tissue reactions and pain associated with daily injections, it is not routinely used.
In many countries there are no commercially available preparations of natural
progesterone approved for use in horses. Natural progesterone is typically prepared
by compounding pharmacies. Several pharmacies in the USA now compound highly
concentrated and long-acting preparations of progesterone. One of these
pharmacies (BET Labs, Lexington, KY) has developed the preparation of a
biocompatible controlled release progesterone-based product that delivers an
adequate amount of progesterone to maintain serum concentrations of progesterone
≥ 2 ng/mL for periods of 7–10 days following one IM injection. The controlled bio
release system used is a proprietary low-viscosity non-aqueous liquid system that
uses an injectable solution designed to deliver the entire dose of progesterone at a
high concentration (300 mg/mL) at a controlled rate for a period of 7–10 days. These
long acting preparations of progesterone despite being non-aqueous do not induce
local inflammatory and tissue swelling at the injection site as often as traditional oil
preparations of injectable progesterone do. An IM administration of a compounded
long-acting formulation of progesterone containing a total dose of 1.5 g progesterone
will maintain blood levels of progesterone above 1.0 ng/mL for approximately 10
days, a level of progesterone sufficient to block oestrous behaviour. In the 2014 FEI
equine prohibited substances list, 17-alpha-hydroxy progesterone is classified as a
controlled substance in the female and a banned substance in the male.
• Altrenogest
Altrenogest or known under the brand name ‘Regu-Mate ® ‘ (manufactured by
Intervet, Millsboro, DE) is a synthetic progestin approved in horses for suppressing
oestrus. Chemically, altrenogest is a 17α-allyl-17β-hydroxy-estra-4, 9, 11-trien-3-one
compound called allyltrenbolone in certain countries. It can probably be considered
as the current ‘gold standard’ for methods of suppressing oestrous behaviour in
mares. Daily oral administration of altrenogest at a dose of 0.044 mg/kg (1 mL per 50
kg [or 110 lb.] of bodyweight) is highly effective for suppressing oestrous behaviour
in mares. (21,22)
Altrenogest has also a number of disadvantages:
It is an oil solution so can be messy to administer
It is expensive
It is inconvenient because of its daily dosing
It is readily absorbed through the skin so individuals handling the solution
should wear gloves.
It is contraindicated for use in mares with uterine inflammation/infection.
There are apparently no long-term deleterious effects on the mare’s general or
reproductive health when treated for prolonged periods (months). In one study by
Shideler et al. mares were treated for 88 days with doses up to five times greater
(0.22 mg/kg) than the recommended label dose (0.044 mg/kg) and showed no signs
of toxicity nor effects on their general health condition. In a subsequent study, 11
mares were treated with oral altrenogest daily for 15, 30, and 60 days so that the
treatment ended in all groups on March 31. No effects on mare’s fertility (first-cycle
pregnancy rate) were documented for all groups. Interestingly, mares treated the
longest became pregnant sooner than control mares. Therefore, performance mares
can be safely treated with altrenogest for prolonged periods and may return to their
original reproductive function once treatment is discontinued. However, one should
Proceedings
of
the
South
African
Equine
Veterinary
Association
Congress
2016
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