The stallion and mare reproductive system

Chapter 13


The stallion and mare reproductive system





Chapter contents



INTRODUCTION


THE STALLION



THE NON-PREGNANT MARE



Congenital abnormalities



Estrous cycle abnormalities



Perineal conformation abnormalities and injuries



Cervical injuries



Venereal infections



Non-venereal aerobic andanaerobic bacterial endometritis


Mycotic endometritis


Uterine immune and/or fluid clearance incompetence


Developmental, functional, degenerative and neoplastic diseases of the ovaries, uterus and fallopian tubes



THE PREGNANT MARE



PARTURITION AND THE POST PARTUM MARE




INTRODUCTION


It has often been suggested that the equine species inherently achieves comparatively low fertility figures and that domestication makes matters worse. With good management, this is not the case. Analysis of the annual Returns for Mares, published by Weatherbys, keepers of the UK and Irish Thoroughbred Stud Book, shows that some well-managed Thoroughbred stallions, with mares indirectly selected for good fertility by fees, regularly achieve conception rates of 90–100% and live foal rates of approximately 80–90%. Even for the total Thoroughbred population of UK and Ireland, there is evidence that conception and live foal rates have increased and that barren mare rates have decreased over the last 20 years, suggesting improvements in management. Data from the same source demonstrate a linear decline in the fertility potential of mares with age. The live foal rate of 4-year-old mares is approximately 75%, whereas that of 20-year-old mares is approximately 50%. Although directly comparable data do not appear to be available from other countries, there is no reason to believe that the results of similarly managed equine populations are significantly different.


An infertile mare is incapable of conceiving when mated by a fertile stallion and is thus, by true definition, a sterile mare. The latter is uncommon, in contrast to the temporary breeding failures that are relatively common, and these mares are more correctly termed subfertile. A barren mare is one that is not pregnant at end of the breeding season, for whatever reason.


Reproductive success is measured in terms of the birth of a healthy foal and failure may thus involve problems that may occur at any stage of the events which lead to that end, i.e. the stallion (libido, ejaculatory or seminal abnormalities), the non-pregnant mare (failure of conception), the pregnant mare (early fetal death, abortion or periparturient fetal death) and/or the management of all these events. The veterinarian must consider all “links” of the reproductive “chain” when aiding management or investigating problems.



THE STALLION



CONGENITAL ABNORMALITIES





Testicular aplasia/hypoplasia


Aplasia is the rare failure of development of one or both testes. In hypoplasia, which is not uncommon in some native pony breeds and other horses, one or both testicles are smaller than expected for age and breed of stallion and may vary in consistency from hard to soft. Diagnosis is made subjectively by palpation or more objectively by measuring the length, width and height of the fully descended testicle though the scrotum with calipers or ultrasonographic imaging. The condition has been associated with cryptorchidism, hermaphroditism and chromosomal abnormalities, e.g. XXY (Klinefelter’s syndrome). Normal fertility (cryptorchidism), reduced fertility or infertility is observed. Collected ejaculates can range from totally aspermic to within normal in vitro parameters. The condition must be differentiated from testicular atrophy.




Cryptorchidism


Stallions with cryptorchidism are colloquially called rigs. Cryptorchidism is the failure of one (unilateral cryptorchidism) or both (bilateral cryptorchidism) testes to descend, with their tunics, though the inguinal canal into the scrotum. Inguinal cryptorchidism (the retained testis is in the inguinal canal) may be temporary or permanent. Abdominal cryptorchidism (the retained testis is in the abdomen) is permanent. In unilateral cryptorchidism, which is most common, sperm production is not necessarily affected and such stallions are often normally fertile. Bilateral, especially abdominal, cryptorchids are infertile. A genetic predisposition is suspected for the condition and therefore many breed societies do not register cryptorchids to be used for breeding.


