To determine the cause of severe megaloblastosis detected at birth and at four weeks in two unrelated infants their bone marrow and liver cells were studied. Both patients had abnormal deoxyuridine suppression tests, corrected to normal by 5-formyl tetrahydrofolic acid. Liver-cell homogenate from one patient had a previously undetectable level of dihydrofolate reductase restored to normal by high cation concentration in the assay. Activity of the liver-cell homogenate from the other patient, which was one quarter of the normal level, was restored to only half normal activity by high cation concentration. Dihydrofolic acid reductase deficiency prevents this conversion of folic acid to tetrahydrofolic acid; the enzyme activity appears to differ in each patient. A satisfactory clinical response in both patients followed parenteral therapy with 5-formyl tetrahydrofolic acid. One sibling in each family died of a similar illness. Autosomal recessive inheritance is probable.

Background Veterinarians are required to interpret the significance of radiographic findings for sale, soundness and future racing performance of weanling and yearling Thoroughbreds. We investigated the prevalence and radiographic appearance of slab fractures of the third (T3) and central tarsal (Tc) bones.Methods Weanling and yearling horses with complete or incomplete T3 or Tc fracture were identified by searching a database of radiographs. The prevalence and radiographic appearance at initial diagnosis and after continued pasture turnout, as well as prognosis for racing, of fractures of T3 and Tc were determined. ResultsFractures were identified in 186 tarsi (184 T3 fracture only, 1 Tc fracture only, 1 Tc and T3 fracture) of 157 horses (126 unilateral T3, 29 bilateral T3, 1 contralateral Tc and T3, 1 unilateral Tc and T3) from 7676 examinations. The prevalence of T3 and Tc fractures was 2.40 (95% CI 2.07, 2.76) and 0.04 (95% CI 0.01, 0.11) per 100 radiographic examinations respectively. Fractures were identified on the D556-65 MPlLO view and occurred by survey examination at 11.1 AE 1.3 months in 85.7% horses. At initial diagnosis, 84.3% of T3 fractures appeared incomplete and involved the distal articular surface. Fracture score improved (P < 0.001), and dorsal modelling (P < 0.001) and osteoarthritis score increased in the distal intertarsal joint (P < 0.001), but not the tarsometatarsal joint, between survey and repository examinations. Fractures healed by repository examination in 71.9% of tarsi if there was > 6 months between examinations. There was no difference in sale price, and horses with T3 fractures had fewer trials when 2 and 3 years old (P = 0.023), yet no difference in other parameters of racing success when 2 or 3 years old compared with controls.Conclusion Tarsal slab fractures can occur in juvenile Thoroughbreds and most heal with continued pasture turnout of > 6 months. Further investigation is required to determine risk factors and before making firm conclusions regarding the optimal management, prognosis for racing and long-term soundness. Figure 2. Radiographic views (D55-65 MPlLO) of the tarsus of weanling or yearling Thoroughbred horses with dorsal modelling of the third (T3) or central (Tc) tarsal bones described as (a) grade 0 (nil), (b) grade 1 (mild), (c) grade 2 (moderate) and (d) grade 3 (severe). Additional changes include: (c) a slab fracture of T3 with marked blurring of the fracture line (stage 3 healing) and (d) a complete Tc fracture. D55-65 MPlLO, dorso-55-65 medial-plantarolateral oblique.

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