Charles F. Scoggin
Resident Farm Veterinary Practice
Charles F. Scoggin
The responsibilities of a resident veterinarian on a breeding farm are limited only by one’s imagination. A thorough knowledge of reproductive management, neonatology, weanling and yearling development, and herd health programs is necessary to optimize the efficiency and success of the operation. The resident veterinarian should also be familiar with treatments for emergent medical and surgical conditions and be able to quickly triage urgent situations. Additionally, a general understanding of athletic training, biosecurity, and nutrition is important in sustaining the most important known commodities on the farm—the horses.
The advantages of serving as a resident veterinarian are numerous. Benefits include a keen awareness of the management practices of the operation; familiarity with each horse residing on the farm; the ability to tailor and render veterinary care based on the veterinarian’s preferences and interests and on the needs of the individual horse; and establishment of a solid professional rapport with the farm’s executives and employees. The resident veterinarian is thus completely in tune with all of the different aspects of the farm, which allows for efficient and timely treatment within the limitations of the operation.
Disadvantages are minor, relative to the advantages. Drawbacks are primarily those faced by other types of solo practitioners, such as decreased interaction with veterinary colleagues, inability to perform advanced procedures in the field because of limited facilities or resources, and a nearly year-round, 24-hour, 7-day-a-week commitment to the farm for all emergencies. Fortunately, these disadvantages can be managed by proper communication and training of staff, recognizing the practical limitations of the operation, and exercising creativity when ideal resources or facilities are not available.
Several topics are discussed in this chapter to provide a general overview of a resident veterinarian’s duties. However, many concepts are beyond the scope of this chapter. A thorough account of breeding farm veterinary practice has been published elsewhere (Knottenbelt et al, 2003). Furthermore, given the dynamic nature of veterinary medicine and the differences in individuals’ personal and professional interests, some content is subjective and based on the author’s personal experiences and preferences in providing veterinary care for a large Thoroughbred breeding operation.
Reproductive Management of Stallions
Proper care of breeding stallions is one of the most important responsibilities of the resident veterinarian. Stallions are a significant source of revenue for the farm in generating breeding fees; are a very active group of horses owing to the frequency of matings; and are exposed to different horses each day, thereby increasing the potential for spread of infectious disease.
Breeding Soundness Evaluations
Maintaining the overall vitality of breeding stallions entails a multifaceted approach. Approximately 60 days before the onset of the breeding season (i.e., commencing December 15), stallions are exposed to an artificial photoperiod consisting of 16 hours of continuous light followed by 8 hours of darkness. Increased light exposure may be useful for priming the hypothalamic-pituitary-testicular axis and stimulating testicular function in preparation for the beginning of breeding season (February 15).
Breeding soundness evaluation (BSE) consists of a physical examination, testicular palpation, testicular ultrasonography (including measurement of testicular size and assessment of testicular echotexture), and serial semen collections and evaluations. It is typically performed before the beginning of the breeding season, but an additional BSE may be necessary if a stallion has a drop in reproductive performance during the breeding season. Semen samples are collected by use of an artificial vagina (AV). Some stallions may not serve an AV, in which case they are allowed to breed a mare in estrus, and a dismount sample is collected and evaluated. Other means of semen collection include aspiration of semen from the vagina of a recently bred mare, use of a condom, and chemical ejaculation. Blood work is commonly performed on each stallion as part of the BSE and typically consists of a complete blood count (CBC), fibrinogen, biochemistry profile, and testing for equine infectious anemia. In addition to screening for underlying disease, these tests are useful for establishing a baseline of normal values should an illness arise later.
Maiden stallions are those that are beginning a career as a breeding stallion and typically have recently been retired from athletic competition. As such, there are both physical and mental adjustments that occur during the transition from performance to breeding. In most cases, the transition period is smooth and uneventful. However, some maiden stallions struggle with this process and experience, for example, an initial drop in body condition, change in demeanor, or foot problems. Fortunately, these issues can be easily mitigated by careful observation and early detection of problems. Concerning the mental adjustments that occur, the most noteworthy change is that a stallion is allowed to express normal stallion behavior, albeit within reason. Stallions must remain respectful to prevent aggression toward human handlers and other horses. It is important to realize that each stallion is an individual, and each new maiden stallion must be treated accordingly.
