Principles of Genetic Inheritance and Define Terms

Genetic inheritance is the passing of genetic information from parent to offspring. This is the way in which traits such as coat colour, height and behavioural characteristics are repeated from generation to generation.

Some definitions of important terms used when describing genetics include:

• Gene: A gene is a segment of deoxyribonucleic acid (DNA) and is the unit which transfers traits. This is the physical unit which is passed from parent to offspring. Each gene contains the data for a specific characteristic. Genes are arranged within chromosomes, and the point on a chromosome where the gene is located is known as the gene locus. Chromosomes are found in the nucleus of the cells of the body, and different species have different numbers of chromosomes. Horses have 32 pairs of chromosomes, which are numbered from 1 to 31, with the final pair identified as X and Y.
  
• Allele: Alleles are versions of the same genes, and a foal will inherit one allele from the mare and one from the stallion.
  
• Homozygous: When a foal inherits the same gene from both parents, it is known as homozygous. If a foal inherits the identical mutated gene for a certain genetic condition, then that foal is at high risk of developing that condition.
  
• Heterozygous: Conversely, when a foal inherits different versions of a gene from each parent, they are known as heterozygous.
  
• Genotype: An individual’s genotype is the specific collection of genes they have or their genetic makeup.
  
• Phenotype: The way in which an individual’s genotype is expressed physically is their phenotype.
  
• Dominant: When a foal receives two different versions of a gene from its parents, the gene which is expressed is known as the dominant gene. It is also possible for two alleles to be different, and both to be expressed and this is known as co‐dominant.
  
• Recessive: In contrast to dominant genes, the allele which is not expressed is known as recessive. This means that the trait for which that gene is coded will not be present in that animal.
  
• Mutation: Changes or disruptions in a DNA sequence are known as mutations. Mutations usually occur due to a mistake in copying of DNA during the cell division process. Mutations can also be caused by exposure to radiation or certain chemicals or viruses. Mutations that occur within the egg or sperm can be passed on to the offspring, and these are known as germ‐line mutations. Mutations which occur elsewhere in the body are not passed on, and these are known as somatic mutations.
  
• Sex‐linked gene: When a gene that controls a characteristic is located on a sex chromosome (x or y chromosome), it is known as sex‐linked. Sex‐linked genes are more frequently present on an x chromosome as x chromosomes are much longer and therefore contain more genes. As females have two x chromosomes, they have the chance of a gene on the other x chromosome being dominant. This means that males more often express sex‐linked traits than females, for example, haemophilia.
  
• Lethal gene: When a gene that is essential for growth and development is mutated in a way that results in the death of the animal, this is known as a lethal gene. Dominant lethal alleles need only be present in one copy to be fatal, whereas identical recessive lethal alleles from each parent must be passed to the offspring to be fatal.

To understand genetic inheritance, Mendel’s first and second laws are used. Mendel’s first law, the law of equal segregation, relates to the chances of specific alleles being passed to a parent’s offspring. The copy of the allele for each gene from each parent is chosen at random, meaning that the chance of an allele being passed to the offspring is 0.5, because one is passed from each parent and each parent has two potential alleles to pass on.

Mendel’s second law, the law of independent assortment, states that the segregation of each gene pair happens independently from other pairs. Genes dictating different characteristics are inherited independently of each other, and this means that whether a foal is chestnut is unrelated to how tall the foal will grow.

To identify how alleles move through generations, a monohybrid cross is used. A monohybrid cross is the study of the inheritance of a single characteristic. The monohybrid cross shows the alleles for one characteristic and the dominant relationship between those alleles. It is commonly demonstrated in a genetic diagram where the dominant allele is represented by a capital letter and the recessive allele by a lowercase letter. Figure 15.1 demonstrates a monohybrid cross for brown coat colour. B represents the dominant allele for a brown coat colour, and b demonstrates the recessive allele for a chestnut coat colour.

Equine Breeding Cycles

From 12 months of age colts can begin to produce spermatozoa and puberty occurs between 18 months and 2 years of age. However, full sexual maturity does not occur until 3–5 years of age in most stallions with some stallions remaining fertile into their 20s. Stallions produce new spermatozoa daily, and it takes approximately 57 days for the sperm to fully mature [1].