Diagnosis is made clinically by careful visual inspection and palpation of the scrotum, if necessary after tranquilization. Both the visual and palpable absence of one or both testicles and the absence of castration scars on the scrotum are indicative of the condition. Serum estrone sulfate levels (>0.2ng/mL) in horses over 3 years old suggest cryptorchidism. The hCG (6000IU) stimulation test provokes a rise in serum testosterone level (>100pg/mL) as measured prior to and 30–120 min after IV injection in cryptorchids and this test should be used in horses younger than 3 yr old, in donkeys and in others where estrone sulfate levels are equivocal. Clinicians are advised to consult the laboratories to which they refer samples for endocrinologic analyses for their reference ranges and interpretations as assays may differ significantly between laboratories. Ultrasonographic examinations may be used to identify a testicle located in the inguinal canal. Laparoscopic examination may be useful to confirm the presence or absence of abdominal testicles in those horses where a reliable history of a previously attempted castration is not known.


Treatment of choice is the localization of the ectopic testis or testes followed by bilateral castration. Abdominally and sometimes inguinally retained testes are now often removed by laparoscopic surgical techniques.






INJURIES TO THE GENITAL ORGANS



Penis and prepuce


Traumatic injuries may result from kicks at mating, the use of stallion rings and brushes, injury by vulvar sutures at mating, incorrectly prepared or used artificial vaginas, and accidents involving gates, fences or sticks. There may be open skin wounds in addition to hemorrhage and edema.


First aid treatment with local applications of cold water and/or crushed ice is recommended. Systemic non-steroidal anti-inflammatory drugs (NSAIDs) and antibiotic therapy are indicated to reduce swelling and to control secondary infection. Dependent edema and swelling may be controlled by the provision of mechanical support with a custom-made sling using women’s tights or similar materials. The stallion should be kept quiet and sexual stimulation avoided; tranquilizers, which cause penile relaxation, are contraindicated.


Complications include lateral or ventral deviation of the penis associated with fibrosis and adhesion formation, damage to the closed vascular system of the corpus cavernosum penis and reduced libido or abnormal mating behavior associated with pain or apprehension.




Testes


Testicular injuries may be caused by kicks received at mating or by accidental injuries involving fences, gates, poles and harness lines. Scrotal edema, hemorrhage, hematomas, lacerations and orchitis may occur and result in transient or permanent disturbance of spermatogenesis, depending on severity. More rarely, anti-sperm antibody production and epididymal obstruction may follow.


Immediate first aid treatment with local applications of cold water and/or crushed ice is recommended. Systemic NSAID and antibiotic therapy is indicated to reduce swelling and to control secondary infection. Ultrasound imaging may monitor the extent of damage, during the acute and healing phases. Where damage to one testis is severe, unilateral castration may be indicated in order to avoid anti-sperm antibody production.





BEHAVIORAL ABNORMALITIES AND EJACULATORY DYSFUNCTION


The events leading to normal mating are regulated by a complex pattern of reflexes that are under hormonal and autonomic nervous control (q.v.). The phenomenon of sexual excitement and enthusiasm before and at mating is called libido. Sexual arousal results in penile erection, teasing, mounting, intromission, thrusting, ejaculation, resting, withdrawal and dismount.


Ejaculation can be divided into emission and ejection. Emission consists of those events leading to the emptying of the contents of the caudae epididymis, ductus deferentia, ampullae, prostate gland and seminal vesicles into the penile urethra. This is accompanied by closure of the neck of the urinary bladder. These events are controlled by the sympathetic nervous system (α-adrenergic mediation). The preganglionic sympathetic nerve fibers that leave the lumbosacral segment of the spinal cord, with their principal connections in the caudal mesenteric plexus, constitute the efferent arch of the emission reflex. Sympathetic stimuli mediated by the pudendal nerve and sacral segment of the spinal cord lead to ejection, i.e. the voiding of formed semen though the penile urethra. Ejection is principally the result of rhythmic contractions of the ischiocavernosus, bulbospongiosus and urethralis smooth muscles. Rhythmic dorsoventral movements of the tail (“flagging”) and contractions of the anal sphincter accompany ejaculation. Five to ten jets of semen, in decreasing volume, sperm concentration and pressure, are produced.



