Nonsurgical contraceptives for dogs and cats (Proceedings)
May 01, 2011
Sterilization = any procedure that makes an individual incapable of reproduction - Castration and vasectomy are reported techniques in males and ovariohysterectomy, ovariectomy, and tubal ligation are reported techniques in females. Castration and ovariohysterectomy are the most common surgical sterilization methods in the United States.
Contraception = the prevention of conception
Reproductive physiology of the bitchThe ovaries contain thousands of follicles, each of which contains an egg or ovum. As each estrous cycle begins, a cohort of follicles is selected to begin development. Development is promoted by release of hormones from the hypothalamus (gonadotropin releasing hormone [GnRH]) and pituitary (follicle stimulating hormone [FSH] and luteinizing hormone [LH]). As the follicle develops, it secretes estrogen, which causes the physical and behavioral signs of early heat, or proestrus. Estrogen levels fall about 9 days after the onset of proestrus; at this time, the bitch will stand to be bred (standing heat or estrus) and a surge of LH is released, causing ovulation. The eggs are released from the follicles into the uterine tube, where fertilization occurs.
The egg ovulated into the oviduct is surrounded by a thick capsule, the zona pellucida, and by a layer of cells from the follicle. Spermatozoa introduced into the reproductive tract of the bitch undergo capacitation, a process involving the acrosome reaction on the head of the spermatozoon and achievement of hypermotility. Capacitated spermatozoa digest the layer of cells surrounding the egg and invade the zona pellucida. As soon as one spermatozoon gets to the inner layer of the zona pellucida, entry of other spermatozoa is blocked by an electrochemical reaction so only one spermatozoon fertilizes each egg. Cell division begins immediately. The developing embryo moves into the uterus within days but does not implant in the uterine wall and develop a placenta until 16 to 18 days after the LH surge.
Drug therapy affects normal hormone secretion, decreasing estrous cycling. Examples include:
a. Progesterone – Megestrol acetate (Ovaban™). This is the only approved estrus-suppressing drug for dogs in the United States. Ovaban can be given either during anestrus (0.25 mg/lb once daily per os x 30 days), to prolong time until the next proestrus begins, or within the first 3 days of proestrus (1.0 mg/lb once daily per os x 8 days), at which point the bitch will go out of heat in 5 to 6 days and will not ovulate at that cycle. If used properly, Ovaban should not cause uterine disease or impact fertility in bitches. Possible side-effects of treatment with any form of progesterone in dogs include weight gain, predisposition to mammary neoplasia and uterine disease and induction of diabetes mellitus. There are no approved estrus-suppressing drugs for queens. The side-effects noted above are significant in queens.
Male Reproductive physiology
Testes of male dogs and cats should be descended into the scrotum by 8 weeks of age and must be descended by 6 months of age for the animal to be considered normal. The testes contain spermatogonia, which will divide to form spermatozoa under the influence of the hormone testosterone. Testosterone secretion is stimulated by GnRH and LH release from the hypothalamus and pituitary, respectively. Spermatozoa are manufactured in the testis but are neither motile nor capable of fertilization until after they pass through the epididymis. Spermatozoa ejaculated at the time of semen collection come from the epididymis.
Sclerosing agents are drugs or compounds that are injected into the testes or epididymes to cause localized inflammation and destroy or scar testicular or epididymal tissue, preventing formation and movement of spermatozoa. Many compounds have been investigated.
In 2002, the FDA approved one compound, Neutersol™, for use in puppies aged 3 to 10 months with testicular width between 10 and 27 mm. The compound (zinc gluconate with arginine) is injected directly into each testis, with dose dependent on testicular width. Sedation may be required but most dogs are reported to tolerate the injection well. Immediate side effects include transient swelling of the testes or scrotum and vomiting. Neutersol™ cannot be used in cryptorchid dogs and should not be used in dogs with scrotal irritation or malformation of the testes or epididymes. In a field study of 224 dogs treated with Neutersol™, 223 were considered to be completely sterilized by 6 to 12 months after injection, based on inability to collect normal semen from these males. Testosterone secretion was decreased but not to as great an extent as with castration. Studies are ongoing for evaluation of safety of the product, efficacy as a contraceptive or sterilant and effect of decreased testosterone concentrations on prostate disease and behavior. Neutersol™ is not consistently available at this time; a similar product, Esterisol™, may be available.
