Fluorosis

does not calcify normally, producing defects in the mature teeth. Cattle are susceptible to dental fluorosis during enamel matrix formation, from approximately 6 months to 3 years of age.

Fluoride accumulation in bone occurs over a prolonged time; osteofluorosis can eventually develop if excessive fluoride is ingested. In cattle the first palpable lesions occur on the medial surface of the proximal third of the metatarsal bones. Later, lesions can be palpated on the mandible, metacarpal bones, and ribs. Radiographically, osteofluorotic bones are thickened with a chalky, roughened, and irregular periosteal surface.

The presence of osteoporosis, osteosclerosis, hyperostosis, osteophytosis, or osteomalacia depends on the amount of fluoride ingested and the duration of exposure to fluorides. The articular surfaces are not involved in osteofluorosis and can be used to differentiate osteofluorosis from osteomyelitis, osteoarthritis, and septic arthritis. The osseous lesions eventually cause intermittent lameness and stiffness, which may affect feed intake, body condition, milk production, and reproduc­tion. Severe dental fluorosis causes reduced feed intake and efficiency, and affected animals are sometimes reluctant to drink cold water.

■ Clinical Pathology and Diagnosis Diagnosis of fluorosis is difficult and complicated by many factors that affect fluoride intake and deposition. Fluorosis may be suspected by history, clinical signs, and physical examination. Radiographic findings of bone disease without evidence of joint involvement are highly suggestive of fluorosis in the live animal. The fluoride concentration in the urine of cattle may help approximate recent fluoride exposure. The diagnostic value of urine fluoride analysis increases as the duration of excess fluoride ingestion increases. Normal cattle have a urine fluoride concentration of about 2 to 6 mg/L (2 to 6 parts per million [ppm]) or less than 10 mg/L (ppm). Cattle exhibiting moderate fluorosis have a urine fluoride concentration of about 15 to 20 mg/L (ppm). Cattle with urine fluoride concentrations of 40 mg/L (ppm) or greater or a urinary fluoride/creatinine ratio of 0.025 : 1 or greater could be suspected of ingesting a diet with a fluoride concentration of 60 ppm or greater.^5,7 The concentration of fluoride in bone is also quite helpful in the diagnosis of fluorosis (Table 38.2). Fluoride content in cancellous bone (e.g., rib, pelvis) is greater than in cortical bone. In addition, fluoride concentrations may vary in different areas of the same bone.

Fluoride concentration of bone is usually expressed as mg/ kg (ppm) of dry, fat-free bone. However, some bone samples are ashed before fluoride determination, so it is critical to note precisely which bone was sampled, how it was prepared, and what part of the bone was analyzed. The metatarsus and metacarpus are typically analyzed for fluoride content. For all practical purposes, fluoride concentration is equal in either of these bones from the same patient. Using sawdust from a longitudinal section of bovine metatarsus (dividing the bone into lateral and medial halves or dorsal and palmar halves) yields virtually the same fluoride concentration as the whole bone.⁸ The fluoride concentration of the fourteenth coccygeal vertebrae (ash basis) is approximately twice that of the meta­carpus (dry, fat-free weight basis). This can be a practical tool for clinical diagnosis in the live animal.

■ TABLE 38.2

Fluoride Concentration in Bones of Dairy Cattle Fed Various Levels of Sodium Fluoride^8,9

■ TABLE 38.3

Long-Term Tolerance of Dietary Fluoride for Cattle¹¹

ppm, mg/kg.

Analysis of dietary fluoride is a valuable adjunct to the diagnosis of fluorosis. The upper safe limit of fluoride in water for livestock is 2 mg/L (ppm).^10,11 This safe limit may not protect against fluorosis in all field situations because of the large number of variables involved in the pathogenesis of fluorosis. Table 38.3 lists the long-term dietary tolerances for cattle. Under field conditions, the clinician must consider all possible sources of fluoride ingestion in evaluating total intake. Because fluoride intake may be intermittent, a low dietary intake at one time may not necessarily eliminate a diagnosis of fluorosis.

■ Treatment and Prognosis No specific treatment is known for ruminants with severe fluorosis, and the prognosis is poor for cattle lame from extensive bony lesions. Animals removed from the offending diet or water may lose 50% of the fluoride from bone within 2 to 5 years, but severe dental damage is irreversible.

■ Prevention and Control Prevention involves avoiding feeds, water, and supplements with excessive fluoride concentra­tions. Feeding aluminum sulfate at 0.5% of the total diet reduces bone fluoride storage by 30% to 40%. However, additional phosphorus must be supplied in the diet, or osteoporosis and possibly spontaneous fractures may occur. Aluminum chloride or calcium aluminate also can be fed to cattle to reduce fluoride absorption. Calcium carbonate added to soils high in fluorides aids in reducing fluoride in forages. Cereal grains do not accumulate fluorides and thus can be helpful in reducing overall fluoride consumption. In circumstances of high fluoride concentrations in water, use of flood irrigation rather than sprinkler irrigation decreases the fluoride content of crops such as alfalfa hay.

The effects of varying levels of dietary protein on long-term fluoride exposure have been examined. In this study the feeding of low-protein diets (75% of recommended amounts) increased the bioavailability of fluoride, but there was no significant influence on the susceptibility to fluorosis by varying the dietary protein concentration from low (75%) to normal (100%) to high (125%).