Although it is possible for a stallion to breed all year‐round, there are several seasonal changes in their reproductive physiology. Production of sperm is affected by daylight hours and reduces during winter months by up to 50%; however, exposure to artificial light has been shown to increase spermatozoa production [2]. Although the sperm output decreases in winter, the quality and motility are unaffected. During the breeding season, a stallion’s testicles are larger and have a greater volume than outside of the breeding season. In some but not all stallions, libido is reduced during the non‐breeding season.

Mares are fertile and can be bred from 4 years of age, reaching peak fertility around 6 years of age. Fertility declines from 15 years onwards, and most mares become completely infertile from 20 years of age. Mares are seasonally polyoestrous; they have an oestrous cycle, which consists of ovulation followed by an interovulatory phase, and this cycle is affected by day length. The phase during which the mare ovulates is called oestrus and during this phase the mare is receptive to the stallion. Oestrus typically lasts 3–6 days, but can last longer at the start of the breeding season, due to lower levels of luteinising hormone (LH). The period between the end of one oestrus cycle and the start of the next oestrus cycle is called dioestrus and is also sometimes known as the luteal phase; this phase lasts 15 days typically but varies based on the length of oestrus to maintain a 21‐day cycle. During the short daylight hours of winter, the mare’s ovaries do not function, and this period is called anoestrus. The transition from anoestrus into the breeding season is called the vernal transition, or alternatively, it may be referred to as a spring transition. Spring transition is triggered when daylight hours increase and subsequently exposure to light increases. This stimulates the hypothalamus to produce gonadotrophic‐releasing hormone (GnRH), which in turn stimulates the pituitary gland to release follicle‐stimulating hormone (FSH), triggering follicle growth in the ovary. The transition back to anoestrus is called the autumnal transition. The hormones involved in the oestrus cycle of the mare can be seen in Figure 15.2.

Ovulation marks the beginning of dioestrus, and the mare stops being receptive to the stallion. Following ovulation, a corpus haemorrhagicum and then a corpus luteum (CL) forms in the ovary, and this secretes progesterone. The high levels of progesterone decrease LH, and ovarian function is reduced so that the mare cannot reproduce during this time. During dioestrus, a group of follicles within the ovary start to develop and, towards the end of dioestrus, one follicle becomes larger than the rest. This is known as the dominant follicle and can usually be identified ultrasonographically approximately a week before ovulation. During dioestrus, the uterus is firm, without oedema and the cervix is tightly closed.

At the start of oestrus, the CL regresses, progesterone levels drop, the uterus becomes soft and oedematous, and the dominant follicle increases in size. Growth of the follicles produces oestrogen, which induces the behavioural characteristics of oestrus and makes the mare receptive to the stallion. These behavioural changes are the key indicators of oestrus, and identifying them can be assisted by the use of a teaser stallion. The mare is introduced to the teaser stallion, usually with a fence or wall between them, to see how the mare responds to his presence. If the mare shows signs of aggression to the stallion, like pinning her ears back or kicking, she is not in oestrus. Conversely, If the mare is receptive to him by lifting her tail and urinating, she is in oestrus. The follicles increase in size, and the largest follicle tends to be the one to ovulate. Immediately before ovulation, the dominant follicle enlarges to peak size and softens. Within the ovary is an ovulation fossa, and when ovulation occurs, the oocyte leaves the ovary via this fossa. The follicle rapidly decreases in size due to the expulsion of fluid, and the cycle begins again.

Anoestrus is the period of the year during which mares do not reproduce. In the United Kingdom and most of the northern hemisphere, this occurs over the winter months and is an evolutionary adaptation to ensure foals are not born in adverse, cold conditions. In countries with less distinct seasons or year‐round warm weather, mares can reproduce at any time of year.

