JPC SYSTEMIC PATHOLOGY

MUSCULOSKELETAL SYSTEM

April 2025

M-T04

 

SIGNALMENT (JPC #1947713): A male Sprague-Dawley rat

HISTORY: This rat was on a chronic 2-year oral toxicity study. Malocclusion and dental fractures were common in treated animals. The surfaces of the incisors were white, thickened or pitted. Frequently, the nasal, maxillary, frontal and parietal bones of treated rats were white, thickened, and rough compared to those of control animals. The changes were bilaterally symmetrical.

HISTOPATHOLOGIC DESCRIPTION: Skull, cross section through maxillary sinuses: Bilaterally, teeth contain disorganized cuboidal to columnar ameloblasts that occasionally pile up 4-5 layers thick and line an enamel layer that has scalloped, fibrillar borders, increased clear space separating the enamel matrix, and numerous variably sized, up to 10µm diameter, basophilic globules. There are multifocal areas with thinning of enamel. The predentin and dentin layers are irregular, thickened 2‑3 times normal, and contain numerous variably sized, up to 20µm diameter, coalescing, basophilic globules. The odontoblastic layer is thickened up to 2 times normal (piling up to 7 layers thick) with increased numbers of vacuolated odontoblasts. Ameloblasts and odontoblasts occasionally contain similar basophilic globules. Diffusely, trabecular bone of the maxilla is mildly thickened (osteosclerosis), the matrix is disorganized with numerous basophilic resting and reversal lines reminiscent of woven bone, the bone has scalloped borders lined by osteoblasts, and there are numerous basophilic globules within the matrix. The medullary spaces are small and contain increased fibrous connective tissue. Multifocally, the subepithelial connective tissue of the floor of the maxillary sinuses contains small aggregates of lymphocytes and plasma cells that form follicles (nasal associated lymphoid tissue).

MORPHOLOGIC DIAGNOSIS: 1. Teeth: Dysplasia, ameloblastic and odontoblastic, chronic, diffuse, moderate, with enamel loss and malformation, dentin malformation, and basophilic globular material within ameloblasts, odontoblasts, dentin, and enamel, Sprague-Dawley rat, rodent.

2. Maxilla: Osteosclerosis, chronic, diffuse, mild, with intratrabecular basophilic globular material.

ETIOLOGIC DIAGNOSIS: Dental fluorosis and osteofluorosis

CONDITION: Fluoride toxicity (fluorosis)

GENERAL DISCUSSION:

• Fluorine: Essential trace element; toxicity can cause characteristic dental and bony changes; ameloblasts and odontoblasts are highly sensitive to fluorine, osteoblasts also affected
  • In excess, fluorine leads to abnormal bone (or tooth) matrix which mineralizes abnormally–end result is a weakened tooth/bone
  • Dental lesions occur when teeth are in the developmental stage; the placental barrier partially protects decidual teeth prevents fluorides from accumulating
  • Principal changes in teeth include “chalky” areas, “mottling”, excessive attrition, and hypoplastic pitting of the enamel
• Fluorosis (chronic fluoride toxicity) most commonly occur in herbivores; cattle are more susceptible than sheep and horses
• Sources of fluorine include pastures or forage contaminated by airborne residues from aluminum manufacturers, phosphate refineries, and similar industrial installations (fluorine uptake by plants); feed supplements, rock phosphates as “licks”, and mineral mixtures with excessive fluoride; drinking well water containing soluble fluorides > 10 ppm; ash/dust from volcanic eruptions contains abundant fluorides
• Fluoride toxicity is enhanced by poor nutrition; alleviated by high dietary intakes of calcium, aluminum, and/or selenium

PATHOGENESIS:

• Fluorine is incorporated into bone matrix during mineralization and will therefore mostly affect young animals at sites of active formation; however, in older animals, fluorine deposition can occur during remodeling; exposure of cattle after 3 years of age typically results in osteofluorosis and not dental fluorosis
• Acute toxicosis results from accidental massive ingestion of fluoride compounds, inhalation of gas anesthetics (methoxyflurane or halothane), and ingestion of rodenticides (dogs/cats) or anti-parasitic compounds (swine/poultry)
• Gastrointestinal irritation (hydrofluoric acid)
• Tetany and hyperesthesia (altered membrane ion exchange)
• Formation of calcium fluoride depletes serum calcium > inhibition of blood coagulation
• Chronic toxicosis results from ingestion of small amounts of fluoride compounds over long periods
• Fluoride is removed rapidly from the blood by renal excretion and deposited in bones and teeth; a small amount is deposited in soft tissues
• Teeth
  • Ingestion of toxic concentrations during odontogenesis (which occurs at 6 to 36 months of age) > incorporation of fluoride into enamel and dentin
  • Fluoride disrupts ameloblasts > irregular enamel matrix, rate of formation, and rate of mineralization > outer layer of enamel is hypomineralized
  • Odontoblasts are also damaged, yet retain the ability to produce dentin after a tooth is fully formed > production of secondary dentin in an attempt to compensate for rapid tooth wear
  • Oxidation of exposed dentin results in dark discoloration
• Bone: Mechanism is unknown
• Can lead to osteosclerosis, osteoporosis, hyperostosis, osteophytosis, or osteomalacia (dependent on amount of fluoride ingested) with resultant abnormal bone formation, altered mechanical properties, increased remodeling, and increased resorption of bone
• Level of toxicity influences osteoblast stimulation (uptake and use in hydroxyapatite); higher levels may be directly toxic
• Fetal fluorosis may develop from fluorine crossing the placental barrier