Drug therapies work by decreasing normal hormone secretion. Examples include:
a. Progesterone – Progestogen compounds may or may not decrease libido and spermatogenesis but definitely will have the same side effects as described in the female. I do not recommend use of progestogens in male dogs.
Immunocontraception is the use of the body's own immune response to decrease fertility while permitting normal sexual behavior. Our goal in companion animals is perhaps more properly termed "immunoneutering" or use of the body's own immune response to decrease fertility and minimize or abolish sexual behavior.
Research has focused on the use of hormones or tissues unique to reproductive function as antigens. Because these tissues are recognized as "self" they are not generally very immunogenic and must be conjugated with a larger, more antigenic molecule to induce an immune response. Most vaccines created with the conjugates also employ an adjuvant, a non-specific immunostimulant.
The final test of any contraceptive or neutering vaccine is that of pregnancy challenge. Vaccines using pZP in dogs cause erratic estrous cycling and do not consistently prevent pregnancy long-term. Vaccines using recombinant canine ZP proteins conjugated to diphtheria toxin in dogs caused a rise in titers and subsequent inhibition of ovarian follicular development but did not prevent estrous cycling and pregnancy in all cases. Most ZP vaccine studies in dogs report at least short-term infertility in more than 75% of cases but were associated with prolonged proestrus bleeding and estrous behavior and with ovarian cystic disease. In cats, vaccines developed using ZP proteins from dogs, cats, mink, and ferrets all were demonstrated to induce a significant, measurable antibody response but did not protect against pregnancy as the antibodies did not bind to the queen's own ZP in vitro. It may be that variation in sperm binding sites on the ZP vary enough between species to minimize the effect of antibodies raised against ZP proteins.
Another reported problem with immunoneutering vaccines evaluated to date is the adjuvant used. In one study in cats, using Freund's complete adjuvant, 7 of 10 cats developed granulomatous reactions at the injection site and in distant tissues including lymph nodes and brain. One of the 10 cats died of a vaccine-associated sarcoma at the injection site, and 3 of 10 suffered from hypercalcemia and compromised renal function. Granulomatous reactions also have been reported at the injection site in dogs. Current work includes variation in adjuvants used and creation of nucleotide vaccines, which introduce small amounts of antigenic material in plasmids and induce both humoral and cellular immunity without use of adjuvants.
A commercial ZP vaccine with Freund's adjuvant (SpayVac®) was available from 2002-2005 through a Canadian company. As of this writing, no ZP vaccine is commercially available for use in companion animal species.
Another "self" antigen used for immunoneutering vaccines is GnRH. This is a highly conserved, very small protein that must be conjugated with a much larger antigen for an immune response to be elicited. In a study in male dogs using GnRH conjugated to tetanus toxoid, rises in antibody titers against the tetanus toxoid but not against the GnRH were demonstrated. A study in bitches using GnRH conjugated to canine distemper virus proteins demonstrated a rise in titers but no inhibition to conception and pregnancy. A recent study in cats using multiple tandem repeats of GnRH conjugated to proteins from Pasteurella showed high titers against GnRH, lack of follicular development, and no estrous cycling or pregnancy for up to 20 months after vaccination. Finally, GnRH conjugated to hemocyanin from the keyhole limpet and adjuvanted with a commercial preparation using Mycobacterium avium (AdjuVac®) has been demonstrated to decrease testosterone and sperm count in male dogs and cats; work in bitches and queens in ongoing.
A commercial GnRH vaccine using AdjuVac® (GonaCon®) is reported to be undergoing registration for use in hoofstock by the USDA. While there are no reports of a commercial vaccine for companion animals as of this writing, a GnRH vaccine of this sort may well be the next immunoneutering vaccine commercially available in the United States.
References available on request.