While the breeding cycle of horses remains fairly consistent, it can be intentionally manipulated to create potentially better breeding outcomes, and it can be unintentionally affected by other factors. As previously discussed, the breeding cycle of both mares and stallions is affected by exposure to daylight and consequently seasonal changes in day length. In some countries, including the United Kingdom, Thoroughbred racehorses and other disciplines compete in age groups with a universal birthday (for example, 1st January for UK Thoroughbreds). This means that it is beneficial for foals to be produced as early in the year as possible, so they are more developed than their later‐born competitors. The relatively short period of long daylight hours within a year means there is a restricted window in which mares can be bred, and if a mare takes multiple attempts to conceive, this time limit may become a problem. For these reasons, systems have been developed to extend the breeding season by exposing horses to artificial light during shorter days, usually to bring forward the start of the breeding season. It appears that daylight is the seasonal factor that most affects the equine breeding cycle rather than ambient temperature. Although an increase in body temperature prior to ovulation has been identified in one study [3], increased ambient temperature does not appear to advance the onset of the breeding season [4]. It is still commonly believed that ambient temperature affects the breeding cycle and without further research, it seems reasonable to assume that both daylight and temperature play a role, as they have been shown to in other species [5].

The breeding cycle may also be affected by food availability and feeding patterns. Minor changes in body condition score do not appear to affect the breeding cycle [4]; however, the ability to access food frequently and trickle feed as horses naturally do in the wild is important. Mares that have constant access to roughage are significantly more reproductively efficient than mares that are fed the same amount of roughage but in restricted time blocks [6]. Access to quality feed in sufficient amounts is especially significant in older mares, past their peak breeding age.

It is possible to manipulate the breeding cycle of the mare with medication to increase the chances of successful breeding. Manipulation of a mare’s cycle can assist in timing ovulation around the availability of the selected stallion, make breeding techniques such as embryo transfer possible, allow more frequent attempts at conception and control the behavioural aspects of oestrous.

Some commonly used hormonal preparations include:

• Prostaglandin: This is used to synchronise oestrus in line with stallion or semen availability. It is also used to shorten the interovulatory period (also known as ‘short‐cycling’) to decrease time to repeat insemination or from foal heat.
  
• Gonadotropin‐releasing hormone: This is used to speed up or induce ovulation.
  
• Human chorionic gonadotropin: This is also used to speed up or induce ovulation.
  
• Altrenogest: This is a synthetic progesterone that halts oestrus effectively keeping the mare in dioestrus until the medication is discontinued.

There are several methods available for breeding horses, including natural covering, artificial insemination and embryo transfer.

Natural covering involves introducing the stallion to the mare when she is in oestrus and receptive. This can be done with both horse’s leads in hand or by turning them out in a paddock. Some mares can be initially aggressive towards the stallion before allowing him to mount her even during oestrus, and may require further restraint such as twitching or hobbling. It may be an appropriate precaution for the mare to wear covering boots; felt boots which cover the hoof and protect the stallion from injury should she kick him. Natural covering has the benefit of being the least time and resource‐consuming breeding method; however, it restricts stallion selection to a smaller number available within travelling distance. For Thoroughbreds, where the offspring is intended to be raced, natural covering is the only method permitted by Weatherbys.

Artificial insemination (AI) involves introducing fresh, chilled or frozen semen into the uterine body as close to ovulation as possible. The mare’s cycle is closely monitored by frequent ultrasound examinations, to ensure insemination occurs close to ovulation, to increase the chance of successful conception. Fresh semen must be used as soon as it is collected. This is only really used when the mare and stallion are at the same stud, but the mare cannot be covered due to her behaviour, risk of injury or presence of a young foal. The decision to use chilled or frozen semen largely depends on the choice of stallion and what is available. When using chilled semen, insemination must take place within 24–48 hours; this restricts selection to stallions within Europe for mares in the United Kingdom. Insemination with frozen semen has the advantage of increasing the number of stallions to choose from. The semen can be transported from anywhere in the world and stored indefinitely. Therefore, it can be ready and available at any time. Frozen semen needs to be inseminated within 12 hours of ovulation, so monitoring for ovulation is frequent. The data on pregnancy success rates is mixed, with some papers [7, 8] reporting lower success rates with frozen semen and some studies [9, 10] finding similar success rates between the two semen types. Factors such as experience with the type of semen for insemination, transportation conditions of the semen, stallion selection and mare reproductive health and anatomy may affect the success rates, and it is not as simple as one semen type is better than another.

