Summary
Successful reproduction is based on complex physiology unique to each species within a taxa. It is important to understand the normal physiology of each species when considering normal or abnormal reproductive function for that species. Many individual species are not well studied resulting in challenges to the practitioner who is entrusted with correcting reproduction problems. When these challenges present themselves, it is best to evaluate all aspects of the individual's history including nutrition, husbandry, health, and reproductive history and subsequently applying a traditional clinical approach to determine what the cause of abnormality may be.
Within veterinary medicine, the practitioner caring for the exotic pet is typically consulted with decreased reproductive success or abnormal reproductive function. This summary includes: 1) basic reproductive physiology, 2) key elements for successful reproduction, 3) problems encountered with reproductive dysfunction, 4) methods to evaluate the reproductive system, and 5) methods to manipulate the reproductive system.
Basic Reproductive Physiology
Both sexes have gonads which function in gametogenesis (production of sperm/egg) and steroidogenesis (steroid hormones, estrogen, and testosterone). These functions are controlled by a peptide hormone called gonadotropin-releasing hormone (GnRH) which is released in a pulsatile pattern from the pituitary.11 GnRH stimulates the production of follicle-stimulating hormone (FSH) and luteinizing hormone (LH).11 Male and female gamete (sperm and egg) production is controlled by FSH and LH which combined are called gonadotropins. These gonadotropins are responsible for reproductive success by driving sperm/egg production and testosterone/estrogen production in mammals, birds and reptiles.4,8,11 Changes in these hormones will affect the ability of the animal to reproduce normally and will also change reproductive behavior.11 Specifically, gonadotropins are produced via a feedback system, an initial surge of GnRH will trigger production of FSH and LH resulting in increased reproductive function, but when the surge is prolonged there will actually be a decrease in FSH and LH resulting in reduced reproductive function over time.11 Changes in circulating hormones result in changes in an animal's reproductive ability. Although this is true for the majority of taxa there are numerous subtle differences for each species.
Physiology in Egg-Laying Species (i.e., Avian and Reptile)
The avian female reproductive tract (left most developed) includes the ovary (produces the ovum), infundibulum (location of fertilization, albumin/chalazae formed), oviduct, magnum (albumin, sodium, magnesium, and calcium added), isthmus (provides inner and outer shell), uterus, vagina, and cloaca.4 The shell and shell membrane are semipermeable allowing gas exchange (i.e., gas anesthesia can move across shell and shell membrane). Stimulation of gonadal growth is controlled by day length in many species where the pineal gland relays information from photoreceptors to result in pulsatile gonadotropin release.11
The avian male reproductive tract includes testicular tissue which produces sperm. Spermatozoa exit the epididymis to ductus deferens running parallel to the ureter. In passerines and budgies, the ductus deferens forms a seminal glomus which enlarges during breeding season.4 Significant enlargement of the testicles can occur during the breeding season of some species. The terminal projector papillae of the ductus deferens results in cloacal folds and phallus.4 This phallus is used for male copulation and is located ventrally in the proctodeum. Psittacines lack a phallus, instead copulation occurs by contact of the everted cloacal wall which contains a raised papilla which transfers semen to the female's everted orifice near the oviduct.
In the reptile female, body stores of calcium, phosphorus, proteins, and globulins are mobilized during yolk production and shell formation of the egg. In reptiles and birds, hypercalcemia occurs in response to hormone production associated with reproductive activity. Female reptiles undergoing active vitellogenesis will typically have elevated measurable cholesterol and triglycerides with elevations in total protein, albumin, and globulin. In reptiles, the gonads of both species are in the caudal third of the body, lateral to the abdominal aorta (bilateral) with the right gonad slightly cranial to the left.7 Size of the gonads varies depending on age and reproductive status. Preovulatory follicles are uniform, round, and anechoic or hyperechoic (ultrasound).7 After ovulation, the ova change shape to be more elliptical, but the wall of the egg should be smooth.