On arrival at the breeding farm, each maiden stallion undergoes a physical examination, and its medical history is thoroughly reviewed. If necessary, treatment and rehabilitation programs are developed and periodically monitored by the veterinarian. The stallion is vaccinated and dewormed according to the farm’s protocol and, if necessary, screened for infectious diseases such as equine viral arteritis (EVA), equine infectious anemia, and contagious equine metritis (CEM). These diseases are further discussed in a later section of this chapter (see Infectious Causes of Reproductive Tract Disease in Stallions).
After a period of adjustment, maiden stallions are introduced to mating by test breeding mares in estrus. This allows for conditioning of the stallion to the farm’s breeding procedure and evaluation of the stallion’s breeding behavior and semen characteristics. Both patience and repetition are required during the test breeding process because mating is a new experience for these stallions. Some stallions may require only a few breeding sessions before they are trained, whereas others need several attempts before they show interest in breeding. Dismount samples are routinely evaluated for the presence of live, motile, and morphologically normal spermatozoa. A stallion is considered trained after the horse willingly breeds a mare in an efficient and proper manner.
Monitoring and Managing Breeding Performance
When the breeding season begins, reproductive performance is closely monitored. The primary means is by periodic evaluation of individual stallion breeding records. The author uses commercial horse farm management system software,1 which enables assessment of numerous reproductive parameters, including conception rates per cycle, session, and day; number of mares bred per day, week, and month; and overall pregnancy rates. Other means of monitoring stallions’ reproductive efficiency are through the use of manual records and graphs as described by Umphenour and colleagues (2011).
The breeding behavior and libido of individual stallions is also carefully observed. Changes could signify the onset of physical or psychological problems, such as musculoskeletal disease, reproductive tract abnormalities, systemic illness, and response to overbreeding. Orthopedic issues, such as stifle or hock arthritis, can adversely affect stallions’ willingness to breed. In some instances, preemptive measures can be taken to manage arthritic conditions, such as judicious use of systemic and local antiinflammatory agents, limiting the size of the stallion’s book of mares, and properly restraining the mare to prevent sudden and forceful movements during mating.
During the past decade, reinforcement breeding has increased in popularity in commercial live-cover breeding operations. This technique entails collection of the drippings from the stallion as he is dismounting the mare, mixing the semen with a suitable extender, and depositing the sample in the uterus of the mare that was just bred by that stallion. Reports have suggested that reinforcement breeding may have several benefits (Blanchard et al, 2006). These benefits include an increase in the total number of spermatozoa available for conception, an increase in spermatozoa longevity within the mare’s reproductive tract, and reduction in the severity of postbreeding endometritis. In return, improvement may be obtained with regard to per cycle and per season pregnancy rates, and it can help decrease the number of return covers. Indications for use of reinforcement breeding include breeding a stallion with historically low sperm numbers or low number of morphologically normal spermatozoa, managing a mare that has failed to conceive on one or more previous cycles, breeding a mare with an anatomic defect that impedes deposition of semen in the uterus, and instances in which the stallion fails to completely ejaculate following intromission.
Infectious Causes of Reproductive Tract Disease in Stallions
In high-scale breeding operations—especially in live cover situations—venereal disease is an important consideration. Aerobic cultures of the prepuce, fossa glandis, and urethra are routinely scheduled for each stallion to monitor for potential pathogens, such as β-hemolytic Streptococcus spp, Escherichia coli, Pseudomonas spp, and Klebsiella spp. Many of these bacteria can be normal or commensal flora of the stallion’s penis, so results should be interpreted with regard to purity and quantity of microbial growth and in relation to the stallion’s reproductive performance. The author prefers to collect cultures after every 10 mares bred by a given stallion, but the frequency of collection is at the discretion of the clinician and varies from farm to farm. If a pure and heavy growth of a potentially pathogenic bacterium is obtained and associated with poor conception rates, further diagnostic testing is necessary to identify the severity of the problem and determine the appropriate treatment. Careful physical examination of the external genitalia, semen evaluation, ultrasonography and endoscopy of the internal reproductive tract, and postbreeding uterine cultures of mares bred by a particular stallion are all methods that can be undertaken to identify the source and severity of the infection.
