Rickets and Metastatic Mineralization in Broilers Associated with Excess Dietary Calcium and Phosphorous

Elevated mortality and culling were reported from 7-day-old broilers in the U.K. Chicks in two affected houses showed clinical signs of weakness and lameness with wet litter. Necropsy findings from dead and culled lame chicks included minimally widened tibiotarsal growth plates, superficial erosions on the gizzard koilin layer, and mildly congested kidneys in several birds. Antibiotic treatment and vitamin D₃ supplementation were commenced, and the feed was replaced. The mortality and culling rate remained elevated for several more days but were confined to the two affected houses. Bacteriologic examination yielded no significant findings. Undersized birds were examined by necropsy 4 days later to investigate deteriorating uniformity. Grossly widened tibiotarsal growth plates were apparent, with suspected osteomyelitis or thickened proventriculi in several birds. Histologic examination of tibiotarsal growth plates revealed mild osteodystrophy and tibial dyschondroplasia. Mineralization or calcium deposits or both were identified histologically and confirmed histochemically within the koilin cuticle of the gizzard, the kidneys, and the proventriculus. No significant findings were noted in the brain, tendon, skeletal muscle, heart, and peripheral nerves. Feed analysis from the initial starter ration revealed that calcium and phosphorous levels were, respectively, approximately three and four times in excess of target levels, with a reduced calcium:phosphorous ratio. The culling rate for lameness and being undersized remained elevated for the duration of the crop, with high overall mortalities of approximately 15%. This is an unusual case of very early lameness due to rickets with metastatic visceral mineralization. The staining characteristics of the mineral deposits also provide interesting clues regarding their chemical nature.