Embryo transfer (ET) is the process of removing an embryo from the biological dam and placing it in the uterus of a recipient mare, to carry the pregnancy and birth the foal. This method can be utilised for mares that are not reproductively fit and able to carry a pregnancy to full term, and give birth or mares with other pre‐existing conditions that preclude this, such as laminitis. The main use of embryo transfer is to produce foals from valuable competition mares while allowing the mare to continue competing. One further benefit is that it allows more than one foal to be bred from one donor mare in a season.

The advantages and disadvantages of natural covering, AI and ET are laid out in Table 15.1.

While it is possible to surgically transfer an embryo laparoscopically via a flank incision, a non‐surgical technique is much more widely used. Successful transfer requires careful planning and chemical manipulation of the donor and recipient mares’ cycles to ensure they are both in oestrus at the same time. Recipient mares are selected based on reproductive and general health, age, size and proven record of carrying foals to term, producing sufficient milk and demonstrating strong maternal instinct. It is beneficial to have several recipient mares available to ensure the synchronisation of cycles. Stallion selection is also important, and semen should be from a stallion with a proven record of high fertility and retaining semen quality when it is chilled or frozen. Most donor mares are artificially inseminated rather than naturally covered and fresh or chilled semen is preferable over frozen in this instance.

Once the donor mare has been successfully inseminated, approximately 1 week from ovulation the embryo is flushed from the donor mare’s uterus via a filter that catches the embryo (Figure 15.3). The embryo is then transferred to a dish, where it is evaluated and washed. The embryo is then transferred into an empty, sterile semen straw and inserted into the uterus of the recipient mare using an insemination gun. In the recipient’s uterus, the embryo is able to develop, as normal, to full term and be delivered and raised by the recipient mare. Recipient mare selection is very important, and using a mare which is significantly different in size can adversely affect foal development; however, once this is accounted for, foals born by ET will grow and progress in the same way a foal born to its biological dam would [11].

With all breeding methods, there are several signs and methods of pregnancy diagnosis in the horse. Behaviourally, the mare should not show further signs of oestrus and not become receptive to the stallion again; however, some mares do. The embryo will travel to the uterus around 6 days after fertilisation and will move between horns, which helps in maternal recognition of pregnancy. Around day 16, the embryo will settle in one place, and this is when pregnancy diagnosis starts to be possible. The most commonly used and accurate method of pregnancy diagnosis in the early stages is a transrectal ultrasound examination of the reproductive tract, and this allows confirmation of pregnancy from 16 days or sometimes even as early as 10 days. Transrectal ultrasound examination is also beneficial for early detection and reduction of twin pregnancies and assessment of foetal viability. Pregnancy may also be diagnosed by transrectal palpation of the reproductive tract and visual inspection of the vagina and cervix. Pregnancy diagnosis via blood test is possible, and the type of test depends on the stage of pregnancy. Equine chorionic gonadotropin (eCG), which is produced by the placenta from 45 to 90 days gestation can be detected in maternal blood. After 90 days of gestation, oestrone sulphate is produced by the foetus itself, and this can be tested from a maternal blood sample.

Despite the effort and cost invested in getting a mare pregnant, sometimes the pregnancy does not reach full term and terminates before the foetus is able to survive outside the uterus. Causes of abortion in horses can be split broadly into infectious and non‐infectious. Bacterial infection is the most common cause of abortion in the mare and occurs when the placenta comes into contact with bacteria. Usually, this is an ascending infection. A fungal infection via the same route is also possible, but it is much less common. Another infectious cause of abortion is equine herpes virus 1 (EHV‐1), which is a virus most commonly transmitted via direct or very close contact with an infected horse. Other rarer infectious causes of abortion are equine viral arteritis (EVA) and equine infectious anaemia (EIA).