Low libido


This most frequent of behavioral disorders is characterized by diminished or a lack of sexual interest and arousal when the stallion is presented with estrous mares. In sexually active stallions, over-use, change of location and management, abusive handling, accidents in the covering yard, unsuitably constructed phantoms and the incorrect use of artificial vaginas can lead to this disorder. It is often seen in young or inexperienced stallions that have been actively discouraged from exhibiting normal sexual behavior during competition careers. The abuse of anabolic steroids during a competition career can lead to breeding shyness and is associated with small testicular size and low levels of circulating androgens (q.v.).


A detailed examination must be performed in order to detect abnormalities such as the presence of stallion rings, painful lesions of the penis and physical lameness, which must be eliminated before presenting the stallion to an overtly estrous mare. Retraining stallions with reduced libido to assume normal mating behavior requires time and patience from experienced handling staff. Once the novice stallion has experienced his first ejaculation he will usually exhibit normal libido and mating behavior. Retraining involves the enhancement of sexual arousal by presenting a variety of estrous mares to the stallion to attain maximum stimulation. Allowing the stallion to run freely with receptive estrous mares will often achieve success. Digital stimulation of the base of the penis, with or without the application of hot compresses may help stimulate ejaculation. Non-steroidal analgesic agents, such as phenyl-butazone, can help overcome musculoskeletal sources of pain.


When these methods are not successful, the additional use of pharmacologic agents may be considered. Human chorionic gonadotropin (hCG), 5000IU administered IV 2 h prior to mating, is often tried first. Shy stallions with low levels of circulating androgens can be treated with gonadotropin-releasing hormone (GnRH). The injection of 50μg GnRH SC 2 h and again 1 h before mating has been suggested.


Anxiety in breeding stallions can be treated using 0.05 mg/kg (not exceeding 20 mg) diazepam, administered slowly IV 5–7 min before mating. Mild transient ataxia may be observed for 3–4 min following injection.




Ejaculatory failure


Ejaculatory failure is seen in stallions that exhibit normal mounting, intromission and thrusting, yet fail to emit and eject semen. Frustration leads to aggressive behavior toward mares and handlers. Ultimately, libido declines and at that stage it may be difficult to differentiate from primary libido failure. Ejaculatory failure may be caused by a variety of conditions including musculoskeletal pain, aorto-iliac thrombosis, pleuritis, orchitis, urethritis and penile injuries (q.v.). Poor semen collection technique may produce a negative experience that may lead to partial ejaculation. Retrograde ejaculation into the bladder has been postulated but has yet to be conclusively confirmed in the stallion.


For the treatment of primary ejaculatory failure (including “retrograde ejaculation”) the administration of 0.01 mg/kg norepinephrine (noradrenaline) IM 15 min prior to mating and 0.015 mg/kg carazolol (a β-adrenergic antagonist) IM 10 min prior to mating has been recommended. Reports suggest that 100–600 mg imipramine PO b.i.d. for a minimum of 2 wk may improve ejaculatory competence. When given in tranquil surroundings, 0.66 mg/kg xylazine IV can induce ejaculation at rest, i.e. without erection or mating, in approximately 25% of stallions. In cooperative stallions this technique can be used to collect semen into a plastic bag, which is tied over the prepuce with a girth strap.




Azoospermia and oligospermia


Azoospermia is a rare condition in which the ejaculate is devoid of spermatozoa. It is usually associated with a bilateral blockage of the ampullae or ductus deferens. Treatment consists of transrectal manual massage of the ampullae, extreme sexual stimulation and repeated encouragement to ejaculate. If the blockages can be relieved by this method, subsequent ejaculates and fertility return to normal.


