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 25-28 September, 2018 | Singapore
of the potential presence of a daily pattern of prolactin secretion, antiprolactinic treatments should best be administered in the morning to achieve a more disruptive effect on prolactin secretion20.
Induction of abortion
The abortion induction properties of antiprolactinic drugs have been well studied for dopamine agonists (cabergoline and bromocriptine), while not as much is known for metergoline. Cabergoline and bromocriptine are effective in terminating pregnancy in dogs when administered at mid-gestation (as prolactin secretion starts around day 25) or later 21,22. When administered after day 40 at oral doses of 5 mcg/kg for 5 days cabergoline is effective in causing abortion in all treated bitches21. If cabergoline administration is started
earlier in pregnancy, at day 25, treatments that are effective later in pregnancy fail in most bitches and the pregnancy continues until terminated by retreatment at day 40. Combination treatment of cabergoline and prostaglandins have been used for induction of late abortion both in bitches and queen11, 12. Also, in our experience alternating PGF and antiprolactinics on consecutive days works well and allows to reduce the dosage of PGF.
Estrus induction
The estrus inducing action of antiprolactinic drugs
was initially thought to be due to the lowering of prolactin concentrations, but studies done at Utrecht have demonstrated that shortening of anestrus occurs irrespective of prolactin concentrations23. All the 3 antiprolactinic products (cabergoline bromocriptine
and metergoline) have been used for oestrus induction in the bitch. Cabergoline and bromocriptine have consistently given positive results, while metergoline’s results have been more variable depending on dosage. Using low metergoline doses (0.1 mg/kg BID) from 100 days after ovulation until the following proestrus, the interoestrous interval will be significantly shortened.
We have used bromocriptine at the dose of 10-25 mcg/ kg in 5 bitches with prolonged anoestrus: 4/5 came
in oestrus within 13-28 days, and all 4 conceived and whelped (unpublished data). In a study on the use of cabergoline (5 mcg/kg, once daily for up to 28 days) in
9 bitches (7 Rough Collies, 1 Shetland sheepdog and 1 English Setter) starting in mid anestrus, fertile oestrus was induced in 10/11 cycles in 24+11 days with a reduction of the interoestrous interval of 1.8+0.2 months24. In our experience, the clinical use of antiprolactinics to induce oestrus has proven to be effective in about 70-80% of cases. Occasionally a bitch may take more than 40-50 days of treatment which may cause the owner to get frustrated and discontinue the treatment.
PROGESTOGENS
Short- and long-acting synthetic progesterone
compounds or progestogens have long been used
in bitches and queens to control reproduction. The only short-acting product currently marketed in most countries of the World is megestrol acetate (MA), available as an oral formulation to determine oestrus suppression (short-term control) in bitches and short- as well as long-term suppression in queens. Long acting progestogens available as veterinary drugs such as medroxyprogesterone acetate (MPA) and proligestone (PROL) are used for oestrus postponement (prolonged control). The mechanism of action by which cyclicity
is blocked involves disruption of pituitary-ovarian communication resulting in lowered LH and FSH release and reduced concentrations of estrogen receptors in target tissues. Duration of effect depends on degree of pituitary responsiveness which increases progressively during anestrus: therefore, early-mid anestrus treatments will be longer lasting than late anestrus treatments. Progestogens act on all target organs of P4 such as uterus (increased endometrial growth and secretion), cervix (closure), motility of reproductive tract (decreased gamete transport), mammary glands (stimulation of growth). Other hormones affected by a (short- or long- acting) progestogen treatments include, estrogens, inhibin and activin which may be decreased, growth hormone (GH) whose secretion by the mammary gland
is increased, and prolactin which is inhibited during treatment and then shows a significant surge once treatment is discontinued. Insulin resistance is mediated by increased GH mammary secretion, and is particularly evident during treatment with MA. Endocrine side
effects are transient and irrelevant when a young and healthy female receives a progestogen treatment with the right dosage and for an appropriate length of time. Unfortunately, overdosing has occurred many times
in the second half of last century both in bitches and queens1,2, and many of these cases have been reported in the literature and cited over and again causing a widespread fear about the use of these compounds.
On the contrary, a large amount of experimental data
is available on the use of progestogens in bitches
and queens which dates back to when these animal species had to be used for approval of marketing of
P4 compounds as human drugs in the ‘60s and ‘70s
of last century. When reading the literature carefully it becomes evident that all the case reports of pyometras, mammary nodules and hypertrophy, diabetes, endocrine imbalances and many others were all due to a) the use of very high dosing, b) too long treatments, or c) choice of the wrong candidate. For instance, treatment during diestrus (when endogenous P4 secretion is active) should be avoided as it could easily cause overdosing even if the correct dosage is used; serum P4 assay
may be useful to identify risk patients, and should be assayed also in queens due to a relevant incidence
of spontaneous ovulations in this species. Diabetic patients or females with a history of irregular cycling,
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43RD WORLD SMALL ANIMAL VETERINARY ASSOCIATION CONGRESS AND 9TH FASAVA CONGRESS






































































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