The most common non‐infectious cause of abortion is twinning, and abortion usually occurs in late gestation due to placental insufficiency to support two pregnancies. It is very rare for twins to be delivered at full term, and even rarer for them to survive, with the chances of both twins being born alive at 1:10,000. Another non‐infectious cause is umbilical cord strangulation, which causes a disruption or complete cessation of the supply of blood, oxygen and nutrients to the foal. Uterine torsion is another cause of late‐term abortion. The exact cause of uterine torsion is unknown, but it can sometimes be corrected by rolling the mare with pressure on her abdomen. Most commonly, surgical correction is required and this can be achieved via flank incision understanding sedation or midline laparotomy under general anaesthesia. Uterine torsion, which is not corrected, can restrict blood and oxygen flow to the foetus, resulting in abortion.

Essentials of Ante and Post‐partum Care

Gestation length in horses is variable but usually lasts between 335 and 342 days and a foal born before 320 days of gestation is considered premature. The equine foetus develops almost completely over the first 107 days as follows:

• Day 15: The primitive beginnings of the optical and nervous system form. Limbs begin to form. Cartilaginous tissue is present.
  
• Day 19: The spinal cord begins to form.
  
• Day 25: The tail starts to develop.
  
• Day 25: Hooves begin to form.
  
• Day 28: The heart is divided into two chambers.
  
• Day 34: The lungs and trachea start to take shape. The limbs are fully formed.
  
• Day 35: Vascularisation of the dermis begins. The forebrain, hindbrain and midbrain are distinguishable.
  
• Day 36: The diaphragm is identifiable.
  
• Day 38: The oesophagus and tongue develop. The heart is fully formed.
  
• Day 40: The nasal cavity, sinuses, nasopharynx, trachea and other structures of the upper respiratory tract develop.
  
• Day 48: Bones begin to ossify.
  
• Day 80: Mammary glands begin to form.
  
• Day 97: Hair is present on the skin.
  
• Day 107: Limbs have prominent joints and hooves.

After this initial rapid development, the foetus is an almost fully formed miniature version of a foal and the rest of gestation is focussed on growth.

Around 16 days from conception the embryo fixes in place in the uterus and develops an extraembryonic membrane surrounding it and this is the chorioallantois. By day 20, the allantois begins to develop over the chorioallantois and quickly grows to encase the embryo and chorioallantois. The allantois then becomes the primary provider of blood supply. From day 40, these membranes begin to attach to the endometrium and the placenta continues to develop to full maturity over the next 100–110 days.

Progesterone is a steroid which is essential in the maintenance of pregnancy, and progesterone levels increase in the first half of gestation before dropping around mid‐gestation. Progesterone drops again immediately prior to parturition, and this induces uterine contractions. The other significant steroid hormone in pregnancy is oestradiol, which rises in the second half of gestation and increases the contractility of the uterus ready for parturition. The CL formed at ovulation is responsible for maintaining pregnancy initially; however, this is taken over by the endometrial cups from day 40. The endometrial cups are a collection of cells formed on the placenta, and they produce large amounts of eCG, which in turn, stimulates the ovaries of the mare to form additional CLs, releasing more progesterone to further support the pregnancy in the first trimester.

It is advised that pregnant mares are kept on the fat side of good condition, with a body condition score of 6–7/10; however, obesity can have a negative impact on the foal, as well as pose a health risk to the mare. Over the first eight months of pregnancy, the mare’s nutritional requirements are the same as that of a non‐pregnant mare, and energy requirements gradually increase over the final three months as the foal grows to full‐term size. Mares in late gestation require an increased amount of dietary protein and adequate provision of calcium and phosphorus for foetal development. Lactating mares have higher energy requirements than during pregnancy as they produce milk at 2–3% of their body weight. Dietary protein requirements are also higher during lactation than pregnancy, but calcium and phosphorus requirements are the same as during late gestation.