Oligospermia, where ejaculates contain smaller than normal numbers of spermatozoa, is most commonly seen in aging stallions in association with testicular atrophy. The spermatozoa that are present in the ejaculate are usually morphologically normal and exhibit normal progressive motility. Affected stallions must be managed carefully by limiting the number of matings per day to a minimum, by careful mare management.



VENEREAL AND OTHER INFECTIONS OF THE GENITAL ORGANS


Venereal infections can be caused by bacteria (Taylorella equigenitalis, Klebsiella pneumoniae, Pseudomonas aeruginosa) or viruses (equine herpesvirus 3, equine viral arteritis) (q.v.) and may be transmitted at natural mating, through artificial insemination or by iatrogenic mechanical transfer. Generalized infectious processes, while they may primarily involve other organ systems, can affect the male reproductive tract. Pyrexia leads to disturbances in spermatogenesis. This results in a reduction in seminal sperm concentration and fertility approximately 4–6 wk after the pyrexia. Infections of the internal genitalia (orchitis, epididymitis, seminal vesiculitis, prostatitis), unless secondary to injury, are unusual in stallions but may result in pain, visible swelling and leukocytes in the ejaculate, which may lead to reduced or absent libido, poor sperm motility/viability and depressed fertility.



Bacterial infections


T. equigenitalis, K. pneumoniae (capsule types 1, 2 and 5) and P. aeruginosa are distinguished from other equine bacterial pathogens (e.g. Streptococcus spp., Staphylococcus spp., Escherichia coli) by their potential to spread venereally and to cause outbreaks of endometritis within groups of previously healthy mares. The stallion’s external genital skin is normally colonized by a diverse microflora of these non-venereal microorganisms. When T. equigenitalis, K. pneumoniae or P. aeruginosa (q.v.) are introduced to and proliferate on the genital skin, the stallion seldom shows clinical signs of illness or abnormal semen quality but may become a mechanical transmitter of infection to mares, which develop endometritis. Treatment therefore aims to eliminate the organism and to re-establish the normal genital skin microflora.


The Horserace Betting Levy Board (HBLB) of UK has produced a highly successful framework for the control of venereal diseases for the Thoroughbred industry by issuing the Common Code of Practice for the Control of Equine Bacterial Venereal Diseases (including Contagious Equine Metritis) and Equine Viral Arteritis in France, Germany, Ireland, Italy and the United Kingdom. This Code is updated on a yearly basis and other breed societies are recommended to follow similar guidelines. Clinicians in other countries are recommended to seek the guidance of their own appropriate industry and veterinary organizations. Semen intended for export must be collected from stallions that fulfill the health requirements of the importing country.


The HBLB Code of Practice requires that swabs are taken from the urethra, urethral fossa, prepuce and pre-ejaculatory fluid of all Thoroughbred stallions in France, Germany, Ireland, Italy and UK, on two occasions not less than 7 days apart, after January 1 and before the start of the mating season. Similar sets of swabs are also taken from the stallion during the breeding season if there is clinical evidence to suggest that venereally transmitted endometritis is affecting his mares (successively mated mares returning to estrus, often early, with vulvar discharges or cytologic and bacteriologic evidence of endometritis). The swabs are submitted for bacteriologic examination by quality assured designated laboratories that examine specifically for T. equigenitalis, K. pneumoniae (capsule types 1, 2 or 5) and P. aeruginosa, in addition to other potential non-venereal pathogens, and provide official certification. Contagious equine metritis (CEM) (q.v.) is a notifiable disease in the UK, France and Ireland.


If one of these three bacterial pathogens is isolated from a stallion, the start of mating is delayed or mating is stopped until the significance of the organism, in terms of its potential to produce outbreaks of true venereal disease, is determined.