Specific Physiology for Mammalian Species
The oviduct is somewhat motile and acquires the egg after ovulation. Fertilization occurs in the oviduct and the zygote moves into the uterus. There is species variation in the shape of the uterus, but broadly the mammalian uterus has two horns (Y shaped) and a body. The cervix acts as a protective barrier to infection. The vagina, vestibulum, and vulva serve as the copulatory tract and the final segment of the birth canal.8
In the male, the reproductive tract contains the testes, epididymis, and ductus deferens. The ductus deferens ascends into the abdominal cavity via the inguinal ring and passes over the dorsal aspect of the bladder to enter the pelvic urethra. The penile urethra are a shared outlet for both semen and urine.8 Sperm found within a urinalysis is normal in a reproductively mature male. The epididymis is involved in sperm maturation and storage prior to ejaculation through the ductus deferens. The ampulla is at the terminal end of the ductus deferens and is more developed in some species over others.8 There are species-specific male accessory glands including seminal vesicles, prostate, and bulbourethral glands which produce seminal plasma (the fluid that carries the sperm) during ejaculation.8
Key Elements for Reproductive Success
When evaluating reproductive success, it is important to understand the expectations of the client/owner. The veterinarian may need to manage these expectations to ensure the client/owner is realistic about what is possible. Suboptimal husbandry, poor nutrition, chronic disease, and age (too young or too old) are the most common cause of decreased reproductive success leading to reproductive problems. It is always good practice to check the sex of individuals before proceeding with advanced diagnostics.
Additional factors to consider are environmental distress. Causes of distress for the reproductive animal may include inappropriate social housing, overcrowding, mixed collection, territorial conspecific, owner disruption, inappropriate pair bond, visible predators. Correcting these sources of distress may improve reproductive success.
Seasonally breeding species must have consistent natural or artificial lighting. Even slight changes in light can affect hormone production. Providing appropriate nest boxes/housing with nesting material will promote reproductive behaviors and successful reproduction.
Hypovitaminosis A is known to reduce reproductive fitness (birds, amphibians, carnivores).1,10,12 In amphibians this results in reduced egg hatchability and tadpole survival. In birds, vitamin A deficiency and toxicity is associated with increased time between clutches, reduced hatchability, increased embryonic mortality, decreased survival of chicks, decreased testes size, poor sperm quality, decreased sexual behavior.
Reproductive Abnormalities in Egg-Laying Species
Egg binding, dystocia, prolapsed oviduct, egg-yolk peritonitis, chronic egg laying, oviduct impaction, oophoritis, salpingitis, metritis, ectopic eggs, cystic hyperplasia of the oviduct, neoplasia, cloacal pathology are described for birds and reptile species. In many cases there are underlying husbandry and management issues that need to be addressed. In egg binding cases, make sure to consider that the egg may be quite valuable prior to determining your methods to remove the egg.
Reproductive Abnormalities in Mammals
Abnormal hormonal cycling, early embryonic loss, late stage abortion, dystocia, uterine rupture, uterine inertia, uterine prolapse, poor maternal care, pyometra, neoplasia. In most cases there are underlying husbandry and management issues that need to be addressed. In an emergency situation, it is typically more successful to save the dam over the offspring, but risks and outcomes should be discussed with the client.
Methods to Evaluate the Reproductive System
The methods of evaluating the reproductive system are similar to diagnostic tools used for other organ systems. Assessing general health is critical. A good baseline exam with complete blood count, serum/plasma chemistry evaluation, urinalysis, and physical exam will provide excellent insight for potential abnormalities. Evaluating ionized calcium will provide information on calcium status for animals presenting with egg binding or dystocia. If the iCa is < 1 mmol/L, then calcium supplementation should be given by injection.
Imaging using ultrasound can be helpful to evaluate reproductive structures7 and is the best method to confirm fetal viability. Although ultrasound is used to confirm pregnancy and fetal count, it is less sensitive at both than radiographic imaging. Multi-projection radiograph imaging can allow for definitive fetal count as well as determination of abnormalities such as ruptured uterus or ectopic egg. CT imaging (with or without contrast) may be more valuable than radiographs for identifying reproductive abnormalities. CT combined with positive contrast placed in the reproductive organs may provide the best diagnostic capabilities within veterinary medicine for truly challenging reproductive abnormalities. Rigid endoscopy (i.e., coelioscopy, laparoscopy) can be used for surgical assessment of the reproductive organs (i.e., cystic ovaries, egg-yolk peritonitis) and can allow for biopsy/sampling of tissues.6
Methods for Manipulating the Reproductive System
If the species is a seasonally breeding species, the control of the reproductive system is nearly exclusively based on photoperiod. Changing the length of day will change the reproductive hormone excretion that controls the reproductive system.