Contagious equine metritis is a venereal disease caused by the bacterium Taylorella equigenitalis. Clinical signs of CEM are more commonly seen in mares (e.g., severe vaginitis, cervicitis, and metritis), compared with stallions. Stallions are generally considered to be asymptomatic carriers but can spread the organism through the act of mating. Measures to prevent and control the spread of T equigenitalis include testing and treating all imported stallions and broodmares and undertaking active surveillance of the disease through a state-mandated program. An example of such a program is that outlined by the Kentucky Department of Agriculture (www.kyagr.com/statevet/equine/CEM.htm). The reader is encouraged to consult his or her local agricultural regulatory body for requirements regarding testing and monitoring for CEM.
Equine viral arteritis is caused by infection with the equine arteritis virus (EAV). A thorough review of EVA is provided (Holyoak et al, 2008) in the Suggested Readings. In breeding operations, the most important source of infection is contact with EAV-infected semen from virus-shedding stallions. Not all stallions exposed to EAV will become shedders, but those that do will harbor the virus in the reproductive tract and thus infect mares exposed to their semen. Other modes of transmission of EVA include inhalation of respiratory secretions, contact with infected fomites, and in utero infection. Clinical manifestations of EVA mimic those of other equine viral diseases and include fever, respiratory disease, vasculitis, and abortion. Abortion storms can occur following a herd outbreak of EVA, especially in a naive population of mares. Given the risk for disease transmission and the financial impact EVA can have on breeding operations, some states have a mandatory screening and vaccination program for breeding stallions. For example, the Kentucky Department of Agriculture requires that all Thoroughbred breeding stallions be vaccinated against EAV before each breeding season (www.kyagr.com/statevet/equine/EVA.htm). In addition, maiden stallions or stallions with an unknown history of vaccination must be screened for EAV antibodies before vaccination. Doing so helps ensure that the vaccine—and not a natural infection—was the cause for seroconversion. Seropositive stallions with no history of vaccination are required to undergo further testing, such as virus isolation from semen samples, to determine whether they are shedding EAV. Because testing and vaccination requirements for EAV may vary with locales, the reader should contact his or her state veterinarian’s office for further guidance.
Coital exanthema, caused by equine herpesvirus-3 (EHV-3), commonly causes superficial lesions on the skin of the stallion’s penis (glans and shaft) and prepuce, as well as on the mare’s vulva. These lesions can be painful and may be a cause of reluctance to breed. Diagnosis is made on the basis of clinical characteristics and lesion location, as well as by performing a polymerase chain reaction assay on samples obtained from active lesions. Equine herpesvirus-3 can be spread venereally or iatrogenically through contact with infected fomites. There is no commercially available vaccine for EHV-3, so the best means of prevention is immediate cessation of breeding. The length of sexual rest needed can vary with the stallion, but a 2- to 4-week quarantine period is often adequate to allow the lesions to heal and reduce future infectivity. Preemptive measures of avoiding virus transmission include careful evaluation of the external genitalia of the mare and stallion and use of proper breeding hygiene. Although coital exanthema is generally considered to be a self-limited disease, a topical antimicrobial agent (e.g., silver sulfadiazine cream or nitrofurazone ointment) can be applied to the lesions to prevent secondary bacterial infections. Information regarding the use of antiviral drugs for treatment of EHV-3 lesions is scarce. Topical acyclovir in the treatment of coital exanthema has been reported, but no mention was made with regard to the frequency or duration of administration. Abreva (docosanol 10% cream)2 is an over-the-counter medication available for treatment of mucocutaneous herpetic lesions in humans. Anecdotally, this medication can be applied topically two to four times daily for 7 days in horses with EHV-3 lesions. The length of infectivity and whether EHV-3 is reactivated from latency are not known at present. Most stallions and mares can resume breeding after the lesions have adequately healed. It is common for lesions to leave permanent areas of depigmented epithelium.
Reproductive Management of Broodmares
Broodmare work on a Thoroughbred stud is perhaps the most significant and intensive activity of a resident veterinarian. The significance of this aspect of the job is born from the necessity to produce foal crops for the next generation of equine athletes, whereas the intensity stems from the interplay of multiple different methods and factors used to optimize conception rates. When coupled with the facts that most mares will only be afforded a single mating per cycle and all mares must carry their own pregnancies, reproductive management of Thoroughbred mares can prove to be a labor-intensive and weighty process. Nevertheless, it is a highly rewarding endeavor, especially when the end result produces a graded stakes winner or other type of top athlete.
