by Dr. Peter Rossdale
Rossdale and Partners Veterinary Surgeons, Newmarket, England
From the Mohill Mare and Foal proceedings, Equine Research Centre
Guelph, Ontario, Canada
The newborn foal is the product of pregnancy described in the first part of my lectures. It is the same individual that was conceived approximately 11 months previously. The normality or otherwise of its development and growth is absolutely crucial to its chances of survival in the newborn environment. Let us consider briefly the changes and environmental experiences of the newborn foal compared with its previous life before birth. These are as follows:
Respiration, i.e. the taking in of oxygen and the getting rid of carbon dioxide, is a process in utero which is conducted through the placenta and, in effect, it is the mare breathing air into her lungs which supplies the fetal bloodstream with oxygen and removes carbon dioxide in exchange between the uterine blood and the placenta.
Outside, the foal adopts the same independent process as the mare when it takes its first breath into its lungs and thereafter, by maintaining the breathing rhythm, continues the exchange throughout its further life.
The basis of the blood circulation in the independent individual is a circulation in which blood is pumped from the left side of the heart to the body and appendages, from whence it returns to the right side from which it is pumped through the lungs returning to the left side of the heart and so on.
In the fetus, the two circulations (body and lungs) are in series and, in addition, incorporate the placenta. The fetal heart pumps this entire circulation of blood through the placenta and only a very small proportion through the lungs which, in fetal life, are non-functioning because they contain no air.
This placental circulation entails the driving of blood through vessels in the cord and placenta extending 200 cm or more, depending upon the length of the cord. This is a remarkable achievement when one considers the size of the fetal heart and the pressure gradients involved in transporting blood through the capillaries of the placenta and through the liver which, in the fetal foal, has no bypass to its liver as in other species, i.e. a ductus venosus.
The fetal circulation, in which blood is directed to the placenta rather than the lungs, is achieved by two special channels through which blood passes, one the venous side (through the foramen ovale) and one on the arterial side (through the ductus arteriosus). These two channels normally close within 24 hours of birth.
Once the cord circulation diminishes or is completely stopped, the circulation must pass through the lungs in the manner of the independent being.
The lungs during fetal life are filled with fluid, but at the first breath, this fluid is replaced by air inhaled through the nostrils into the windpipe. The expansion of the small air sacs is an essential process to the maintenance of post natal life.
The fluid present in the lungs at this time is partly absorbed, partly escaping up the windpipe (trachea) and partly forms a thin film of moisture around the lining of the air sacs. It is this film which contains the substance surfactant that is produced by special cells in the lungs throughout life and enables the lungs to remain expanded. It reduces the surface tension of the fluid lining which would otherwise cause the air sacs to collapse and the lungs to lose most, if not all, their air.
This situation occurs in cases of prematurity and dysmaturity when the surfactant system is either underdeveloped or becomes impaired by abnormal processes, the so-called respiratory distress syndrome or hyaline membrane disease of infants sometimes encountered in newborn foals.
Normal breathing rhythm is established within seconds of delivery, although the lungs may not fill with air to the extent that is present in an adult until several hours or days after birth. There is a gradual development of capacity based on connective tissue and elastic development accompanying use.
Another dramatic change is that of digestion, the means of receiving nourishment and of getting rid of waste products. In the fetus, this is achieved through the placenta. It is the mare’s digestive processes which break down nutrients into simple forms of sugars, protein and fat, and these simple compounds are transferred by diffusion or actively across the placenta for the benefit of the fetal foal.
Once the foal is born, it has to undertake the digestive process for itself, imbibing milk and other nutritional products which are digested in its own gut before being absorbed by processing into simpler compounds as previously undertaken by the mare herself.
Excretion in the fetal foal is performed by a combination of passing a small amount of urine from the kidneys into the allantois (through the urachus) and, to a lesser extent, into the amniotic sac. However, most of the function of excretions of the kidneys is performed through the placenta. Faeces are formed and stored in the fetal hind gut, i.e. meconium. Following birth, this excretion is performed solely through the kidneys and through the hind gut in the form of faeces expelled from the rectum.
All these changes involve quite complicated reactions of enzymes and peptide hormones in the gut, kidneys and liver. These changes are not as dramatic as occur in the lungs and circulation, but are just as necessary over the longer term.
Balance and Activity
The foal lies on its side (or on its back) during uterine life. It makes movements with its limbs and turns from side to side. It also uses its muscles in breathing movements which are characteristic of fetal mammals.
The activity of the fetus is in preparation for its extra uterine life when it has to use its muscles and nervous system to co-ordinate its actions. In precocious species, such as the horse, this activity is equivalent to that of the adult within hours of birth, i.e. the foal has to get up and stand and subsequently gallop alongside its dam.
The movements of the fetus may, therefore, be regarded as a preparation for the movements that have to be undertaken soon after birth if the foal is to be able to survive by getting to its feet, following the dam and sucking from the udder. All of this has to be done against the background of balance. It is remarkable that in the uterus the foal is protected from gravity and does not stand, whereas within two or three hours of birth, it is achieving these functions in an increasingly co-ordinated manner.
Thermoregulation in the fetus is largely controlled by the mare. The fetus undergoes very little energy producing activity and what is produced has to be eliminated through the mare. In the newborn, the fetus is subjected to much lower temperatures and has to maintain its own body temperature.
The foal’s rectal temperature decreases slightly after birth from a peak recorded during the first hour. At age 24 hours, the rectal temperature is about 38.4°C. Respiratory quotients are high at birth declining by age 24 hours indicating the high levels of carbohydrate metabolism forming the basis for energy and heat production in the newborn foal.
The Newborn Environment
The atmosphere into which the foal is born is very different from that within the mare. In the mare, it has been in a sterile environment free from bacteria and viruses. In the newborn environment, the foal experiences the challenge of microbes, dust and hard surfaces which may bruise and predispose it to infection etc.
The portholes of entry for microbes are through the mouth, the airways or the umbilicus. Dust may be inhaled into the lungs causing irritation.
Recognising the Normal Newborn Foal
Experience gives us an intuitive sense of whether or not an individual is normal – the way it gets to its feet and goes to the mare’s udder and the manner and frequency of its sucking, its stance during inactivity, either standing or laying on its side or its brisket.
Recognising the Sick or Abnormal Foal
Departures from the normal are ways in which we recognise that some disease or abnormal condition is present in the individual.
The significance of departures from the normal is that they have to be recognised and the diagnosis then made as to what is causing the abnormality. This is essentially a veterinary task. A veterinary surgeon will bring various aids and tests to bear in order to make a diagnosis, for example, blood biochemistry and haematology, ultrasound scanning of the abdomen and chest, and, of course, the clinical signs which with experience indicate what type of abnormality may be present in the first place.
It is the lay attendant’s task to identify in the first instance that something is wrong. This is a particularly essential responsibility in the newborn because deterioration can occur quite rapidly compared with the adult or older animal.