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EQUINE VETERINARY EDUCATION / AE / FEBRUARY 2018
Fluid therapy Fluid therapy may be required to treat both hypovolaemia and dehydration. Hypovolaemic foals demonstrate tacky mucous membranes, increased eyelid tenting, prolonged capillary refill time, decreased mentation, and cool extremities. Decreased frequency of urination is present, but may be missed by inexperienced personnel. Correction of hypovolaemia is best accomplished by boluses of fluids at 20 mg/kg bwt. The foal is assessed after fluid administration, and bolus fluids should be repeated every 20 min until the signs of hypovolaemia or a total of four boluses have been administered. If fluids alone do not correct hypovolaemia, more intensive therapy is likely to be needed, and the foal should be referred (Palmer 2004). Many sick foals are hypoglycaemic when first examined by the ambulatory practitioner because of limited energy reserves and decreased nursing. Correction of hypoglycaemia can be life- saving. If bolus fluids are needed, they should contain very small amounts so as not to cause hyperglycaemia. Supplemental glucose in resuscitation fluids should not exceed 6.6 mg dextrose/kg bwt/min (Speir and Meagher 1989).
Once hypovolaemia is corrected, fluids for the first 24 h
are administered to maintain blood glucose concentrations and to meet fluid requirements. Initially, foals may be started on 5% glucose to provide 4 mg glucose/kg bwt/min, and then glucose concentration may be increased to 8 mg glucose/kg bwt/min provided the foal maintains glucose tolerance (Palmer 2004). A point-of-care glucometer should be used to monitor blood glucose and adjust glucose administration to maintain blood glucose levels between 4.0 and 5.5 mmol/l (Austin 2013). It is not possible to meet caloric needs by glucose infusion alone, but total parental nutrition would rarely be undertaken in the field. After stabilisation, fluid rates can either be given at a
basic 2–4 ml/kg bwt or use the Holiday-Segar formula (Table 2) to calculate a ‘dry’ rate based upon the basal metabolic rate of the neonate (Axon and Wilkins 2015).
Estimated ongoing losses are then added to the calculated fluid needs. Resuscitation fluids should be any fluid with strong ion difference such as Normosol or lactated Ringer’s solution. These fluids help restore normal anion gap and correct metabolic acidosis, but long-term use of replacement fluids to supply maintenance requirements in foals will result in water retention and a ‘jelly belly’ appearance because of the foal’s limited ability to excrete sodium. Therefore, with the exception of foals with diarrhoea, maintenance fluids should have restricted sodium. Restricted sodium-containing fluids include Normosol-M, 5% dextrose, and a combination of 5% dextrose and lactated Ringer’s solution (Axon and Wilkins 2015). Fluids with a high chloride concentration, such as normal saline, can have a significant acidifying effects and
TABLE 2: Holliday-Segar formula for maintenance fluid calculations
Bodyweight (kg)
1–10
11–20 >20
Total © 2016 EVJ Ltd
Fluid requirement (ml/kg bwt/day)
100 50 25
Cumulative total for 50 kg foal (ml)
1000 500 750
2250 ml/day
are not appropriate for routine resuscitation. Electrolytes should be evaluated with a biochemistry panel after resuscitation, and electrolyte abnormalities should be addressed slowly over time to prevent adverse neurological events. Potassium supplementation should be considered in any foal that is not being fed and has no predisposing factors that trigger hyperkalaemia such as ruptured bladder; however, intravenous supplementation is challenging and potentially dangerous without a fluid pump or experienced personnel to monitor the rate of fluid administration. If the foal will tolerate even small amounts of enteral fluids, oral supplementation would be safer. After the basic needs of the equine neonate have been addressed, the clinician will need to provide specific therapy for medical problems that are evident on clinical examination or identified on initial laboratory assessment.
Therapy of specific diseases
Diseases of the immune system One of the most common problems treated in the field is failure of passive transfer (FPT). Multiple tests are available and many can be performed stall-side to assess the adequacy of passive transfer of antibodies once the foal is aged 8–24 h. If FPT is identified when a foal is aged <18 h, colostrum supplementation can be utilised. After age 18 h, gut closure has occurred, and plasma must be administered to elevate serum immunoglobulin levels. Failure of passive transfer may be identified in otherwise
normal foals, and treatment is dependent on antibody levels, experience of the clinician, farm history, and wishes of the owners. All foals with complete FPT (<4 g/l) should receive additional antibodies. Foals that are determined to be at increased risk for disease should receive supplemental IgG if measured values are <8 g/l. If the foal is to be insured, the veterinarian and owner should check insurance company requirements to ensure stipulations for binding of the insurance are met. A single litre of plasma is expected to increase serum IgG levels in a 50 kg foal by 2–3 g/l. Plasma products should always be thawed in a water bath to prevent destruction of essential proteins prior to administration. After treatment, all foals should have IgG levels checked again in 12–24 h to ensure adequate antibody levels. Reactions are rare with the administration of most commercial plasma products provided a blood administration set is used for administration. Initial fluid rates should be 0.5 ml/kg bwt and gradually increased to 40 ml/kg bwt/h once the clinician is comfortable that adverse effects are not occurring (Sellon 2006). Plasma administration should be slowed or stopped if reactions such as muscle trembling, piloerection, increased heart and respiratory rates, colic, or blanching of mucous membranes are observed. Plasma must be stopped if signs recur when administration is resumed (Sellon 2006). Neonatal isoerythrolysis is an immune disease that is much
less commonly encountered, and is the most common cause of icterus in foals. Neonatal isoerythrolysis occurs primarily in multiparous mares that are A orQ negative and have become sensitised to A or Q antigens during previous pregnancies. Anti- A or Q antibodies are then secreted into the colostrum and cause lysis of the foal’s red blood cells if the foal has inherited either A or Q antigens. Clinical signs include depression, tachypnoea, tachycardia, icterus and occasionally
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