Page 1355 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

1330 CHAPTER 14

VetBooks.ir 14.1 PHYSIOLOGICAL DIFFERENCES
BETWEEN THE NEONATAL FOAL
AND THE MATURE HORSE

There are several physiological differences that are
important when assessing and treating neonatal
foals. Newborn foals have a larger total body water
content (70–75% compared with 60% in the older
animal) and a larger extracellular fluid volume (394
± 29 ml/kg) and plasma volume (94.5 ± 8.9 ml/kg)
Fig. 14.1 Post-mortem specimen illustrating fatal at 2 days, falling to an extracellular fluid volume
multiple rib fractures. of 348 ± 45 ml/kg and a plasma volume of 61.8 ±
5.9 ml/kg by 4 weeks. Young foals are particularly
susceptible to water loss because they have a high
be identified by the presence of pain, crepitus or surface area to volume ratio and the kidneys are
an abnormal chest contour (Fig. 14.1). The most less able to compensate. The equine neonatal kid-
common location for rib fractures is at the ven- ney also has a reduced capacity for sodium excre-
tral portion of the chest just behind the elbows. tion compared with that of an adult horse, which
Auscultation of the lungs should reveal relatively means that foals are intolerant of a high sodium
loud bronchovesicular sounds over all lung fields; load. Hepatic function is not mature for the first
however, it is an insensitive indicator of pulmonary 7–14 days of life, which may slow drug metabolism.
disease in foals. There are significant differences in the pharmaco-
Auscultation of the heart should be performed, kinetics of some drugs in the neonatal foal. Dose
and the rate and rhythm assessed. The heart rate is rates of some antibiotics are higher, and others have
very labile in the young foal and if the rate is rapid, a shorter dosing interval. Consequently, it is impor-
it is worth re-evaluating once the foal has relaxed. tant to use foal-specific doses for drugs, especially
Transient arrhythmias are common in newborn antibiotics (Table 14.2). It is important to consider
foals. A grade I–IV left-sided holosystolic murmur the age of the foal, because from approximately
is often audible for the first 2–3 days, associated with 1 month of age drug metabolism becomes similar
the patent ductus arteriosus. A flow murmur may be to that of the adult. Foals have immature colonic
audible at the base of the heart on the left side and this function and do not ferment food material in their
may persist for many days. It is important to check hindgut. In general, this makes young foals more
for signs of cardiovascular compromise if a murmur tolerant of the administration of oral antimicrobials
is detected. Signs of severe congenital cardiac disease than adult horses. Neonatal foals are metabolically
include exercise intolerance, cyanosis and abnormal- less stable, with poorly established homeostatic
ities in peripheral pulses. Any indication of cardio- mechanisms, and therefore they are susceptible
vascular compromise merits further investigation, to disturbances in blood glucose and serum elec-
including radiography and echocardiography. trolyte levels. The foal is dependent on frequent
The abdomen should be evaluated for distension ingestion of milk to maintain blood glucose, fluid
or a ‘tucked up’ appearance. The umbilical remnants and electrolyte requirements. Thermoregulatory
should be palpated for thickening or dampness. The function is less able to compensate for changes in
latter may indicate a patent urachus. Any umbili- environmental temperature. Foals are agamma-
cal hernia should be evaluated. Normal foals pass a globulinaemic at birth and immunologically naïve,
large quantity of dark brown meconium. Once this is with reduced levels of many of the components of
passed, paler milk faeces should be observed. the non-specific immune system. Antibody levels
Limbs should be evaluated for joint effusion and and effective neutrophil function are dependent on
flexural and angular limb deformities. adequate transfer of colostral immunity.