intravulval. A useful technique for training personnel how to find the site for intramuscular injection is to hold the ear between your thumb and first finger at a point halfway along the base of the ear and one-third to one-half the way towards the tip of the ear (Figure 1). If the animal is facing forward, when the ear is folded back onto the neck (Figure 2) the point were the thumb touches the neck is the correct site for injection (Figure 3). Ensure that a needle of the correct size is used and do not forget to tell them that the needle should enter at right angles (Figure 4).

For intravulval injection, the injection is given externally at the vulva-cutaneous junction and directed about 30 degrees cranially (Figures 5). The maximum needle size is half-inch 20 g (smaller is better). This is a very vascular area, so avoid vulva-mucosal injection.

When deciding whether to inject in the neck or the vulva, you should consider the volume of injection and the biochemical characteristics of the hormone. As a rule, I consider 1 mL to be the maximum volume to be injected into the vulva. The rationale for using a vulval injection is to allow a reduction in the injected dose (and therefore volume). However, before a dose reduction is considered, you must be aware of the biochemical properties of the hormone. The targeted reproductive tissues are either the uterus (and possible mammary gland) and the ovary. It is likely that an intravulval injection will result in higher concentrations of hormone in the venous system of the reproductive tract. Therefore, if the target is the uterus (or mammary gland), you can consider a lower-dose intravulval injection. A hormone that fits here is oxytocin. I agree with Dr. Steve Henry in that I can think of no reason to inject more than 10 IU oxytocin into a sow and, if injected intravulvally, I recommend a dose of 5 IU (0.25-0.5 mL). If the targeted tissue is the ovary, further considerations are the half-life of the hormones and whether they are subject to counter-current exchange between the uterine vein and ovarian artery. If the hormone is not counter-current exchanged, it will reach the ovary via the systemic circulation and so the route of administration is not relevant.

If the product requires that a relatively large volume be injected (e.g., PG600^(R), which requires a 5-mL injection), an intramuscular route is appropriate. Also, both PMSG and hCG have a long half-life so the route of injection is unlikely to influence local concentrations for a significant period; even if it did, these proteins will not be subject to counter-current exchange so a high uterine vein concentration would not influence ovarian artery concentrations. Therefore, reducing the administered dose of gonadotropins is not appropriate and the manufacturer's recommendation should be followed.

A further question is when to use a split-dose (i.e., initially injecting half the dose followed some hours later by the remainder of the dose). This approach works very well when using a prostaglandin for farrowing induction. However, it is important to remember that each of the split-dose prostaglandin injections will induce farrowing because the prostaglandin is counter-current exchanged between the uterine vein and ovarian artery. Because the prostaglandin is metabolized relatively quickly, the second injection serves only to reinforce the luteolytic signal and so induce the 15%-25% of sows that fail to respond to the first injection. When injecting gonadotropins (e.g., PG600^(R)) the half-life of the hormones is such that a physiologically meaningful change in circulating hormone patterns (i.e., a sort of pulsitility) will not be observed.

In summary:

• For intravulval injection, do not exceed 0.5-inch, 20-g needle
• For intravulval injection, do not inject more than 1.0 mL
• Do not exceed 10 IU oxytocin IM; reduce the dose to 5 IU if injecting intravulvally
• If the target tissue is the ovary (not uterus or mammary gland), reduced-dose intravulval injection is appropriate only if the hormone is lipid based and subject to countercurrent exchange. This is true for prostaglandins but not true for gonadotropins such as PG600^(R).
• Gonadotropins should be injected intramuscularly at the manufacturer's recommended dose.

A split-dose regime will only work when each of the low dose injections is efficacious. The second injection merely reinforces the signal and works in those few animals where the first injection was inadequate (i.e., it works just like a clean-up boar). Prostaglandins can be administered as a split-dose but the gonadotropins (PMSG, hCG) can not.