PASTURE POISONS
Nearly 20 years ago this month I attended a mare who had been in labour for over an hour without birthing her foal – not a normal thing for horses and an emergency! When I arrived, the mare did not attempt to rise (worrying) and I could see pain and shock in her eyes. I began the internal examination with both of us lying on the ground. As I inserted my arm I was met with a narrowing, an abnormal structure – a painful pelvic fracture – worse still, warm swelling that was rapidly increasing in size ……the sharp bone fragments had ruptured the aorta and the mare was rapidly losing consciousness. On two mornings of the same week, I was called early by owners who’d awoken to find their horses acutely, non-weight-bearing, 3-legged lame. One spontaneously fractured its femur, the other had fractured the near fore radius. The outcomes for all 4 (including the unborn foal) were devastating.
The mind seems to hold onto fragments and memories of the most challenging, puzzling and unsolved cases – maybe hoping to one day understand more about why and how things happen. My mind slipped back through time to my first years in practice with Doug English – valued by both owners and his peers as one of Australia’s most experienced and respected equine veterinarians. There were a few cases that had always had us scratching our heads, questioning and lying awake at night. I remembered several young, healthy horses that sustained fractured ribs when we anaesthetised them for surgery; a couple of others whose mandibles had fractured when dental gags were removed – normally a risk-free procedure, and a couple of race horses who presented with the same injury when feeding from metal hay racks. Rushing through my thoughts too, came ‘Bubbles’ - my daughter’s trick-riding horse who, after a show, arrived home with a fractured hock. ‘Bubbles’ was a lovely, beautiful, reliable and skilled trick-riding/vaulting horse. She had performed perfectly and been 100% sound when we loaded her into the truck for the trip home. Polo ponies all from one family who over a month all became lame with suspensory and tendon avulsions and strains; an aged mare (Emma) who’d lost so much condition and was so lame that she could barely walk. Lucky for Emma, that same month I saw a Shetland pony whose head was the elliptical shape of a football – all natural curves and ridges lost. I was called because he was overweight and reluctant to move. Examination found lameness in all four feet, but no signs of laminitis. It was the big head that finally brought together all these unrelated, seemingly disparate cases.
Oxalates. Nutritional secondary hyperparathyroidism (NSH). Fibrous osteodystrophy. Only 20% of horses present with the nasal, facial and mandibular changes that we recognise as ‘bighead’. Working with Dr Oliver Liyou and Prof Garry Wilson over several years, we tested various formulas that would address the oxalate-induced calcium deficiency in a block form, and a palatable powder for horses that were hand-fed. The results were Calsorb Forte® and Bone Formula Forte®. In the 10 years since, many purported big-head supplements have appeared – some made by marketing companies, others by various feed companies. Some are produced in accredited, government-approved, quality-controlled and audited laboratories – others not. Some respect regulations of the APVMA regarding ‘claims’ to treat or prevent diseases, others don’t; some stay within legislated selenium levels, others breach them. Some are purported to have been formulated by vets – who can’t be contacted.
Confusion has been spread on the influence of oxalates on magnesium absorption – which in mineral balance studies has been shown to be unaffected by oxalates*. Some companies who are unaware of the research claim that magnesium is affected, which clouds understanding. Likewise the research on 2.5kg of molasses mixed with DCP and/or lime showed that horses still developed bighead on the supplement and a doubling of the dose to 5kg per horse, per week was required**. Others suggest a common respiratory condition of thoroughbred racehorses, known as ‘roaring’, is secondary to bighead. Roaring is due to paralysis of the laryngeal nerve, and is corrected by appropriate surgery on the vocal cord or nerve graft. It is unrelated to bone osteoporosis – but the stories are repeated often enough that they are believed. Although estimates can be found in textbooks and feeding guides, to be clinically applicable and meet ‘best practice’, nutritional advice on medical conditions requires veterinary knowledge and understanding of pathophysiology – why and how things happen.
Have a look at the December 2018, January and February 2019 issues of Hunter 2 Hawkesbury Equestrian News, Equestrian News and Lifestyle, Hills & Hawkesbury Equestrian News or Highlands 2 Hawkesbury Equestrian News for more useful tips on poisons and toxins that can occur in plants paddocks.
EQUINE CLINICAL NUTRITION
Dr Jennifer Stewart
