History

A 1-week-old female commercial brown chick died following a sudden onset of lethargy and weakness. The carcass was submitted for necropsy. The chick was 1 of 300 similarly affected 1-week-old chicks that died in a flock of 37,000 organic laying hens. Multiple other chick carcasses were also submitted for evaluation. Among the affected chicks, clinical signs developed over a 2-day period. All affected chicks were lethargic, had head tilts that progressed to paralysis, and ultimately died. No clinical signs among the chicks upon their arrival from the hatchery 5 days earlier were reported.

Clinical and Gross Findings

On gross evaluation, the chick was emaciated with a Gregory-Robbins body condition score of 1/3 (determined on the basis of keel protuberance and the development of breast muscles).¹ The long bones were pliable (Figure 1) and fractured easily. The beak and claws were similarly pliable. Internal examination revealed a soft keel, ribs, and vertebral column. The crop and gizzard were full of feed material. No gross enlargement of the thyroid or parathyroid glands was present. There were no other notable findings during the gross evaluation. The other chicks submitted for necropsy had similarly pliable bony structures.

Photograph of the tarsometatarsus of an approximately 1-week-old commercial brown chick that died within 2 days after a sudden onset of lethargy and weakness. Notice that the bone is pliable and easily bends at the diaphysis when manual pressure is applied. Bar = 1 cm.

Citation: Journal of the American Veterinary Medical Association 258, 10; 10.2460/javma.258.10.1087

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Photograph of the tarsometatarsus of an approximately 1-week-old commercial brown chick that died within 2 days after a sudden onset of lethargy and weakness. Notice that the bone is pliable and easily bends at the diaphysis when manual pressure is applied. Bar = 1 cm.

Citation: Journal of the American Veterinary Medical Association 258, 10; 10.2460/javma.258.10.1087

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Photograph of the tarsometatarsus of an approximately 1-week-old commercial brown chick that died within 2 days after a sudden onset of lethargy and weakness. Notice that the bone is pliable and easily bends at the diaphysis when manual pressure is applied. Bar = 1 cm.

Citation: Journal of the American Veterinary Medical Association 258, 10; 10.2460/javma.258.10.1087

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Histopathologic Findings

Histologically, the physeal cartilage of all examined long bones was diffusely expanded with a wide zone of hypertrophy, and trabeculae of cartilage extended deep into the metaphysis, compared with histologic features of long bone specimens from an unaffected, age-matched bird that died of unrelated causes (Figure 2). Cartilage and bony trabeculae were multifocally surrounded by a moderate amount of homogeneous, pink-staining, amorphous material (osteoid seams). The periosteum was diffusely thickened by fibrous connective tissue. A modified tetrachrome special stain was applied to differentiate between mature mineralized bone (red appearance) and osteoid (dark blue appearance). There was little red-staining mature lamellar bone in the metaphysis and diaphysis (Figure 3). Numerous retained cartilaginous trabeculae were present, extending deep into the medulla of the metaphysis. The cartilaginous trabeculae were intermittently lined by blue-staining osteoid material. Multifocally throughout the examined sections, there were multinucleated osteoclasts (≤ 8 in a single hpf [400×]) surrounding scalloped trabeculae of bone. This was in contrast to the findings from a healthy chick of the same age, which had considerably more mature lamellar bone and increased structural organization within the metaphysis. Sections of the vertebrae of the affected chicks were not examined.

Photomicrographs of decalcified sections of the tarsometatarsus at the level of the physis and metaphysis from the chick in Figure 1 (A) and an unaffected age-matched healthy chick (for comparison; B). In a section from the affected chick, the zone of hypertrophy is thickened with multifocal retained cartilaginous trabeculae that extend into the underlying metaphysis and are surrounded by a moderate amount of homogeneous, amorphous material. In both panels, H&E stain; bar = 1,000 μm.

Citation: Journal of the American Veterinary Medical Association 258, 10; 10.2460/javma.258.10.1087

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Photomicrographs of decalcified sections of the tarsometatarsus at the level of the physis and metaphysis from the chick in Figure 1 (A) and an unaffected age-matched healthy chick (for comparison; B). In a section from the affected chick, the zone of hypertrophy is thickened with multifocal retained cartilaginous trabeculae that extend into the underlying metaphysis and are surrounded by a moderate amount of homogeneous, amorphous material. In both panels, H&E stain; bar = 1,000 μm.

