A. Fontbonne, DVM, MSc, DECAR
Ass. Pr., Animal Reproduction--Alfort National Veterinary College (Paris)
Over the past 5 to 10 years, many new drugs have been applied to canine, and sometimes feline, reproduction. Some are not officially approved for use in companion carnivores; their use is only experimental at the moment and further clinical trials are necessary. Due to differences in availability of these drugs in various countries, the frequency of application and the general knowledge of these compounds may vary in a large extend.
The purpose of this lecture is to review the new and future applications of these compounds in canine and feline theriogenology. Note that the uses of anti-progestins are described in a different specific lecture.
1. Uses of Hormones in Canine and Feline Reproduction
1.1 Gonadotrophin Releasing Hormone (GnRH) and Analogs
GnRH is an hypothalamic decapeptide. In GnRH agonists one or two amino-acids have been substituted in the GnRH chemical formulation. GnRH agonists stimulate production and release of gonadotropins from the pituitary with different potencies. Conversely, GnRH agonists, when used at sustained doses, reversibly inhibit the gonadal axis, by down-regulating the anterior pituitary GnRH receptors, after a short period of stimulation.
1.1.1 In Females
In bitches, as in cows, GnRH has been proposed in the treatment of cysts (luteinization).
More interestingly for everyday veterinary practice, GnRH agonists implants may be used in a near future for the suppression of reproductive function in bitches. They induce a prolonged and reversible estrus suppression in adult bitches. However, many treated bitches first come in heat, due to the primary activating effect of these drugs. This problem is not entirely solved at the moment. Some studies have showed that the concomitant use of progestins may lower this adverse reaction, but not suppress it completely.
A new indication of the use of GnRH agonists may also be the postponement of puberty without any estrus response at the time of implantation, without any alteration of growth nor weights during treatment. In the following heats after cessation of treatment, ovulation and luteal function seem to occur normally.
Estrus induction in the bitch represents a very promising use of these hormones. Constant infusion or release of GnRH analogs via minipumps or subcutaneous implants may lead to estrus induction and pregnancies, provided that the GnRH administration is stopped after ovulation, to prevent premature luteal failure. Deslorelin has been manufactured in Australia as a biodegradable subdermal implant. It has been showed to induce a rapid and synchronous estrus. In order to be able to easily remove the implants to prevent secondary luteal failure, deslorelin implants may be inserted just beneath of the vestibular mucosa on the inside of the vulva lips.
1.1.2 In Males
For a long time, GnRH agonists have been proposed for the treatment of cryptorchidism. New indications in the male are:
Suppression of reproductive function in male dogs: males implanted with deslorelin long-acting implants show a decrease of serum testosterone to basal values. After removal of the implant, spermatogenesis reappears and the fertility is restored.
Control of prostatic disease: a significant 50% decrease in the prostatic volume from week 6 to week 44 is observed.
Reduction of aggressiveness: some studies tend to show that the prolonged use of deslorelin implants may reduce the aggressiveness in male dogs and cats.
Gonadotrophins include the pituitary gonadotrophins, luteinizing hormone (LH) and follicle stimulating hormone (FSH), and the nonpituitary gonadotrophins, human chorionic gonadotrophin (hCG) and equine chorionic gonadotrophin (eCG), which is also called pregnant mare's serum gonadotrophin (PMSG).
Note that repeated injections of exogenous gonadotrophins may lead to the production of antibodies against FSH or LH, which may cross react with the endogenous and exogenous hormones and lead to a subsequent decreased response to stimulation or subsequent infertility.
1.2.1 In Females
hCG is well known for ovulation induction in queens and bitches. However, its efficacy depends on the time of treatment. It does not work if the follicles are not fully mature in a pre-ovulatory stage.
Estrus induction in bitches may not be a good indication of the use of gonadotropins. Luteal failure occurs frequently and, therefore, the pregnancy rates remain low. Furthermore, ultrasonographic and histological studies have demonstrated that ovarian follicular dynamics differ widely from those at spontaneous estrous cycles. In felids, gonadotropins display many side effects, like the development of ancillary follicles that secrete estrogen which may lower the conception rate.
Luteinization of follicular cysts is also a bad indication. Veterinarians may be aware that this treatment which enhances the production of progesterone by the ovaries, after a prolonged estrogen secretion by an ovarian cyst, may lead to a CEH-pyometra in the treated bitch.
1.2.2 In Males
Protocols for challenge testing for testicular tissue or treatment of cryptorchidism have been described.
1.3 Prolactin Antagonists
These hormonal compounds are dopamine agonists compounds which act on D2-dopamine receptors of the lactotropic cells of the pituitary gland (cabergoline bromocriptine) or antiserotoninergic compounds (metergoline). The usual indications of the use of these compounds are Galactostasis and Pseudocyesis. However, in the recent years, new indications have been described:
Pregnancy termination in the bitch and in the queen.
Estrus induction in bitches: Dopamine agonists (Bromocriptine, Cabergoline) have been used successfully to induce estrus in bitches. Their action is not fully understood. It does not act solely in reducing the level of serum prolactin. They may also directly stimulate the hypothalamic pituitary axis. The stage of treatment appears to be important, as fertile estrus are more likely to be obtained if the bitches are treated during late anestrus. In case of prolonged anestrus (>8months), a 3 weeks daily treatment with oral cabergoline (5μg/kg) induces fertile estrus within one month in 70% of treated bitches.
Treatment of pyometra in bitches and queens.
Pre-treatment of mammary tumours: most of the mammary tumours are discovered and surgically treated during diestrous period, where the mammary gland is enlarged and often partially filled with milk. A 5-7 days cabergoline treatment just before surgery will facilitate the surgical procedure, help in detecting the mammary lesions, lower the post-surgical reaction and accelerate the recovery of the bitch or queen. Furthermore, due to the decreased progesteronemia, there may be a higher immunity in the operated bitches or queens.