Two strains of T. equigenitalis (streptomycin sensitive and resistant) have been identified but both have clinically demonstrated their potential to produce outbreaks of true venereal disease in mares. The stallion is treated daily for 5 days by teasing him to penile erection and then thoroughly washing the penis and prepuce with chlorhexidine surgical scrub (4% weight/volume), rinsing and drying, and then applying 0.2% nitrofurazone soluble ointment (if available), particularly packing the urethral fossa and diverticulum. Following treatment, it is recommended that a normal genital skin microflora should be established as soon as possible, and an individually prepared bacterial broth culture of specifically selected common equine genital commensals may be applied.


Many strains of K. pneumoniae, identified by capsule type, have been isolated and only capsule types 1, 2 and 5 have clinically demonstrated their potential to produce outbreaks of true venereal disease in mares. Capsule types 7 and 68 and many others have been isolated from the genitalia of individual stallions and mares but have not clinically demonstrated their potential to cause true venereal disease. They have sometimes been isolated from equine feces, urine and other non-genital sites. Therefore, further specialized laboratory examination of the isolate is required before the decision is made to treat the stallion.


If capsule types 1, 2 or 5 are isolated (or another capsule type if it is demonstrated to be causing true venereal disease), the stallion is treated daily for 7 days by teasing him to penile erection and then thoroughly washing the penis and prepuce with non-antiseptic soap and water to remove all the accumulated smegma. Following this, his penis and prepuce are washed with an aqueous solution of 1% hypochlorite solution, rinsing and drying, and then applying soluble gentamicin ointment, particularly packing the urethral fossa and diverticulum. Some stallions may show signs of local sensitivity to this treatment with inflammation and discomfort. In such cases, care must be taken to keep the penile skin supple to prevent cracking. Following treatment, it is recommended that a normal genital skin microflora should be established as soon as possible, and an individually prepared bacterial broth culture of specifically selected common equine genital commensals may be applied. K. pneumoniae is frequently difficult to treat and repeated attempts may be required before success is proven.


Many strains of P. aeruginosa (q.v.), identified by serotype, have been isolated, but no association has been proven with their potential to produce outbreaks of true venereal disease in mares. P. aeruginosa has been isolated from individual mares with endometritis and from equine feces, urine and other non-genital sites, but the organism has also been isolated in outbreaks of true venereal disease. The clinician therefore has no alternative but to treat all isolates as potential venereal disease risks, unless a test-mating program can be arranged. Even if this shows that the isolate is not being transmitted to the test mares, it is probable that the organism will cause endometritis in mares who are “susceptible” to infection and these may be very difficult to treat.


The stallion is treated daily for 10 days by teasing him to penile erection and then thoroughly washing his penis and prepuce with non-antiseptic soap and water to remove all the accumulated smegma. Following thorough drying, his penis and prepuce are thoroughly sprayed with 1% silver nitrate solution. Following treatment, it is recommended that a normal genital skin microflora should be established as soon as possible, and an individually prepared bacterial broth culture of specifically selected common equine genital commensals may be applied. P. aeruginosa is frequently difficult to treat and repeated attempts may be required before success is proven. Recently, success has been achieved following local treatment of enrofloxacin-sensitive strains of P. aeruginosa with 50 mg/mL enrofloxacin. The 10% (100 mg/mL) injection preparation is diluted 50% with water and massaged liberally and thoroughly into the erect penile and preputial skin, daily for at least 7 days.


After the stallion has been treated for any of these three organisms, three sets of urethral, urethral fossa, prepuce and pre-ejaculatory fluid swabs are taken at intervals of not less than 7 days to confirm removal of the organism. The first swab should not be taken until at least 7 days after treatment has been completed. The stallion should then be test mated to at least three mares before the normal mating program is started or resumed.



Viral infections



Equine herpesvirus 3 (EHV-3, coital exanthema)

EHV-3 infection (q.v.) causes the development of small vesicles on the penis and the prepuce up to 10 days after mating a carrier mare. Stallions may exhibit generalized symptoms of lethargy, anorexia and pyrexia, libido is depressed and they may be unwilling to mate mares. The vesicles form pustules before eroding and then ulcerating and becoming secondarily infected with bacteria resulting in purulent crust formation and discharge from the surface of the penis. This may be confused with excessive smegma formation.