Citation: Journal of the American Veterinary Medical Association 258, 10; 10.2460/javma.258.10.1087

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Photomicrographs of decalcified sections of the tarsometatarsus at the level of the physis and metaphysis from the chick in Figure 1 (A) and an unaffected age-matched healthy chick (for comparison; B). In a section from the affected chick, the zone of hypertrophy is thickened with multifocal retained cartilaginous trabeculae that extend into the underlying metaphysis and are surrounded by a moderate amount of homogeneous, amorphous material. In both panels, H&E stain; bar = 1,000 μm.

Citation: Journal of the American Veterinary Medical Association 258, 10; 10.2460/javma.258.10.1087

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Photomicrographs of decalcified sections of the tarsometatarsus at the level of the physis and metaphysis from the chick in Figure 1 (A) and an unaffected age-matched healthy chick (for comparison; B). In the section from the affected chick, a modified tetrachrome stain reveals that there is little mature lamellar bone (areas stained bright red). Multifocal retained cartilaginous trabeculae are lined by a small amount of osteoid material (areas stained deep blue). Cartilage appears pale blue, and RBCs appear yellow-orange. In both panels, modified tetrachrome stain; bar = 500 μm.

Citation: Journal of the American Veterinary Medical Association 258, 10; 10.2460/javma.258.10.1087

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Photomicrographs of decalcified sections of the tarsometatarsus at the level of the physis and metaphysis from the chick in Figure 1 (A) and an unaffected age-matched healthy chick (for comparison; B). In the section from the affected chick, a modified tetrachrome stain reveals that there is little mature lamellar bone (areas stained bright red). Multifocal retained cartilaginous trabeculae are lined by a small amount of osteoid material (areas stained deep blue). Cartilage appears pale blue, and RBCs appear yellow-orange. In both panels, modified tetrachrome stain; bar = 500 μm.

Citation: Journal of the American Veterinary Medical Association 258, 10; 10.2460/javma.258.10.1087

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Photomicrographs of decalcified sections of the tarsometatarsus at the level of the physis and metaphysis from the chick in Figure 1 (A) and an unaffected age-matched healthy chick (for comparison; B). In the section from the affected chick, a modified tetrachrome stain reveals that there is little mature lamellar bone (areas stained bright red). Multifocal retained cartilaginous trabeculae are lined by a small amount of osteoid material (areas stained deep blue). Cartilage appears pale blue, and RBCs appear yellow-orange. In both panels, modified tetrachrome stain; bar = 500 μm.

Citation: Journal of the American Veterinary Medical Association 258, 10; 10.2460/javma.258.10.1087

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Morphologic Diagnosis and Case Summary

Morphologic diagnosis: failure of endochondral ossification with diaphyseal and metaphyseal cartilaginous trabeculae and periosteal fibrosis.

Case summary: hypovitaminosis D (rickets) in a 1-week-old commercial brown chicken.

Comments

In poultry, rickets is a common nutritional deficiency caused by vitamin D₃ or phosphorus imbalances in the diet that result in abnormal ossification and skeletal malformations in growing birds.² Feed formulation errors and unbalanced diets account for most outbreaks of rickets in flocks of young chicks.³ In the case described in the present report, development of rickets among chicks was attributed to the failure to add a vitamin mineral premix to the feed, and the issue resolved as soon as vitamin mineral premix was included in the chicks' diet. In many cases, it can be difficult to determine the causal nutritional deficiency of rickets; therefore, analysis of the feed for vitamin and mineral content is typically recommended to obtain a definitive diagnosis.⁴ In the case described in the present report, it was not possible to determine the causal nutritional deficiency of rickets solely from gross and histopathologic lesions; however, feed analysis confirmed a vitamin D deficiency.

In young birds, clinical signs of rickets develop in the first 1 to 4 weeks after hatching, and affected chicks have slow growth, progressive lameness, and occasionally swollen, enlarged hocks.^4,5,6 If deficiencies are not quickly corrected, death may occur shortly after clinical signs become apparent. Young broiler chicks are particularly at risk, and the morbidity rate may reach as high as 50% in affected flocks.⁷ Consistent with the gross findings in the bird of the present report, young affected birds have weak bones, beaks, and claws, all of which appear soft and rubbery and bend easily.⁵ Beading or thickening of the ribs at the costochondral junction, termed rachitic rosary, is considered a classic necropsy finding for this condition.³ In addition, the physes of long bones in affected animals appear segmentally enlarged and thickened and often replace the normal metaphyseal architecture.⁸ The parathyroid glands may also be enlarged in young birds or older layers.⁹ In contrast to young chicks, older laying hens with rickets typically develop cage layer fatigue, in which osteoporosis develops and causes pathological long bone fractures and soft-shelled eggs because of hypocalcemia.² Affected layers may be found lying on their sides with extended legs.