The objective of this section is to provide a general summary of broodmare management on a commercial breeding farm. Five topics are discussed: estrus detection, reproductive examinations, timing of breeding, postbreeding management, and pregnancy evaluations. This information is not meant to serve as an exhaustive review of mare reproductive physiology; an abundance of veterinary-related publications devoted to reproduction in mares is available, as is a plethora of opinions regarding “ideal” breeding strategies. More comprehensive information can be found in textbooks or current periodicals, and the reader is referred to these resources for additional discussion on mare theriogenology.
Mares are seasonally polyestrous long-day breeders. When exposed to a stallion, they will naturally display signs of estrus or “heat” coincident with the follicular phase of the estrous cycle. Common signs of estrus include assuming lordosis posture, urination, and eversion of the clitoris. Examples of estrus detection include individual teasing, group teasing, observation in pasture, and simulation of breeding. A detailed account of estrus detection on a Thoroughbred breeding farm is provided (McCue et al, 2011). Mares are teased early in the morning with a teaser stallion, which interacts with mares individually in their stalls for approximately 15 to 30 seconds. At least two people assist with teasing to allow for careful evaluation of the mares’ behavior. Restraint may be necessary for those that are shy, have a foal by their side, or have a previous history of poor estrus behavior.
The importance of a good estrus detection program is threefold. First, it identifies mares that are in behavioral estrus and ready to be bred. These mares will be placed on the veterinary list to be evaluated that same day to determine the optimal time for mating. Second, it detects mares that are not in heat. Lack of receptivity is a valuable observation because it may indicate that a mare has recently ovulated (e.g., if she was in heat the day before), suggest that the mare is anestrous or in diestrus, or be used as a secondary sign of pregnancy if the mare was previously bred. Lastly, daily teasing can be used as an adjunct for identifying mares with abnormal estrous cycles (i.e., those that have irregular estrus behavior patterns), mares that are no longer pregnant, and mares that have experienced premature luteolysis secondary to endometritis. These mares will also be placed on the veterinary list for further evaluation to identify abnormalities.
Not all mares readily display signs of heat, despite having other characteristics indicating they are suitable for mating, such as cervical softening, uterine edema, and presence of a dominant ovarian follicle. For this reason, it is important to maintain good reproductive records to enable review of previous breeding performance. Mares with a history of poor estrus behavior can be teased more thoroughly, observed for a longer period of time, or be restrained with a twitch or lip chain to better detect their suitability for breeding. Also, some mares may not show characteristic signs of estrus and instead simply show a change in their demeanor or willingness to interact with a teaser stallion. It is for these reasons that the veterinarian’s familiarity with individual mares is invaluable in determining the optimal time for breeding.
The frequency of teasing depends on the status of the mares. Most foaling mares are teased beginning 25 to 28 days after foaling unless a foal-heat breeding is desired, in which case they are teased beginning 6 to 8 days after parturition. It is important to note that several Thoroughbred breeding sheds do not allow mares to be bred on foal heat before May 1. Barren and maiden mares are teased daily beginning in late January.
Simulation breeding or test jumping is commonly performed on maiden mares before a live cover. It is carried out in a safe area, such as a grass lawn or breeding shed. The mare is properly restrained, and a teaser stallion, fitted with a leather apron to prevent intromission, is allowed to mount the mare to simulate the act of mating. Test jumping is important because it conditions the maiden mare to the process of a live cover and allows the broodmare and stallion managers to gauge the mare’s breeding behavior. Successive jumps are performed until the mare stands quietly for mounting.
Synchronization of estrus is an important consideration in the breeding of Thoroughbreds. This results in part from the universal birth date of January 1 for Thoroughbreds registered in North America. Barren and maiden mares are exposed to an artificial photoperiod consisting of 16 hours of light and 8 hours of dark beginning on December 1. Most mares begin cyclic activity within 60 days of the initiation of light therapy. Other means of estrus synchronization include administration of oral altrenogest (Regumate3; 0.044 to 0.088 mg/kg, PO, every 24 hours) or a combination of progesterone (150 mg, IM, every 24 hours) and estradiol-17β (10 mg, IM, every 24 hours) in oil (P&E4—50 mg progesterone and 3.33 mg estradiol-17β per 1 mL; total dose of 3 mL, IM, every 24 hours) for 10 consecutive days. Mares are examined on the last day of P&E treatment, and a luteolytic dose of prostaglandin-F2α is administered on that day. Successive evaluations are performed at the discretion of the veterinarian, and mares are bred on detection of a dominant follicle. An ovulatory agent (discussed later) is regularly administered in conjunction with breeding. Mares frequently are ready for breeding 5 to 9 days after the last dose of P&E.