It is recommended that the pregnant mare should be moved to the foaling location no later than 4 weeks before her due date. This is important not only to ensure the mare is relaxed and settled into her environment before foaling, but also to expose her to the pathogens endemic to that location, so that antibodies are formed and ready to be passed on to the foal. The mare should also be checked for the presence of a Caslick’s. A Caslick’s procedure involves suturing closed the vulval lips to prevent contamination of the reproductive tract. Once the sutures are removed or have dissolved, the vulva remains sealed and this should be cut well in advance of foaling to prevent injury. During the final 4 weeks of pregnancy, the mare should be checked daily for signs of impending foaling.

These signs include:

• Increased udder size
  
• Relaxing of the pelvic ligaments and vulva
  
• Vulval discharge
  
• Secretions or ‘waxing’ of the teats

If the mare produces and drips or streams milk before the foal is born, it is important to make provisions for supplementary feeding of colostrum as soon as the foal is born. Prematurely streaming of milk means the limited amount of colostrum produced at the beginning of milk production is likely to run out before the foal is born.

Normal parturition involves three distinct stages. Stage one can last between 30 minutes and 4 hours and is the preparatory stage for the expulsion of the foal. During stage one, the mare will become restless, and this can be mistaken for colic. The mare will repeatedly lay down and stand back up, sweat up, lift and swish her tail and flank watch. During this stage, the mare should be prepared by wrapping her tail and washing her vulva and udder. The handler should be prepared that the mare may be unpredictable during this time and take care when preparing the mare not to become trapped or injured as she lays down or stands up.

Stage two begins with the rupture of the chorioallantois and the expulsion of a large volume of allantoic fluid. This stage is fast‐moving and should take no more than 30 minutes. The mare will usually stay lying down throughout most of stage two and will push as her uterus contracts to expel the foal. Immediately after the rupture of the chorioallantois, one of the foal’s front feet will appear followed by the other and this slight difference in position of the two forelimbs allows the shoulders to pass through the mare’s pelvis more easily. The forefeet are closely followed by the nose. The front legs, head and neck are delivered first, then the torso and hindlimbs. It is advisable to stay close by to observe but not intervene unless necessary. The mare will usually rest for a while, often with the foal’s hind feet still in the vagina. The umbilical cord should be left to break naturally as the mare stands. Cutting the cord should be avoided as this may result in haemorrhage.

Stage three of parturition involves the expulsion of the foetal membranes, and this should occur between 30 minutes and 3 hours after foaling. The placenta should be tied up above the hocks to prevent the mare stepping on it. Failure of this process is known as retained foetal membranes and can be due to uterine inertia and hormonal dysregulation. Retained foetal membranes are a potential source of uterine infection and, if left untreated, can lead to septicaemia, metritis, endotoxaemia, laminitis and death. Treatment may include uterine lavage and oxytocin administration to induce uterine contractions. When caring for patients with retained foetal membranes it is important to monitor closely for signs of infection or septicaemia such as increased temperature and dark red mucous membranes. Digital pulses should also be checked regularly and comfort levels assessed, in order to identify the signs of laminitis.

The two most common complications during foaling are premature placental separation (also known as ‘red bag’ delivery) and dystocia. Premature placental separation is observed as a failure of the chorioallantois to rupture and the appearance of the red velvety chorioallantois rather than the bluish‐white membranes of the amnion. This is a life‐threatening emergency for the foal as oxygen delivery has been compromised. The chorioallantois must be cut immediately, using blunt‐ended scissors, to allow safe delivery of the foal.

Dystocia is difficulty foaling and can be due to incorrect positioning of the foal, malformation of the foal, oversize of the foal or delivery of twins. Dystocia is a time‐sensitive and life‐threatening emergency for both the mare and the foal. Once the cause of dystocia has been established, the most appropriate intervention can be selected. In some cases of foetal malpositioning, it may be possible to manipulate the foal into the correct position with the mare standing. Alternatively, the mare may be anaesthetised to allow controlled vaginal delivery. The mare is positioned in dorsal recumbency with the hind end elevated by hoisting the hindlimbs upwards (also known as the Trendelenburg position). The foal can then be pushed back into the birth canal and manipulated into the correct position. Ropes or chains can be attached to the foal’s limbs and head to pull the foal out. If the foal cannot be delivered vaginally, then caesarean section is indicated. This requires general anaesthesia and a midline laparotomy to incise the uterus and remove the foal via the abdomen. Caesarean section requires a large, experienced team and a well‐equipped operating theatre. Alternatively, if the foal is dead or not viable, then fetotomy may be performed. This involves using embryotomy wire on handles to dissect and remove the dead foetus in sections.