Vitamin D is found in several biologically available forms, but vitamin D₃ (cholecalciferol) is considered the best supplemental form to use in feed mixtures.⁷ Vitamin D₃ is the precursor of 1,25-dihydroxycho-lecalciferol, the hormone that stimulates calcium and phosphorus absorption in the small intestine.⁸ Poultry have the ability to synthesize vitamin D₃ in their nonfeathered skin areas but require adequate exposure to UV light to do so.² In most production environments, natural light is reduced; therefore, the birds must rely exclusively upon dietary sources.² The required amount of dietary vitamin D depends on several factors. The calcium-to-phosphorus concentration ratio of the diet, UV light exposure, and vitamin D concentration in the diet consumed by hens can all affect vitamin D₃ requirements of their offspring.¹⁰ According to the National Research Council, growing broilers and layers up to 18 weeks of age require at least 200 U of vitamin D₃/kg of feed; producing layers require at least 300 U of vitamin D₃/kg added to the diet.¹¹ Some studies have also found that increasing the amount to 4,000 U of vitamin D₃/kg of feed greatly reduces the presence of osteochondral disease in flocks.⁴

When animals have insufficient phosphorus to form hydroxyapatite, the mineral component of bone, exuberant amounts of osteoid accumulate as a reaction to normal biomechanical forces.⁸ Phosphorus deficiencies and hypovitaminosis D are associated with contrasting lesions. Whereas phosphorous deficiencies may affect the zone of hypertrophy in long bones, hypovitaminosis D is more likely to affect the zone of proliferation located closer to the physis. An adequate amount of phosphorus in the body is necessary to induce apoptosis of chondrocytes in the zone of hypertrophy.¹⁰ Therefore, a phosphorus deficiency is likely to result in retention of chondrocytes, thereby increasing the thickness of the cartilaginous tongues within the growth plate. Osteoid is the organic component of bone and is composed mostly of type 1 collagen and ground substance.⁷ Osteoid lacks the strength of mineralized bone; therefore, exuberant osteoid accumulation results in increased pliability of the long bones. Histologically, the failure of mineralization manifests as clusters of degenerated chondrocytes and persistent trabeculae of hypertrophic chondrocytes in sites of endochondral ossification with unmineralized osteoid covering irregular, thickened metaphyseal trabeculae. Incomplete fractures, or infractions, occasionally occur. Vitamin D deficiency may also result in fibrous osteodystrophy (in which secondary hyperparathyroidism causes increased osteoblast and osteoclast activities) and replacement of mineralized bone with fibrous connective tissue. A modified tetrachrome stain is a histochemical stain that differentiates among various tissues, including mineralized bone and unmineralized osteoid, and is useful for highlighting osteoid accumulation in cases of rickets.¹² For the chick of the present report, the modified tetrachrome staining of sections of long bone specimens revealed numerous seams of deep blue–staining osteoid surrounding the red-staining hyperplastic cartilaginous tongues and metaphyseal trabeculae. These results were consistent with the findings of the histologic evaluation of H&E-stained long bone sections.

In recent years in the United States, there have been precipitous increases in the numbers of backyard poultry flocks and small-scale, organic poultry operations, especially with the advent of the farm-to-table movement. In some instances, flock owners may forgo purchasing more expensive certified-organic feeds and instead elect to mix their own feeds or augment commercial feed with alternative nutritional sources. Such homemade diets are often nutritionally unbalanced and deficient. Therefore, when a veterinarian suspects a nutrition-associated disease in a flock, feed analysis should be performed to rule out mineral and vitamin deficiencies.² The increasing popularity of backyard poultry and the increasing number of households that view chickens as companion animals mean that many general practitioners may have clients who seek their advice regarding poultry nutrition. In these situations, general practitioners can provide simple advice to clients by suggesting the use of a balanced poultry feed appropriate for the age and use of the birds. Properly formulated commercial poultry feed can easily be found at any local animal feed store.