1.4 Prolactin Agonists
Dopamine agonists, which promote prolactin secretion, include metoclopramide and phenothiazines. Metoclopramide may be successfully used in case of hypogalactia to enhance milk production after a normal parturition or a surgical caesarean section in the bitch.
Often used by veterinarians to prevent or suppress cyclicity in bitches and queens, progestins may bring side effects. High doses of progestins or repeated or prolonged exposure to moderate doses of progestins may increase the incidence of uterine pathology, including the CEH-pyometra complex, increase the secretion of Growth Hormone and thus the risk of acromegalic changes, increase the risk of local skin alterations when administered parentally, and promote weight gain (Romagnoli 2003). Their role in promoting mammary tumours after prolonged use remains unclear. For all these reasons, we do not recommend repeated progestins treatments in females, especially in breeding bitches.
1.6 Anti-androgenic Drugs
This term refers to various drugs that exert different actions leading to an anti-androgenic effect. They are mostly used for the treatment of prostatic diseases in dogs including canine benign prostatic hyperplasia.
Delmadinone acetate, cyproterone acetate or finasteride are the mostly known anti-androgenic drugs.
Recently, experimental trials using Osteone acetate (0.25 mg/kg daily for 7 days) showed a significant clinical improvement in dogs suffering from prostatic hyperplasia without any effect on libido or sperm quality.
1.7 Estradiol and Derivatives
Due to a significantly increased risk of induced CEH-pyometra after estrogenic induced abortion, we no longer recommend the use of estrogens to treat mismating in the bitch and queen.
In our practice in theriogenology, we use estrogens only for treating vaginitis in bitches. Estriol (0.5 to 1mg PO daily) for 30 days may be a successful adjuvant therapy of prepuberal vaginitis in young bitches, or to the treatment of vaginitis in adult spayed bitches.
Most anti-estrogenic drugs are receptor blockers, such as clomifene and tamoxifen citrate. However, they may also induce an estrogenic response, due to their partially agonist effect. The relative estrogenic-antiestrogenic effect depends on the species, organ and tissue considered. In bitches, tamoxifen seems to add mostly as an estrogen agonist.
Potential applications in treatment of prostatic diseases in dogs are currently being studied by researchers. But they remain experimental at the moment.
Just before parturition, or before a caesarean section, late-term glucocorticoids may enhance the viability of pups by enhancing the maturation of fetal lungs, as it has been showed in humans. However precise data are missing to confirm this hypothesis.
1.10 Prostaglandin F2α and Derivatives
Natural or synthetic PGF2α are rarely capable of inducing luteolysis in early pregnancy (before day 15 to 20) and therefore have been proved to be efficient abortive agents.
Natural prostaglandins are even better tolerated by queens than by dogs.
New indications of prostaglandins may be the reduction of the duration of inter-estrous intervals in the bitch.
2. New Facts About the Use of Antibiotics in Canine Reproduction
2.1 In the Bitch
Very few reports are available on the qualitative and quantitative analysis of vaginal bacterial microflora of healthy bitches and comparative in bitches with reproductive disorders. Some authors involved aerobic bacteria in some cases of infertility. However, many authors showed that the aerobic bacterial population in the vagina is the same in fertile and infertile bitches. On the other hand, Hirsch et al. (1977) found that the bacterial counts were significantly higher in bitches with a vaginal discharge and with fewer species cultured than on the vagina of normal bitches. Watts et al. (1996) showed that the bacteria found in the uterus during pro-oestrus and oestrus were the same than the bacteria species cultured in the cranial portion of the vagina.
A recent study (Lévy et al. 2006) characterized the aerobic bacteria found in the anterior vagina of breeding bitches suffering from different reproductive disorders, and analysed the sensitivity of these bacteria various anti-infectious agents.
The bacterial species most often isolated from bitches in order of decreasing frequency (Figure 1) were Escherichia coli (27/56 bitches, 48.2% of positive bitches, of which 11 hemolytica E. coli and 16 no hemolytica E. coli), Streptococcus canis (17/56 bitches, 30.6% of positive bitches), Staphylococcus intermedius (14/56 bitches, 25%), Pseudomonas aeruginosa (6/56 bitches, 10.7%), (3/56 bitches, 5.4%) and Proteus sp.(2/56 bitches, 3.6%). Staphylococcus aureus, Ralstonia pickettii, Pseudomonas fluorescens, Flavimonas oryzihabitans, Enterococcus sp. and Citrobacter koseri were isolated only once each.
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Figure 1. Bacterial findings after swabbing from the cranial vagina of 56 bitches suffering from different reproductive disorders.
These bacteria showed a relatively high level of resistance to usual anti-infectious agents. This may be due to the overuse of anti-infectious agents by dog breeders, who often practice auto-medication.
Figure 2. Sensitivity to various antibiotics of predominant aerobic vaginal bacteria.
2.2 In the Male Dog
The main difficulty about the use of antibiotics in the male concerns the treatment of prostatitis, as the prostate is not readily available to anti-infectious agents. Therefore, treatments have to be given for a long period (often > 1 month).
In a recent study conducted at the Alfort Veterinary College (Paris-France) in 35 dogs suffering from prostatitis, bacterial identification was performed by ultrasound-guided tissue biopsy, and the bacterial sensitivity to various antibiotics was recorded. Surprisingly, anti-infectious agents commonly stated as being ideal in the case of prostatic infection, like sulfamides in combination with trimethoprim, had a moderate activity against the bacteria that were identified in these dogs.
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Figure 3. Sensitivity to various antibiotics of predominant aerobic bacteria found in the prostate.