TYPICAL CLINICAL FINDINGS:

• Acute Intoxication:
• Gastroenteritis, convulsions, lethargy, collapse, death
• Chronic Intoxication:
• Mottling and abrasion of teeth
• Intermittent lameness, shifting lameness, loss of production
• Periosteal hyperostosis (demonstrated via radiography)
• > 6 ppm fluorine in the urine

TYPICAL GROSS FINDINGS:

• Acute intoxication:

Gastrointestinal ulceration and other nonspecific gross lesions

• Chronic intoxication: Pathognomonic lesions involve the teeth and bones
• Teeth: (Note: Dental lesions develop only when fluorosis is present and the tooth is being formed; once the tooth is fully formed, it is not affected by fluorosis)
  • In mild cases, presence of small foci with a dry, chalky appearance compared to the normal glistening surface of the enamel
  • Hypoplastic enamel evidenced by punctuate pits or horizontal grooves
  • Excessive attrition
  • In severe cases, all enamel in affected teeth may be chalky, opaque, and show varying degrees of yellow, dark brown, or black discoloration (virtually pathognomonic for fluorosis)
  • Lesions are bilateral and usually affect incisors first, followed by premolars and molars
  • Teeth exhibit enamel hypoplasia, irregular wear, malocclusion, and fractures
  • Lesions may develop in the deciduous teeth of calves exposed during gestation
  • Bone lesions (“osteofluorosis”):
  • Lesions are generalized but not uniform
  • Lesions occur rapidly with high levels of exposure
  • Lesions occur first on the medial surface of the proximal third of the metatarsal and later on the mandible, metacarpals, and ribs
  • Pelvis, vertebrae, and other bones of the distal pelvis are also affected
  • Chalky-white and brittle bones with increased outer circumference due to periosteal hyperostosis in severe cases
  • In chronically infected animals, fracture of the medial claw is common, leading to lameness and a “cross-legged” stance
  • Typically, no joint involvement

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Acute intoxication: Gastrointestinal ulceration and necrosis, nephrosis, and other nonspecific microscopic changes
• Chronic intoxication
• Teeth:
  • Ameloblasts are disorganized and small, with enamel malformation, and hypoplasia
  • Outer layer of enamel is hypomineralized; incremental lines in the enamel are disrupted and the normal subsurface pigment band of bovine incisors is distorted
  • Odontoblasts are disorganized and vacuolated and have excessive production of predentin and globular dentin
  • Fibrosis of pulp cavity with ectopic bone formation
  • Cementum hyperplasia
  • Bone:
  • Lesions are variable and are often influenced by age, species, fluoride compound, and toxic dose
  • Mottled osteons and lamellae with abnormal and tangled osteocytes
  • Ground bone sections may display brown discoloration of osteons, similar to enamel
  • Sections of cortex may have a mottled appearance with lamellae in some osteons showing discoloration with others appearing normal
  • Osteosclerotic changes: Bony trabeculae are thick, dense, and have sharp heavy outlines, with reduced inter-trabecular marrow spaces
  • Osteoporotic changes: Endosteal resorption enlarges the medullary cavity and may involve the laminar periosteal bone; this may result in hyperostosis

ADDITIONAL DIAGNOSTIC TESTS:

• Radiographs
• Serum and urine fluoride levels

DIFFERENTIAL DIAGNOSIS:

• Discolored teeth in cattle:
  • Congenital porphyria: Deficiency of uroporphyrinogen III cosynthetase results in production of uroporphyrinogen I and coproporphyrinogen I (photodynamic); teeth are pink; not discolored or mottled, pitted, or eroded
  • Tetracycline administration: especially during mineralization; toxic to ameloblasts; deposited into dentin during mineralization; yellow teeth
  • Osteogenesis imperfecta: Defect in dentin production with marked thinning of dentin and irregular dentinal tubule formation; teeth are pink due to the visibility of dental pulp through the thin crown
• Bone lesions in cattle: Osteomyelitis, osteoarthritis, and septic arthritis all affect the articular surface (osteofluorosis does not)

COMPARATIVE PATHOLOGY:

• Guinea pigs fed diets high in fluorine:
  • Vitamin C and calcium deficiencies may aggravate condition
  • Deformed and irregular molars and incisors cause profuse salivation (“Australian slobbers”); brittle bones and exostosis
  • Lesions: Impaired dentin and enamel formation, excessive wear
• Dogs:
  • Ingestion of rodenticide sodium monofluoroacetate (Compound 1080)
  • Overdose with fluorinated anesthetic gases (methoxyflurane or halothane)
• Large animal:
  • Acute fluoride toxicosis is rare; chronic fluoride toxicosis (fluorosis) results in dental abnormities (more frequently in cattle) and osteofluorosis as previously described in addition to spontaneous fractures in ruminants
  • Common sources of exposure: Contaminated soil, water, forage, supplements in excess, volcanic fluoride-containing ash contaminating environment
    • Sodium monofluoroacetate (sodium fluoroacetate or 1080 in its pesticide form) is a derivative of fluoroacetic acid (organofluorine compound); this is a highly regulated pesticide that occasionally poisons animals; major lesions are cardiac hemorrhage and myofiber degeneration
• Pigs, chickens: May result from improper use of sodium fluoride anti-parasitic compounds used to treat internal parasites (swine) and external parasites (chickens)
• Pet birds: Chronic fluorine toxicity causes osteopetrosis
  • Gross lesions: Thickened cortical bone, obliteration of marrow cavity
  • Microscopic lesions: Similar to vitamin A and D deficiency (which are not documented in pet birds)
• Young growing dogs and pigs (and presumably other species): Lesions resemble rickets due to the inhibition of mineralization when fluoride is present in high doses; ends of long bones and the costochondral junctions are enlarged while the physes are increased in depth, softer than normal, and yield to the pressure of weight bearing
• Horses: Case report of chronic dental and skeletal fluoride toxicosis in two horses exposed to fluorosis from consumption of well water (Kelly et al., JVDI 2020)
• Rabbits: Fluoride toxicosis results in characteristic osseous proliferations of the extremities and mandible
  • Microscopic lesions: Periosteal and endosteal hyperostosis, marked (adenomatous) proliferation of gastric mucosa and marked proliferation of branching villous projections in the gastric and duodenal mucosa
  • Reported in growing rabbits in two European rabbitries from increased fluoride levels in pelleted feed; outbreaks have been reported in Mexico
• Rodentia (to include hamsters): Pathognomonic periosteal hyperostosis occurs first along the metatarsals (medial) and progress to the mandible, metacarpals, ribs and may affect all bones in severe/chronic cases
  • Gross: Brittle, chalky white bones, pathological fractures (joints spared of pathology); enamel hypoplasia during tooth development (dull, dry, chalky, hypomineralized and discolored teeth with pitting and grooves) and excessive wearing first affecting incisors
  • Microscopic lesions: Small ameloblasts, disorganized and vacuolated osteoblasts; abundant globular dentin and hypomineralization of outer enamel layer
  • Hamster Parvovirus (HaPV) is differential diagnosis for enamel hypoplasia in young Syrian hamsters; concurrent testicular hypoplasia and cerebral malacia
• Reported in a hyrax fed guinea pig chow suggesting species sensitivity
  • Gross: Red-brown discoloration of bone and teeth
  • Microscopic lesions: Periosteal new bone formation, enlargement of medullary cavity, replacement with coarse woven cancellous bone
• Bats: Reported in captive Rousettus aegyptiacus, Pteropus giganteus, and poliocephalus associated with high levels of fluoride in the diet
  • Gross lesion: Periarticular, asymmetric bony proliferations of the wings (most common), hindlimbs, sternum, ribs and mandible
  • Microscopic lesions: Subperiosteal nodules of new woven bone well demarcated from the underlying cortical bone with some degree of resorption
  • Differential diagnosis for bony proliferative on the leading edge of the wing lesions in captive bats is trauma
• Frogs: Fluorosis has been considered a contributor a metabolic bone disease-like lesions in those exposed to a water supply with added fluoride
• Cervidae: Susceptible to fluorosis through forage and drinking water contaminated with high levels of fluoride in the soil by rain splash or wind, or volcanic ashes (tephra); gross and microscopic lesions are similar to what has been previously described; characteristic gross lesions in teeth and bones, and high levels of fluoride in bone confirms a diagnosis of fluorosis
• Marsupials (Eastern Grey Kangaroos): Formation of multiple, large, smooth exostoses over the diaphysis of long bones (especially the tibia, fibula, and metatarsi) and degenerative joint disease most commonly affecting the knee, hock, and metatarsophalangeal joints (periarticular osteophytes, articular cartilage erosion/ulceration, synovial hyperplasia, and joint capsule fibrosis); animals displayed similar changes in the enamel as other species (discoloration, loss or hypoplasia)
• NHP:
  • Lesions with experimental fluorosis in rhesus macaques include abnormal bone if deficient in L-ascorbic acid
  • Studies with fluoride revealed that this mineral:
    • Can protect against osteoporosis
    • Can lead to osteomalacia in combination with dietary calcium deficiency

REFERENCES:

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2. Barthold SW, Griffey SM, Percy DH. Pathology of Laboratory Rodents and Rabbits. 4th ed. Ames, IA: Blackwell Publishing; 2016: 246, 315.
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