Immunity is short-lived but the condition often recurs with less severe clinical signs. The stallion can become a symptomless carrier with periods of typical herpesvirus recrudescence.


Diagnosis is made on the basis of typical clinical signs, often following the occurrence of symptoms in mares covered by the stallion. The definitive diagnosis may be confirmed during the acute stage by isolating the virus from swabs and scrapings taken from the edge of erosions. Histologically, typical herpesvirus inclusion bodies can be demonstrated from fresh lesions and herpesvirus particles can be seen with an electron microscope. Serologic examinations can be used to confirm exposure to the virus. Complement fixing and neutralizing antibodies reportedly peak 14–21 days after infection. The former decline by 60 days while the latter can be demonstrated for (at least) up to 1 yr.


Mating should be suspended for 10 days, during which time treatment is aimed at controlling secondary bacterial infection. Topical disinfection with povidone-iodine surgical scrub, followed by the application of sodium fusidate ointment, is indicated in severe cases. In all but severely affected stallions, lesions have healed by 10 days to 2 wk after infection, and mating may resume. The infection has no direct effect on the fertility of either stallions or mares.



Equine viral arteritis (EVA)

EVA (q.v.), which is spread via respiratory and venereal infection, is caused by a member of the Togaviridae family. While clinical outbreaks of a disease similar to EVA are described in the veterinary literature of the last century, the virus was first isolated from an outbreak in the USA in 1953. Until 1993, the clinical disease had not been reported in the UK or Ireland and the level of seropositivity seen in these horse populations was <1%.


Epidemiologic evidence obtained from serologic studies performed internationally suggests that the virus is being spread globally through increased transport of some populations of horses and semen. Standardbreds, Thoroughbreds and other breeds have been affected by outbreaks in North America and elsewhere. Serologic studies of Warmblood breeds in various European countries have shown significant levels of exposure. The virus is spread by aerosol via the respiratory route and in semen at mating or insemination.


Typical cases show marked pyrexia, conjunctivitis, serous nasal discharge, head and limb edema, periorbital swelling and skin rashes. Stallions can develop edema of the scrotum and prepuce. Hematologic examinations, performed during the acute phase, reveal leukopenia. The virus may be isolated from peripheral leukocytes, nasopharyngeal swabs and urine during the acute phase. Serologic confirmation can be obtained using serum neutralizing (titers >1:4) or complement fixation tests from serum samples taken during the acute and convalescent stages of the disease. Approximately one third of infected stallions become permanent seminal “shedders” and therefore persistent carriers of the disease. Infected and symptomless seropositive stallions should be screened by seminal virus isolation.


Treatment is symptomatic and supportive and most horses recover, with the exception of some pneumonic foals. Prevention is based on identification of acute cases and symptomless carriers, followed by isolation and segregation.


In Kentucky, stallions are examined serologically prior to the start of the breeding season and seropositive stallions are test mated to determine if they are seminal “shedders” of the virus. Shedding stallions can only be mated to seropositive or vaccinated mares. A safe and effective but live vaccine is available in North America but is not licensed for use in many other countries, including the UK. It has been shown to be helpful in limiting the spread of the disease in outbreaks. A formalin-fixed vaccine is licensed for use in Germany and is available in the UK for use under the terms of a Government Animal Test Certificate (ATC), and under similar arrangements in Ireland and France, and is widely used for breeding stallions. A similarly prepared formalin-fixed vaccine has been shown to protect stallions in Japan from becoming semen shedders.