Once the placenta is passed, it should be checked thoroughly to ensure it is intact and healthy. The placenta should be handled with gloves and immediately disposed of after inspection, as anatomical clinical waste. If any part of the placenta is retained in the mare, endometritis can develop. This is an inflammation of the uterus, which can become septic, and lead to the development of septicaemia and laminitis if left untreated. The mare should be monitored closely postpartum for complications. Regular physical examination can help identify early signs of issues. The udder should be checked for milk. One sign of inadequate milk production, and need for medication such as domperidone, is a foal that constantly butts at the udder, repeatedly attempts to nurse and does not lie down to rest after nursing. A serious potential complication postpartum is uterine artery rupture, which can cause fatal haemorrhage. Signs of uterine artery rupture include pale mucous membranes and abdominal pain. The condition is diagnosed based on rectal palpation of the uterus and abdominal ultrasound examination. Uterine tear or rupture is also a possible postpartum complication and presents as general colic signs. If left untreated; this can lead to peritonitis. Uterine prolapse is a rare complication post‐foaling and is easily identified by the appearance of the uterus protruding from the vulva. Mares are at increased risk of many gastrointestinal lesions post‐foaling, and close monitoring for signs of colic is advisable.

Key Differences Between the Major Body Systems of Neonates, Foals and Adult Horses

There are many key differences between neonates, foals and adults, and it is important not to think of foals as identical, smaller versions of adult horses. A comparison of clinical parameters for neonates, foals, weanlings, and adult horses can be found in Table 15.2.

Differences can be identified within the body systems as follows.

Development of Immunity in Equine Neonates

Foals are born almost entirely agammaglobulinaemic meaning insignificant amounts of immunoglobulins are present in neonatal serum before ingesting colostrum. Passive immunity is essential for the newborn foal, as it has virtually no active immunity at this stage. Transfer of passive immunity from the mare occurs via ingestion of colostrum. The mare produces colostrum for the first 12–24 hours after parturition before milk production takes over. This coincides with the foal’s ability to absorb immunoglobulins and other sources of passive immunity via the temporary permeability of the gut. Colostrum contains immunoglobulins immunoglobulin G (IgG) and, in smaller quantities, immunoglobulin A (IgA) and immunoglobulin E (IgE). There are seven types of IgG, all of which have slightly differing functions for immunity. Peak immunoglobulin concentrations are reached at 8–12 weeks of age for most immunoglobulins; however, some IgG types do not reach adult levels until after one year of age [14]. Colostrum also contains inflammatory cytokines, which have a poorly understood but significantly positive effect on the development of immunity in equine neonates.

While exposure to environmental antigens is important in the development of immunity in the equine neonate, consideration must be made for foals born in a hospital environment or admitted within the first few weeks of birth. The neonate’s immature immune system leaves it extremely vulnerable to nosocomial infections when housed in a practice where multiple, potentially antibiotic‐resistant pathogens are present. Close attention to cleanliness and hygiene is essential, and it is not unjustified to establish a protocol for reverse barrier nursing of all neonatal inpatients. As a minimum, gloves must be worn at all times when handling neonates; equipment must be sterilised between patients (especially feeding equipment such as bottles and teats), and disinfectant foot dips/mats should be used when entering and leaving the stable.

Vaccination of the mare during late gestation is recommended for its benefits of increasing immunity in the foal. Tetanus vaccination 4 weeks before parturition allows placental transfer of immunity to tetanus, to protect the vulnerable neonate after birth. Tetanus antitoxin may also be administered to the foal immediately after birth to increase protection. Tetanus antitoxin is used short‐term as a treatment or for a very short period of protection, whereas tetanus vaccination is used to prevent tetanus infection in the long term. Influenza vaccination of the mare at the same time as tetanus vaccination is recommended. Vaccination against EHV is required three times during pregnancy; this initially protects the mare from the virus and subsequently prevents abortion. Another EHV vaccination at nine months of pregnancy allows placental transfer of immunity to the foetus. Some veterinary surgeons recommend vaccination against rotavirus in populations with larger numbers of foals, where exposure to rotavirus is higher.

