Nutritional Secondary Hyperparathyroidism in Horses
What is it?
Nutritional Secondary Hyperparathyroidism, more commonly known as Bighead or Bran’s disease, is a severe calcium deficiency.
Bighead was first noticed in horses owned by distillery workers that were fed a high proportion of wheat bran and other grain by products from the brewing process to their horses. Many cereal grains contain incorrect dietary calcium to phosphorus ratio, putting these horses in a calcium deficiency state.
During periods of calcium shortage, horses will mobilise calcium (and phosphorus) from their bones to keep blood calcium at normal levels. If this occurs over long periods of time, horses can mobilise so much calcium and phosphorus that their bones become weak and fibrous. Much of this calcium is taken from the facial and pelvic bones, which become fragile and fibrous connective tissue develops. This fibrous tissue causes the facial bones to swell, giving them a ‘big head’ appearance. Young horses are most prone to this facial swelling as their bones have not completely formed and hardened.
Causes
Two main diets are the main culprits of calcium deficiency:
- Incorrect Calcium to Phosphorus ratio
Incorrect calcium to phosphorus ratio can be present in diets with too much phosphorus. Too much phosphorus in a horse’s diet can block calcium absorption, eventually leading to a phosphorus deficiency. This is most common in horses fed complete grain diets high in cereal grains and ingredients like wheat bran, pollard, rice bran and copra that are high in phosphorus.
- Oxalate pastures
The most common diets associated with Bighead in QLD include forages with high levels of oxalates. Oxalates are chemical compounds that bind to calcium in the digestive system making the calcium unavailable for absorption. So even if grasses contain plenty of calcium, horses cannot access it, eventually causing a deficiency.
For pastures to be safe, the oxalate levels must not be more than double the calcium levels. This means that the calcium to oxalate ratio should be greater than 0.9:1. Pastures are the highest in oxalates in wet, warm weather where the pasture is in its growing stages.
Some of the pastures, their total Oxalate levels, and Calcium to Oxalate ratios are listed below:
| Grass species | Total Oxalate | Calcium to Oxalate Ratio |
| Setaria | 2 | 0.09 |
| Buffel Grass | 1.42 | 0.22 |
| Pangola | 0.92 | 0.37 |
| Green Panic | 0.81 | 0.32 |
| Kikuyu | 0.75 | 0.29 |
Symptoms
The more noticeable facial swelling appears most commonly in young horses whose facial bones haven’t fully formed and hardened. Other symptoms can include:
- Illthrift, loss of top-line and poor hoof quality
- Unevenness of movement or ‘shifting lameness’
- Hard, bony lumps and splints
- Increased susceptibility to fractures under stress
- Poor tooth quality, loose teeth, difficulty chewing
- Onset of sway back, osteoporosis, arthritis
- Calcium withdrawal from the skeleton has progressed to the point the bone matrix has been replaced with softer connective tissue, starting with the bones of the skull. Swelling of the jaws and nasal bones grossly deform the head
Prevention and Treatment
Prevention is always better than treatment, and Bighead can be easily avoided given correct nutrition.
If incorrect calcium causes your Bighead: phosphorus ratio adding additional calcium to the diet to correct the imbalance can ensure that you avoid this problem. Calcium: Phosphorus ratio should be kept above 1:1 for all horses. The ideal ratio for young horses is between 1-3:1, and mature horses is 1-6:1. Calcium can be added to the diet by feeding limestone, commercial supplements, or adding high calcium forages like lucerne and clover.
If high oxalate grasses pose a threat to your horses, you must feed enough calcium in the diet to bind the oxalates and provide your horse with a sufficient amount of absorbable calcium. It is also important to supplement your horse with phosphorus as much of QLD is deficient. The easiest way to prevent Bighead in horse grazing pastures high in oxalates is to feed a commercial supplement specifically formulated for these circumstances.
Top Country Solutions
Top Country has a range of EQ supplements specifically targeted towards preventing Bighead in horses grazing buffel grass and other pastures high in oxalates. Each of our supplements is designed to be fed adlib in the paddock and contain a full suite of essential vitamins and minerals to keep your animals in top condition. I usually break down our bighead licks into two seasons – wet season licks and dry season licks.
Wet Season Lick
In pastures, the oxalate content increases in the wet season when the grass is green, growing, and lush. This is when you should be especially vigilant in providing your horses with a bighead supplement.
| Wet Season Licks | Pasture Conditions | Horse Risk of Developing Bighead |
| Top EQ Premium Protect | Lush green pastures, primarily buffel grass or other high oxalate pastures | For horses at high risk of developing Bighead, clinically diagnosed in the early stages of Bighead |
| Top EQ Tropical | Green pastures, primarily buffel grass or other high oxalate pastures | For horses at high-medium risk of developing Bighead |
Top EQ Premium Protect
Our Premium protect supplement is a top of the range bighead supplement that not only contains limestone as a calcium source to bind to the oxalates in the digestive system but also contains chelated calcium. Chelated calcium is a calcium source that is bound to an organic molecule that protects it from being bound by oxalates in the digestive system. This makes the calcium more bioavailable and readily absorbed. Top country chooses to include 19% of the total calcium content as chelated calcium to make our product safe enough to be fed adlib in the paddock. Suppose your horses are at high risk of developing Bighead or have been clinically diagnosed in the early stages. In that case, Top EQ Premium Protect is the most suitable product for your horses.
Top EQ Tropical
The Tropical range is formulated especially to be fed on tropical pastures. This supplement contains 18% calcium and 5% phosphorus, so it is suitable to feed horses on pastures that are at a high risk of developing Bighead. The limestone in our tropical range is included at a high enough rate that although some of the calcium will be bound to the oxalates, much of the calcium in the supplement will be absorbed and be able to be utilised by the animal. If your horses are continually grazing pastures such as buffel grass, then the tropical range will suit you.
Dry Season Licks
The oxalate content in pastures usually decreases as pastures hay off and lose much of their nutritional value. It is still important to provide your horses with a high calcium supplement at this stage in the season, but it is also important to feed an adequate amount of protein to sustain your horses through the dry season.
| Dry Season Licks | Pasture Conditions | Horse Risk of Developing Bighead |
| Top EQ Calphos | Dried off pastures, high oxalate pastures mixed with safe pastures Pastures that are lacking in protein | For horses at medium risk of developing Bighead |
| Top EQ | Dried off pastures Pastures lacking protein | For horses are at low risk of developing Bighead |
Top EQ Calphos
The Calphos range of supplements is designed for horse owners concerned about Bighead in their horses and want to give their animals an extra hand up through the dry season. The 2:1 calcium: phosphorus ratio will still provide your horses with enough calcium to prevent them from developing Bighead while also providing them with much needed protein to sustain them through the dry season
— Sara Darrow, Nutritionist