Artificial insemination using transported EVA-contaminated semen can result in the infection of the recipient mare and transmission to other in-contact horses via respiratory spread. Mare owners importing semen to UK and Ireland should establish the antibody status of the donor stallion before importation so that they do not import EVA to these highly susceptible horse populations. (See also the Common Code of Practice for the Control of Equine Bacterial Venereal Diseases (including Contagious Equine Metritis) and Equine Viral Arteritis in France, Germany, Ireland, Italy and the United Kingdom and the British Equine Veterinary Association’s Code of Practice for Artificial Insemination.)


EVA is a notifiable disease in the UK and Ireland.



Protozoal infection



Trypanosoma equiperdum

Dourine (q.v.) is a venereal disease that was prevalent in western Europe during the earlier part of the 20th century but has not been reported to occur there for many years. It is a notifiable disease in many countries. The disease is still seen in horses in Asia, Africa, South America, southern and eastern Europe and Mexico. It is transmitted in semen at natural mating or insemination.


Clinical signs appear from 5 to 6 days to several weeks after infection and include pyrexia, anorexia, edema of the genitalia, discharge from the urethra and characteristic raised urticarial skin plaques (2–10 cm in diameter) that appear on the lower parts of the body and then disappear within hours. If these plaques persist they leave depigmented areas. Small pustules develop in waves on the penis and prepuce. They ulcerate and heal slowly leaving slightly elevated unpigmented scars reminiscent of those seen following EHV-3 infection (q.v.). Penile and generalized muscular paralysis can develop, leading eventually to emaciation, lameness and death. Differential diagnosis includes EHV-3 and the paralytic form of EHV-1. Diagnosis is confirmed serologically with complement fixation test or demonstration of the organism in smears of exudative fluid. Treatment is attempted with quinapyramine sulfate (3 mg/kg SC) but it is not known whether recovered stallions are safe for breeding purposes; eradication is therefore the best policy.



NEOPLASTIC DISEASES OF THE GENITAL ORGANS


Tumors situated on the penis, prepuce and scrotum can cause pain, hemospermia and loss of libido. Squamous cell carcinoma (q.v.) is the most commonly seen penile tumor of horses, often arising from the areas of the urethral process at the tip of the glans, with a predilection for unpigmented skin. The tumor ulcerates and then proliferates and can spread to regional lymph nodes. If removed early in its course the condition may be cured, and surgical debulking followed by laser or cryosurgery has been found to be particularly useful. Where local recurrence occurs, penile amputation is indicated. The diagnosis can be confirmed by histologic examination.


Melanomas, fibromas, sarcoids, hemangiomas and fibropapillomas can be encountered on the skin of the penis, prepuce and scrotum, as they can elsewhere on the body (q.v.).


Tumors of the testis and epididymis are usually unilateral and benign or of low malignancy. They include seminomas (mainly encountered in older stallions), lipomas, Sertoli and interstitial cell tumors. Dermoid cysts, teratomas and adenocarcinomas have also been reported. Diagnosis is facilitated with the aid of ultrasound imaging. Tumors must be differentiated from abscesses or hematomas. Testicular biopsies, not performed without risk, may be indicated to reach a definitive diagnosis. Treatment for seminoma, the most commonly seen equine testicular neoplasm, is unilateral castration, i.e. removal of the affected testicle.



SEMINAL CHARACTERISTICS AND MORPHOLOGY


The evaluation of semen samples is required for so-called “breeding soundness” examinations, infertility investigations and evaluation for suitability for preservation using chilling or freezing. Complete ejaculates can be obtained, from most but not all stallions, with artificial vaginas or condoms. Incomplete samples can be obtained from a stallion’s penis or a mare’s vagina or uterus immediately after dismount. These incomplete samples are of limited value only, but can confirm the presence of live sperm. Ideally, in order to obtain optimal information, a stallion should be allowed to ejaculate several times during the fortnight preceding collection. A 2–3 day rest should then be allowed before collecting two ejaculates spaced 1 h apart. When fertility problems occur with popular stallions during the breeding season it is seldom possible to organize 2 days of rest.


