Seasonal changes in serum calcium, phosphorus, and vitamin D concentrations in llamas and alpacas

Objective - To evaluate the interaction of season and age on serum calcium, phosphorus, and vitamin D₃ concentrations in llamas and alpacas. Animals - 23 clinically normal llamas and 7 alpacas. Procedures - Animals were assigned to 1 of the 3 following groups on the basis of age at the start of the study: adult (age, ≥ 24 months; n = 8), yearling (> 12 but < 20 months; 5), and neonate (< 6 months; 17). Twelve serum samples were obtained at monthly intervals. Calcium, phosphorus, and vitamin D₃ concentrations were measured, and the calcium-to-phosphorus concentration (Ca:P) ratio calculated. Effect of season and age on each of these variables was determined. Results - Vitamin D₃ concentrations varied significantly as a function of season; the highest and lowest concentrations were detected September through October and February through March, respectively. The seasonal decrease in vitamin D₃ concentration was significantly greater in neonates and yearlings, compared with adults. Serum phosphorus concentration decreased as a function of age, with the most significant seasonal change detected in the neonate group. The Ca:P ratio in neonates varied between 1.1 and 1.3 except during winter months when it increased to ≥ 2.0. Conclusions and Clinical Relevance - Mean vitamin D₃ concentration varied by > 6 fold in neonatal and yearling llamas and alpacas and > 3 fold in adult animals as a function of season. These results support the hypothesis that seasonal alterations in vitamin D₃ concentrations are a key factor in the development of hypophosphatemic rickets in llamas and alpacas.