Common Conditions Affecting the Neonatal Foal

Neonatal Maladjustment Syndrome (NMS)

NMS, also known as Hypoxic Ischaemic Encephalopathy (HIE) or dummy foal syndrome, is one of the most common reasons for hospitalisation of the equine neonate in the first few days of life [15]. The exact pathogenesis of the disease is still not fully understood; however, increasing research is being carried out, and more information is becoming available. The primary pathway of NMS appears to be a compromise to the oxygen supply to the foetus or neonate either through premature placental separation, or a prolonged or abnormal parturition. This causes cerebral hypoxia and consequent ischemia. Recent research has identified the role of a neurosteroid in NMS and has found that increased plasma progestogen concentrations in affected foals have a role in suppressing the central nervous system. This suppression is what creates the depression and dull demeanour associated with NMS [16]. Development of the Madigan foal squeeze technique is based on this premise, and the foal is squeezed with a rope around the thorax for 20 minutes to mimic the process of passing through the birth canal, which starts the neurological process that lead to the reduction in neurosteroid levels (Figure 15.4). A similar syndrome exists in human neonates, which has been more thoroughly researched and disappointingly, the only intervention which has been found to have a consistent positive effect on outcome is therapeutic hypothermia [17]. As seen in foals, intensive nursing care, prevention of infection, symptomatic treatment and time are the key factors in supporting a human baby with NMS.

Madigan foal squeeze step‐by‐step

• Make a fixed loop at the end of a long rope
  
• Place the rope around the foal, between the front legs and over the withers ending with the fixed loop on the withers
  
• Pass the rope around the foal’s thorax and half hitch it behind the fixed loop
  
• Repeat this a second time further back on the thorax
  
• Pull all of the loops snug and take the remaining rope behind the foal
  
• Pull on the end of the rope until the foal lies down
  
• Maintain the same pressure for 20 minutes then remove the rope

Meconium Retention

Meconium retention is a common condition of the equine neonate, occurring more frequently in colts than fillies, where the first faeces of the foal, containing waste material from foetal life, are retained in the GI tract. Usually, it is relatively easy to treat, and the first (and often second) attempt at resolving meconium retention is administration of a phosphate enema. Some breeders and veterinary professionals choose to administer a phosphate enema after birth routinely as a preventative measure. Soapy water enemas are also utilised by some practitioners and can be very effective. Failure of phosphate or soapy water enema to resolve meconium impaction requires administration of a retention enema. This procedure consists of passing a foley catheter rectally and administering an acetylcysteine solution, which is left in place for 20–30 minutes with the foal in lateral recumbency and the hind end raised (Figure 15.5). This usually requires sedation of the foal although the author has found it useful to perform retention enema during a foal squeeze procedure, if both are required, as foals tend to become very relaxed during the squeeze. In some cases, meconium impaction cannot be resolved medically and surgery to remove the impaction via midline laparotomy is required.

Septicaemia

Septicaemia, commonly referred to as sepsis, is one of the most common causes of disease in neonatal foals, and if left untreated it will inevitably be fatal. Bacteria, or less commonly viral pathogens, enter the bloodstream and spread throughout the body to the organs and joints. The most common entrance points for pathogens are the umbilicus, GI tract, intravenous catheter sites or wounds. Septicaemic foals may become depressed, hyperthermic, tachycardic, tachypnoeic and progress to recumbency. Septicaemia often results in synovial infections affecting multiple joints (also known as joint ill), and without systemic antimicrobial treatment, aggressive lavage and local antimicrobial administration to the affected joints, a performance and life‐limiting lameness is a distinct possibility. Blood culture and culture of synovial fluid of affected joints (if any) are important for pathogen identification. Antimicrobial sensitivity testing should be utilised to ensure correct antimicrobial selection.

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