Surgical methods for augmenting the reproductive system are used frequently. Ovaries and testicles are critical organs for hormone production and control reproductive behavior. Removal of these organs will result in behavioral changes and manifest in a broad range of presentations from altered social interaction with conspecifics (i.e., flock or herd) or with humans in the same household. These behavioral changes are not always desirable and should be discussed with the client prior to surgery.
Surgeries such as tubal ligation, uterine removal, and vasectomies do not impact hormone production. These surgeries may be more technically demanding, but may have a more pleasant result for the client. Removing reproductive organs (infundibulum, uterus) but leaving the gonad (ovary) will not prevent ovulation and should be used cautiously with an egg-laying species.
Progestins (synthetic progesterone: melengestrol acetate [MGA], Regu-Mate® [altrenogest], Depo-Provera) given to mammalian females can decrease reproductive success by preventing ovulation via negative feedback and will also prevent sperm passage by interrupting sperm/ova transport.1 Exogenous progestins often cause behavioral changes mimicking behavioral signs of estrus. Although progestins are effective contraceptives across mammalian taxa, there are various associated side effects to consider. Reproductive pathology is a known complication of long-term progestin supplementation in carnivores (mammary carcinoma, pyometra, cystic ovaries, endometrial hyperplasia, diabetes mellitus, hydrometra, uterine mineralization) and androgenic effects (i.e., aggression, pelage darkening) can occur in other species. Equids and hoofstock have fewest side effects from progestins where studied.1,2 Melengestrol acetate (MGA) milled in feed is suitable for bovids, giraffes, cervids, camelids, and hippos but should not be used in suids or equids.1,2
Progestins are also used for luteal insufficiency to maintain a pregnancy in mammals. Outside of domestic species (i.e., dog, horse, swine), this use of progestin has mixed results and is poorly studied. In non-domestic species, progestins can assist in pregnancy in some species, but can also cause loss of pregnancy in other species.
Oxytocin causes smooth muscle contraction improving milk letdown and uterine contraction. It relies on normal calcium within the muscle to be effective. The uterine contractions are profound and can be strong enough to cause uterine rupture, severe pain, and exhaustion in the dam (mammal, reptile, and avian). Oxytocin should not be given if the calcium is low.
Lupron, leuprolide acetate, is a short-acting GnRH analog. It causes a brief increase in LH/FSH, estrogen/testosterone, but ultimately downregulates the system resulting in a decrease in circulating hormones. This drug is used in avian species with mixed results. The major challenge for this drug is that the length of action is variable among species and not easily predictable.9
Deslorelin is a long-acting gonadotropin-releasing hormone (GnRH) implant. It is commercially available, but at this point it is poorly studied for most species. Early evidence has shown it can often reduce the sperm and egg production of mammalian and avian species.5,9 However, the length of reproductive disruption is not easily predicted and varies dramatically among species. Use of deslorelin in prepubertal or peripubertal animals may permanently disrupt reproductive function. There is also evidence that GnRH agonists may correct uterine pathology that is complicated by hormone stimulation.3
General dosing: Deslorelin acetate implants (4.7-mg implant) in birds have an average duration of action of approximately 3 months.9 The 9.5-mg deslorelin implants may achieve a longer duration of action with species differences.9 Leuprolide acetate duration is about 2 to 3 weeks.5,9
References
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6. Hanse M, Krautwald-Junghanns ME, Reitmeier S, Einspanier A, Schmidt V. Testicular biopsy in psittacine birds (Psittaciformes): comparative evaluation of testicular reproduction status by endoscopic, histologic, and cytologic examination. J Avian Med Surg. 2013;27:247–257.
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