Big Head is not a new disease – it has been recognised in many species and called “animal osteomalacia” by Vegetius in 400AD. It is also called nutritional secondary hyperparathyroidism or osteodystrophia fibrosa.
It was first described in horses in 1851 – a 2 year old horse in Switzerland was described as having “….slow troubled chewing and uniform swelling extending backwards from the last molars. The teeth were freely moveable and the bone appeared like pumice stone”. This is as true today as it was then!
It was also reported to be prevalent in the 1900’s in horses fed large amounts of bran by-product and was also known as “bran disease” or “miller’s disease”.
Over the last 30 years, it has been recognised as a continuing problem in horses grazing high oxalate pastures particularly in Germany, Japan, Switzerland, USA and Australia.
Certain grasses contain high levels of calcium oxalate crystals – the calcium in this form is unavailable for absorption in the horse gut and thus affects their utilisation of the mineral. Grasses with more than 0.5% oxalate are capable of causing Big Head.
The disease occurs when horses are grazed on pure or near pure swards of tropical grasses. They include:
Setaria, Kikuyu, Buffel, Green Panic, Signal, Guinea, Pangola
Common Grasses in this area (Samford region) are:
Setaria is particularly prevalent in the Samford area as it was planted for cattle pasture when this was a dairy area.
The oxalate content of grasses is highest under conditions of rapid growth -they can reach levels as high as 6% or more.
What happens in the horse?
As a result of lack of absorption of calcium from the grass, it is passed straight out in the manure and the body’s levels of calcium fall. In response to the decreased calcium the parathyroid gland releases parathyroid hormone (PTH).
The function of PTH is to prevent calcium from reaching dangerously low levels, it does this by:
- Increasing phosphate excretion and resorbing calcium from the urine.
- Acts on the gut to increase uptake from the feed – on oxalate grasses there is minimum for them to absorb
- Stimulates the release of small amounts of calcium from the bone – thus affecting bone density.
Affected horses also enter a state of phosphorous deficiency as it is excreted. The bones become weakened and have fibrous tissue deposited within them – the clinical signs then appear. These can be quite variable from case to case.
- Ill Thrift
Horses have harsh coats and lose body condition even while grazing pasture. Some are seen eating dirt – pica.
Usually an insidious, shifting lameness with a general tenderness of the joints or a stiff stilted gait. They may only be mildly lame and there may not be any physical findings. There may be creaking of the joints. Some horses prefer to canter rather than trot and some are reluctant to move. Badly affected horses may go down and be unable to get up. Sometimes horses are found with a catastrophic fracture without any prior warning of a problem.
- Swelling of facial bones.
The most severe changes occur in the skull bones – jaws, maxilla, mandible and nasal bones – hence the name BIG HEAD.
Swelling is usually bilateral. They may include swellings of the maxilla over the cheek teeth (see below), a ridge over the nasal bones or of the mandible.
Affected horses may present with upper airway breathing difficulty, a watery nasal discharge or watery eye discharge (from occlusion of the nasolacrimal duct through the skull). Chewing problems may occur when there is resorption around the teeth.
Regions of Skull Affected by Big Head are the nasal bone, the upper jaw (Maxilla) and lower jaw (Mandible)
Physical Effect on the Bones.
At least 30% of bone mineral must be removed to see radiographic changes so this is late in the course of the disease; the radiographic changes appear faster in the skull then the limbs.
At post mortem, the affected bones may be easily cut with a knife. Microscopically they comprise of mostly fibrous tissue with occasional fragments of bone. There may be fractures of the long bones and the joint surfaces are pitted.
Do all the horses on the pasture get it?
Some or all horses may get it – there is 1 -100% prevalence. Mares and young horses (weanlings to 7 years of age) are more susceptible because of their higher requirement for calcium. Time of onset after grazing high oxalate pasture is 2 to 9 months.
Treatment and Prevention.
The horses graxing oxalate grasses need to be supplemented with calcium and phosphorous; this needs to be in a palatable form and safe to be fed in the long term. The recommended calcium to phosphorous ratio is 3:1. Many of the supplements available do not provide enough phosphorous e.g. Dolomite.
Feeding of good quality Lucerne hay is a good start but to meet the possible deficiency, the average horse would require 20kgs of hay a week.
It takes 4 to 12 months to remineralise the bones once supplementation is commenced.
There is quite a lot of controversy over management with varying opinions on the best supplementation to use. Some of the commercially available supplements recommended for Big Head are actually deficient in the calcium and phosphorous requirements. There has also been varying results with the different supplementations available and often each case needs to be judged on its individual merit. There are often many factors that affect the actual intake of such supplementation and the result they have. Therefore seeking advice from your veterinarian with local knowledge is advisable.
Daily supplementation seems to have better results than weekly supplimentation.
- Dr Jennifer Stewart’s Fortified Calcium Block and Bone Formula.
Dr Stewart is an equine veterinarian and consultant nutritionist who has formulated Big Head supplements designed for Australia. She is also performing ongoing research.
- Ross McKenzie’s Recipe.
This has been used for years in this area by Dr Brian Sheahan and Dr Judy Law with good results although there may some question about maximum absorption.
For a 500kg horse:
1/3 kg ground limestone
2/3kg dicalcium phosphate (DCP)
1.5kg molasses (to make it more palatable)
This total should be fed over a week. IF IT IS being used as a treatment THEN IT CAN BE INCREASED.
SPEAK TO A VET IF YOU THINK YOUR HORSE MAY HAVE BIG HEAD!
Please do not hesitate to contact any of the large animal veterinary surgeons at Samford Valley Veterinary Hospital should you have any questions about Big Head.