Normal ejaculates are gray to opalescent in appearance, odorless and do not contain clots, blood or urine. Total volume varies according to age, time from the preceding ejaculate and degree of sexual excitement. The proportion of gel to gel-free fractions varies between stallions and especially with degree of sexual excitement. The gel is removed and the gel-free volume of semen is measured. Gel-free semen volumes >40 mL can be considered normal in 500–600 kg stallions.


Sperm concentration can be measured with a hemocytometer chamber or a spectrophotometer calibrated for stallion semen. Depending on the ejaculation frequency and timing of ejaculations prior to collection, sperm concentrations can vary between 50 and 700 × 106 spermatozoa per mL. The total number of sperm per ejaculate is then obtained by multiplying the sperm concentration with the gel-free volume.


Initial motility of spermatozoa is usually estimated subjectively, within 5 min of collection, on a microscope slide at 37°C. Overall motility should range from 60% to 80% with 40–70% progressive motility. Dilution in semen extender will increase these figures due to decreased viscosity. These estimations are subjective and vary from one observer to another. More objective computerized motility analyzers are available for equine use.


As an estimate of sperm longevity, raw sperm kept at 37°C should not show significant decreases in motility over the first 30 min. Motility can be comparatively estimated periodically until <10% sperm are alive, when the semen is maintained in dark and airtight conditions at room temperature (22°C), but precise reference data have not been published. Semen extended in equal volumes of a standard skim milk and glucose extender and kept at 4°C should maintain sperm motility of greater than 40% when warmed to 37°C after 24 h. Correlation between the fertility of fresh raw semen and motility of extended chilled semen at 24 h has not been satisfactorily established. The pH of semen immediately after collection should be 7.2–7.6.


Sperm morphology may be examined by viewing unstained fixed slides under phase contrast or differential interference microscopy. Alternatively, spermatozoa can be stained following fixation and examined by conventional light microscopy. The classical nigrosin/eosin and some other stains allow differential staining of those sperm which were considered live and those that were dead at the time of collection. There are special acrosome stains. Hematoxylin and eosin stain permits the examination for leukocytes and primitive spermatogenic cells. Primary sperm abnormalities are those acquired during spermatogenesis, secondary abnormalities occur during maturation, storage in the epididymis and ejaculation, and tertiary abnormalities occur after collection. Sperm are classified, according to morphologic features, as normal sperm, abnormal heads, detached heads, proximal mid-piece droplets, distal mid-piece droplets, abnormal mid-pieces, and abnormal tails. Further morphologic features can be ascertained using electron microscopic examinations. A normal equine ejaculate should contain a minimum of 50–55% morphologically normal sperm. Sperm chromatin assays are available at specialized laboratories.


No precise correlation between individual seminal characteristics and fertility exists as mare and managerial factors are at least as important, if not more important, for results obtained in a commercial breeding program. The evaluation of repeated semen samples combined with history, physical examination and behavioral observations must be considered prior to making conclusions and recommendations concerning an individual stallion’s fertility. The total number of normal motile spermatozoa that a stallion can repeatedly produce is probably the most useful parameter that can be determined by semen analysis, in terms of the prediction of fertility potential. The numbers and fertility potential of the mares that he is expected to mate or inseminate are very important as are the managerial and veterinary supervision of the stallion and his mares.



THE NON-PREGNANT MARE



CONGENITAL ABNORMALITIES





ESTROUS CYCLE ABNORMALITIES


The normal mare is seasonally polyestrous, with transitional periods of varying length before and after winter anestrus. Normal cyclic ovulatory estrous periods occur, varying with individuals, between February/March/April and September/October/December in the northern hemisphere. The normal estrous period is 3–5 days of estrus (sexually receptive, follicular phase), with ovulation occurring at or near its end, followed by 14–16 days of diestrus (sexually non-receptive, luteal or progestational phase).



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Jul 8, 2016 | Posted by in EQUINE MEDICINE | Comments Off on The stallion and mare reproductive system

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