Nonsurgical contraceptives for dogs and cats (Proceedings)

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 bitch

The 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.

Pharmacologic agents

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.

     b. Androgens – The only androgen approved for use in dogs (Cheque™) is no longer available from the manufacturer. This compound never was approved for use in breeding bitches. Other androgens described for estrus suppression are forms of testosterone. Possible side-effects include vaginitis, hypertrophy of the clitoris and liver disease. Mibolerone cannot be used to suppress estrus in queens as the effective dose is nearly at the level of the toxic dose. Hepatotoxicity and thyrotoxicosis are reported in treated queens.

     c. GnRH agonists – These drugs mimic the action of GnRH, causing release of FSH and LH. Initially this will cause estrus but continued treatment with the drug will shut down the system, with no further estrous cycling. These drugs have been demonstrated to prevent estrus in bitches for up to 27 months and have not been associated with decreased fertility. These drugs are not approved for use in dogs or cats in the United States.

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

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.

Pharmacologic agents

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.

     b. Estrogen – Estrogen compounds can be used to induce azoospermia (lack of spermatozoa in the ejaculate) in dogs. However, toxic effects of estrogen include induction of squamous metaplasia in the prostate and pancytopenia. I do not recommend the use of estrogens in male dogs.

     c. GnRH agonists – These drugs mimic the action of GnRH, causing release of FSH and LH. Initially this will cause an increase in testosterone secretion but continued treatment with the drug will shut down the system, decreasing testosterone secretion and spermatogenesis. These drugs have been shown to be effective for up to 2 years in dogs and often are used in exotic species. These drugs are not approved for use in dogs in the United States.

Immunocontraception

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 ZP is made of up three glycosylated proteins that are highly conserved between species. Immune response generally is greater to those ZP proteins from animals far removed from a given species phylogenetically. The abundance of pig ovaries available in the United States at abattoirs has made porcine ZP (pZP) a popular candidate. However, pZP does not cause an immune response in all species, presumably because of lack of enough common antigenic sites to stimulate an immune response. Researchers have tried ZP from other species and again have seen variable titres after injection, and variable binding of antibodies formed to reproductive tract tissues. Little cross-reactivity with non-reproductive tissues has been identified, suggesting these vaccines are not detrimental to other organ systems in treated animals and in animals that may unwittingly be exposed, as might occur if oral vaccines were used in baits to control feral dog or cat populations.

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.