Routine methods used for reproductive evaluations are palpation per rectum, transrectal ultrasonography, and vaginal speculum examination. Combined with observing the mare’s estrus behavior, these techniques allow for determining the best time for mating, and a breeding request is submitted to the booking agent. Additionally, most Thoroughbred breeding sheds require a certificate stating the following: (1) the mare is sound for breeding, and (2) a uterine culture performed before breeding was negative for aerobic bacterial growth after 48 hours. The known exceptions are first-trip foaling mares, in which case uterine cultures are not required. Because of the requirement for a negative uterine culture, it is ideal to perform the initial evaluation well in advance of the desired breeding date to finalize culture results before the scheduled day of mating. Further evaluations may be performed in individual cases, on the basis of each mare’s history. At the very least, mares are reevaluated the day before the planned breeding day to ensure they are still suitable for breeding. All mares are examined the day after breeding to detect ovulation and perform any necessary postbreeding treatments.
Several adjunctive methods are available for reproductive evaluation. These include endometrial cytology and biopsy, manual vaginal examination, and calculation of the Caslick index (length of vulva in centimeters × angle of declination in degrees). A thorough physical examination and clinicopathologic testing (e.g., CBC and biochemistry) can be performed in mares with suspected systemic disease, which can have a deleterious effect on fertility. Testing for endocrine disease (e.g., pituitary pars intermedia dysfunction) and endogenous serum progesterone concentrations can be used in mares with irregular estrous cycles or prolonged anestrous.
Imported mares require additional testing. In the Commonwealth of Kentucky, all imported mares must undergo CEM testing and treatment before being bred to a stallion. Rules may vary from one locale to another, so the reader is encouraged to contact his or her agricultural department for information regarding CEM testing requirements.
Timing of Breeding
In most cases, mares are booked several days in advance of the planned breeding date. Doing so greatly increases the chances of obtaining the desired breeding date, especially if they are to be bred to a busy stallion. Consequently, the clinician must be precise with regard to his or her findings and must be able to predict the desired day for breeding to optimize the chances for conception. Precision is accomplished by sequential reproductive evaluations and administration of ovulatory agents. Criteria for breeding include detection of a dominant follicle or follicles (diameter ≥35 mm), good uterine edema, and adequate cervical relaxation. It is important to remember that each mare is an individual, and access to past reproductive history can be incredibly useful in identifying the optimal time for cover, especially in mares who “fail to read the book” regarding normal reproductive behavior and physiology.
In most live-cover situations, the goal is to breed a mare while she is in estrus with a dominant follicle and have her ovulate within 24 to 48 hours of breeding. Synchronization of ovulation is most commonly accomplished by use of an ovulatory agent. The two most commonly used ovulatory agents are the gonadotropin-releasing hormone agonist, deslorelin acetate (SucroMate5; 1 mL [1.8 mg/mL], IM), and human chorionic gonadotropin (Chorulon6; 1500 to 3300 IU, IV or IM). Timing of administration is at the discretion of the clinician, but these agents are most commonly administered the day before breeding or on the day of breeding. The interval to ovulation has been reported to be shorter for mares receiving human chorionic gonadotropin (35.9 h ± 2.8 hours), compared with deslorelin acetate (40.7 ± 3.2 hours). In most live-cover situations, this variation is of little consequence, and the clinician uses the agent that he or she is most familiar with. It is important to document the agent and quantity used so that doses remain consistent and can be determined from the mare’s record.
When a mare fails to ovulate in a timely manner (e.g., 48 to 72 hours after the initial breeding), a second breeding or “double” will be requested for that cycle. Second covers can be difficult to obtain, especially in stallions with large mare books. Nevertheless, they can be requested and are periodically granted. The clinician should not rely on doubles, but instead should strive to limit the number of breedings necessary to one cover per cycle.