A cohort study of nutritional secondary hyperparathyroidism in horses in Hawai’i.

Research Overview

• This study investigated the prevalence and treatment of nutritional secondary hyperparathyroidism (NSHP), also called Bighead Disease, in horses grazing on pastures in Hawai’i, where typical calcium-rich legume forage is scarce and high oxalate grasses are prevalent.
• The study evaluated calcium and parathyroid hormone levels before and after calcium supplementation to assess treatment effectiveness in affected horses.

Introduction and Background

• Equine nutritional secondary hyperparathyroidism (NSHP) is a metabolic bone disease caused by diets with imbalanced calcium to phosphorus ratios, leading to excessive parathyroid hormone secretion and bone resorption.
• Historically common, NSHP has become rare in many regions due to the availability of fortified feeds and nutrient-rich legume forage.
• Hawai’i is unique because legume forage is scarce and local pastures contain grasses with high oxalate content, which can bind calcium and reduce its bioavailability, potentially causing NSHP despite natural grazing.

Study Design and Methods

• A cohort of 17 adult horses was studied; all were imported from the continental United States to the Waiki’i region of Hawai’i.
• These horses grazed exclusively on kikuyu grass pastures, known for high oxalate levels.
• Baseline plasma ionized calcium and parathyroid hormone (PTH) concentrations were measured to identify horses with NSHP.
• Two treatment groups were formed for calcium supplementation using gastroprotectant formulations:
  • Group 1 consisted of 8 horses receiving Supplement 1.
  • Group 2 consisted of 5 horses receiving Supplement 2.
• Treatment effects were evaluated after eight months by re-measuring plasma calcium and PTH levels.

Results and Findings

• At baseline, 9 out of 17 horses were diagnosed with NSHP, while 8 were normal.
• In Group 1:
  • 1 NSHP horse’s condition resolved after treatment.
  • 4 NSHP horses remained affected despite supplementation.
  • 2 horses without NSHP stayed unaffected.
  • 1 previously unaffected horse developed NSHP during the study.
• In Group 2:
  • 2 NSHP horses resolved their condition after treatment.
  • 1 NSHP horse remained affected.
  • 2 horses without NSHP remained unaffected.
• Statistically significant differences between groups were observed post-treatment:
  • Ionized calcium levels differed significantly (p = 0.0063).
  • Parathyroid hormone concentrations differed significantly (p = 0.0236).

Interpretation and Conclusions

• NSHP remains prevalent in horses grazing on high oxalate pastures without access to fortified feeds, such as in Hawai’i.
• Oxalate-rich forages likely cause calcium binding and toxicosis, contributing to the disease’s persistence.
• Calcium supplementation via gastric protectant formulations can be effective in correcting mineral imbalances and resolving NSHP in some horses but is not universally successful.
• Careful and ongoing monitoring of calcium and parathyroid hormone levels is necessary to evaluate disease status and treatment success.
• The study highlights the importance of regional diet availability and mineral content in managing equine metabolic diseases.

Implications for Practice

• Horse owners and veterinarians in regions lacking legume forage should be vigilant for NSHP signs.
• Dietary management, including the provision of calcium supplements tailored to individual needs, may prevent or treat NSHP.
• Regular blood testing is essential to track calcium and PTH levels and adjust treatments accordingly.
• Further research may be needed to identify optimal supplements and management strategies for horses in oxalate-rich environments.