A cohort study of nutritional secondary hyperparathyroidism in horses in Hawai'i

Equine nutritional secondary hyperparathyroidism (NSHP, Bighead Disease) is a consequence of diets with abnormal calcium: phosphorus ratios. With the widespread availability of fortified feeds and legume forage, the disease has largely disappeared. The Hawaiian Islands are unique in that legume forage is largely unavailable and pastures have high oxalate concentrations. This was a cohort study that included seventeen adult horses imported from the continental US to the Waiki'i region, Hawai'i that grazed on kikuyu grass pastures. Plasma ionized calcium and parathyroid hormone concentrations at baseline and after eight months were evaluated. Calcium supplementation was provided via gastroprotectant formulations with Group 1 (eight horses) receiving supplement 1 and Group 2 (five horses) receiving supplement 2. 8/17 horses were normal and 9/17 horses were diagnosed with NSHP. In Group 1, 1 NSHP horse resolved, 4 NSHP horses remained affected, 2 non-NSHP horses remained unaffected, and 1 non-NSHP horse developed disease. In Group 2, 2 NSHP horses resolved, 1 NSHP horse remained affected, and 2 non-NSHP horses remained unaffected. A statistically significant difference between treatment cohorts was found in post-treatment ionized calcium (p-value =0.0063) and parathyroid hormone (p-value =0.0236) concentrations. The results of this study demonstrate that NSHP remains a common disease in regions without consistent access to fortified feeds and with high oxalate grasses leading to oxalate toxicosis. Calcium supplementation may be effective in some horses, but continued monitoring is required.